Modeling optimal irrigation scheduling under conjunctive use of canal water and poor quality groundwater in semi-arid region of northwestern India

Abstract

In the state of Punjab, India available water resources are inadequate to meet the irrigation needs of the crops. Optimal irrigation scheduling includes allocation of limited water supply to several crops so, as to maximize the net benefits and reduce the stress of the crops during its growing season. Dynamic programming technique of optimization has been adopted for seasonal allocation of water for multiple crops (Wheat, Barley, Mustard and Gram). The stochastic nature of canal water releases of Golewala distributary for 20 years (1982-2001) was estimated by gamma distribution. Based on this expected values of canal water releases were computed as 3766.41, 4138.76, 4422.2, 4674.5 and 4918.95 hectare – meter (ha-m) corresponding to 10%, 20%, 30%, 40% and 50% risk levels of canal water releases in the distributary. The conjunctive use of canal water along with bad quality ground water offers sustainable water allocation option based on water production function. The seasonal allocation is done corresponding to different combinations of canal water and ground water at different risk levels of canal water. The seasonal water has been further redistributed on weekly basis by making use of dated water production functions and soil water balance equation. The potential evapotranspiration was estimated by Penman Montieth method and actual evapotranspiration was estimated on the basis of soil moisture balance in the study area. Economic co-efficient, crop areas, and crops growth stage stress effects are included in the mathematical formulation at both levels. The weekly allocation takes into account the initial moisture content along with limitations in terms of channel capacity, available water supply and soil storage capacity. The allocation of water was 97% and 3% for wheat and mustard crop respectively. Model did not allocate water to barley and gram crops in the catchment area. The seasonal water was redistributed on weekly basis with different risk levels of potential evapotranspiration. The weekly allocation of water varied from 0 – 22.5 mm for 10% risk level of evapotranspiration. The risk level of evapotranspiration did not much affect the allocation and varied from 278.08– 79.01 for full season. The net returns for 10% and 50% risk levels of canal water and 30% ground water were 8.51% and 32.42% higher than existing net returns observed in the command area. The increase in the ground water amount beyond 30% tends to have an adverse effect on the yield of the crops.