Study on adequacy of traditional water yield forecasting methods focusing on two dry zone basins in Sri Lanka

Abstract

The majority of dry zone basins are still ungauged in Sri Lanka, and this has led to uncertainties in the planning and development of water management infrastructure. The Irrigation Guideline Method of Sri Lanka (IGM) has been recommended to estimate the basin yield, but there is no appropriate evidence to evaluate the accuracy of the estimation. To fulfil this gap and for reliable yield estimation, a proper comparison should be undertaken incorporating the available water yield models. The IGM and HEC-HMS models were selected and compared by using the observed streamflow data of Kirindi Oya and Maduru Oya in the present study. Model parameters were identified by an extensive literature survey. Daily and 75% probable rainfall data were considered as the input data for the models and the model results were compared with the observed streamflow data. The evaluation has been carried out by using the flow hydrograph, annual water balance, flow duration curves, runoff coefficients and the MRAE value as an indicator. According to comparisons, it can be concluded that the IGM is still the best model for yield estimations in the dry zone watersheds and it was identified that rainfall act as the key factor for the variation of the yield.