Assessing the compound impacts of climate change and land use dynamics on future groundwater availability in dry land areas of Sri Lanka

Abstract

Climate change and land use and land cover (LULC) change are the primary challenges that disrupt groundwater availability. While numerous studies have examined their individual impacts, a critical gap exists in understanding their combined impact on groundwater availability. To address this gap, the present study assesses the compound impacts of climate change and LULC change in the Kumbukkan Oya catchment, a dryland area in southeastern Sri Lanka. The analysis uses hydro meteorological data, groundwater levels, borehole logs, and catchment characteristics such as land use, soil type, and elevation. Climate data were obtained from 1989 to 2011, while the groundwater level data and hydrological data were used from 2022 to 2024 at a mostly daily time step. Groundwater flow was then numerically modeled with MODFLOW. Projected future climate was obtained from the CNRM-CM6-1-HR General Circulation Model (GCM) and downscaled to the local scale using a stochastic Random Forest (RF) machine learning algorithm. Land-use data of 2018 and projected for future conditions were employed in assessing LULC dynamics. According to the result, the model captures the seasonal patterns of decline and recovery, particularly in the upstream, although some overestimation occurred in mid-catchment areas. Future projections indicate a decrease in seasonal rainfall compared to the baseline period, with reduced average rainfall but still frequent extreme events. The future period total rainfall will be 1,552 mm, and the observed total of 1,733 mm. Based on the projected results, groundwater levels are expected to decline by approximately 0.43 m to 0.65 m compared to observed con ditions. Therefore, simulation reveals that the Kumbukkan Oya catchment has some extent of resilience in the groundwater level relative to the projected climate and land use change. However, localized tendencies in the fall in groundwater levels suggest the need for site-specific adaptation responses, particularly over vulnerable recharge zones.