Strategies for the Management of Conjunctive use of Surface Water and Groundwater Resources in Semi-arid Areas

Abstract

"This report presents the results of a modeling study carried out to evaluate the long-term effects of a different quality of irrigation water on root zone salinity. The simulations were performed for the Rechna Doab (subbasin of the Indus Basin), Punjab, Pakistan by using 15 years of actual rainfall and climatic data. Rechna Doab covers approximately 2.8 million hectares of cultivated land. Groundwater quality in Rechna Doab varies from north to south. In the upper part of the doab, groundwater is relatively fresh (EC = 1.0 dSm-1), in the middle, there are several pockets where groundwater quality is marginal (EC = 1.5 - 2.7 dSm-1) and in the lower part of the doab groundwater is highly saline (EC > 2.7 dSm-1). In all these areas, groundwater is mixed with canal water in different ratios without knowing the consequences of any quality hazard. For model simulations, groundwater of these three qualities (i.e., 1.0 dSm-1, 1.5 dSm-1, and 3.0 dSm-1) was mixed with canal water in four different ratios i.e., 0 percent, 25 percent, 50 percent and 75 percent. In total 12 different scenarios were generated. The resultant water quality of each scenario was used as input to the model to study the long-term effects of this water quality on crop production and soil salinization.

"The simulation results indicate that in fresh groundwater areas, farmers present irrigation practices i.e., mixing groundwater and canal water with a 1:1, which ratio provides sufficient leaching to push salts below the root zone, thereby minimizing the risk of reduction in crop production. The direct use of fresh groundwater for irrigation will accumulate salts at shallow depths in the root zone, therefore, the risk of an upward movement of salts due to capillary action during a dry year will be high. Under these conditions, occasional leaching with canal water will be necessary to maintain favorable salt balance in the root zone."