The Interplay between Land Use Changes and Hydrological Processes in the Upper Offin Basin of Ghana

Abstract

The transformation of the food system is intricately linked to the effective management of land and water resources, particularly in regions where diverse land uses compete for limited space. The upper Offin sub-basin serves as a prime example of this complexity, where agricultural, mining, and agroforestry practices fight for arable land, influencing the local food system and changes in hydrological processes. This study aims to comprehend the flow paths, the status of water resources, and land use changes in the agroforestry-dominated landscape of the upper Offin basin in Ghana. To assess historical land use patterns, Landsat images were utilized, alongside trend analyses of past hydro-climatic variables and a Thornthwaite-based water balance incorporating inputs from remote sensing and secondary data spanning from 1981 to 2022. Furthermore, the study instrumented an upland Mankran watershed in the upper Offin, where citizen scientists measured basic hydrological variables in three landscape positions—such as daily rainfall, streamflow rates, and groundwater levels—and water quality parameters (nitrate, phosphate, and mercury) from June to October 2023. The analysis revealed that annual and monthly rainfall exhibited minimal changes over the study period (1981–2022). Forest areas experienced a general decrease, while croplands and built-up areas increased between 2008 and 2021, impacting water balance components. Actual evapotranspiration (AET) based on the water balance model and WaPOR data demonstrated a decreasing trend, while streamflow at the basin outlet increased from 1986 to 2012. The runoff coefficient and the hydrological simulation based Thornthwaite-based water balance demonstrated that subsurface flow dominated the runoff processes, constituting approximately 20% of the average annual rainfall. This is also supported by the nitrate concentrations only peaked in rivers in June, while agricultural wells exhibited consistently high concentrations throughout the rainy period, suggesting leaching through subsurface flow. Phosphate concentrations increased in streams as the rainy period progressed, mirroring well concentrations, and mercury concentrations were low in surface water but four times higher in groundwater, indicating further the subsurface flow dominance. This study provides crucial insights for informed decision-making regarding the hydrological processes amid changing landscapes for sustainable agriculture and biodiversity preservation in the region. The emphasis on subsurface flow dominance underscores its significance in potential transport mechanisms for water quality within the landscape. Landscape management interventions must consider the role of subsurface flow to safeguard environmental resources, enhance water quality, and protect human health.