Deptt. of Irrigation and Drainage Engg. Irrigation Depths & Irrigation Efficiencies Shri M. U. Kale Assistant Professor Deptt. of Irrigation and Drainage Engg.

Net Irrigation Requirement: The net irrigation requirement is the depth of irrigation water, exclusive of precipitation, carry over soil moisture or groundwater contribution or other gains in soil moisture i.e. required consumptively for crop production. It is the amount of irrigation water required to bring the soil moisture level in the effective root zone to field capacity. Thus it is the difference between the field capacity and soil moisture content in the soil moisture content in the root zone before starting irrigation.

This may be obtained by following relationship: Where, d=net amount of water to be applied during an irrigation, cm Mfci=field capacity moisture content in the ith layer of the soil, per cent Mbi=moisture content before irrigation in the ith layer of the soil, per cent Ai= bulk density of the soil in the ith layer Di=depth of the ith layer of soil,cm, within the root zone,and n=number of soil layers in the root zone D.

In drawing up the seasonal or monthly irrigation requirement for a given crop or cropping pattern the main variables composing the field water balance include: Crop water requirement as determined by climate and crop characteristics, Contribution from precipitation, Groundwater , and Carry over of soil water.

Gross irrigation requirement: The total amount of water applied throughout irrigation is termed as gross irrigation requirement. In other words, it is net irrigation requirement plus losses in water application and other losses. The gross irrigation requirement (IR) at the field head, for instance can be determined as follows:

In which, IR=seasonal gross irrigation requirment at the field head, cm D=net amount of water to be applied, cm E(application) =water application efficiency, and N=number of irrigation in season.

Irrigation frequency: Irrigation frequency refers to number of days between irrigations during periods without rainfall. It depends on the consumptive use rate of crop and on the amount of available moisture in the crop root zone. It is the function of crop, soil and climate. Sandy soils must be irrigated more often than fine textured deep soil.

Irrigation period: Irrigation period is the number of days that can be allowed for applying one irrigation to a given design area during the peak consumptive use period of crop being irrigated. It is the basis for irrigation system capacity and equipment design. The irrigation system must be so designed that the irrigation period is not greater than the irrigation frequency.

Irrigation efficiency: Irrigation efficiency indicates how efficiently the available water supply is being used on different methods of evaluation. Loss of irrigation water occurs in the conveyance and distribution system, non-uniform distribution of water over the field, percolation below crop root zone and with sprinkler irrigation evaporation from the spray and retention of water on the foliage. The losses can be held to a minimum by adequate planning of the irrigation system, proper design of the irrigation method, adequate land preparation and efficient operation of the system.

1. Water conveyance efficiency: This term is used to measure the efficiency of water conveyance system with the canal network, water courses and field channels. In which, Ec=water conveyance efficiency, per cent Wf=water delivered to the irrigated plot, Wd=water diverted from the source

2. Water application efficiency: A measure of how efficiently water is applied to the field is the water application efficiency. In which, Ea=water application efficiency, per cent Ws=water stored in the root zone of the plants Wf=water delivered to the field (at field supply channel)

3. Water storage efficiency: This concepts relates how completely the water needed prior to irrigation has been stored in the root zone during the irrigation. it is defined as In which, Es=water storage efficiency, per cent Ws=water stored in the root zone during irrigation Wn=water needed in the root zone prior to irrigation.

4. Water distribution efficiency: Not only the application of the right amount of water to the field but also the uniform distribution over the field is important. Water distribution efficiency indicates the extent to which water is uniformly distributed along the run. It is given as

In which, Ed=water distribution efficiency, per cent d=average of depth of water stored along the run during the irrigation from d. y=average numerical deviation

5.Water use efficiency: The water utilization by the crop is generally described in terms of water use efficiency (kg/ha-cm or q/ha-cm). It can be defined in following ways: Crop water use efficiency: It is the ratio of crop yield (Y) to the amount of water depleted by the crop in the process of evaporation (ET). Water use efficiency

Field water use efficiency: It is the ratio of (Y) to the total amount of water used in the field (WR). Field water use efficiency

6. Project efficiency: Project efficiency indicates the effective use of irrigation water source in crop production. It is the percentage of irrigation water that is stored in the soil and is available for consumptive use by crops. 7. Operational efficiency: It is the ratio of the actual project efficiency of an ideally designed and managed system using the same irrigation method and facilities.

8. Economic efficiency: Economic efficiency is the ratio of the total production (net or gross profit) attained with the operating irrigation system, compared to the total production expected under ideal condition. This parameter is a measure of the overall efficiency, because it relates the final output to input.

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