Streamflow generation in a drying catchment outside Bangalore, India

Abstract:

The causes of a dramatic reduction in surface flows in the TG Halli Catchment near Bangalore, India in the past forty years remain largely uncertain. The development of conceptual and process models is currently hampered by a fundamental lack of understanding of runoff generation mechanisms, and whether these mechanisms changed in concert with declining surface flow. We used stable water isotopes as tracers, measuring their concentrations in rainfall, soil water, and surface runoff. The isotopic profiles of runoff were very similar to rainfall, and displayed no evidence of mixing with soil water, leading us to conclude that infiltration excess runoff generation mechanisms were dominant, consistent with observed sharp rising and falling limbs in the regional hydrograph. This interpretation was supported by observations from nearly 100 surficial wells, which confirmed that water tables remained well below the level of the stream channel during the monsoon season. Infiltrometer measurements across different land uses in the catchment indicated broadly similar rates of infiltration capacity, suggesting that land use is unlikely to have altered the frequency with which infiltration excess runoff occurs. Changes in the occurrence of large storms over the 40 year period in which streamflow declines were observed are not sufficient to explain the decline in streamflow, if the runoff generation mechanism had not also changed during this period. We suggest that the results are consistent with the interpretation that a change in the dominant runoff generation mechanism has occurred simultaneously with the reductions in flow in the TG Halli catchment, potentially induced by the widespread expansion of groundwater pumping in the region.