Climate change and ecohydrological dynamics of Upper Nile basin in Ethiopia: the Tekeze River basin case
Baye, Samuale Tesfaye
Climate change is affecting society and environment across every continent, particularly water limited regions which are sensitive to disturbances due to limited available water resources. Recent accelerated climate variation has aggravated existing environmental problems in the Nile River Basin, particularly regions in the East Africa that are caused by the combination of land degradation processes and recurrent droughts. East Africa including Ethiopia is one of the most vulnerable region to climate variability and change given the region has experienced some of the worst drought events in the past several decades. In Ethiopia, particularly in the northern Ethiopia, the relations between climate, land degradation, hydrological processes and ecosystem production are quite complex. The variability in climate, topography, lithology, vegetation cover and land management may imply a considerable variation in the hydrological and ecological processes at both basin and regional scales. These variabilities together with frequent occurrence of droughts pose major challenges for water and ecosystems management in the area.
Although dealing with challenges that climate variability and change poses and crafting appropriate adaptation and mitigation mechanisms are crucial in the region, hydro-climatic variability and change is not yet well understood and the resulting climate, hydrology and ecology interactions and feedbacks are highly uncertain in this region. Thus, a better understanding of hydrological and ecological responses to climate variability and change is essential for water and land managers and decision makers to develop appropriate mitigation and adaptation strategies to sustain the broad variety of ecosystem services. This study aims at better understanding dynamics of ecohydrological processes under past to future climate variabilities and change and their effects on the resilience of ecosystems at different spatiotemporal scales in Tekeze River basin of the upstream parts of the Nile basin, northern Ethiopia. An integrated approach with combination of multiple statistical analysis of hydro-climatic data, remote sensing data, field observation, laboratory analysis and hydrological and ecological modelling have been developed and conducted at various spatial and temporal scales in the basin.
We started by identify and characterize trends, change points and cross-correlations of precipitation, temperature, streamflow and net primary productivity (NPP) for the period 1980–2014. The study focused on Geba sub-basins, as a case study in the Tekeze River basin, for in-depth analysis on observed hydro-climatic data and NPP. The results showed both significant increasing and decreasing trends for temperature, and no statistically significant trends for precipitation at most of the stations, both over the years and over each season, except for a few stations. Trends of annual and seasonal monthly mean streamflow for most stations showed decreasing trends, except for the dry season. While the trend analysis for NPP indicated a significant increasing in the entire Geba sub-basin and all catchments in the sub-basin. The analysis from correlation coefficients of the relationships between the mean values and coefficient of variations (CVs) for streamflow, NPP and climate variables indicated streamflow, however, was most strongly correlated with precipitation, and NPP was controlled mostly by precipitation and maximum temperature. We also evaluated the controlling factors, and found that climate variabilities and human interventions were the most important factors governing ecohydrological dynamics in the Geba sub-basin.
To understand the large-scale hydro-climatic variabilities of the region, we investigate the spatiotemporal variation of recent past to future hydro-climatic variabilities and change for and within Tekeze River basin. We also present an evaluation of the performance of several Coupled Model Intercomparison Project (CMIP5) General Circulation Models (GCMs) in simulating the historical and future patterns in temperature, precipitation and streamflow over the study region.
The trend analysis of the entire Tekeze River basin temperature and precipitation revealed a significant increasing trend in temperature and a slight increasing non-significant trend in precipitation. Although the majority of upper region stream gauging stations showed decreasing trends in streamflow, the trend at the basin-wide outlet is instead showed an increasing trend. This suggests that other factors, such as human activities and topographic settings in the upper region possibly cause this difference. The analysis also showed the superiority of the multi-model ensemble means compared with individual GCM output, and suggests that selection of GCMs with well-defined evaluation criteria can be important in this region of northern Ethiopia, in its sub-river basins, and in other similarly complex regions. Using the multi-model ensemble mean, the results of GCM projections for the 21st century showed a gradual reductions and considerable variations in streamflow attributed to the combined effect of increasing temperature and decreasing precipitation.
The spatial and temporal variabilities of NPP, actual evapotranspiration (ETa) and water use efficiency (WUE = NPP/ETa) for different sub-basins, bioclimatic zones and land cover types was assessed in the Tekeze River basin. The NPP, ETa and WUE values showed appreciable differences amongst the nine sub-basins, five bioclimatic zones and four land cover types. Results presented that all NPP, ETa and WUE showed an increasing trends both annually and seasonally, in the period from 1982−2014. An increase in NPP due to an increase in vegetation coverage is the primary driving force for the increase in WUE, in view of rather stable variation of ETa during the study period.
The analysis for bioclimatic zones revealed that semi-arid zones had higher average WUE than humid zones. From the perspective of various land cover types, high WUE values were mostly distributed in woodland vegetation cover type in the centre and south, and low WUEs were mostly in the east parts of the basin, which were dominated by shrubland and cropland. It was also found that the spatiotemporally changes in climatic factors as temperature and precipitation, and human activities as ecological restoration and soil and water conservation measures were the dominant driving factors for changes in NPP and ETa, and thus in WUE. Human activities since the 2000s, however, have had much larger effects than climatic factors in this basin.
An integrated modeling approach that combined hydrology and ecosystem models was devised and used to examine the implications of climate variabilities and change on water and carbon cycles and their coupling processes in the basin. A physically based ecohydrological model by coupling SWAT and Biome-BGC was developed and used to evaluate how water balance components (e.g. water yield and ETa) and ecosystem productivity (e.g. NPP) and the water-carbon coupling processes are responded to the recent past (1980–2009) and future (2010–2099) climate condition based on an ensemble mean of 30 downscaled GCMs under the two RCPs scenarios, RCP4.5 and RCP8.5.
The results revealed that the annual water balance components and NPP varied spatially across the basin during 1980–2009. Water balance components such as ETa, surface runoff and groundwater flow at the basin scale and in most parts of the basin tended to increase from 1980 to 2009, but not significantly. While, the average annual NPP of the entire basin and its sub-regions of the basin gradually decrease and this corresponds to the spatiotemporal variabilities and changes of temperature and precipitation in the basin. The spatial and inter-annual variability of water balance components and ecosystem productivity is also expected to increase as a result of high variability and change rates of increasing temperature and decreasing precipitation over the region during the twenty-first century. The correlation analysis also showed that the water and carbon cycles are tightly coupled in the basin. The highest correlation coefficient was found during the past period, and it tended to be lower during the future periods under all RCPs.
Using time series of WUE data from the output of CASA and Biome-BGC ecosystem models the spatiotemporal variations in ecohydrological resilience to drought during the past (1982–2010) and future (2011–2100) periods were examined. WUE used as an indicator of ecohydrological resilience, before, during and after drought events. We compiled proxies of WUE data, namely, Biome-BGC model based WUE (WUE-BGC, potential ecosystem WUE) and CASA model based WUE (WUE-CASA, actual ecosystem WUE) data, to evaluate the role of human activity as ecological restoration and soil and water conservation practices for ecohydrological resilience to drought events.
The analysis for Biome-BGC and CASA models based WUE data showed most parts of the basin were not resilient enough to drought during the past and for future periods. The ecohydrological resilience exhibited a congruent behaviour across sub-regions, bioclimates and land covers. Resilience indices of WUE-CASA, resistance and recovery, exhibited higher increasing trend than WUE-BGC based resilience and varied over the drought events, which accompanies an expansion in the area of high resistance and recovery. These observations indicated increased ecohydrological resilience over the basin, which is represented by the increased human interventions through ecological restoration and soil and water conservation activities.
Overall, results from this study verified that climate variables as temperature and precipitation are strongly varied both spatially and temporally in the basin. Consequently, the response of streamflow and ecosystem productivity across the investigated sub-basins were also spatiotemporally diverse. The main mechanisms behind such complexities are related to the combined effect of climatic factors and human activities as ecological restoration and soil and water conservation measures. The projected increased variability in precipitation conjunction with increasing temperature trends, if sustained into the next century, may portend significant impacts on the water budgets and ecosystem productivity, with consequences for water and carbon cycling and water-carbon coupling processes in the region. The study also highlighted that ecosystems in most parts of the basin are not resilient enough to drought during the past and for future climatic conditions. This provides evidence for the vulnerability of the region to persistent drought and potential loss of ecosystems function with future climate change. Human activities such as, ecological restoration and soil and water conservation measures, as an ecological engineer, in combination with their recovery, enhancing ecosystem functioning during and after drought events, and improve ecohydrological resilience within a basin. This implies that the existence of a compensation mechanism that increases resilience and points to the importance of human activities for understanding of the dynamic ecohydrological processes in the variable and changing climate. Thus, it is an urgent need to take more effective strategies than the current approaches of environmental rehabilitation programmes in the region in order to increase the capacity of ecosystems to regulate their functioning in the face of future hydro-climatic disturbances.