Assessing the impact of land use/land cover and climate change on environmental flow requirement in river systems with multiple uses

Abstract

Water resources systems like the R. Namatala, shaped by the hydrological cycle, involve complex processes impacted by both human activities and climate change. The pressures of commercialization, industrialization, and increasing water demands exert immense stress on water resources with 65% of the world's rivers now at risk. Diminished river flows, driven by more water withdrawals, land use changes, and climate change, are compromising environmental flow requirements and threatening ecosystem health. Challenges such as inadequate water management practices and limited data on river systems hinder the maintenance of ecological flows, leading to unsustainable water resources exploitation. Consequently, reliance on flow estimates that overlook essential ecological factors worsens the threat to freshwater ecosystems. The purpose of this study was to assess the impact of land use change and climate change on environmental flow (e-flow) requirements. Specifically, the study; i) determined the current environmental flow requirements, ii) examined land use change trends and, iii) determined the impact of land use / land cover and climate change trends on the environmental flow requirements of R. Namatala. Three approaches were that included; (1) Hydrological method, (2) Hydraulic method and (3) Holistic approach were selected to estimate the current E-flow requirements. Land use trends between 1995 to 2023 were determined using image classification tools in google earth engine and trends for 2023 to 2040 were projected using Terrset Land Change Modeler. Hydrologiska Byråns Vattenbalansavdelning (HBV) model was used to determine impact of Land use/land cover and climate change on environmental flow under RCP 2.6 and RCP 8.5. River Namatala had a mean annual flow of 2.65  0.08 m3/s obtained over a period of 73 years. The river normally experiences low flows in the months of December to March and high flows in the months of April to June. Average annual Low flows of 0.65  0.45 m3/hr and max average annual flows of 18.41  8.79 m3/hr. The current E-flow requirement for R. Namatala was determined based on the flows over a period of 73 years, consideration of the different water demands. The estimated current e-flow requirement at the outlet of R. Namatala catchment was 1.072 , 1.036 and 1.103 m3/s as determined from the hydrological, hydraulic and holistic methods, respectively. The results from the historical land use/ land cover trends estimated over a period of 26 years (1995 – 2020) indicated an incremental dominance by cropland (0.471% yr-1), followed by grassland (0.158% yr-1) at the expense of forestland (-0.466% yr-1) and wetlands (-0.176% yr-1). The projected LULC trends estimated over a period of 17 years (2023 – 2040) indicated that forestland would have the predominant increment (0.263% yr-1) followed by cropland (0.071% yr-1) while grasslands would decrease (-0.343% yr-1). Utilizing projected land use data and climate change most likely scenario - RCP 2.6, the projected e-flow requirement was estimated to be 0.880, 1.082 and 1.1591 m3/s for the hydrological, hydraulic and holistic methods, respectively. On the other hand, for the most unlikely scenario - RCP 8.5, e-flows were estimated as 0.910, 1.076 and 1.153 m3/s for the hydrological, hydraulic and holistic methods, respectively. The study concluded that land use together with climate change will have an incremental impact to the future e-flow requirement for R. Namatala of between 4.53% and 5.08% as predicted by the most unlikely and most likely scenarios, respectively. Further to this, the ecosystem is foreseen to have water quantity challenges due to abstractions with months of December to February cited as critical months with reduced flows. Government interventions, including wetland gazettement and forest protection, contributed to increased forest cover but there was a reduction in cropland, a factor that could in itself contribute to food scarcity in the area. Climate projections under RCP 2.6 and RCP 8.5 indicated increased precipitation and seasonal flow shifts, with minimal variation in environmental flow requirements. The study assumed socio-economic dependence of the communities on the river based on the communities’ proximity and accessibility to the river. The e-flow determination also depended on measured flows and no groundwater component. It is thus recommended that further socio-economic and groundwater assessments, improved water management, enhanced policy enforcement, and continued climate-focused research are done to ensure sustainable catchment management. With there being no universally accepted e-flow determination method, this study notes a minimum e-flow requirement of 0.880 m3/hr as determined using the hydrologic method but recommends a value of 1.153 m3/hr as determined using the holistic approach for sustainable exploitation of the water resource since the holistic approach considers a number of ecosystem functions that are not considered by the other methods.