Using the depleted fraction to manage the groundwater table in irrigated areas

The depleted fraction, defined as the ratio of ETactual over total inflow (P Vc), relates parameters of the water balance of an irrigated area with each other in such a way that the (water) manager obtains information on the rate of change of water stored in the area (soil moisture and groundwater). If the annual average of the depleted fraction equals about 0.6 water storage in the area is stable, while water is stored for lower values of the depleted fraction. If the value of the depleted fraction exceeds about 0.6, the volume of water stored in the area decreases. This decrease is partly due to natural drainage and partly due to capillary rise into the root zone of the irrigated crop. Despite this capillary rise, the actual evapo-transpiration drops below the potential ET-value. For most crops, a decrease of ET by about 25 percent would result in a higher productivity in terms of yield per cubic meter water. However, the yield per hectare (and thus farm income) would decrease. Management of an irrigation system is recommended in such a way that the monthly values of the depleted fraction range is between 0.5 and 0.8. Such a management rule would provide sufficient water for leaching (at the 0.5 side of the range) and provide high crop yield per unit water consumed (at the 0.8 side).