Impact of Land Use/Cover and Climate Change on Water Balance in the Ouémé River Basin, Benin

Abstract

The influence of land use/cover (LULC) and climate changes on hydrological processes are not fully understood in data-scarce regions such as the Ouémé River Basin, Benin. However, this is crucial for sustainable water management, infrastructure design, flood mitigation, and improving food security. This study assessed the impact of historical and projected future LULC changes, and climate change on hydrological components: surface runoff, lateral flow, aquifer recharge, and evapotranspiration. Using Landsat images from 1986, 2000, 2015, and 2023, LULC in the basin were categorised through a supervised classification in Google Earth Engine into Forests, Savanna, Settlements/bare lands, Water bodies, and Agricultural lands. The classified LULC maps were used to estimate water balance in the Soil and Water Assessment Tool (SWAT) model. LULC was then projected for 2030, 2063, and 2100 using the Cellular Automata-Markov projection, and future climate from 2021-2100 was projected using the ACCESS-CM2 model precipitation and ensemble mean temperature from five CMIP6 climate models. These were subsequently used to project future water balance, and a linear regression was used to examine the relationship between LULC, climate, and water balance components. The 1986 LULC map showed that the basin was predominantly Savanna (70 %), but these and Forests have reduced in 2023 by 24 % and 4 %, respectively; however, Settlements/bare lands and Agricultural lands have increased by 1 % and 27 %, respectively. These LULC changes resulted in increased surface runoff (32 mm/y) from 1986 to 2023, with a reduction in lateral flow (6 mm/y), baseflow (5 mm/y), aquifer recharge (22 mm/y), and evapotranspiration (6 mm/y). The highest rates of change were observed between 2015 and 2023. The projected future LULC changes under the business-as-usual scenario showed continued expansion of Settlements/bare lands and Agricultural lands at the expense of Forests and Savanna. The projected future climate under the “Sustainability” scenario, SSP1-2.6 showed gentler variability in rainfall patterns from 2021 – 2100, and more drastic changes in precipitation and temperature under the “Regional Rivalry” scenario, SSP3-7.0. The projected future climate resulted in an increase in surface runoff and a reduction in subsurface flow where temperature and precipitation increased; however, where precipitation reduced, surface and subsurface flow reduced. The combined future LULC and climate change increased surface runoff and reduced subsurface flow at a potential lower than LULC change alone and higher than climate change alone, with trends mirroring precipitation change patterns. The Partial Least Squares regression revealed that water balance in the basin was more sensitive to changes in Settlements/bare lands and Forests than Savanna and Agricultural land, and more sensitive to temperature than precipitation changes. The combined effect of increased runoff and reduced subsurface flows will increase the risk of people displacement and destruction of properties during flooding, and limit water availability for domestic and farming activities. Floods and droughts would lead to higher crop failures. Thus, sustainable land management practices such as reforestation, green urban spaces, conservation agriculture, and rainwater harvesting systems are recommended to limit runoff, reduce floods, and enhance water availability for domestic, industry and food production.