This report presents the various methodologies for correct assessment of irrigation return flow, which is one of the most significant components in the water balance of irrigation command areas. A part of the water applied to the irrigation fields percolates deep to recharge the groundwater and is known as irrigation return flow. In this report methodologies have been presented for assessment of irrigation return flow through soil moisture modeling, water balance approach, experimental determination and groundwater modeling.
The irrigation return flow depends upon the geological setup of the irrigation command, soil moisture characteristics, meteorological parameters, crop types, method of irrigation and depth to water table. Application of fertilizers and leaching requirement of soil salts may result in application of more irrigation water leading to more irrigation return flow.
In the absence of any studies, the irrigation return flow is usually taken as 35 % of the water applied for irrigation in case of canal irrigation and 30 % in case of groundwater irrigation. But this is only an approximate estimate. the quantification of irrigation return flow should be made both at micro scale (by process level model) and macro scale.
The micro scale quantification will be useful in ascertaining the return flow corresponding to soil, crop type and method of irrigation. The treatment of the investigation at macro scale will help verify the accuracy of computation made at micro scale.
Process level models such as evaluation of seepage from a canal reach, evapotranspiration from the root zone, infiltration-redistribution and field tailwater by themselves have only marginal utility in evaluating irrigation return flow unless they are used in the fabrication of a macro level model study. The input data for macro level study can be obtained from process level models.
Deep percolation, which represents a major component of irrigation return flow can be estimated using soil moisture modeling for each soil group and crop type. Using this estimate of deep percolation and other components of recharge and groundwater withdrawal, the water level fluctuation can be simulated using a distributed-groundwater flow model for the command area.
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