Understanding the Hydrological Response of a Forested Watershed Garhwal Himalayas, Uttarakhand, India

Abstract

The hydrological dynamics of dense forest cover catchments are highly complicated. Precipitation, soil storage water, actual evapotranspiration, surface runoff, base flow, and soil erosion are the most frequently used hydrologic components. Precipitation is a critical input into the process of hydrology and plays a vital role in understanding watershed response. Field observations using hydro-meteorological and hydrological data are required to comprehend the behavior of forested watersheds. Specifically, the baseflow of various rivers have been declining in the lesser Himalayan region. Further, sediment transport from high mountainous to lowland areas has significantly increased, which an essential geomorphological process. Both baseflow reduction and the increase in sediment transport require attention in the current times. In addition to these above processes understanding, actual evapotranspiration and groundwater recharge estimations are very less understood, and difficult to measure for forested watersheds in the mountainous regions. This study aims to understand the hydrological responses of a forested watershed, i.e., the Kempty watershed located in Garhwal Himalayas, Uttarakhand, India, using different hydrological tools and methods. Data have been received for five years and nine years from the meteorological and hydrological stations within the Kempty micro-catchment (January 2015 through September 2020). The monsoon is the major contributor every year to the seasonal and annual rainfall pattern. Analysis showed that the annual rainfall days varied widely from 66 to 185, so much during the monsoon season. This study also analyzed the streamflow discharge components, specifically the baseflow using a Hydro-Office (BFI+3) hydrograph separation model over the 16 months. The outlet monthly average discharge values reduce to 0.027 and rise to 0.203 m3/s in June and August 2020, respectively. The stream flow and storm flows at the outlet have shown a weak relationship with the rainfall. Other factors like rainfall intensity, duration, and watershed parameters influence their relationship. The baseflow analysis explained that the peaks of baseflow had been seen during monsoon periods and an average base flow index value at 0.92, showing much of the streamflow comes from the subsurface flow. As a result, the runoff coefficient for June-July-August (JAS) 2019 and JAS 2020 is 0.43 and 0.39, respectively. Seasonal and annual amounts of rainfall and streamflow strongly influenced annual suspended sediment flux. Mostly during the rainy season, 82% of the yearly load is being generated and only 6 peak flow events accounted for 40%. Monthly denudation rates for viii | the given study period range between 0.13 to 1.67 mm/year in August and December, respectively. The average monthly denudation rate is at 0.73mm/year. Actual evapotranspiration (AET) is estimated and compared using soil water balance, sink term, and soil water reduction (SWR) method from the five-year study period from the monthly recorded volumetric water content. Water balance, sink term, and soil water reduction methods estimated average monthly AET as 259.03 mm, 232,57 mm, and 330.61 mm respectively. The temporal variability of estimates of AET showed that water balance and sink term estimated similar values, except for insignificant changes. In contrast, the soil water reduction method estimated much higher than the other two methods. Since the SWR method showed higher temporal variability, we took the water balance estimates to calculate the groundwater recharge. Groundwater recharge estimated on average of 195mm per month using the water balance method. The monthly average of change in Ground water storage (ΔGW) is obtained 148mm. The present study shows that during the rainy season period, precipitation was the biggest factor in a watershed hydrologically properly working, while forests are providing significant services via control of water balance, soil moisture, and sediment through various forest-scale mechanisms in the Garhwal Himalayas.