In irrigation water management, it is necessary to evaluate the irrigation practices from the time water leaves the source till it is utilized by the plants. The concept of efficiency which is an input output relationship is applied to irrigation practice. The objective of this analysis is to identify the areas where improvement is necessary in order to achieve better utilization of the irrigation water. There are several ways in which irrigation efficiencies are expressed.
The movement of water through an irrigation system can be considered as three separate operations: conveyance, distribution, and field application. The efficiencies of water use in each of these operations are defined as follows.
The conveyance efficiency can be expressed as –
1. Distribution Efficiency:
This is the efficiency of the water distribution canals and conduits supplying water from the conveyance network to individual fields. The intake from the conveyance network and the output from these values to the fields can be measured, and the efficiency of distribution can be assessed. The distribution efficiency can be expressed as –
2. Field Application Efficiency:
The field application efficiency ea’ is made up of two parts :
1. The efficiency of the water transport system in the field; and
2. The efficiency with which the water is applied—a method efficiency. It is, by the definition, the relation between the quantity of water furnished at the field inlet and the quantity of water needed to maintain the soil moisture above some target level required for the crop.
The application efficiency is also defined as the ratio of the required depth to the average applied depth. In level basins, as there is no surface runoff, the average depth applied to the field equals the average infiltrated depth.
3. Tertiary Unit Efficiency:
The tertiary unit efficiency, eu, is the combined efficiency of the water distribution system and of the water application process, i.e., it is the efficiency with which water is distributed and consumptively used within the tertiary unit. The tertiary unit efficiency can be expressed as –
This value represents the efficiency of the entire operation between river diversion or source of water and the rootzone of the crops grown in the irrigated area.
The evaluation of irrigation water use at crop production level is based on some relation between crop yield and evapotranspiration, and between crop yield and depth of water applied to the field. In temperate zones with supplementary irrigation, the mass of marketable crop produced with or without irrigation varies from year to year; and so does the evapotranspiration. Evaluation studies in these zones should be supported by a statistical analysis of weather factors. This is especially important if the objective is to select an economically justified investment level for the irrigation system and its related equipment.
In (semi) arid zones, where water is a limiting factor, evapotranspiration and plant growth are related more to water availability than to weather factors. In zones where irrigation is the only way to produce a crop, it is relatively easy to evaluate water use at crop production level. ICID recommends the use of the following terms.
Yield – ET Ratio, which is given as –
Where, Wf = mass of irrigation water furnished to the field (Vf in Eq. 15.67).
For meaningful comparison of data, water use efficiency (WUE) should be calculated based on water consumed during specific periods of the crop production cycle, such as from planting to harvest or harvest to harvest. Accordingly, WUE can be expressed as –
4. Consumptive Use Efficiency:
The consumptive use efficiency is a measure of the water that has been actually used by the plant from the total water that is depleted from the rootzone. This is expressed as –
The consumptive use efficiency is very useful in the evaluation of irrigation methods and the crop response to irrigation. For example, in furrow irrigation a wider spacing may result in considerable amount of water being evaporated from the ground surface. Consumptive use efficiency is also affected by the soil texture, distribution of roots within the soil profile, and variation of soil moisture within the rootzone.
In evaluating irrigation methods, an aspect that is to be taken into consideration is the placement of the water where it can be most efficiently utilized by the plants. The drip method of irrigation takes this into consideration, and hence it usually gives high consumptive use efficiencies.
5. Water Use Efficiency:
This term is the same as WUE. It is often used to relate the crop yield with water use. Two terms are commonly used for this purpose. One is the ‘crop water use efficiency’ which is the ratio of the crop yield (y) to the amount of water depleted by the crop in the process of evapotranspiration (ET).
The other term is the ‘field water use efficiency’ which is the ratio of crop yield (y) to the total amount of water used in the field (WR). The yield may be expressed as kg or quintals or tons and ET or WR may be expressed in terms of ha cm or m3. The yield of the crop refers to the marketable produce. However, sometimes if the dry matter is important, the dry matter yield could also be considered.