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 above requeriments 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 plantas is lost by transpiration and evaporation from the plant surface. This, for all practical purposes, the water requirement of crops is equal to the evapotranspiration requirement. ET}c. Crop evapotranspiration is mainly determined by climatic factors and hence can be estimated with responsable accuracy using meteorological data. An extensive review of this subject and guidelines for estimating ET}c 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 impotant farm practices aimed at optimizing crop production when treated sewage effluent is used as irrigation water will be discussed in this chapter.
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:
Ya ETa
( 1 - ____ ) = ky ( 1 - _______) (12)
Ym ETm
|
| 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:
Table 20
presents some basic features of selected irrigation systems as reported by Doneen and Westcot (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 most root zone is called leaching and that portion of the irrigation water which mobilizes the excess of salts is called the leaching fraction. LF.
Leaching depth of water leached below the root zone Fraction = ______________________________________________ (13) (LF) depth of water applied at the surface |
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.
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