Impact of rainfall variability and anthropogenic activities on streamflow changes and water stress conditions across Tapi Basin in India

Water security is a major concern in recent years, especially, in semi-arid and arid regions, which are adversely impacted due to extensive anthropogenic and climatic changes across the globe. Present study investigates the long-term changes in streamflows, their quantitative attribution and socio-environmental implications for a climatically sensitive Tapi basin in west central India. The abrupt and gradual changes in streamflows have been detected over the period 1973-2013 using non-parametric Pettit's and Mann-Whitney U tests; and Modified Mann-Kendall, Spearman's Rho and Sen's slope estimator tests respectively. The streamflow analyses revealed heterogeneity across the basin, wherein the tributaries exhibited higher variability in annual runoff compared to the Tapi River. The spatial analyst tool of ArcGIS has been used to graphically demonstrate the spatial variability in rainfall and streamflow trends across the basin. The streamflows exhibited predominantly decreasing trends across the basin, except in the upper reaches of the Tapi River. The decreasing trends in streamflow are largely attributed to anthropogenic activities such as changes in the land use patterns viz., increase in agriculture area, construction of minor and medium hydraulic structures, decrease in forest cover, rapid urbanization, excessive groundwater extraction and streamflow regulations. The anthropogenic activities played a major role in declining streamflow trends in the Tapi basin, contributing about 68%-99% (average 85%) decline in total runoff, compared to rainfall variability, which contributed around 1%-32% (average 15%) decrease in total runoff. The analysis of water stress levels at sub-catchment scale exhibited significant rise in percentage of population living in the basin facing chronic water shortage (i.e., <1000 m³/capita/year), from 34.0% in year 1981 to 63.2% in year 2011, mostly in semi-arid regions. The reduction in water availability clearly highlighted the deteriorating water quality and stream ecology of the Lower Tapi River in Surat city, India.