Water balance in the complex mountainous terrain of Bhutan and linkages to land use

• Temperature based Hargreaves equation calibrated its parameters to the Bhutanese condition and it resulted comparable values to those estimated by Penman Monteith (PM) method with full set of data.
• Multiple regression models are developed for seasonal and annual reference evapotranspiration (ETo) and water balance.
• ETo is significantly related to station elevation and latitude whereas water balance is related to station slope, latitude and longitude.
• Positive water balance areas are dominated by broadleaf forests and water balance with low or negative areas are with coniferous forests.

Study Region: BhutanStudy Focus: Located in the Himalayas with elevation ranging 100–7550 m and with an area equivalent to Switzerland, Bhutan has great biodiversity despite its small area. A monsoon-dominated climate causes generally wet summer and dry winter. Bhutan is highly dependent of climatic conditions for its developmental activities. Using multiple regression analysis we have established models to predict the evapotranspiration (ETo) and water balance and test the linkage to vegetation and land cover using meteorological data from 70 weather stations across Bhutan. Temperature-based ETo equations were evaluated in reference to the Penman-Monteith (PM) method and a calibrated Hargreaves (H) equation was used for computing the ETo. New Hydrological Insights for the Region. The calibrated Hargreaves equation gave good estimates of average daily ETo comparable to the PM ETo. The spatial variation in PM ETo is linked to variation in sunshine hours in summer and temperature in other seasons. Seasonal and annual ETo was mainly affected by elevation and latitude, which is linked to temperature and sunshine duration. Precipitation and water balance correlated positively with the Southern Oscillation Index (SOI) while ETo correlated negatively. Our models for predicting ETo and water balances performed clearly better than the global CRU gridded data for Bhutan. A positive water balance is found in broadleaf forest areas and small or negative water balance for coniferous forests.