Comparative Hydrological Dynamics and Water Security in Sundarijal Watershed: A RHESSys Modeling Approach for Broadleaf and Conifer Forests

In order to restore steep, mountainous watersheds from the effects of forest loss and significant land degradation, Nepal has developed an active program of reforestation. This has been realized through the development of forest conservation areas, and the devolvement of forest governance to local communities to develop sustainable forest management practices. Reforestation has occurred both by the growth of native tropical broadleaf forest (Sal) and by pine plantation. We investigate the impacts of both Sal and pine expansion and growth on coupled watershed ecohydrology, and particularly water security issues of springs, which are the major source for local communities, and downstream freshwater flows. The Sundarijal watershed covers ~38 square kilometers within Shivapuri National Park and is a pivotal water source for local communities and populations downstream in the Kathmandu Valley. The period between 1970 and 2000 witnessed extensive deforestation due to the demand for wood fuels, urbanization, and agricultural expansion. Recognizing the environmental consequences, the area was designated a protected zone in 2000, prompting reforestation initiatives for ecological restoration. This study utilizes the RHESSys (Regional Hydro-Ecological Simulation System) model, simulated from 1998 to 2017, to analyze hydrologic and ecosystem recovery. The protective measures implemented after 2000 led to an increase in forested areas, resulting in decreased streamflow. Notably, a peak in streamflow between 1998 and 2000, attributed to more bare lands and deforestation, was followed by a decline up to 2011. Two scenarios are explored: one featuring a tropical broad-leaved forest (Sal Forest) and another with a conifer forest (Pine Forest). Comparative investigation reveals broadleaf forests exhibit higher streamflow, while conifer forests display higher evapotranspiration and deeper water table depths. These hydrological distinctions emphasize the diverse impacts of forest species on the local water cycle. Broadleaf forests emerge as crucial players in terms of water security. Their lower water use results in greater groundwater recharge and increasing spring water source reliability. This enhanced water availability holds profound implications for communities in the watershed area, fostering sustainable water management and bolstering considerations of water security, including availability, access, quality, sustainability, resilience, and equity. Our study sheds light on the intricate relationship between reforestation efforts, forest types, and ecohydrological dynamics in the Sundarijal watershed. It provides valuable insights for watershed management strategies, emphasizing the importance of broadleaf forests in ensuring water security for surrounding communities. The research offers a comprehensive overview of the complex interplay between environmental interventions, forest ecosystems, hydrological processes, and water security acknowledging the historical context of deforestation and protective measures that have shaped the hydrological patterns in the region.