Predicting Water Availability in Irrigation Tank Cascade Systems: The Cascade Water Balance Model

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"In the dry zone of Sri Lanka and in other similar regions, better water management in irrigation tank cascade systems is vital in achieving higher productive use of available water. To develop and implement management practices aimed at improving effective use of water, studies leading to the development of models that can predict available tank water in irrigation tank cascade systems are invaluable. This report presents a water balance model Cascade that can predict tank water availability in the Thirappane tank cascade system in Anuradhapura, Sri Lanka. The model determines tank water availability on a daily basis, for the purpose of improving productive use of the water resources in a tank cascade system. It represents the physical system using a node-link system configuration, and incorporates water balance components of different types of irrigation tanks including rainfall runoff, rainfall on tank, evaporation of tank water, tank seepage and percolation, irrigation water release, spillway discharge and return flow from upstream tanks. An important feature of the Cascade model is that it employs a modified runoff coefficient method for estimating runoff from rainfall, which incorporates an Antecedent Precipitation Index (API) function as an indicator for catchment wetness, providing a simplified method for the representation of the nonlinear runoff-generation process. The model calculates tank seepage and percolation based on functions derived from an analysis of the observed tank water reduction during time periods without rainfall. "The Cascade model was calibrated using field data collected at four tanks over a period of 21 months, which represented different agrometeorologic conditions encountered at the Thirappane tank cascade system under both maha (wet) and yala (dry) growing seasons. The model was applied over a 10-year period for predicting tank water availability for rice crops in the Thirappane tank cascade system. The results demonstrated the applicability of the model for evaluating feasibility of a cropping scenario, and thus the potential use of the model in the development of management options for minimizing the effects of water shortage on crops. The model provided valuable insights into the processes that determine tank water balance, and clearly manifested the relative magnitudes of the tank water balance components and their temporal variations. Further, it demonstrated the availability of water from upstream tanks as return flow in the immediately downstream tanks, and thus the increased potential usage of water resources facilitated by the tank cascade system. Using a relatively modest input and a simple water balance modeling approach, the Cascade model provides a valuable means to determine water availability in the Thirappane tank cascade system. The model has shown its potential to become a useful tool in the process of optimizing usage of the limited water resources in tank cascade systems for improving agricultural production."



water resources--models, agriculture, irrigation, simulations