Modeling the Impact of Climate and Land Use/Land Cover Change on Hydrological Responses in Lake Tana Basin, Ethiopia

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Date

2020-06

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Addis Ababa University

Abstract

Environmental pressure resulting from interlinked climate and land use land cover (LULC) change is increasingly threatening water resources. Evaluating the effect of climate and LULC change on watershed hydrology has become an important research topic for developing appropriate strategies for the water resources sector. Recently, hydrological impact assessments focused on separate impacts of either LULC or climate change. However, both LULC and climate have been changed significantly since the mid-20th century, and in most parts of the world, including Ethiopia, the change will continue in the future. Hence, the focus of this study was to model the impact of LULC and climate change on hydrologic processes. Additionally, the climate model’s sensitivity to landuse data and land surface model (LSM) was investigated. The study was conducted in the Lake Tana basin which located in northern Ethiopia. The Lake Tana basin is the headwater of the Upper Blue Nile basin having a catchment area of 15,140 km2 . This study mainly focused on the major tributaries rivers including Gilgel Abay, Gumara, Ribb, and Megech watershed. LULC changes are one of the main human-induced factors influencing the hydrological process. The SWAT hydrological model was calibrated and validated using static land-use (SLU) and dynamic land-use (DLU) setup to evaluate the impact of LULC changes on the hydrological processes and parameters in Gumara watershed (case study watershed). The SLU setup used single land-use data (1985), whereas the DLU setup used four land-use data (1985, 1995, 2005, and 2015). Results from the LULC study showed that expansion of agriculture (11.1%) and decrease of forest (2.3%) and shrub-land (8.8%) occurred between 1985 and 2015. SWAT model with DLU setup showed a slightly higher performance than SLU setup, particularly during the calibration period. The LULC data for 2015 showed an overall increase in surface runoff (11.6 mm) and peak flow (2.4 m3 /s) and a decrease in evapotranspiration relative to 1985 LULC data. The incorporation of DLU into the SWAT model results in a more realistic representation of changes in temporal land-use, thus improving the accuracy of estimation of temporal and spatial hydrological processes. Therefore, hydrological modelers should take into account the temporal dynamics of LULC data to improve model simulation performance. Besides LULC change, climate change could be a threat to the water resource sector. In order to accurately simulate climate data, the Weather Research and Forecasting (WRF) model parameters were chosen on local bases, in particular land-use data and LSM. The result indicated that vii simulations of temperature and rainfall were sensitive to the choice of LSM and land-use data. The combination of updated new land-use (NLU) with Rapid Update Cycle (RUC) and Thermal Diffusion (TD) produced very small cold bias (0.27 °C) and warm bias (0.20 °C) for maximum and minimum temperature, respectively, whereas rainfall simulation with NLU and Noah configuration produced the lowest mean bias (2.39 mm/day). The WRF model had limitations in terms of detection ability during the occurrence of heavy rainfall. Overall, results suggested that the application of updated land-use data substantially improved the performance of the WRF model in simulating temperature and rainfall. The study would provide valuable support in identifying suitable LSM and land-use data that can accurately predict the climate variables in the Blue Nile basin. Lake Tana basin is vulnerable to climate change and variability. Climate data for the baseline (2005-2015) and future period (2045-2055) under two Representative Concentration Pathways (RCP) scenarios (RCP4.5 and RCP8.5) were simulated using the WRF model. The SWAT model was used to investigate the impacts of climate change on the four main tributary watersheds of the Lake Tana basin: Gilgel Abay, Gumara, Ribb, and Megech. The result showed that projected changes in rainfall vary with seasons and watershed under both scenarios. On average, under RCP4.5 and RCP8.5 scenarios, the mean annual rainfall may increase by 7.9% and 21.1%, respectively. Minimum temperature may increase by 1.4 °C and 1.9 °C while maximum temperature may increase by 1.4 °C and 2.4 °C under RCP4.5 and RCP8.5 scenarios, respectively. Climate change under RCP4.5 and RCP8.5 scenarios can increase streamflow by 7.2% and 33% and evapotranspiration by 11.2% and 15.2%, respectively. The findings provide valuable insights to implement appropriate water management strategies to mitigate and adapt the negative impacts of climate change and variability. The effect of LULC and climate change on Gumara watershed hydrology was assessed using projected LULC and climate data. Three future LULC scenarios (BAU, EFL, and EIC) were developed using Land-use Change Evaluation Module in QGIS, based on hypothetical scenarios. Projected climate data were simulated using the WRF model under the RCP4.5 and RCP8.5. The result showed that BAU scenario can increase surface runoff by 5.1% and decrease base-flow by 6.5% without altering streamflow and evapotranspiration noticeably. On the contrary, EIC and EFL scenarios can decrease streamflow by 12.5% and 5.2%, and surface runoff by 7.9% and 10%, viii respectively, and increase evapotranspiration by 4.9% and 8.9%, respectively. Climate change under RCP8.5 can increase streamflow, surface runoff and evapotranspiration significantly by 34.3%, 51.8%, and 12.2%, respectively. The simulated SF, SR and ET may increase significantly under the combination of all three land-use and RCP8.5 scenarios. The findings suggested that climate change may have a greater effect on hydrologic responses than land-use change. The expansion of agriculture (BAU) and the wetter climate (RCP8.5) would exacerbate flooding, while the expansion of irrigation and forest offset SF increase. The findings from this study can be useful to decision-makers and planners in the design of adaptive measures to LULC and climate changes.

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Keywords

Dynamic land-use, Static land-use, SWAT, Updated land-use, LSM, WRF, land-use change, climate change, hydrological response, Lake Tana basin

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