A model for the planning and operation of heterogeneous irrigation schemes in semi-arid regions under rotational water supply
This research is aimed at developing the method for efficiently using the water in irrigation schemes in semi-arid regions. These irrigation schemes are often short of water to irrigate entire culturable command area (CCA) with maximum water requirement of different crops and are characterised with different weather patterns, soils and the possibility to grow several crops. The CCA of these schemes is also large with several users or units, each having different characteristics. The previous research in this field was mostly either on optimum allocation of the resources considering the irrigation scheme as a whole or on evaluating the performance of the irrigation scheme for certain irrigation schedules for different units in the scheme. However in such schemes optimum allocation of resources (land and water) to different crops and their distribution over different units is important (optimum allocation plan, OAP). In the present study, the method and a computer model are developed to prepare OAPs for these irrigation schemes under rotational water supply, by incorporating the concepts of deficit irrigation and productivity and equity in the optimisation process. The previous research stressed the importance of equity observed in different ways but seldom adopted in optimum allocation of resources. Therefore this method includes the preparation of OAPs while observing equity in allocation of land and water resources and distribution of crop production and net benefits. The developed model, Area and Water Allocation Model (A WAM), consists of four phases each one for generating irrigation strategies, preparing irrigation programme for each irrigation strategy, screening irrigation programmes and allocating resources optimally to different crops in different units. The A W AM estimates the irrigation water requirement, crop yield and net benefits by simulating the various process in the irrigation scheme, produces the OAPs at preseason planning with different scenarios of productivity and equity and management options, develops the steady OAP by considering the temporal variability in the weather and modifies the allocation plan optimally during the intraseasonal operation of the irrigation scheme. A W AM operates in seven different modes to achieve this. These are simulation, calibration, generation, optimisation, planning, operation and evaluation. The A W AM was applied to Nazare Medium Irrigation Project (medium lmgation scheme) in semi-arid region of Maharashtra State, India to evaluate the existing practice of irrigation (fixed depth irrigation), full depth irrigation and deficit irrigation for obtaining the OAPs. The practice of deficit irrigation was found to be beneficial over the existing approach and full depth irrigation. The OAPs at preseason planning are obtained for several alternatives and compared. The OAPs were obtained for different equity criteria. The productivity and equity were found to be inversely related. The method is proposed to obtain the stable OAP with A WAM by considering several years' data. The present research contributes towards efficient utilisation of water in the irrigation scheme by incorporating the deficit irrigation and productivity and equity in obtaining OAPs, developing the methods to obtain the steady OAP and modifying the allocation plan optimally during the intraseasonal operation of the irrigation scheme.