Water resources availability and low flow discharge analysis of two selected river basins in the dry zone under changing climate conditions

Abstract

Climate change is the most significant challenge to achieving sustainable development and it is one of the most discussed topics today due to the frequent changes triggered around the globe as a result of both manmade and natural causes. Water resources and river flow are among the main hydrologic parameters often affected by climate change. In the present study, the predicted climate change variations are derived from literature and the HEC-HMS rainfall-runoff model is used to investigate impending climate impacts using two dry zone basins. For Kirindi Oya and Maduru Oya basins, the model was calibrated using data period from 2001/2008 and validated from 2009- 2015. The MRAE (Mean Ratio Absolute Error), NSE (Nash-Sutcliffe Efficiency), R2 (Coefficient of Determination) and PBIAS (Percentage of BIAS) were used as objective functions. Even though the main study area is the dry zone of Sri Lanka, the analysis included the other climatic zones, mainly the wet zone as well, as they have a direct or indirect impact on the river flows in the dry zone. Synthetic climate change scenarios based on predicted climate variations were considered for future streamflow generation. In this semi distributed model for Kirindi Oya Basin, the MRAE and NASH values were found to be 0.79 and 0.62 for calibration and 0.70 and 0.60 for validation, respectively. For Maduru Oya Basin, the MRAE and NASH values were 1.10 and 0.50 for calibration and 5.10 and 0.24 for validation. Model performance is low for Maduru Oya Basin and Kirindi Oya Basin shows satisfactory results. Based on flow hydrograph and flow duration curve analyses, it was established that the low flows are slightly underestimated indicating that the model baseflow component has to be further refined. The research findings indicate that HEC-HMS in combination with Standardized Precipitation Index (SPI) analysis is a useful tool in low flow analysis and assessment of water resources availability. Global climate change data for precipitation and evaporation were used for observing the future changes in river flow. Different climatic scenarios were adopted and percentage change in flow was analysed both graphically and statistically to derive basin-specific streamflow response to climate change. The study results were useful in gaining better insight into how climate change affects water resources and low flow conditions in the dry zone basins in Sri Lanka and elsewhere with similar hydrologic conditions.