Climate Change and Optimal Watershed Management Scenarios in the Jemma Sub-basin of Upper Blue Nile Basin, Ethiopia
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Date
2020-06
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Addis Ababa University
Abstract
This dissertation develops climate scenarios, climate impact scenarios and prioritize watershed
management alternatives which can provide optimal benefits of adaptation under different climate
scenarios. First, it characterizes the historical (baseline) climate that was a frontier for future climate
scenarios. The baseline climate (1981-2014) unfolds an increase in the annual and main rainy season
(June-September) rainfall and annual maximum temperature (TMAX) and minimum temperature
(TMIN). Different rainfall and temperature extreme indices corroborate a steady increase of rainfall and
temperature extreme events in the baseline climate. To develop future climate scenarios, first it was
essential to identify Regional Climate Models (RCMs) which can simulate the historical climate of the
study area. Accordingly, this study has identified that Global Climate Models (GCMs) dynamically
downscaled through CCLM4 and REMO modeling schemes were better in simulating the historical
(1981-2005) mean rainfall and the distribution of rainfall events. However, the RCM simulations
overestimated and underestimated rainfall in high elevation (>2800m.a.s.l) and lower elevation
(<2300m.a.s.l) areas, respectively.
Thus, it was worthwhile to adjust such biases through robust statistical bias correction method before
using RCM simulations to develop climate scenarios and climate impact scenarios. The intercomparison
of different statistical bias correction methods under different metrics revealed comparable performance
in adjusting mean rainfall, TMAX and TMIN simulations of RCM. Nonetheless, most bias correction
methods struggle to adjust the frequency and intensity of RCM simulations. Distribution mapping bias
correction method was superior in adjusting the frequency and intensity of RCM simulations such as the
wet day probabilities and the 90th percentiles of rainfall and temperature. Subsequently, RCM simulations
bias adjusted using distribution mapping method were used to develop future climate scenarios. Future
climate scenarios developed from the ensemble mean of statistically bias corrected RCM outputs
designate a likely decrease of rainfall whilst steady increase of TMAX and TMIN under all future climate
scenarios. The future climate will be also characterized by temperature and rainfall extreme events. Along
with climate scenarios, climate impact scenarios were developed using the multi-model ensemble mean
and multi-gauge calibrated and validated hydrological model. All climate impact scenarios describe a
steady decline of surface runoff, water yield and an increase in loss of water through evapotranspiration.
As a response, multi-criteria decision analysis system which straddles climate scenarios, climate impact
scenarios, the stakeholders’ perspectives and other biophysical settings was used to identify watershed
management alternatives for optimal climate adaptation. The multi-criteria analysis establishes water
harvesting structures as the most prioritized watershed management alternatives for climate adaptation
under the scrutiny of different criteria. The potential of water harvesting structures in reducing climate
change impacts was evaluated using GIS and hydrological model. Water harvesting, particularly, in-situ
water harvesting structures will significantly (≤0.05) reduce surface runoff and thereby significantly
increase soil water under baseline and future climate scenarios. There will be an insignificant change on
the streamflow due to the realization of water harvesting structures under all climate scenarios. Therefore,
effecting water harvesting structures at highly and optimally suitable watersheds will increase water
availability and strengthen watershed-based climate adaptations under different climate change scenarios.
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Keywords
climate change, climate models, downscaling, statistical bias correction, RCPs, extremes, SWAT model, watershed management, water harvesting, optimal scenarios, Jemma sub-basin, Blue Nile Basin, Ethiopia