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