SALINIZATION AND CROP TRANSPIRATION OF IRRIGATED FIELDS IN PAKISTAN PUNJAB

In Punjab, Pakistan, scarcity of canal water has lead to a rapid growt h of the number of tube wells, pumping poor quality groundwater. In re cent years, concerns have been expressed about the threat of salinizat ion through the increasing use of tube well water for irrigation purpo ses. This paper reviews research results of a model study on the impac t of irrigation practices, i.e., irrigation quantity, quality and freq uency, on soil salinity and crop transpiration of cotton and wheat in a conjunctive use environment. Four fields with a different soil textu re and irrigation water quality were selected and monitored during thr ee cropping seasons. The agro-hydrological model SWAP93 was used to an alyse the water flow and salt transport for the prevailing conditions. In the calibration process, the simulated water and salt balance appe ar to be sensitive to rooting depth, crop factors, actual soil evapora tion and the soil retention function. Two concepts were used to accoun t for field-scale heterogeneity of water infiltration and preferential flow. Taking into account this heterogeneity, the percolation losses increase, while the leaching of salts decreases. The calibrated model was used to evaluate the effects of various irrigation practices on sa linization and crop transpiration. The irrigation water quantity, in t erms of under-irrigation, has a greater impact than the quality of the irrigation water. Irrigation frequency may have a significant impact on the water and salt balance, especially on sandy soils. Finally, lon g term effects of current irrigation practices are discussed, in relat ion to the different soils in the study area. Soil texture appears to affect crop transpiration and salinization considerably. (C) 1997 Else vier Science B.V.