Reconstructing surface hydrological change from 1973 to 2010 in the Arkavathy watershed near Bangalore, India

Population growth, urbanization, and intensification of agriculture have placed increasing pressure on hydrological systems in south India. Man-made lakes known as "tanks" have been used for centuries to capture monsoon-season streamflow for irrigation. Many of these tanks have dried since the 1970s, but the hydrological changes associated with the drying of tanks are not well understood. We focus on the the Arkavathy watershed near Bangalore, India, where humans activities have modified the landscape through groundwater depletion, watershed development programs (designed to encourage groundwater recharge), and urbanization. We estimated tank water extent in over 1000 tanks in Landsat images from 1973 to 2010 using an automated classification algorithm with sub-pixel unmixing. Classification error was small in pixels containing only water or land, but higher in pixels containing a mix of water and land. At the tank level, errors in water extent were unbiased and classification accuracy improved with increasing tank size. We aggregated water extent in clusters of at least 15 tanks, and used a statistical model to estimate the temporal changes in each cluster after accounting for the interannual variability of precipitation. The results revealed a range of drying and wetting, with land use playing an important role in hydrological changes. Groundwater-irrigated agriculture was associated with a reduction in tank water extent over the study period. Tanks within and downstream of urban areas exhibited mixed results. In some cases, urbanization led to increased tank water extent (likely due to urban water imports), while in other cases tank water was diminished due to encroaching construction. In general, there was an increase in tank water associated with natural land cover. In this region, where historical hydrological data is scarce, classification and analysis of tanks can be a useful tool in understanding long-term hydrological change.