5.1 Conditions for successful irrigation
5.2 Strategies for managing treated wastewater on the farm
5.3 Crop selection
5.4 Selection of irrigation methods
5.5 Field management practices in wastewater irrigation
5.6 Planning for wastewater irrigation
5.1.1 Amount of water to be applied
5.1.2 Quality of water to be applied
5.1.3 Scheduling of irrigation
5.1.4 Irrigation methods
5.1.5 Leaching
5.1.6 Drainage
Irrigation may be defined as the application of water to soil for the purpose of supplying the moisture essential for plant growth. Irrigation plays a vital role in increasing crop yields and stabilizing production. In arid and semi-arid regions, irrigation is essential for economically viable agriculture, while in semi-humid and humid areas, it is often required on a supplementary basis.
At the farm level, the following basic conditions should be met to make irrigated farming a success:
- the required amount of water should be applied;
- the water should be of acceptable quality;
- water application should be properly scheduled;
- appropriate irrigation methods should be used;
- salt accumulation in the root zone should be prevented by means of leaching;
- the rise of water table should be controlled by means of appropriate drainage;
- plant nutrients should be managed in an optimal way.
The above requirements are equally applicable when the source of irrigation water is treated wastewater. Nutrients in municipal wastewater and treated effluents are a particular advantage of these sources over conventional irrigation water sources and supplemental fertilizers are sometimes not necessary. However, additional environmental and health requirements must be taken into account when treated wastewater is the source of irrigation water.
It is well known that more than 99 percent of the water absorbed by plants is lost by transpiration and evaporation from the plant surface. Thus, for all practical purposes, the water requirement of crops is equal to the evapotranspiration requirement, ETc. Crop evapotranspiration is mainly determined by climatic factors and hence can be estimated with reasonable accuracy using meteorological data. An extensive review of this subject and guidelines for estimating ETc, prepared by Doorenbos and Pruitt, are given in Irrigation and Drainage Paper 24 (FAO 1977). A computer program, called CROPWAT, is available in FAO to determine the water requirements of crops from climatic data. Table 19 presents the water requirements of some selected crops, reported by Doorenbos and Kassam (FAO 1979). It should be kept in mind that the actual amount of irrigation water to be applied will have to be adjusted for effective rainfall, leaching requirement, application losses and other factors.
Irrigation water quality requirements from the point of view of crop production have been discussed in Chapter 2. The guidelines presented are indicative in nature and will have to be adjusted depending on the local climate, soil conditions and other factors. In addition, farm practices, such as the type of crop to be grown, irrigation method, and agronomic practices, will determine to a great extent the quality suitability of irrigation water. Some of the important farm practices aimed at optimizing crop production when treated sewage effluent is used as irrigation water will be discussed in this chapter.
Table 19: WATER REQUIREMENTS, SENSITIVITY TO WATER SUPPLY AND WATER UTILIZATION EFFICIENCY OF SOME SELECTED CROPS
|
Crop |
Water requirements (mm/growing period) |
Sensitivity to water supply (ky) |
Water utilization efficiency for harvested yield, Ey, kg/m3 (% moisture) |
|
Alfalfa |
800-1600 |
low to medium-high |
1.5-2.0 |
|
Banana |
1200-2200 |
high |
plant crop: 2.5-4 |
|
Bean |
300-500 |
medium-high |
lush: 1.5-2.0 (80-90%) |
|
Cabbage |
380-500 |
medium-low |
12-20 |
|
Citrus |
900-1200 |
low to medium-high |
2-5 |
|
Cotton |
700-1300 |
medium-low |
0.4-0.6 |
|
Groundnut |
500-700 |
low |
0.6-0.8 |
|
Maize |
500-800 |
high |
0.8-1.6 |
|
Potato |
500-700 |
medium-high |
4-7 |
|
Rice |
350-700 |
high |
0.7-1.1 |
|
Safflower |
600-1200 |
low |
0.2-0.5 |
|
Sorghum |
450-650 |
medium-low |
0.6-1.0 |
|
Wheat |
450-650 |
medium high |
0.8-1.0 |
Source: FAO(1979)
To obtain maximum yields, water should be applied to crops before the soil moisture potential reaches a level at which the evapotranspiration rate is likely to be reduced below its potential. The relationship of actual and maximum yields to actual and potential evapotranspiration is illustrated in the following equation:
(12)
where:
Ya = actual harvested yield
Ym = maximum harvested yield
ky = yield response factor
ETa = actual evapotranspiration
ETm = maximum evapotranspiration
Several methods are available to determine optimum irrigation scheduling. The factors that determine irrigation scheduling are: available water holding capacity of the soils, depth of root zone, evapotranspiration rate, amount of water to be applied per irrigation, irrigation method and drainage conditions.
Many different methods are used by farmers to irrigate crops. They range from watering individual plants from a can of water to highly automated irrigation by a centre pivot system. However, from the point of wetting the soil, these methods can be grouped under five headings, namely:
i. Flood irrigation - water is applied over the entire field to infiltrate into the soil (e.g. wild flooding, contour flooding, borders, basins, etc.).ii. Furrow irrigation - water is applied between ridges (e.g. level and graded furrows, contour furrows, corrugations, etc.). Water reaches the ridge, where the plant roots are concentrated, by capillary action.
iii. Sprinkler irrigation - water is applied in the form of a spray and reaches the soil very much like rain (e.g. portable and solid set sprinklers, travelling sprinklers, spray guns, centre-pivot systems, etc.). The rate of application is adjusted so that it does not create ponding of water on the surface.
iv. Sub-irrigation - water is applied beneath the root zone in such a manner that it wets the root zone by capillary rise (e.g. subsurface irrigation canals, buried pipes, etc.). Deep surface canals or buried pipes are used for this purpose.
v. Localized irrigation - water is applied around each plant or a group of plants so as to wet locally and the root zone only (e.g. drip irrigation, bubblers, micro-sprinklers, etc.). The application rate is adjusted to meet evapotranspiration needs so that percolation losses are minimized.
Table 20 presents some basic features of selected irrigation systems as reported by Doneen and Westcot (FAO 1988).
Table 20: BASIC FEATURES OF SOME SELECTED IRRIGATION SYSTEMS
|
Irrigation method |
Topography |
Crops |
Remarks |
|
Widely spaced borders |
Land slopes capable of being graded to less than 1 % slope and preferably 0.2% |
Alfalfa and other deep rooted close-growing crops and orchards |
The most desirable surface method for irrigating close-growing crops where topographical conditions are favourable. Even grade in the direction of irrigation is required on flat land and is desirable but not essential on slopes of more than 0.5%. Grade changes should be slight and reverse grades must be avoided. Cross slops is permissible when confined to differences in elevation between border strips of 6-9 cm. Water application efficiency 45-60%. |
|
Graded contour furrows |
Variable land slopes of 2-25 % but preferable less |
Row crops and fruit |
Especially adapted to row crops on steep land, though hazardous due to possible erosion from heavy rainfall. Unsuitable for rodent-infested fields or soils that crack excessively. Actual grade in the direction of irrigation 0.5-1.5%. No grading required beyond filling gullies and removal of abrupt ridges. Water application efficiency 50-65%. |
|
Rectangular checks (levees) |
Land slopes capable of being graded so single or multiple tree basins will be levelled within 6 cm |
Orchard |
Especially adapted to soils that have either a relatively high or low water intake rate. May require considerable grading. Water application efficiency 40-60%. |
|
Sub-irrigation |
Smooth-flat |
Shallow rooted crops such as potatoes or grass |
Requires a water table, very permeable subsoil conditions and precise levelling. Very few areas adapted to this method. Water application efficiency 50-70%. |
|
Sprinkler |
Undulating 1->35% slope |
All crops |
High operation and maintenance costs. Good for rough or very sandy lands in areas of high production and good markets. Good method where power costs are low. May be the only practical method in areas of steep or rough topography. Good for high rainfall areas where only a small supplementary water supply is needed. Water application efficiency 60-70 %. |
|
Localized (drip, trickle, etc.) |
Any topographic condition suitable for row crop farming |
Row crops or fruit |
Perforated pipe on the soil surface drips water at base of individual vegetable plants or around fruit trees. Has been successfully used in Israel with saline irrigation water. Still in development stage. Water application efficiency 75-85 %. |
Source: FAO (1988)
Under irrigated agriculture, a certain amount of excess irrigation water is required to percolate through the root zone so as to remove the salts which have accumulated as a result of evapotranspiration from the original irrigation water. This process of displacing the salts from the root zone is called leaching and that portion of the irrigation water which mobilizes the excess of salts is called the leaching fraction, LF.
(13)
Salinity control by effective leaching of the root zone becomes more important as irrigation water becomes more saline.
Drainage is defined as the removal of excess water from the soil surface and below so as to permit optimum growth of plants. Removal of excess surface water is termed surface drainage while the removal of excess water from beneath the soil surface is termed sub-surface drainage. The importance of drainage for successful irrigated agriculture has been well demonstrated. It is particularly important in semi-arid and arid areas to prevent secondary salinization. In these areas, the water table will rise with irrigation when the natural internal drainage of the soil is not adequate. When the water table is within a few metres of the soil surface, capillary rise of saline groundwater will transport salts to the soil surface. At the surface, water evaporates, leaving the salts behind. If this process is not arrested, salt accumulation will continue, resulting in salinization of the soil. In such cases, sub-surface drainage can control the rise of the water table and hence prevent salinization.
5.3.1 To overcome salinity hazards
5.3.2 To overcome toxicity hazards
5.3.3 To prevent health hazards
Success in using treated wastewater for crop production will largely depend on adopting appropriate strategies aimed at optimizing crop yields and quality, maintaining soil productivity and safeguarding the environment. Several alternatives are available and a combination of these alternatives will offer an optimum solution for a given set of conditions. The user should have prior information on effluent supply and its quality, as indicated in Table 21, to ensure the formulation and adoption of an appropriate on-farm management strategy.
Basically, the components of an on-farm strategy in using treated wastewater will consist of a combination of:
- crop selection,
- selection of irrigation method, and
- adoption of appropriate management practices.
Furthermore, when the farmer has additional sources of water supply, such as a limited amount of normal irrigation water, he will then have an option to use both the effluent and the conventional source of water in two ways, namely:
- by blending conventional water with treated effluent, and
- using the two sources in rotation.
These are discussed briefly in the following sections.
Table 21: INFORMATION REQUIRED ON EFFLUENT SUPPLY AND QUALITY
|
Information |
Decision on irrigation management |
|
Effluent supply | |
|
The total amount of effluent that would be made available during the crop growing season. |
Total area that could be irrigated. |
|
Effluent available throughout the year. |
Storage facility during non crop growing period either at the farm or near wastewater treatment plant, and possible use for aquaculture. |
|
The rate of delivery of effluent either as m3 per day or litres per second. |
Area that could be irrigated at any given time, layout of fields and facilities and system of irrigation. |
|
Type of delivery: continuous or intermittent, or on demand. |
Layout of fields and facilities, irrigation system, and irrigation scheduling. |
|
Mode of supply: supply at farm gate or effluent available in a storage reservoir to be pumped by the farmer. |
The need to install pumps and pipes to transport effluent and irrigation system. |
|
Effluent quality | |
|
Total salt concentration and/or electrical conductivity of the effluent. |
Selection of crops, irrigation method, leaching and other management practices. |
|
Concentrations of cations, such as Ca++, Mg++ and Na+. |
To assess sodium hazard and undertake appropriate measures. |
|
Concentration of toxic ions, such as heavy metals, Boron and Cl-. |
To assess toxicities that are likely to be caused by these elements and take appropriate measures. |
|
Concentration of trace elements (particularly those which are suspected of being phyto-toxic). |
To assess trace toxicities and take appropriate measures. |
|
Concentration of nutrients, particularly nitrate-N. |
To adjust fertilizer levels, avoid over-fertilization and select crop. |
|
Level of suspended sediments. |
To select appropriate irrigation system and measures to prevent clogging problems. |
|
Levels of intestinal nematodes and faecal coliforms. |
To select appropriate crops and irrigation systems. |
Not all plants respond to salinity in a similar manner; some crops can produce acceptable yields at much higher soil salinity than others. This is because some crops are better able to make the needed osmotic adjustments, enabling them to extract more water from a saline soil. The ability of a crop to adjust to salinity is extremely useful. In areas where a build-up of soil salinity cannot be controlled at an acceptable concentration for the crop being grown, an alternative crop can be selected that is both more tolerant of the expected soil salinity and able to produce economic yields. There is an 8-10 fold range in the salt tolerance of agricultural crops. This wide range in tolerance allows for greater use of moderately saline water, much of which was previously thought to be unusable. It also greatly expands the acceptable range of water salinity (ECw) considered suitable for irrigation.
The relative salt tolerance of most agricultural crops is known well enough to give general salt tolerance guidelines. Table 22 presents a list of crops classified according to their tolerance and sensitivity to salinity. Figure 13 presents the relationship between relative crop yield and irrigation water salinity with regard to the four crop salinity classes. The following general conclusions can be drawn from these data:
i. full yield potential should be achievable with nearly all crops when using a water with salinity less than 0.7 dS/m,ii. when using irrigation water of slight to moderate salinity (i.e. 0.7-3.0 dS/m), full yield potential is still possible but care must be taken to achieve the required leaching fraction in order to maintain soil salinity within the tolerance of the crops. Treated sewage effluent will normally fall within this group,
iii. for higher salinity water (more than 3.0 dS/m) and sensitive crops, increasing leaching to satisfy a leaching requirement greater than 0.25 to 0.30 might not be practicable because of the excessive amount of water required. In such a case, consideration must be given to changing to a more tolerant crop that will require less leaching, to control salts within crop tolerance levels. As water salinity (ECw) increases within the slight to moderate range, production of more sensitive crops may be restricted due to the inability to achieve the high leaching fraction needed, especially when grown on heavier, more clayey soil types,
Figure 13: Divisions for relative salt tolerance ratings of agricultural crops (Maas 1984)
Table 22: RELATIVE SALT TOLERANCE OF AGRICULTURAL CROPS
|
TOLERANT | |
|
Fibre, Seed and Sugar Crops | |
|
Barley |
Hordeum vulgare |
|
Cotton |
Gossypium hirsutum |
|
Jojoba |
Simmondsia chinensis |
|
Sugarbeet |
Beta vulgaris |
|
Grasses and Forage Crops | |
|
Alkali grass |
Puccinellia airoides |
|
Alkali sacaton |
Sporobolus airoides |
|
Bermuda grass |
Cynodon dactylon |
|
Kallar grass |
Diplachne fusca |
|
Saltgrass, desert |
Distichlis stricta |
|
Wheatgrass, fairway crested |
Agropyron cristatum |
|
Wheatgrass, tall |
Agropyron elongatum |
|
Wildrye, Altai |
Elymus angustus |
|
Wildrye, Russian |
Elymus junceus |
|
Vegetable Crops | |
|
Asparagus |
Asparagus officinalis |
|
Fruit and Nut Crops | |
|
Date palm |
Phoenix dactylifera |
|
MODERATELY TOLERANT | |
|
Fibre, Seed and Sugar Crops | |
|
Cowpea |
Vigna unguiculata |
|
Oats |
Avena sativa |
|
Rye |
Secale cereale |
|
Safflower |
Carthamus tinctorius |
|
Sorghum |
Sorghum bicolor |
|
Soybean |
Glycine max |
|
Triticale |
X Triticosecale |
|
Wheat |
Triticum aestivum |
|
Wheat, Durum |
Triticum turgidum |
|
Grasses and Forage Crops | |
|
Barley (forage) |
Hordeum vulgare |
|
Brome, mountain |
Bromus marginatus |
|
Canary grass, reed |
Phalaris, arundinacea |
|
Clover, Hubam |
Melilotus alba |
|
Clover, sweet |
Melilotus |
|
Fescue, meadow |
Festuca pratensis |
|
Fescue, tall |
Festuca elatior |
|
Harding grass |
Phalaris tuberosa |
|
Panic grass, blue |
Panicum antidotale |
|
Rape |
Brassica napus |
|
Rescue grass |
Bromus unioloides |
|
Rhodes grass |
Chloris gayana |
|
Grasses and Forage Crops | |
|
Ryegrass, Italian |
Lolium italicum multiflorum |
|
Ryegrass, perennial |
Lolium perenne |
|
Sudan grass |
Sorghum sudanense |
|
Trefoil, narrowleaf birdsfoot |
Lotus corniculatus tenuifolium |
|
Trefoil, broadleaf |
L. corniculatus arvenis |
|
Wheat (forage) |
Triticum aestivum |
|
Wheatgrass, standard crested |
Agropyron sibiricum |
|
Wheatgrass, intermediate |
Agropyron intermedium |
|
Wheatgrass, slender |
Agropyron trachycaulum |
|
Wheatgrass, western |
Agropyron smithii |
|
Wildrye, beardless |
Elymus triticoides |
|
Wildrye, Canadian |
Elymus canadensis |
|
Vegetable Crops | |
|
Artichoke |
Helianthus tuberosus |
|
Beet, red |
Beta vulgaris |
|
Squash, zucchini |
Cucurbita pepo melopepo |
|
Fruit and Nut Crops | |
|
Fig |
Ficus carica |
|
Jujube |
Ziziphys jujuba |
|
Olive |
Olea europaea |
|
Papaya |
Carica papaya |
|
Pineapple |
Ananas comosus |
|
Pomegranate |
Punica granatum |
|
MODERATELY SENSITIVE | |
|
Fibre, Seed and Sugar Crops | |
|
Broadbean |
Vicia faba |
|
Castorbean |
Ricinus communis |
|
Maize |
Zea mays |
|
Flax |
Linum usitatissimum |
|
Millet, foxtail |
Setaria italica |
|
Groundnut/peanut |
Arachis hypogaea |
|
Rice, paddy |
Oryza sativa |
|
Sugarcane |
Saccarum officinarum |
|
Sunflower |
Helianthus annuus palustris |
|
Grasses and Forage Crops | |
|
Alfalfa |
Medicago sativa |
|
Bentgrass |
Agrostisstoloniferapalustris |
|
Bluestem, Angleton |
Dichanthium aristatum |
|
Brome, smooth |
Bromus inermis |
|
Buffelgrass |
Cenchrus ciliaris |
|
Burnet |
Poterium sanguisorba |
|
Clover, alsike |
Trifolium hydridum |
|
Grasses and Forage Crops | |
|
Clover, Berseem |
Trifolium alexandrinum |
|
Clover, ladino |
Trifolium repens |
|
Clover, red |
Trifolium pratense |
|
Clover, strawberry |
Trifolium fragiferum |
|
Clover, white Dutch |
Trifolium repens |
|
Corn (forage) (maize) |
Zea mays |
|
Cowpea (forage) |
Vigna unguiculata |
|
Dallis grass |
Paspalum dilatatum |
|
Foxtail, meadow |
Alopecurus pratensis |
|
Grama, vlue |
Bouteloua gracilis |
|
Lovegrass |
Eragrostis sp. |
|
Milkvetch, Cicer |
Astragalus deer |
|
Oatgrass, tall |
Arrhenatherum, Danthonia |
|
Oats (forage) |
Avena saliva |
|
Orchard grass |
Dactylis glomerata |
|
Rye (forage) |
Secale cereale |
|
Sesbania |
Sesbania exaltata |
|
Siratro |
Macroptilium atropurpureum |
|
Sphaerophysa |
Spaerophysa salsula |
|
Timothy |
Phleum pratense |
|
Vetch, common |
Vicia angustifolia |
|
Vegetable Crops | |
|
Broccoli |
Brassica oleracea botrytis |
|
Brussel sprouts |
B. oleracea gemmifera |
|
Cabbage |
B. oleracea capitata |
|
Cauliflower |
B. oleracea botrytis |
|
Celery |
Apium graveolens |
|
Corn, sweet |
Zea mays |
|
Cucumber |
Cucumis sativus |
|
Eggplant |
Solanum melongena esculentum |
|
Kale |
Brassica oleracea acephala |
|
Kohlrabi |
B. oleracea gongylode |
|
Lettuce |
Latuca sativa |
|
Muskmelon |
Cucumis melon |
|
Pepper |
Capsicum annum |
|
Potato |
Solanum tuberosum |
|
Pumpkin |
Cucurbita peop pepo |
|
Radish |
Raphanus sativus |
|
Spinach |
Spinacia oleracea |
|
Squash, scallop |
C. pepo melopepo |
|
Sweet potato |
Ipomoea batatas |
|
Tomato |
Lycopersicon lycopersicum |
|
Turnip |
Brassica rapa |
|
Watermelon |
Citrullus lanatus |
|
Fruit and Nut Crops | |
|
Grape |
Vitis sp. |
|
SENSITIVE | |
|
Fibre, Seed and Sugar Crops | |
|
Bean |
Phaseolus vulgaris |
|
Guayule |
Parthenium argentatum |
|
Sesame |
Sesamum indicum |
|
Vegetable Crops | |
|
Bean |
Phaseolus vulgaris |
|
Carrot |
Daucus carota |
|
Okra |
Abelmoschus esculentus |
|
Onion |
Allium cepa |
|
Parsnip |
Pastinaca sativa |
|
Fruit and Nut Crops | |
|
Almond |
Prunus dulcis |
|
Apple |
Malus sylvestris |
|
Apricot |
Prunus armeniaca |
|
Avocado |
Persea americana |
|
Blackberry |
Rubus sp. |
|
Boysenberry |
Rubus ursinus |
|
Cherimoya |
Annona cherimola |
|
Cherry, sweet |
Prunus avium |
|
Cherry, sand |
Prunus besseyi |
|
Currant |
Ribes sp. |
|
Gooseberry |
Ribes sp. |
|
Grapefruit |
Citrus paradisi |
|
Lemon |
Citrus limon |
|
Lime |
Citrus aurantifolia |
|
Loquat |
Eriobotrya japonica |
|
Mango |
Mangifera indica |
|
Orange |
Citrus sinensis |
|
Passion fruit |
Passiflora edulis |
|
Peach |
Prunus persica |
|
Pear |
Pyrus communis |
|
Persimmon |
Diospyros virginiana |
|
Plum: Prune |
Prunus domestica |
|
Pummelo |
Citrus maxima |
|
Raspberry |
Rubus idaeus |
|
Rose apple |
Syzgium jambos |
|
Sapote, white |
Casimiroa edulis |
|
Strawberry |
Fragaria sp. |
|
Tangerine |
Citrus reticulata |
Source: FAO (1985)iv. if the salinity of the applied water exceeds 3.0 dS/m, the water might still be usable but its use may need to be restricted to more permeable soils and more salt-tolerant crops, where high leaching fractions are more easily achieved. This is being practised on a large scale in the Arabian Gulf States, where drip irrigation systems are widely used.
If the exact cropping patterns or rotations are not known for a new area, the leaching requirement must be based on the least tolerant of the crops adapted to the area. In those instances, where soil salinity cannot be maintained within acceptable limits of preferred sensitive crops, changing to more tolerant crops will raise the area's production potential. If there is any doubt about the effect of wastewater salinity on crop production, a pilot study should be undertaken to demonstrate the feasibility of irrigation and the outlook for economic success.
A toxicity problem is different from a salinity problem in that it occurs within the plant itself and is not caused by water shortage. Toxicity normally results when certain ions are taken up by plants with the soil water and accumulate in the leaves during water transpiration to such an extent that the plant is damaged. The degree of damage depends upon time, concentration of toxic material, crop sensitivity and crop water use and, if damage is severe enough, crop yield is reduced. Common toxic ions in irrigation water are chloride, sodium, and boron, all of which will be contained in sewage. Damage can be caused by each individually or in combination. Not all crops are equally sensitive to these toxic ions. Some guidance on the sensitivity of crops to sodium, chloride and boron are given in Tables 23, 24 and 25, respectively. However, toxicity symptoms can appear in almost any crop if concentrations of toxic materials are sufficiently high. Toxicity often accompanies or complicates a salinity or infiltration problem, although it may appear even when salinity is not a problem.
The toxic ions of sodium and chloride can also be absorbed directly into the plant through the leaves when moistened during sprinkler irrigation. This typically occurs during periods of high temperature and low humidity. Leaf absorption speeds up the rate of accumulation of a toxic ion and may be a primary source of the toxicity.
In addition to sodium, chloride and boron, many trace elements are toxic to plants at low concentrations, as indicated in Table 10 in Chapter 2. Fortunately, most irrigation supplies and sewage effluents contain very low concentrations of these trace elements and are generally not a problem.
However, urban wastewater may contain heavy metals at concentrations which will give rise to elevated levels in the soil and cause undesirable accumulations in plant tissue and crop growth reductions. Heavy metals are readily fixed and accumulate in soils with repeated irrigation by such wastewaters and may either render them non-productive or the product unusable. Surveys of wastewater use have shown that more than 85 % of the applied heavy metals are likely to accumulate in the soil, most at the surface. The levels at which heavy metals accumulation in the soil is likely to have a deleterious effect on crops are discussed in Chapter 5. Any wastewater use project should include monitoring of soil and plants for toxic materials.
From the point of view of human consumption and potential health hazards, crops and cultivated plants may be classified into the following groups:
Table 23: RELATIVE TOLERANCE OF SELECTED CROPS TO EXCHANGEABLE SODIUM
|
Sensitive |
Semi-tolerant |
Tolerant |
||||
|
Avocado |
Carrot |
Alfalfa |
||||
|
|
(Persea americana) |
|
(Daucus carota) |
|
(Medicago sativa) |
|
|
Deciduous Fruits |
Clover, Ladino |
Barley |
||||
|
Nuts |
|
(Trifolium repens) |
|
(Hordeum vulgare) |
||
|
Bean, green |
Dallisgrass |
Beet, garden |
||||
|
|
(Phaseolus vulgaris) |
|
(Paspalum dilatatum) |
|
(Beta vulgaris) |
|
|
Cotton (at germination) |
Fescue, tall |
Beet, sugar |
||||
|
|
(Gossypium hirsutum) |
|
(Festuca arundinacea) |
|
(Beta vulgaris) |
|
|
Maize |
Lettuce |
Bermuda grass |
||||
|
|
(Zea mays) |
|
(Lactuca sativa) |
|
(Cynodon dactylon) |
|
|
Peas |
Bajara |
Cotton |
||||
|
|
(Pisum sativum) |
|
(Pennisetum typhoides) |
|
|
(Gossypium hirsutum) |
|
Grapefruit |
Sugarcane |
Paragrass |
||||
|
|
(Citrus paradisi) |
|
(Saccharum officinarum) |
|
|
(Brachiaria mutica) |
|
Orange |
Berseem |
Rhodes grass |
||||
|
|
(Citrus sinensis) |
|
(Trifolium alexandrinum) |
|
(Chloris gayana) |
|
|
Peach |
Benji |
Wheatgrass, crested |
||||
|
|
(Prunus persica) |
|
(Mililotus parviflora) |
|
(Agropyron cristatum) |
|
|
Tangerine |
Raya |
Wheatgrass, fairway |
||||
|
|
(Citrus reticulata) |
|
(Brassica juncea) |
|
(agropyron cristatum) |
|
|
Mung |
Oat |
Wheatgrass, tall |
||||
|
|
(Phaseolus aurus) |
|
(Avena sativa) |
|
(Agropyron elongatum) |
|
|
Mash |
Onion |
Karnal grass |
||||
|
|
(Phaseolus mungo) |
|
(Allium cepa) |
|
(Diplachna fusca) |
|
|
Lentil |
Radish |
|
||||
|
|
(Lens culinaris) |
|
(Raphanus sativus) |
|||
|
Groundnut (peanut) |
Rice |
|||||
|
|
(Arachis hypogaea) |
|
(Oryza sativus) |
|||
|
Gram |
Rye |
|||||
|
|
(Cicer arietinum) |
|
(Secale cereale) |
|||
|
Cowpeas |
Ryegrass, Italian |
|||||
|
|
(Vigna sinensis) |
|
(Lolium multiflorum) |
|||
|
|
Sorghum |
|||||
|
|
(Sorghum vulgare) |
|||||
|
Spinach |
||||||
|
|
(Spinacia oleracea) |
|||||
|
Tomato |
||||||
|
|
(Lycopersicon esculentum) |
|||||
|
Vetch |
||||||
|
|
(Vicia sativa) |
|||||
|
Wheat |
||||||
|
|
(Triticum vulgare) |
|||||
Source: Adapted from data of FAO-Unesco (1973); Pearson (1960); and Abrol (1982).
i. Food crops
- those eaten uncooked
- those eaten after cooking
ii. Forage and feed crops
- direct access by animals
- those fed to animals after harvesting
Table 24: CHLORIDE TOLERANCE OF SOME FRUIT CROP CULTIVARS AND ROOTSTOCKS
|
Crop |
Rootstock or Cultivar |
Maximum permissible Cl- without leaf injury1 |
|
|
Root zone (Cle) (me/l) |
Irrigation water (Clw)2 3 (me/l) |
||
|
|
Rootstocks |
|
|
|
Avocado (Persea americana) |
West Indian |
7.5 |
5.0 |
|
Guatemalan |
6.0 |
4.0 |
|
|
Mexican |
5.0 |
3.3 |
|
|
Citrus (Citrus spp.) |
Sunki Mandarin |
25.0 |
16.6 |
|
Grapefruit |
|
|
|
|
Cleopatra mandarin |
|
|
|
|
Rangpur lime |
|
|
|
|
|
|||
|
Sampson tangelo |
15.0 |
10.0 |
|
|
Rough lemon |
|
|
|
|
Sour orange |
|
|
|
|
Ponkan mandarin |
|
|
|
|
|
|||
|
Citrumelo 4475 |
10.0 |
6.7 |
|
|
Trifoliate orange |
|
|
|
|
Cuban shaddock |
|
|
|
|
Calamondin |
|
|
|
|
Sweet orange |
|
|
|
|
Savage citrange |
|
|
|
|
Rusk citrange |
|
|
|
|
Troyer citrange |
|
|
|
|
Grape(Vitis spp.) |
Salt Creek, 1613-3 |
40.0 |
27.0 |
|
Dog Ridge |
30.0 |
20.0 |
|
|
Stone Fruits (Prunus spp.) |
Marianna |
25.0 |
17.0 |
|
Lovell, Shalil |
10.0 |
6.7 |
|
|
Yunnan |
7.5 |
5.0 |
|
|
Cultivars |
|
|
|
|
Berries (Rubus spp.) |
Boysenberry |
10.0 |
6.7 |
|
Olallie clackberry |
10.0 |
6.7 |
|
|
Indian SUmmer |
5.0 |
3.3 |
|
|
Raspberry |
|
|
|
| Grape(Vitis spp.) |
Thompson seedless |
20.0 |
13.3 |
|
Perlette |
20.0 |
13.3 |
|
|
Cardinal |
10.0 |
6.7 |
|
|
Black Rose |
10.0 |
6.7 |
|
|
Strawberry (Fragaria spp.) |
Lassen |
7.5 |
5.0 |
|
Shasta |
5.0 |
3.3 |
|
1 For some crops, the concentration given may exceed the overall salinity tolerance of that crop and cause some reduction in yield in addition to that caused by chloride ion toxicities.2 Values given are for the maximum concentration in the irrigation water. The values were derived from saturation extract data (ECe) assuming a 15-20 percent leaching fraction and ECd = 1.5 ECw.
3 The maximum permissible values apply only to surface irrigated crops. Sprinkler irrigation may cause excessive leaf bum at values far below these.
Source: Adapted from Maas (1984).
Table 25: RELATIVE BORON TOLERANCE OF AGRICULTURAL CROPS1
|
VERY SENSITIVE (<0.5 mg/l) | |
|
Lemon |
Citrus limon |
|
Blackberry |
Rubus spp. |
|
SENSITIVE (0.5-0.75 mg/l) | |
|
Avocado |
Persea americana |
|
Grapefruit |
Citrus X paradisi |
|
Orange |
Citrus sinensis |
|
Apricot |
Prunus armeniaca |
|
Peach |
Prunus persica |
|
Cherry |
Prunus avium |
|
Plum |
Prunus domestica |
|
Persimmon |
Diospyros kaki |
|
Fig, kadota |
Ficus carica |
|
Grape |
Vitis vinifera |
|
Walnut |
Juglans regia |
|
Pecan |
Carya illinoiensis |
|
Cowpea |
Vigna unguiculata |
|
Onion |
Allium cepa |
|
SENSITIVE (0.75-1.0 mg/l) | |
|
Garlic |
Allium sativum |
|
Sweet potato |
Ipomoea batatas |
|
Wheat |
Triticum eastivum |
|
Barley |
Hordeum vulgare |
|
Sunflower |
Helianthus annuus |
|
Bean, mung |
Vigna radiata |
|
Sesame |
Sesamum indicum |
|
Lupine |
Lupinus hartwegii |
|
Strawberry |
Fragaria spp. |
|
Artichoke, Jerusalem |
Helianthus tuberosus |
|
Bean, kidney |
Phaseolus vulgaris |
|
Bean, lima |
Phaseolus lunatus |
|
Groundnut/Peanut |
Arachis hypogaea |
|
MODERATELY SENSITIVE (1.0-2.0 mg/l) | |
|
Pepper, red |
Capsicum annuum |
|
Pea |
Pisum sativa |
|
Carrot |
Daucus carota |
|
Radish |
Raphanus sativus |
|
Potato |
Solanum tuberosum |
|
Cucumber |
Cucumis sativus |
|
MODERATELY TOLERANT (2.0-4.0 mg/l) | |
|
Lettuce |
Lactuca sativa |
|
Cabbage |
B. oleracea capitata |
|
Celery |
Apium graveolens |
|
Turnip |
Brassica rapa |
|
Bluegrass, Kentucky |
Poa pratensis |
|
Oats |
Avena sativa |
|
Maize |
Zea mays |
|
Artichoke |
Cynara scolymus |
|
Tobacco |
Nicotiana tabacum |
|
Mustard |
Brassica juncea |
|
Clover, sweet |
Melilotus indica |
|
Squash |
Cucurbita pepo |
|
Muskmelon |
Cucumis melo |
|
TOLERANT (4.0-6.0 mg/l) | |
|
Sorghum |
Sorghum bicolor |
|
Tomato |
L. lycopersicum |
|
Alfalfa |
Medicago sativa |
|
Vetch, purple |
Vicia benghalensis |
|
Parsley |
Petroselinum crispum |
|
Beet, red |
Beta vulgaris |
|
Sugarbeet |
Beta vulgaris |
|
VERY TOLERANT (6.0-15.0 mg/l) | |
|
Cotton |
Gossypium hirsutum |
|
Asparagus |
Asparagus officinalis |
1 Maximum concentrations tolerated in soil water without yield or vegetative growth reductions. Boron tolerances vary depending upon climate, soil conditions and crop varieties. Maximum concentrations in the irrigation water are approximately equal to these values or slightly less.Source: Maas (1984)
iii. Landscaping plants:
- unprotected areas with public access
- semi-protected areas
iv. Afforestation plants:
- commercial (fruit, timber, fuel and charcoal)
- environmental protection (including sand stabilization)
In terms of health hazards, treated effluent with a high microbiological quality is necessary for the irrigation of certain crops, especially vegetable crops eaten raw, but a lower quality is acceptable for other selected crops, where there is no exposure to the public (see Table 8 in Chapter 2). The WHO (1989) Technical Report No. 778 suggested a categorization of crops according to the exposed group and the degree to which health protection measures are required, as shown in Example 4.
|
EXAMPLE 4 - CATEGORIZATION OF CROPS IN RELATION TO EXPOSED GROUP AND HEALTH CONTROL MEASURES Category A: - Protection required for consumers, agricultural workers, and the general public, - Includes crops likely to be eaten uncooked, spray-irrigated fruits and grass (sports fields, public parks and lawns); Category B: - Protection required for agricultural workers only, - Includes cereal crops, industrial crops (such as cotton and sisal), food crops for canning, fodder crops, pasture and trees, - In certain circumstances some vegetable crops might be considered as belonging to Category B if they are not eaten raw (potatoes, for instance) or if they grow well above ground (for example, chillies), in such cases it is necessary to ensure that the crop is not contaminated by sprinkler irrigation or by falling on to the ground, and that contamination of kitchens by such crops, before cooking, does not give rise to a health risk. |
The different types of irrigation methods have been introduced in Section 5.1.4. Under normal conditions, the type of irrigation method selected will depend on water supply conditions, climate, soil, crops to be grown, cost of irrigation method and the ability of the farmer to manage the system. However, when using wastewater as the source of irrigation other factors, such as contamination of plants and harvested product, farm workers, and the environment, and salinity and toxicity hazards, will need to be considered. There is considerable scope for reducing the undesirable effects of wastewater use in irrigation through selection of appropriate irrigation methods.
The choice of irrigation method in using wastewater is governed by the following technical factors:
- the choice of crops,
- the wetting of foliage, fruits and aerial parts,
- the distribution of water, salts and contaminants in the soil,
- the ease with which high soil water potential could be maintained,
- the efficiency of application, and
- the potential to contaminate farm workers and the environment.
Table 26 presents an analysis of these factors in relation to four widely practised irrigation methods, namely border, furrow, sprinkler and drip irrigation.
Table 26: EVALUATION OF COMMON IRRIGATION METHODS IN RELATION TO THE USE OF TREATED WASTEWATER
|
Parameters of evaluation |
Furrow irrigation |
Border irrigation |
Sprinkler irrigation |
Drip irrigation |
|
1 Foliar wetting and consequent leaf damage resulting in poor yield |
No foliar injury as the crop is planted on the ridge |
Some bottom leaves may be affected but the damage is not so serious as to reduce yield |
Severe leaf damage can occur resulting in significant yield loss |
No foliar injury occurs under this method of irrigation |
|
2 Salt accumulation in the root zone with repeated applications |
Salts tend to accumulate in the ridge which could harm the crop |
Salts move vertically downwards and are not likely to accumulate in the root zone |
Salt movement is downwards and root zone is not likely to accumulate salts |
Salt movement is radial along the direction of water movement. A salt wedge is formed between drip points |
|
3 Ability to maintain high soil water potential |
Plants may be subject to stress between irrigations |
Plants may be subject . to water stress between irrigations |
Not possible to maintain high soil water potential throughout the growing season |
Possible to maintain high soil water potential throughout the growing season and minimize the effect of salinity |
|
4 Suitability to handle brackish wastewater without significant yield loss |
Fair to medium. With good management and drainage acceptable yields are possible |
Fair to medium. Good irrigation and drainage practices can produce acceptable levels of yield |
Poor to fair. Most crops suffer from leaf damage and yield is low |
Excellent to good. Almost all crops can be grown with very little reduction in yield |
Source: Kandiah (1990b)
A border (and basin or any flood irrigation) system involves complete coverage of the soil surface with treated effluent and is normally not an efficient method of irrigation. This system will also contaminate vegetable crops growing near the ground and root crops and will expose farm workers to the effluent more than any other method. Thus, from both the health and water conservation points of view, border irrigation with wastewater is not satisfactory.
Furrow irrigation, on the other hand, does not wet the entire soil surface. This method can reduce crop contamination, since plants are grown on the ridges, but complete health protection cannot be guaranteed. Contamination of farm workers is potentially medium to high, depending on automation. If the effluent is transported through pipes and delivered into individual furrows by means of gated pipes, risk to irrigation workers will be minimum.
The efficiency of surface irrigation methods in general, borders, basins, and furrows, is not greatly affected by water quality, although the health risk inherent in these systems is most certainly of concern. Some problems might arise if the effluent contains large quantities of suspended solids and these settle out and restrict flow in transporting channels, gates, pipes and appurtenances. The use of primary treated sewage will overcome many of such problems. To avoid surface ponding of stagnant effluent, land levelling should be carried out carefully and appropriate land gradients should be provided.
Sprinkler, or spray, irrigation methods are generally more efficient in terms of water use since greater uniformity of application can be achieved. However, these overhead irrigation methods may contaminate ground crops, fruit trees and farm workers. In addition, pathogens contained in aerosolized effluent may be transported downwind and create a health risk to nearby residents. Generally, mechanized or automated systems have relatively high capital costs and low labour costs compared with manually-moved sprinkler systems. Rough land levelling is necessary for sprinkler systems, to prevent excessive head losses and achieve uniformity of wetting. Sprinkler systems are more affected by water quality than surface irrigation systems, primarily as a result of the clogging of orifices in sprinkler heads, potential leaf burns and phytotoxicity when water is saline and contains excessive toxic elements, and sediment accumulation in pipes, valves and distribution systems. Secondary wastewater treatment has generally been found to produce an effluent suitable for distribution through sprinklers, provided that the effluent is not too saline. Further precautionary measures, such as treatment with granular filters or micro-strainers and enlargement of nozzle orifice diameters to not less than 5 mm, are often adopted.
Localized irrigation, particularly when the soil surface is covered with plastic sheeting or other mulch, uses effluent more efficiently, can often produce higher crop yields and certainly provides the greatest degree of health protection for farm workers and consumers. Trickle and drip irrigation systems are expensive, however, and require a high quality of effluent to prevent clogging of the emitters through which water is slowly released into the soil. Table 27 presents water quality requirements to prevent clogging in localized irrigation systems. Solids in the effluent or biological growth at the emitters will create problems but gravel filtration of secondary treated effluent and regular flushing of lines have been found to be effective in preventing such problems in Cyprus (Papadopoulos and Stylianou 1988). Bubbler irrigation, a technique developed for the localized irrigation of tree crops avoids the need for small emitter orifices but careful setting is required for its successful application (Hillel 1987).
Table 27: WATER QUALITY AND CLOGGING POTENTIAL IN DRIP IRRIGATION SYSTEMS
|
Potential Problem |
Units |
Degree of Restriction on Use |
|||
|
None |
Slight to Moderate |
Severe |
|||
|
Physical |
|||||
|
|
Suspended Solids |
mg/l |
< 50 |
50- 100 |
> 100 |
|
Chemical |
|||||
|
|
pH |
|
< 7.0 |
7.0 - 8.0 |
> 8.0 |
|
|
Dissolved Solids |
mg/l |
< 500 |
500-2000 |
> 2000 |
|
|
Manganese |
mg/l |
< 0.1 |
0.1 - 1.5 |
> 1.5 |
|
|
Iron |
mg/l |
< 0.1 |
0.1 - 1.5 |
> 1.5 |
|
|
Hydrogen Sulphide |
mg/l |
< 0.5 |
0.5 - 2.0 |
> 2.0 |
|
Biological |
maximum |
|
|
|
|
|
|
Bacterial populations |
number/ml |
< 10000 |
10 000 - 50 000 |
> 50000 |
Source: Adapted from Nakayama (1982)
When compared with other systems, the main advantages of trickle irrigation seem to be:
i. increased crop growth and yield achieved by optimizing the water, nutrients and air regimes in the root zone,ii. high irrigation efficiency - no canopy interception, wind drift or conveyance losses and minimal drainage losses,
iii. minimal contact between farm workers and effluent,
iv. low energy requirements - the trickle system requires a water pressure of only 100-300 k Pa (1-3 bar),
v. low labour requirements - the trickle system can easily be automated, even to allow combined irrigation and fertilization (sometimes terms fertigation).
Apart from the high capital costs of trickle irrigation systems, another limiting factor in their use is that they are only suited to the irrigation of row crops. Relocation of subsurface systems can be prohibitively expensive.
Clearly, the decision on irrigation system selection will be mainly a financial one but it is to be hoped that the health risks associated with the different methods will be taken into account. As pointed out in Section 2.1, the method of effluent application is one of the health control measures possible, along with crop selection, wastewater treatment and human exposure control. Each measure will interact with the others and thus a decision on irrigation system selection will have an influence on wastewater treatment requirements, human exposure control and crop selection (for example, row crops are dictated by trickle irrigation). At the same time the irrigation techniques feasible will depend on crop selection and the choice of irrigation system might be limited if wastewater treatment has already been decided before effluent use is considered.
5.5.1 Water management
5.5.2 Land and soil management
5.5.3 Crop management and cultural practices
Management of water, soil, crop and operational procedures, including precautions to protect farm workers, play an important role in the successful use of sewage effluent for irrigation.
Most treated wastewaters are not very saline, salinity levels usually ranging between 500 and 200 mg/l (ECw = 0.7 to 3.0 dS/m). However, there may be instances where the salinity concentration exceeds the 2000 mg/l level. In any case, appropriate water management practices will have to be followed to prevent salinization, irrespective of whether the salt content in the wastewater is high or low. It is interesting to note that even the application of a non-saline wastewater, such as one containing 200 to 500 mg/l, when applied at a rate of 20,000 m3 per hectare, a fairly typical irrigation rate, will add between 2 and 5 tonnes of salt annually to the soil. If this is not flushed out of the root zone by leaching and removed from the soil by effective drainage, salinity problems can build up rapidly. Leaching and drainage are thus two important water management practices to avoid salinization of soils.
Leaching
The concept of leaching has already been discussed. The question that arises is how much water should be used for leaching, i.e. what is the leaching requirement? To estimate the leaching requirement, both the salinity of the irrigation water (ECw) and the crop tolerance to soil salinity (ECe) must be known. The necessary leaching requirement (LR) can be estimated from Figure 14 for general crop rotations reported by Ayers and Westcot (FAO 1985). A more exact estimate of the leaching requirement for a particular crop can be obtained using the following equation:
(14)
where:
LR = minimum leaching requirement needed to control salts within the tolerance (ECe) of the crop with ordinary surface methods of irrigationECw = salinity of the applied irrigation water in dS/m
ECe = average soil salinity tolerated by the crop as measured on a soil saturation extract. It is recommended that the ECe value that can be expected to result in at least a 90% or greater yield be used in the calculation.
Figure 14 was developed using ECe values for the 90% yield potential. For water in the moderate to high salinity range (>1.5 dS/m), it might be better to use the ECe value for maximum yield potential (100%) since salinity control is critical in obtaining good yields. Further information on this is contained in Irrigation and Drainage Paper 29, Rev. 1 (FAO 1985).
Figure 14: Relationship between applied water salinity and soil water salinity at different leaching fractions (FAO 1985)
Where water is scarce and expensive, leaching practices should be designed to maximize crop production per unit volume of water applied, to meet both the consumptive use and leaching requirements. Depending on the salinity status, leaching can be carried out at each irrigation, each alternative irrigation or less frequently, such as seasonally or at even longer intervals, as necessary to keep the salinity in the soil below the threshold above which yield might be affected to an unacceptable level. With good quality irrigation water, the irrigation application level will almost always apply sufficient extra water to accomplish leaching. With high salinity irrigation water, meeting the leaching requirement is difficult and requires large amounts of water. Rainfall must be considered in estimating the leaching requirement and in choosing the leaching method.
The following practices are suggested for increasing the efficiency of leaching and reducing the amount of water needed:
i. leach during cool seasons instead of during warm periods, to increase the efficiency and ease of leaching, since the total annual crop water demand (ET, mm/year) losses are lower,ii. use more salt-tolerant crops which require a lower leaching requirement (LR) and thus have a lower water demand,
iii. use tillage to slow overland water flow and reduce the number of surface cracks which bypass flow through large pores and decrease leaching efficiency,
iv. use sprinkler irrigation at an application rate below the soil infiltration rate as this favours unsaturated flow, which is significantly more efficient for leaching than saturated flow. More irrigation time but less water is required than for continuous ponding,
v. use alternate ponding and drying instead of continuous ponding as this is more efficient for leaching and uses less water, although the time required to leach is greater. This may have drawbacks in areas having a high water table, which allows secondary salinization between pondings,
vi. where possible, schedule leachings at periods of low crop water use or postpone teachings until after the cropping season,
vii. avoid fallow periods, particularly during hot summers, when rapid secondary soil salinization from high water tables can occur,
viii. if infiltration rates are low, consider pre-planting irrigations or off-season leaching to avoid excessive water applications during the crop season, and
ix. use one irrigation before the start of the rainy season if total rainfall is normally expected to be insufficient for a complete leaching. Rainfall is often the most efficient leaching method because it provides high quality water at relatively low rates of application.
Drainage
Salinity problems in many irrigation projects in arid and semi-arid areas are associated with the presence of a shallow water table. The role of drainage in this context is to lower the water table to a desirable level, at which it does not contribute to the transport of salts to the root zone and the soil surface by capillarity. What is important is to maintain a downward movement of water through soils. van Schilfgaard (1984) reported that drainage criteria are frequently expressed in terms of critical water table depths; although this is a useful concept, prevention of salinization depends on the establishment, averaged over a period of time, of a downward flux of water. Another important element of the total drainage system is its ability to transport the desired amount of drained water out of the irrigation scheme and dispose of it safely. Such disposal can pose a serious problem, particularly when the source of irrigation water is treated wastewater, depending on the composition of the drainage effluent.
Timing of irrigation
The timing of irrigation, including irrigation frequency, pre-planting irrigation and irrigation prior to a winter rainy season, can reduce the salinity hazard and avoid water stress between irrigations. Some of these practices are readily applicable to wastewater irrigation.
In terms of meeting the water needs of crops, increasing the frequency of irrigation will be desirable as it eliminates water stress between irrigations. However, from the point of view of overall water management, this may not always produce the desired results. For example, with border, basin and other flood irrigation methods, frequent irrigations may result in an unacceptable increase in the quantity of water applied, decrease in water use efficiency and larger amounts of water to be drained. However, with sprinklers and localized irrigation methods, frequent applications with smaller amounts may not result in decrease in water use efficiency and, indeed, could help to overcome the salinity problem associated with saline irrigation water.
Pre-planting irrigation is practised in many irrigation schemes for two reasons, namely: (i) to leach salts from the soil surface which may have accumulated during the previous cropping period and to provide a salt-free environment to germinating seeds (it should be noted that for most crops, the seed germination and seedling stages are most sensitive to salinity); and (ii) to provide adequate moisture to germinating seeds and young seedlings. A common practice among growers of lettuce, tomatoes and other vegetable crops is to pre-irrigate the field before planting, since irrigation soon after planting could create local water stagnation and wet spots that are not desirable. Treated wastewater is a good source for pre-irrigation as it is normally not saline and the health hazards are practically nil.
Blending of wastewater with other water supplies
One of the options that may be available to farmers is the blending of treated sewage with conventional sources of water, canal water or ground water, if multiple sources are available. It is possible that a farmer may have saline ground water and, if he has non-saline treated wastewater, could blend the two sources to obtain a blended water of acceptable salinity level. Further, by blending, the microbial quality of the resulting mixture could be superior to that of the unblended wastewater.
Alternating treated wastewater with other water sources
Another strategy is to use the treated wastewater alternately with the canal water or groundwater, instead of blending. From the point of view of salinity control, alternate applications of the two sources will be superior to blending. However, an alternating application strategy will require duel conveyance systems and availability of the effluent dictated by the alternate schedule of application.
Several land and soil management practices can be adopted at the field level to overcome salinity, sodicity, toxicity and health hazards that might be associated with the use of treated wastewater.
Land development
During the early stages of on-farm land development, steps can be taken to minimize potential hazards that may result from the use of wastewater. These will have to be well planned, designed and executed since they are expensive and, often, one time operations. Their goal is to improve permanently existing land and soil conditions in order to make irrigation with wastewater easier. Typical activities include levelling of land to a given grade, establishing adequate drainage (both open and sub-surface systems), deep ploughing and leaching to reduce soil salinity.
Land grading
Land grading is important to achieve good uniformity of application from surface irrigation methods and acceptable irrigation efficiencies in general. If the wastewater is saline, it is very important that the irrigated land is appropriately graded. Salts accumulate in the high spots which have too little water infiltration and leaching, while in the low spots water accumulates, causing waterlogging and soil crusting.
Land grading is well accepted as an important farm practice in irrigated agriculture. Several methods are available to grade land to a desired slope. The slope required will vary with the irrigation system, length of run of water flow, soil type, and the design of the field. Recently, laser techniques have been applied to level land precisely so as to obtain high irrigation efficiencies and prevent salinization.
Deep cultivation
In certain areas, the soil is stratified, and such soils are difficult to irrigate. Layers of clay, sand or hard pan in stratified soils frequently impede or prevent free movement of water through and beyond the root zone. This will not only lead to saturation of the root zone but also to accumulation of salts in the root zone. Irrigation efficiency as well as water movement in the soil can be greatly enhanced by sub-soiling and chiselling of the land. The effects of sub-soiling and chiselling remain for about 1 to 5 years but, if long term effects are required, the land should be deep and slip ploughed. Deep or slip ploughing is costly and usually requires the growing of annual crops soon after to allow the settling of the land. Following a couple of grain crops, grading will be required to re-establish a proper grade to the land.
Several cultural and crop management practices that are valid under saline water use will be valid under wastewater use. These practices are aimed at preventing damage to crops caused by salt accumulation surrounding the plants and in the root zone and adjusting fertilizer and agrochemical applications to suit the quality of the wastewater and the crop.
Placement of seed
In most crops, seed germination is more seriously affected by soil salinity than other stages of development of a crop. The effects are pronounced in furrow-irrigated crops, where the water is fairly to highly saline. This is because water moves upwards by capillarity in the ridges, carrying salts with it. When water is either absorbed by roots or evaporated, salts are deposited in the ridges. Typically, the highest salt concentration occurs in the centre of the ridge, whereas the lowest concentration of salt is found along the shoulders of the ridges. An efficient means of overcoming this problem is to ensure that the soil around the germinating seeds is sufficiently low in salinity. Appropriate planting methods, ridge shapes and irrigation management can significantly decrease damage to germinating seeds. Some specific practices include:
i. Planting on the shoulder of the ridge in the case of single row planting or on both shoulders in double row planting,ii. Using sloping beds with seeds planted on the sloping side, but above the water line,
iii. Irrigating alternate rows so that the salts can be moved beyond the single seed row.
Figure 15 presents schematic representations of salt accumulation, planting positions, ridge shapes and watering patterns.
5.6.1 Central planning
5.6.2 Desirable site characteristics
5.6.3 Crop selection issues
Government policy on effluent use in agriculture will have a deciding effect on what control measures can be achieved through careful selection of site and crops to be irrigated with treated effluent. A decision to make treated effluent available to farmers for unrestricted irrigation or to irrigate public parks and urban green areas with effluent will remove the possibility of taking advantage of careful selection of sites, irrigation techniques and crops in limiting the health risks and minimizing environmental impacts. However, if a Government decides that effluent irrigation will only be applied in specific controlled areas, even if crop selection is not limited (that is, unrestricted irrigation is allowed within these areas), public access to the irrigated areas will be prevented and some of the control measures described in Chapter 2 can be applied. Without doubt, the greatest security against health risk and adverse environmental impact will be achieved by limiting effluent use to restricted irrigation on controlled areas to which the public has no access but even imposing restrictions on effluent irrigation by farmers, if properly enforced, can achieve a degree of control.
Cobham and Johnson (1988) have suggested that the procedures involved in preparing plans for effluent irrigation schemes are similar to those used in most forms of resource planning and summarized the main physical, social and economic dimensions as in Figure 16. They also indicated that a number of key issues or tasks were likely to have a significant effect on the ultimate success of effluent irrigation, as follows:
i. organizational and managerial provisions made to administer the resource, to select the effluent use plan and to implement it,ii. the importance attached to public health considerations and the levels of risk taken,
iii. the choice of single-use or multiple-use strategies,
iv. the criteria adopted in evaluating alternative reuse proposals,
v. the level of appreciation of the scope for establishing a forest resource.
Adopting a mix of effluent use strategies is normally advantageous in respect of allowing greater flexibility, increased financial security and more efficient use of the wastewater throughout the year, whereas a single-use strategy will give rise to seasonal surpluses of effluent for unproductive disposal. Therefore, in site and crop selection the desirability of providing areas for different crops and forestry so as to utilize the effluent at maximum efficiency over the whole yearly cycle of seasons must be kept in mind.
The features which are critical in deciding the viability of a land disposal project are the location of available land and public attitudes. Land which is far distant from the sewage treatment plant will incur high costs for transporting treated effluent to site and will generally not be suitable. Hence, the availability of land for effluent irrigation should be considered when sewerage is being planned and sewage treatment plants should be strategically located in relation to suitable agricultural sites. Ideally, these sites should not be close to residential areas but even remote land might not be acceptable to the public if the social, cultural or religious attitudes are opposed to the practice of wastewater irrigation. The potential health hazards associated with effluent irrigation can make this a very sensitive issue and public concern will only be mollified by the application of strict control measures. In arid areas, the importance of agricultural use of treated effluent makes it advisable to be as systematic as possible in planning, developing and managing effluent irrigation projects and the public must be kept informed at all stages.
The ideal objective in site selection is to find a suitable area where long-term application of treated effluent will be feasible without adverse environmental or public health impacts. It might be possible in a particular instance to identify several potential sites within reasonable distance of the sewered community and the problem will be to select the most suitable area or areas, taking all relevant factors into account. The following basic information on an area under consideration will be of value, if available:
- a topographic map,
- agricultural soils surveys,
- aerial photographs,
- geological maps and reports,
- groundwater reports and well logs,
- boring logs and soil test results,
- other soil and peizometric data.
At this preliminary stage of investigation it should be possible to assess the potential impact of treated effluent application on any usable aquifer in the area(s) concerned. The first ranking of sites should take into account other factors, such as the cost and location of the land, its present use and availability, and social factors, in addition to soil and groundwater conditions.
The characteristics of the soil profile underlying a particular site are very important in deciding on its suitability for effluent irrigation and the methods of application to be employed. Among the soil properties important from the point of view of wastewater application and agricultural production are: physical parameters (such as texture, grading, liquid and plastic limits, etc.), permeability, water-holding capacity, pH, salinity and chemical composition. Preliminary observation of sites, which could include shallow hand-auger borings and identification of vegetation, will often allow the elimination of clearly unsatisfactory sites. After elimination of marginal sites , each site under serious consideration must be investigated by on-site borings to ascertain the soil profile, soil characteristics and location of the water table. Peizometers should be located in each borehole and these can be used for subsequent groundwater sampling. A procedure for such site assessment has been described by Hall and Thompson (1981) and, if applied, should not only allow the most suitable site among several possible to be selected but permit the impact of effluent irrigation at the chosen site to be modelled. When a site is developed, a long-term groundwater monitoring programme should be an essential feature of its management.
Normally, in choosing crops, a farmer is influenced by economics, climate, soil and water characteristics, management skill, labour and equipment available and tradition. The degree to which the use of treated effluent influences crop selection will depend on Government policy on effluent irrigation, the goals of the user and the effluent quality. Government policy will have the objectives of minimizing the health risk and influencing the type of productivity associated with effluent irrigation. Regulations must be realistic and achievable in the context of national and local environmental conditions and traditions. At the same time, planners of effluent irrigation schemes must attempt to achieve maximum productivity and water conservation through the choice of crops and effluent application systems.
A multiple-use strategy approach will require the evaluation of viable combinations of the cropping options possible on the land available. This will entail a considerable amount of survey and resource budgeting work, in addition to the necessary soil and water quality assessments. The annual, monthly and daily water demands of the crops, using the most appropriate irrigation techniques, have to be determined. Domestic consumption, local production and imports of the various crops must be assessed so that the economic potential of effluent irrigation of the various crop combinations can be estimated. Finally, the crop irrigation demands must be matched with the available effluent so as to achieve optimum physical and financial utilization throughout the year. This process of assessment is reviewed by Cobham and Johnson (1988) for the case of effluent use in Kuwait, where afforestation for commercial purposes was found to offer significant potential in multiple-use effluent irrigation.
