Conjunctive use of surface and groundwater to improve food productivity in a restricted area

Abstract

This thesis presents alternate policy decisions based on technical strategies to operate minor and medium irrigation schemes with integrated conjunctive use of surface and groundwater to improve groundwater systems in a restricted area for the economic pumping for agricultural and domestic water use, by optimizing the use of groundwater and surface water. A groundwater simulation model was formulated using integrated finite difference method to carryout this research. Unlike finite difference method the integrated finite difference method can be formulated to any shape of catchment by connecting perpendicular bisectors of the observation points through out the catchment. An electronic spread sheet model was developed for groundwater system in integrated finite difference method and was applied to a selected restricted catchment area of about 185.23 km2in Vavuniya, for testing its validity after calibration. Forty one domestic dug wells were identified as observation wells among the available domestic/agro wells within the study area of 185.23 krrr', to represent the aquifer. This study area was divided into forty one Thiessen polygons by connecting the perpendicular bisectors of adjoining observation wells. A groundwater simulation model was formulated for this polygonal net work using integrated finite difference method in spreadsheet. The model was calibrated for the period 1997 to 2001 having eight seasons. The recharging period of eight months was taken as from 1st October to 31st May of the following year and discharging period of four months was taken as from 1st June to so" September. By this calibration the hydro geological stress parameters such as Transmissibility, Storage coefficient, Recharge coefficients for irrigation tank, irrigation field, rainfall and the Withdrawal factor for agro and domestic pumping were found using an optimization technique. A complete water balance study for each polygon for each season was carried out. Forty one error models have been prepared for the water balance, for each polygon for all the seasons. To avoid the negative and positive errors getting cancelled, the squares of all seasonal errors were added and minimized with suitable constraint. Practicable ranges for Transmissibility, Storage coefficient, Recharge coefficients and Withdrawal factor were given during minimization as constraint. MAT was used for this optimization. This model in spreadsheet, calibrated by error optimization technique, validated and recalibrated was used to predict the system behavior for various operational policies of the selected restricted groundwater catchment. Water levels were predicted for changes in operational policy of minor and medium irrigation schemes by forgoing certain percentage of cultivation, boundary treatment to reduce the transmissibility in steps, and combination of both. The economic feasibility was analyzed by taking the energy saved in pumping of raised groundwater as a benefit and boundary treatment cost and income loss due to change in operational policy of minor and medium irrigation schemes by forgoing certain percentage of cultivation as cost. The present worth of benefit and cost for various interest rate and project life period were calculated and compared. Change in operational policy of minor and medium irrigation schemes by forgoing one third of the cultivation under them or keeping one fourth of the storage of minor and medium irrigation schemes at any time together with 40% - 50% reduction in boundary permeability will recover an average of 60% to 70% of the loss of water table in any consecutive season in almost 95% of the area under consideration.