Streamflow trends of Kelani river basin in Sri Lanka (1983-2013)

Assessment of hydro-climatic trends is useful in water resource planning, especially in river basin scale. This study investigated the trends of streamflow in the Kelani river and its association with rainfall over 31 years period. Streamflow and rainfall were assessed using Innovative Trend Analysis (ITA) and also Mann Kendall test (MK) with Sen’s slope estimator. The association between streamflow and average catchment rainfall was tested using Spearman’s rho correlation coefficient (ρ). Both ITA and MK tests confirmed the decreasing annual and seasonal streamflow trend from mid-stream to downstream of the Kelani river basin. A decreasing trend of rainfall was recorded at 75% and 63% stations during South West Monsoon (SWM) and Second Inter-Monsoon (SIM) respectively. However, annual, North East Monsoon (NEM) and First Inter-Monsoon (FIM) rainfall showed an increasing trend at 63%, 88% and 100% stations respectively. There was a significant association between streamflow and catchment rainfall (p<0.05) for 70% of stations suggesting that the variation of streamflow is mainly attributed to the variation of catchment rainfall. The decreasing trend of streamflow and rainfall during SWM and SIM towards the downstream area with the increasing temperature trend indicate a drying tendency of the Kelani river basin over the study period.