Spatio-temporal distribution of groundwater recharge under climate change in the Namngum++ river basin in lower Mekong region

Abstract

Groundwater meets the water needs of one third of global population. The erratic nature of rainfall, likely to be exacerbated by climate change further compounded by population growth and urbanization, underscore the crucial role of groundwater in water resources management in Namngum++ river basin in Lower Mekong. Climate change affects groundwater by altering different hydrological processes such as evapotranspiration, infiltration and most importantly the ground water recharge. This study attempts to quantify groundwater recharge in the basin and how climate change is likely to impact it in three stress periods (2030s, 2060s and 2090s) under socio-economic pathways (SSP 2-4.5 and SSP 5-8.5) scenarios. Precipitation, maximum temperature and minimun temperature datasets from four global climate models (GCMs) under coupled model inter comparison project phase 6 (CMIP6) were linearly bias corrected. The Soil and Water Assessment Tool (SWAT), a semi-distributed model, was then employed to assess climate change impact on groundwater recharge. The model performance was very good with coefficient of determination and Nash Sutcliffe Efficiency of 0.72 and 0.70 for calibration period (2000–2010) and 0.80 and 0.77 for validation period (2011–2015) respectively. The hydrological response unit (HRU) level groundwater recharge for four GCMs were individually computed then ensembled by averaging across stress periods. Precipitation, minimum temperature and maximum temperature are likely to increase by 8.20%, 1.40 °C and 1.60 °C for SSP2-4.5 and 11.15%, 1.85 °C and 1.96 °C for SSP5-8.5 respectively. Groundwater recharge is likely to increase for 2060s and 2090s by 9.31% and 8.31% respectively in SSP2-4.5 and decrease by −3.25% in 2030s while increase up to 2.23% in 2060s and finally decreasing by −1.10% in 2090s in SSP5-8.5. The differences in the direction of change for two scenarios are attributed to increase in evapotranspiration due to elevated temperature, as well as increased stream flow yielded by increased precipitation in SSP5-8.5 compared to SSP2-4.5. While this study acknowledges uncertainties that permeate every step, it still presents a potential future scenario for water management.