A transient model approach to improve on-farm irrigation and drainage in semi-arid zones

A transient model approach is introduced to improve design procedures for subsurface drainage systems in relation to different irrigation management strategies to increase crop productivity and environmental sustainability in the water scarce environment of Pakistan. The water flow and solute transport model, SWAP, is used to evaluate the impact of irrigation and drainage on crop transpiration, soil salinity and groundwater table behavior taking 15 years of actual weather data. Model calibration improves considerably when field determined soil water retention curves were used. The reference evapotranspiration calculated by the Priestly-Taylor method appears to be physically more realistic than the Penman-Monteith method because the latter ignores the feed-back mechanism of vapor pressure deficit on stomatal closure. For the Fourth Drainage Project (FDP) conditions, a zero flux at the bottom of the soil profile was found to be a suitable bottom boundary for further model simulations.For semi-arid areas, the selection of a proper drain depth is more critical than drain spacing. A drain depth of 2.2 m is found to be optimal for the multiple cropping system of the FDP-area. This drain depth gave the best results with regard to crop yields, soil salinity and groundwater table control at rather low drainage intensity ( q drain /Δh ), resulting from a drain spacing of 500 m. Long-term model simulations covering a period of 15 years show that the present FDP drainage system has been designed at too high drainage intensity. If no operational and maintenance constraints are present, the FDP-area could be drained with 25 percent less drainage intensity.Under shallow groundwater table conditions, reduced irrigation applications can save up to 25 percent of the canal water each year. This strategy will produce reasonably high crop yields (relative transpiration T act /T pot > 0.90) and limit field percolation losses. For either conjunctive use or use of tubewell water alone, reduced irrigation applications will not be sufficient and additional supplies would be required for leaching the salts from the root zone. It must be recognized that during relatively dry years drainage is not a guaranteed success. In the absence of a drainage system, leaching of salts by means of poor quality irrigation water will not be suitable. For these areas other options like growing more salt tolerant crops should be considered. Reduced irrigation inputs is a proper short-term solution, however, in the long run drainage systems associated with adjusted irrigation planning seems necessary.Under average conditions, the effect of irrigation schedule flexibility on crop yields is insignificant. However, compared to a fixed schedule and when un-restricted water supplies would be available, the productivity of water ( Y act /I rr ) for the on-demand schedule would be up to 30 percent higher, but at the cost of salinity build up. The average annual water use by the on-demand schedule is 20 percent lower than the fixed schedule, which would result in 30 percent lower drainage volumes and 15 percent lower recharge to the groundwater. In the absence of sufficient canal supplies, necessary infrastructure and management facilities in the Indus basin, moving towards a demand-based system would neither be economically feasible nor socially acceptable. Therefore the emphasis should be on reducing irrigation water application and constructing drainage in conjunctive water use areas.Key-words : drainage design, irrigation, crop productivity , soil salinity, sustainability , integrated water management, transient modeling, semi-arid zones, Pakistan.