This study was conducted to investigate the impact of changes in water management on water and salinity problems and crop production at field and basin level by analysing several probable scenarios. First, a simplified water and salinity basin model (WSBM) was developed for a quick analysis of river basin processes and was combined with the comprehensive field-scale model, SWAP (soil–water–atmosphere–plant). The WSBM model was calibrated and used for water resources analyses in Zayandeh Rud basin in central Iran. Observed and simulated stream flows were similar, proving that the model could be used for scenario analyses. Yield functions for cotton were developed with SWAP, including the impact of water quantity and quality on crop yields and field water and salinity balances. Three scenarios were considered. The first scenario analysed the effect of more efficient irrigation techniques on the basin water resources, where it was assumed that farmers would not accept lower water allocations if they invested in these more efficient techniques. Therefore, return flows would decrease and less water would be available for downstream users. It was concluded that the effect on the downstream irrigation schemes was dramatic, with a 22% decrease in yield. Obviously, upstream yields would increase. A second scenario was defined where the effect of an increase in water extraction for the town of Esfahan was evaluated. In terms of basin-scale water quantity aspects, this increased extraction was negligible as extractions were relatively low and return flows high. The last scenario was developed to study the additional releases required from the reservoir to provide sufficient water for expansion of the tail-end Rudasht irrigation scheme. If no restriction were imposed on water quality, additional releases from the reservoir would be limited. However, if salinity levels were not to exceed 2�dS/m, mean annual water release requirements from the reservoir would increase from 52 to 64 m3/s, and peak requirements during the irrigation season would increase from 85 to 112 m3/s. In this case, the crop yield would increase from 66% (for the baseline scenario) to 73%. Finally, it was concluded that the methodology and the models developed were useful for a swift and transparent analysis of past, current, and future water and salt resources, and to perform scenario analyses.
Uniform and lateral preferential flows under flood irrigation at field scale
