ASSESSMENT AND MANAGEMENT OF POOR QUALITY WATERS FOR CROP PRODUCTION - A SIMULATION-MODEL (SWAM)

A simulation model has been prepared for assessing water quality to ju dge its suitability for irrigation. When water is classified as poor q uality water (saline/sodic/saline-sodic) utilizing standard norms for Indian agro-climatic conditions, the model determines the potential of the water for direct application. Further, it also evaluates manageme nt strategies based on conjunctive use of fresh and saline waters. For this purpose, the model requires water quality data, crop data, soil data and rules established in the expert system rule-base. Data are co mpiled in data files which can be updated. For conjunctive use of sali ne and fresh waters, an irrigation scheduling sub-model has been modif ied to include a soil salinization-desalinization module based on laye r-wise equilibrium theory. The module was independently tested using f ield data. The model SWAM has been successfully tested using data from a number of field experiments. Sodic waters of 16.2 meg l(-1) residua l sodium carbonate would require 2.73 t ha(-1) of gypsum for each 20 c m of water applied to the soil. Field observations usually attest to t his requirement. Likewise, saline water of 16 dS m(-1), when applied t o a wheat crop in conjunction with fresh water of 0.5 dS m(-1) would y ield optimally in case two saline water irrigations are followed by on e fresh water irrigation in a normal rainfall year with an initial soi l salinity of 2.98 dS m(-1). Some more useful data sets are analyzed a nd compared with results from field experiments. In our opinion, the m odel which is based upon recent guidelines can be applied to the class ification of waters and their management. The minor changes necessary to apply the model to other conditions can be easily carried out.