Abstract:Hydrological modeling is a commonly used tool by water resource planners to simulate the hydrological response in a basin due to precipitation for the purpose of management of basin water. With the increasing demand for limited water resources in every basin, careful management of water resources becomes more important. The Deduru Oya river in Sri Lanka supplies water to number of new and ancient irrigation systems and the management of water resources in the Deduru Oya river basin, which has an area of 2620 km 2 , is important for optimum utilization of water for these irrigation systems. This paper describes a case study of continuous rainfall-runoff modeling in part of the Deduru Oya basin with intra-basin diversions and storage irrigation systems using the Hydrologic Engineering Center -Hydrologic Modeling System (HEC-HMS) version 3.0.1 to estimate runoff in the Deduru Oya river.Long term daily rainfall data at several rain gauging stations, evaporation, land use and soil data in the river basin, daily river runoff at a stream gauging station, intra-basin diversions from the river into a storage reservoir, irrigation releases from the reservoir and drainage flow returned to the river from irrigation systems were used to set up the HEC-HMS model. Five-layer soil moisture accounting loss method, Clark unit hydrograph transformation method, and recession base flow method of the HEC-HMS model were used. Temporally varying irrigation water uses, storages and losses in the basin were taken into account in the analysis. The results depict the capability of HEC-HMS to reproduce stream flows in the basin to a high accuracy with averaged computed Nash Sutcliffe efficiencies of 0.80. The study demonstrates potential HEC-HMS application in flow estimation from tropical catchments with intra-basin diversions and irrigation storages. The model developed is a tool for water management in the Deduru Oya river basin.

Flood warning and effective radial gate operations of Deduru Oya reservoir are important in flash flood controlling and reducing the impact of flash floods. Future rainfall forecast is essential to obtain runoff forecast into the reservoir and for the hydrological modelling. A limited number of studies have been conducted on weather forecasting in Sri Lanka. This study proposes a weather forecasting model for the basin using Weather Research and Forecast (WRF) model. WRF model was optimized by doing a series of simulations with multiple microphysics and cumulus physics parametrization schemes. Optimum physics combinations were identified considering two extreme rainfall events on 2 November 2015 and 16 May 2016, and the results were validated for the extreme rainfall event on 1 September 2017. WSM3 and Betts-Miller-Janjic cumulus schemes were found to be the optimum parameterization schemes for microphysics and cloud microphysics, respectively. The developed model can be used for weather forecasting in the region.

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Rainfall in DeduruOya basin has a significant temporal variation and thus the DeduruOya carry flash floods during rainy season and very low flows during dry season. TheDeduruOya reservoir under construction at the upstream of the existing RidiBediElaanicut will be useful to regulate discharge of the DeduruOya for better utilizing the basin water resources especially for irrigation. The multipurpose DeduruOya reservoir project with a reservoir of a capacity of 75 Million Cubic Meters (MCM), augments water resources in 136 existing tank based irrigation systems in the DeduruOya Left Bank through a Left Bank (LB) canal and also diverts water to the Iginimitiya tankin the MeeOya basin through a Right Bank (RB)transbasin canal. This study develops a model for water management in LB canal development area and for the assessment of diversion requirement from the DeduruOya reservoir through the LB Canal to supplement LB irrigation demand. Hydrological Engineering Center-Hydrological Modeling System (HEC-HMS) is used for runoff estimations and CROPWAT model is used to estimate crop water requirements. Water Evaluation And Planning (WEAP) model is used for water balance simulations in DeduruOya LB canal development area and to calculate water requirements from LB canal for the period of recent 10years. The study reveals that the annual water requirement from the LB canal for 100% cropping intensity in the proposed 3000 ha irrigable area in LB canal development area varies from 26 MCM to 41 MCM.