Chapter 3

Improving water-use efficiency

In general, the term efficiency is used to quantify the relative output obtainable from a given input. Referring to the use of water in irrigation, efficiency may be defined in various ways, depending on the nature of the inputs and outputs to be considered. An economic criterion of efficiency, for example, might be the financial return obtained from irrigation in relation to the investment made in the water supply. The problem here is that costs and prices fluctuate from year to year and vary widely from place to place. Another problem is that some of the costs of irrigation and some of the benefits cannot easily be quantified in tangible economic or financial terms, especially where a market economy is not yet fully developed. Often, only the short-term costs and immediate benefits are seen, whereas the long-term advantages or disadvantages are not fully realized at the outset. How is it possible to assess the economic value, for instance, of saving the population of a region from the potential effects of a drought, if the probability or severity of future drought events is not known? To some degree, therefore, it is necessary to operate in a state of uncertainty.

In more restricted technical terms, what irrigation engineers often call conveyance efficiency is defined as the net amount of water delivered to a farm, as a fraction of the amount taken from some source. The difference between the two amounts represents the seepage and evaporative losses incurred en route from source to field. Not generally considered in the term conveyance efficiency is the possible loss of water quality through pollution - such as that caused by wading animals or by human use of the canal water for washing and waste disposal.
The term on-farm application efficiency or field application efficiency generally refers to the fraction of the water volume applied to a farm or a field that is "consumed" by a crop, relative to the amount applied. Crop consumption consists of the amount of water actually absorbed by the crop, most of which is generally transpired to the atmosphere (only a small fraction, often less than 1 percent, being retained in the vegetative biomass). There is much evidence that, in a given climate, the growth of many crops is directly related to the amount of water they transpire. The explanation is that both carbon dioxide (CO₂) for photosynthesis and transpiration occur concurrently through the same stomatal openings in the leaves, so the two processes should be roughly proportional.
In actual practice, however, the volume of water reported to be consumed in the field consists of evapotranspiration rather than of transpiration alone. Evapotranspiration includes, in addition to the amount of water transpired by the plants, the amount evaporated directly from the soil surface without being taken up by the plants. In addition, evapotranspiration
often includes the amount of water intercepted by the foliage (e.g. under overhead sprinkler irrigation) and evaporated without ever entering either the soil or the plant. The reason why the term evapotranspiration is taken to be consumptive use is that, in practice, direct evaporation is difficult to measure separately from transpiration, so the two terms are lumped together merely for the sake of convenience.
Clearly, however, much of the water evaporated without entering the plant is consumed non-productively. Therefore, any method of irrigation that minimizes evaporation (but not transpiration) is likely to increase the efficiency of water utilization by the crop. Some of the irrigation methods described in this publication are capable of doing just that: they introduce water directly into the root zone without sprin-kling the foliage or wetting the entire soil surface. Such partial-area irrigation methods offer the additional benefit of keeping the greater part of the soil surface (between the rows of crop plants) dry. This discourages the growth of weeds, that would otherwise not only compete with crop plants for nutrients and moisture in the root zone and for light above ground, but also hinder field operations and the control of pests.
Even with total evapotranspiration considered as consumptive use, field application efficiency in most traditional irrigation schemes is still very low: typically less than 50 percent and often as low as 30 percent. Excessive application of water generally entails losses due to surface runoff from the field as well as to deep percolation below the root zone within the field. Both runoff and deep percolation losses are difficult to control under flood or furrow irrigation, where a large volume of water is applied all at once. They can, however, be minimized where a controlled volume of water is applied at a slow rate over an extended period of time directly to the root zone.
Even with the best irrigation practices, however, field application efficiency values cannot attain 100 percent. Nor should that be the aim, since a certain fraction of the water applied must be allowed to seep downwards and leach the salts that would otherwise accumulate in the root zone.¹ However, with careful management, field water application efficiency values approaching 90 percent are possible, and values of 80 percent are practicable, by some of the methods described in this publication.
A word of warning is in order at this point. No irrigation method or technology in itself guarantees the attainment of high efficiency. How the system is operated is all important. With poor management, even the most sophisticated system can result in water loss and inefficiency. Only knowledgeable, experienced and caring management can ensure that appropriate irrigation systems achieve their full potential benefits (Figure 8).

FIGURE 8
Typical crop root distributions

Quite different from strictly technical criteria of efficiency is the physiological index, known as crop water-use efficiency. The relevant measure here is the response of the crop to irrigation, not in percentage terms but as total biomass produced (above-ground dry matter) per unit mass of water taken up by the crop. Since, as mentioned above, well over 90 percent of the water taken up by plants in the field is normally transpired, crop water-use efficiency is in effect the reciprocal of what has long been known as the transpiration ratio. The latter is defined as the ratio of the amount of water transpired to the amount of dry matter produced (tonnes per tonne). That ratio can be of the order of 1 000 or more in a dry climate of high evaporative demand.
An alternative way to characterize crop water-use efficiency is in terms of the marketable crop produced per unit volume of water. This expression is identical to the above-ground biomass in the case of crops grown and harvested for forage, but it is quite different where the marketable product is only the fruit, seed or fibre. Generally, but not always, the yield of such products is proportional to total growth, hence also to transpiration.
To maximize crop water-use efficiency, by either of the above criteria, it is necessary both to conserve water and to promote maximal growth. The former requires minimizing losses through runoff, seepage, evaporation and transpiration by weeds. The latter task includes planting high-yielding crops well adapted to the local soil and climate. It also includes optimizing growing conditions by proper timing and performance of planting and harvesting, tillage, fertilization and pest control. In short, raising water-use efficiency requires good farming practices from start to finish.

Finally, all the above indexes of efficiency may be combined in a single concept, the overall agronomic efficiency of water use, F_ag:

where P is crop production (total dry matter or the marketable product, as the case may be) and U is the volume of water applied.
As only a fraction of the applied water is actually absorbed and utilized by the crop, it is necessary to consider the various components of the denominator U:

U = R + D + E_p + E_s + T_w + T_c (2)

where R is the volume of water lost by runoff from the field, D the volume drained below the root zone (deep percolation), E_p the volume lost by evaporation during the conveyance and application to the field,² E_s the volume evaporated from the soil surface (mainly between the rows of crop plants), T_w the volume transpired by weeds, and T_c the volume transpired by the crop. All these volumes pertain to the same unit area.

Accordingly:

Under flood irrigation as commonly practised in river diversion schemes, excessive water application often results in considerable runoff, evaporation from open water surfaces and transpiration by weeds. In the experience of the author, these losses commonly amount to 20 percent or even 30 percent of the water applied. In addition, the loss of water due to percolation below the root zone may be of the order of 30 percent or even 40 percent of the water applied. Consequently, the fraction actually taken up by the crop is often below 50 percent and may even be as low as 30 percent.

FIGURE 9
The water balance of a field

If runoff and direct evaporation of free water are prevented, and if evaporation from the soil surface is minimized (as under partial-area irrigation that avoids wetting the areas between rows) and weeds are effectively controlled; and if, furthermore, water is applied in measured quantities commensurate with crop requirements so as to avoid excessive percolation, all the losses can be reduced to less than 20 percent of the water applied. Irrigation efficiency can then attain or even exceed 80 percent.
Finally, and no less important, the numerator of the equation (namely, the yield attainable) can be greatly enhanced by judicious selection of crops and varieties, optimal fertilization and tillage and proper timing of planting and harvesting. All in all, the agronomic efficiency of water use in irrigated farming can be significantly increased relative to the low efficiency characteristic of traditional practice.

¹ Irrigation water, even if it is of high quality, invariably contains some salts, and these are mostly left behind as crop roots absorb water from the soil.
² Evaporation may take place from exposed bodies of water in the case of surface irrigation, or from wind-drift and intercepted water in the case of sprinkler irrigation.