Modelling Trends in Streamflow and Groundwater Storage in an Agricultural Basin: A Case Study of Himayat Sagar Catchment, India
The most visible signs of hydrologic change in a catchment can be observed from trends in streamflow and groundwater levels. Increasing irrigation and associated groundwater extraction, as well as interception of runoff by watershed structures (Hydrological Structures) in the Himayat Sagar (HS) catchment, India have caused serious declines in streamflows (- 3.03 mm/year) and groundwater levels (-0.22 m/year) over the past two decades. This paper reports modelling of the effects of changes in different catchment characteristics and separates their individual impacts on streamflow and groundwater with the aim of providing insights into the causes of hydrologic change. This paper first addresses whether the trends in streamflows and change in groundwater level/storage in the HS catchment can be captured by modelling the major observed changes in the catchment. It then examines, how important increases in hydrological structures and land use change are in causing the observed trends. Finally it examines the relative importance of declining Potential Evapotranspiration (PET) due to decreased wind speed compared with other drivers of change. The analysis was carried out in a semi-distributed manner using a HRU based surface runoff model combined with a lumped groundwater model in individual sub catchments called the Landuse Catchment Assessment Tool (LCAT). The model was calibrated on a relatively stable period of landuse and then verified over a period of rapid change. Variations in monthly streamflows were modelled very well during the calibration period (1980-89,E=0.84) and also performed satisfactorily during the verification period (1990-07, E=0.58), which was subject to significant landuse change. Trends in streamflows and groundwater storage were found to be primarily due to increasing irrigation and associated groundwater extraction (contribution to streamflow rate of change = -2.45 mm/year; groundwater levels rate of change = -0.15 m/year). Interception by watershed structures and reduction in PET due to wind speed on streamflows were estimated to be -0.18 mm/year and +0.20 mm/year each respectively. Overall, irrigation and groundwater extraction in Himayat Sagar catchment has already exceeded its sustainable limits and further expansion would further impact streamflows and groundwater levels.