Ecohydrology of Sikkim Himalayas and its Future under a Changing Climate

Abstract

Changing climate and its interaction with rapid land-use and land-cover (LU&LC) changes are major threats to water security in subtropical mountains like the Himalaya. The temperature rise and changes in precipitation regimes are likely to negatively impact the springs and streams, the principal water resources for over 90 % of the Himalayan population. However, inadequate climatic data and lack of regional studies limit our understanding of climate change and variability on spring and stream hydrology in the Himalaya. My work addresses this lacuna using an ecohydrological framework for understanding the linkages and feedbacks between vegetation, climate, and water in Sikkim Himalaya, a representative of the Eastern Himalaya biodiversity hotspot. I focus on three critical aspects of Himalayan ecohydrology: (a) characterising the drivers of vegetation-water use and its impact on hydrological fluxes; (b) investigating the relative influence of vegetation, climate, and hydrogeology on flows in streams and springs and (c) potential changes in regional hydrology in the context of climate variability and change. I collected high-resolution discharge and microclimate data for five streams (1st – 3rd orders) and three springs in Sikkim between 2011-2017. The elevation (1100 – 4200 masl) and rainfall (1400 – 4900 mm) gradients across the study covered sub-tropical, temperate evergreen, and sub-alpine conifer forests.

In a first from Himalaya, I show that stand transpiration significantly controls lean-season streamflow in a wet-temperate forest. Sunlight and vapour pressure deficit were the primary drivers of diurnal and seasonal transpiration in winters and summers, respectively. However, the impact of vegetation on flows varied between forest types and eco-climatic regimes. Streams from sub-alpine forests showed more stable wet and lean season flows than temperate broad-leaved forests. Snowmelt and sub-surface pathways influenced seasonal flows from sub-alpine forests, whereas high transpiration demand dominated temperate forest streams. Conversely, aquifer properties governed seasonal springflows, whereas LU&LC influenced diurnal variability in discharge and net recharge to groundwater from rainfall. Lastly, I illustrate the potential of mapping spatio-temporal patterns in rainfall across Sikkim using a combination of ground observations and gridded-precipitation products (GPPs). Strong diurnal cycles in rainfall are observed peaking at late-night in monsoon and afternoon in summer. Declining trends in monsoon rainfall and rainy days and an increase in summer rainfall are reported. The frequency of high-intensity (< 100 mm d-1) events and the size of large storms have increased in southern and eastern Sikkim, most vulnerable to natural hazards like landslides and flash floods.

Climate change projections for the next century, from published literature, project an increase in summer and monsoon rainfall, a decrease in snowfall, and warmer days and nights in Sikkim. Overall, my study suggests that the positive impact of warming and increased moisture on forest productivity is likely offset by reduced sunlight availability. Additionally, the likely shift in peak forest productivity window to warmer winters, along with reduced snowfall, will induce significant water stress in forest ecosystems in Sikkim. Consequently, temperate forests may experience higher flood risk in summer and monsoon, whereas lean season flows from sub-alpine and alpine forests may show further decline. Karst and fracture springs, and springs with smaller storage, show higher vulnerability to future changes in rainfall and LU&LC. My results could inform management and policy initiatives for prioritising the conservation of catchments of springs and streams in Sikkim Himalaya under a changing climate.