Impact of Climate and Land Use/Cover Changes on the hydrology of Lake Hawassa Watershed (Central Ethiopian Rift):

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Addis Ababa University


It is generally concurred that climate of the world is changing or at least its variability is increasing and that is bringing significant hydrologic influences on water resources. Consequently, the variation has transformed the way hydrologists view prediction of future hydrologic parameters. The impact to water resources is not only limited to climate variations however. Land use changes have large impacts on these resources as well. Therefore, impact assessment on water resources is a compound effect of natural phenomena and manmade alterations to the environment. Although these modifications have a profound impact on many aspects of the environment, wetlands, especially lakes, are among the most significantly affected. Lake Hawassa is one of the extensively impacted Ethiopia Rift Valley Lakes. Perhaps the most notable recent changes in the Hawassa watershed were the Lake Hawassa level rise and the flooding of Hawassa town in 1998. Tikur Wuha River discharge increase and the decline thereby disappearance of Lake Cheleleka was attributed to the increasing siltation caused by alarming deforestation of the eastern catchment. The impacts were reported to be of both natural and anthropogenic origin. However, little was done to scientifically address these effects. The investigation entail understanding the hydrologic regime shift caused by the climate variability in the recorded meteorological parameters and anthropogenic factor as in the land use land cover change in the watershed. Therefore, this thesis focuses in testing and detecting the presence of significant trend in hydro-meteorological variables, identifying the amount, distribution and time of land use land cover changes, assess the morphometric change in Lake Hawassa and evaluate the dominant hydrological processes using coupled surface and groundwater modeling framework. Mass and double mass curves analyses of rainfall in four stations (namely Hawassa, Haisawita, Yirba and Wendo Genet) within and nearby the watershed showed that there is no trend in the rainfall of the area while Tikur Wuha River flow at Tikur Wuha Bridge and Dato village depicted jumps. Trend and homogeneity test of Lake Hawassa level, rainfall, temperature and flow at Tikur Wuha by Mann-Kendall and Pettitt’s test revealed that all had a trend (at 5% significant level) and nonhomogeneity characteristics except rainfall. Change point analysis illustrated the year 1987-1988 was the year where most of the hydrometeorological parameters showed changes. As a result, Landsat images taken in the years 1973, 1987 2003 and 2019 were selected for image classification and the period from 1973 to 1987 and 1988-2003 were taken as statistically stationary periods for model development (calibration and validation) and study the impacts. The results of hydrometeorological analyses were the bases for land use land cover change analysis and hydrological modeling activities. Land use and land cover maps were derived from ground truthing and satellite imagery by supervised image classification technique. The analysis identified six major land use land cover forms (Built up, Cultivated, grassland, grassed wetland, shrub and forested land and water body). The change investigation displayed vii cultivated land was the dominant land use form in Hawassa watershed. Built up area increased by 188% while shrub and forested lands diminished by almost 23% during 1973-2003. Recently, urban areas increased by over 800% proving to be the fastest growing land use type. The land use changes in the only perennial Tikur Wuha River catchment also followed similar patterns with the entire Hawassa watershed. In this study, a bathymetric map was prepared using advances in global positioning systems, portable sonar sounder technology, geostatistics, remote sensing and geographic information system (GIS) software analysis tools with the aim of detecting morphometric changes against the first extensive hydrographic map of Lake Hawassa in 1999. Results showed that the surface area of Lake Hawassa increased by 7.5% in 1999 and 3.2% in 2011 from that of 1985. Between 1999 and 2011, while water volume decreased by 17%, silt accumulated over more than 50% of the bed surface has caused a 4% loss of the Lake’s storage capacity. The sedimentation patterns identified may have been strongly impacted by anthropogenic activities including urbanization and farming practices located on the northern, eastern and western sides of the lake watershed. The investigation also demonstrated geostatistical modeling approach to be a rapid and cost-effective method for bathymetric mapping. Finally, a coupled surface and groundwater modeling system, MIKE SHE, suitable to model lake watersheds effectively, was used to diagnose the responses of Tikur Wuha catchment in Hawassa watershed to LULC changes and climate variability. Two models based on the Bridge and Dato village stations flow data were developed to tackle the huge difference between the two data sets. The models were calibrated and validated, and were able to capture the dominant runoff processes and streamflow dynamics of the catchment. Streamflow simulations and water balance assessment indicated that, evapotranspiration accounted 85%, while the other components represented 22% of the total rainfall the area received. This showed that the watershed had given off its reserves to satisfy the water balance of the hydrologic components. The models demonstrated that climate variability was found to have impacted unsaturated zone storage but have smaller impact on the rest of the water balance components in the watershed during the study period. Tikur Wuha River flow and the components of the catchment water balance were adversely modified by land use land cover changes; especially evapotranspiration, overland flow, unsaturated zones storage, base flow to river and the saturated drain to river components. These indicated that impacts in the watershed are reversible with the proper catchment management supported by sound land use policies. Annual water balance was moderately affected by the changes while streamflow was most susceptible to land use change for both models of the catchment. Simulated streamflow indicated that the Bridge model deteriorated with time while the Dato model simulated well but failed to distinguish land use impacts. The study highlighted the importance of soil and water conservation interventions in the various LULC classes particularly in the agricultural land use systems. The study showed the gaps in streamflow data accuracy and emphasized on the reassessment of the stations condition.



Change Detection, Bathymetry, MIKE SHE, Land Use Land Cover (LULC), Lake Hawassa