Assessment of the natural flow regime and its variability in a tributary of Ganga River: Impact of land use and land cover change

Abstract

Accurate assessment of hydrological processes in a catchment including the estimation of virgin or natural flow regime of the rivers is critical to maintaining healthy river ecosystems, particularly in the developing nations that currently face the challenges of limited quantity and poor quality of available water. In this work, we aim to estimate the natural river flow and its variability due to land use and land cover (LULC) alterations over a period of ∼43 years (1970–2013–14) in the Ramganga River Basin (RRB), a sub-basin of the most fertile and densely populated basin in the world i.e., the Ganga River Basin. Hydrological modeling of the RRB was performed using the Soil and Water Assessment Tool (SWAT) and its performance was evaluated by performing statistical analysis (e.g. coefficient of determination and Nash–Sutcliffe efficiencies). The simulated flows exhibited good agreement with the observed flows at the three flow monitoring stations in the RRB - Katghar, Chaubari, and Dabri. Upon achieving a good model performance, the natural flows for the 2013-14 LULC scenario were estimated by removing all existing flow interventions (e.g. dams and barrages) and irrigation usage. Average annual natural flows were observed to be higher by at least ∼40% than the average annual existing flows (i.e., flows estimated with current managed conditions) for the three stations. Historical LULC information for the basin was obtained from the object-based image classification of CORONA satellite images (1970). The annual average natural flows for the 1970 scenario were observed to be lower (3–12%) than the 2013-14 scenario which could be attributed to a significant increase in urban settlements and a reduction in the area coverage by barren land and shrubs and bushes. Overall, our work provides valuable insights into the natural flow regime and its variability due to anthropogenic activities and long-term LULC alterations in the RRB which may inform future strategies for sustainable water resources management in the basin.