Spatial characterization of long-term hydrological change in the Arkavathy watershed adjacent to Bangalore, India

Abstract. The complexity and heterogeneity of human water use over large spatial areas and decadal timescales can impede the understanding of hydrologic change, particularly in regions with sparse monitoring of the water cycle. In the Arkavathy watershed in south India, surface water inflows to major reservoirs decreased over a 40 year period during which urbanization, groundwater depletion, modification of the river network, and changes in agricultural practices also occurred. These multiple, co-varying drivers along with limited hydrological monitoring make attribution of the causes of water scarcity in the basin challenging, and limit the effectiveness of policy responses. We develop a novel, spatially distributed dataset to understand hydrologic change by characterizing trends in surface water area in nearly 1700 rainwater harvesting and irrigation structures known as tanks. Using an automated classification approach with subpixel unmixing, we classified water surface area in tanks in Landsat images from 1973 to 2010. The classification results compared well with a reference dataset of water surface area of tanks (R2 = 0.95). We modeled water surface area of 42 clusters of tanks in a multiple regression on simple hydrological covariates and time, and found distinguishable trends in water surface area in different regions of the watershed. Agricultural areas with considerable groundwater irrigation exhibited the strongest drying. Urban land use was associated with intra-urban drying, likely due to tank encroachment, and downstream periurban wetting, likely due to increased urban effluents. Disaggregating the watershed-scale hydrological response via remote sensing of surface water bodies over multiple decades yielded a spatially resolved characterization of hydrological change in an otherwise poorly monitored watershed. This approach presents an opportunity for understanding hydrological change in heavily managed watersheds where surface water bodies integrate upstream runoff and can be delineated using satellite imagery.