This report presents "Cascade", a water balance model 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 and provides 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 model's applicability for evaluating feasibility of a cropping scenario, and thus its potential use in 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. (Author abstract, modified)