Evaluating drainage design parameters for the Fourth Drainage Project, Pakistan by using SWAP model: Part II - modeling results

This paper presents the results of model simulations to evaluate drainage design parameters for the Fourth Drainage Project (FDP), Punjab, Pakistan. The SWAP model was applied to compute the effects of land drainage (12 combinations of drain depth and spacing) on soil moisture conditions in the root zone and their effect on crop yield and soil salinization. For the conditions considered, the selection of drain depth is found to be more critical than that of drain spacing. Deeper drains perform technically better in relation to crop growth and soil salinization. The optimum drain depth for the multiple cropping system of the FDP-area was found to be 2.2 m. This drain depth will produce reasonably good crop yields at rather low drainage intensity while keeping the root zone salinity within acceptable limits. This drainage design also maintained the groundwater table depth below the root zone throughout the growing season. The outcome of this study reveals that the drainage design criteria applied for the FDP is rather conservative with high drainage intensity. The FDP-area can effectively be drained with a 25 percent lower drainage intensity (qdrain/Δh) provided no operational or maintenance constraints are present. However, the final decision on the optimum combination of drain depth and drain spacing would require a thorough economical analysis. The non-steady state approach proved successful in analyzing the complex interactions between irrigation and drainage components. It is a valuable tool to optimize the design of drainage systems against crop yields and soil salinization.