|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 4 - Water supply and demand|
Estimation of crop water needs, a basic factor in irrigation design, is by no means as straightforward as might be assumed. Actual water consumption (evapotranspiration), is influenced by climatic factors, including air temperature, humidity, radiation, cloud cover, and wind, and by the nature of the plant itself including its stage of growth. It is also influenced by the amount of moisture in the soil at the time (soil moisture tension). In the face of this number of factors, values for many of which are frequently not known, simplified approximate methods of estimation are commonly used. These employ a limited number of parameters, for instance air temperature and number of daylight hours only, or the measured evaporation from an open pan, as the basis for estimation. Alternatively, approximate estimations of values of climatic factors for which actual measured values are not available are inserted in more general formulae. "Plant factors", the water-consuming characteristics of each particular type of plant at each stage of growth, are based on field observations for which generally-accepted tabular data are available. There is, of course, a more direct method of water-use estimation, which measures water abstraction from a lysimeter containing soil and the growing plant. However, the difficulties of using the Iysimeter have limited its application to basic research.
Values of consumptive use obtained by the various methods of estimation vary widely. A comparison between actual measured water use and estimates made by eighteen different methods was given in the 19 74 report on Irrigation Water Requirements by the Irrigation and Drainage Division of the American Society of Civil Engineers. The investigation was related to alfalfa and grass crops, grown at ten stations in varying climate situations. The two most commonly used methods of estimation, Penman and Modified Blaney Criddle, gave results ranging from 14% low to 30% high (Penman), and 46% low to 35% high (Modified Blaney Criddle), compared with actual measurements. A.S.C.E has issued a further comprehensive report on the same subject (Jensen 1990).
A widely used reference for the estimation of crop water requirements is the Irrigation and Drainage Paper No. 24 (Revision of 1977) of the Food and Agriculture Organization of the U.N. (Doorenbos 1977). This covers the Penman, Blaney Criddle, Radiation, and Pan-evaporation methods of estimation and extends their applicability by calculating coefficients based on climatic factors not otherwise included in the estimation (particularly for the latter three methods, Penman is already comprehensive). However, estimates prepared by the four methods still differ substantially.
The estimates of consumptive use discussed above refer to "optimum" conditions, i.e. with unrestricted availability of water at plant roots or virtually zero soil moisture tension. These are the basic E to values. The customary use of the word "optimum" in this situation is misleading, in that such moisture conditions while possibly optimizing vegetative growth may not result in optimum economic use of water.
The effect of restricting the availability of soil moisture on plant growth is an important issue with respect to two questions. First, can less than "optimum" amounts of irrigation be used without significantly reducing crop yields, and second, how do the fluctuations in soil moisture tension between conventional periodic irrigations affect yields (Jensen 1990, Hillel 1987).
Research relevant to these two questions continues, but work to date indicates that any reduction in transpiration imposed by soil moisture stress automatically reduces the rate of vegetative growth in an approximately linear fashion, and as a corollary, cycling the soil moisture in the root zone from field capacity down to near wilt point, a basic feature of conventional irrigation practice, inevitably adversely affects yields.
However, the above conclusions must be treated with caution, in view of the results of extensive field station trials, which indicate that crop yields can be highly responsive to irrigation at critical stages of plant development, but that with-holding irrigation between such stages for periods of a month or more (with inevitable stress) has little effect on yields. This is notably true for certain crops and less so for others. Moreover, cycling of soil moisture in the root zone is an unavoidable feature of all irrigation systems (other than trickle or sprinkler), and the question of period between irrigations, which affects the range in soil moisture tension, has considerable implications on system design. More data is needed on the relationship between range of soil moisture tension between irrigations and crop yields.
Added to the level of uncertainty regarding crop water use is field efficiency, a factor involving considerable approximation. Consumptive use refers to water use at the plant. Field efficiency is the ratio between the amount of water consumptively used by the crop and the amount applied at the outlet to the field. Factors contributing to field inefficiency are percolation beneath the reach of the plant root system, evaporation from areas not occupied by the crop, seepage from distribution furrows, spillage from the end of the field, and non-uniformity in distribution of water on the field (i.e. some areas receiving more than sufficient and some less). Some elements contributing to field inefficiency are not, in fact, a loss to the project. Seepage below the root zone may fill a necessary leaching function (unless this is provided seasonally by monsoon rains) or may be recovered by groundwater development. Spill from the end of the field may be used elsewhere in the system. However, these elements contribute to the amount of water which must be applied at the field boundary.
Values of field efficiency are simply judgement figures. They may vary from an upper limit of some 80% to a more generally applicable range of 70-75%, and be much lower in less inadequately managed systems. One procedure which largely avoids the need for separate estimation of field efficiency is to base the estimation of crop water needs on field station data on irrigation requirements at the field boundary (which includes field inefficiency). Such data usually gives crop production under a range of seasonal water applications and irrigation schedules, in particular relating time of watering to stage of plant growth.
Thus, estimation of crop water requirements by conventional formulae inevitably involves considerable approximation. Estimates using different, but well accepted, formulae are likely to differ by 25% or more. Calculation of basic Eto figures for consumptive use under "optimum" soil moisture conditions is a necessary step, as a point of reference. However, for actual project design the use of agricultural field station data is preferable, if such data is available. If it is necessary to extrapolate, the ratio of Eto values for that station and for the project area can be used.
Because of the differences likely to be obtained in consumptive use estimates using different but reputable approaches, it is most desirable that agreement be reached in this respect between the agencies concerned with formulation and appraisal of a particular project. It is preferable to avoid a situation in which a government agency, or a consultant, carries out detailed designs and prepares cost estimates for a project, only to find at appraisal that the prospective financing organization disagrees with the basic assumptions regarding water requirements.