To ensure sustainable agriculture, the conditions under which crops have to grow should remain stable over a prolonged period. For crops to grow optimal, water stress should be limited as much as possible. This occurs when the actual evapotranspiration is close to the potential evapotranspiration. At the same time degradation of the soil (salt accumulation), mining of the groundwater aquifer and the negative influence of drainage water on the downstream environment should be avoided. Water resources therefore should be used wisely and efficiently. To manage these water resources, the water balance of the irrigated area should be known.
The water balance of an irrigated area shows three sources of water: precipitation, groundwater inflow and river (surface) water diversion. Part of this “inflowing” water leaves the area as evapotranspiration; the remainder will flow to downstream areas either via surface streams (drains) or as groundwater. If the summed inflow exceeds the outflow, part of the water will be stored within the irrigated area and the groundwater table will rise. This increased storage may cause water logging and the related salinity. If, on the other hand, the summed outflow exceeds the inflow, the groundwater table will drop. In first instance this will reduce the availability of capillary water to crop growth. Depending on factors like soil type and depth to the groundwater table, capillary rise can be as much as several millimeters of water per day which becomes available to the plants. With continued mining of groundwater, this water resource will be depleted and may even lead to sea water intrusion. To avoid the above problems it is recommended to manage irrigation water in such a way that the groundwater table remains stable from year to year.
When irrigation water is applied, salts (but also pesticides and fertilisers) enter the soil. To avoid the accumulation of salts (in the root zone of the crops), the annual summed inflow of water should exceed the summed outflow by about 10 to 20 percent. Accumulation of salts can be tolerated during dry months provided that they will be leached during wet months. The summed outflow is the actual evapotranspiration (the real evapotranspiration) plus drainage and groundwater outflow.
The amount of water, which is applied to an irrigated area, influences the groundwater table during the year. Besides the type of crop, other factors may influence the groundwater table: effective rainfall, irrigation efficiencies (application methods, water conveyance, water delivery), soil type, and the reuse of water by pumping. These factors can also influence the availability of capillary water.
The CRIWAR model
The model is intended to be used in irrigated areas to calculate the irrigation water requirements of cropping patterns for various stages of crop development throughout the crops’ growing season. It further evaluates the efficiency of irrigation water use and reviews the annual fluctuation of the groundwater table. A strategy module can be used to determine water saving options.
The model consists of two parts. Part one deals with the Crop Irrigation Water Requirements. Part two contains the Strategy module. It uses an innovative method: using the depleted fraction to manage the groundwater table in an irrigated area.