It is generally concurred that climate of the world is changing or at least its variabilityrnis increasing and that is bringing significant hydrologic influences on water resources.rnConsequently, the variation has transformed the way hydrologists view prediction ofrnfuture hydrologic parameters. The impact to water resources is not only limited tornclimate variations however. Land use changes have large impacts on these resourcesrnas well. Therefore, impact assessment on water resources is a compound effect ofrnnatural phenomena and manmade alterations to the environment. Although thesernmodifications have a profound impact on many aspects of the environment, wetlands,rnespecially lakes, are among the most significantly affected.rnLake Hawassa is one of the extensively impacted Ethiopia Rift Valley Lakes. Perhapsrnthe most notable recent changes in the Hawassa watershed were the Lake Hawassarnlevel rise and the flooding of Hawassa town in 1998. Tikur Wuha River dischargernincrease and the decline thereby disappearance of Lake Cheleleka was attributed tornthe increasing siltation caused by alarming deforestation of the eastern catchment. Thernimpacts were reported to be of both natural and anthropogenic origin. However, littlernwas done to scientifically address these effects. The investigation entail understandingrnthe hydrologic regime shift caused by the climate variability in the recordedrnmeteorological parameters and anthropogenic factor as in the land use land coverrnchange in the watershed. Therefore, this thesis focuses in testing and detecting thernpresence of significant trend in hydro-meteorological variables, identifying thernamount, distribution and time of land use land cover changes, assess thernmorphometric change in Lake Hawassa and evaluate the dominant hydrologicalrnprocesses using coupled surface and groundwater modeling framework.rnMass and double mass curves analyses of rainfall in four stations (namely Hawassa,rnHaisawita, Yirba and Wendo Genet) within and nearby the watershed showed thatrnthere is no trend in the rainfall of the area while Tikur Wuha River flow at Tikur WuharnBridge and Dato village depicted jumps. Trend and homogeneity test of LakernHawassa level, rainfall, temperature and flow at Tikur Wuha by Mann-Kendall andrnPettittâ€™s test revealed that all had a trend (at 5% significant level) and nonhomogeneity characteristics except rainfall. Change point analysis illustrated the yearrn1987-1988 was the year where most of the hydrometeorological parameters showedrnchanges. As a result, Landsat images taken in the years 1973, 1987 2003 and 2019 werernselected for image classification and the period from 1973 to 1987 and 1988-2003 wererntaken as statistically stationary periods for model development (calibration andrnvalidation) and study the impacts. The results of hydrometeorological analyses werernthe bases for land use land cover change analysis and hydrological modelingrnactivities.rnLand use and land cover maps were derived from ground truthing and satelliternimagery by supervised image classification technique. The analysis identified sixrnmajor land use land cover forms (Built up, Cultivated, grassland, grassed wetland,rnshrub and forested land and water body). The change investigation displayed rnviirncultivated land was the dominant land use form in Hawassa watershed. Built up arearnincreased by 188% while shrub and forested lands diminished by almost 23% duringrn1973-2003. Recently, urban areas increased by over 800% proving to be the fastestrngrowing land use type. The land use changes in the only perennial Tikur Wuha Riverrncatchment also followed similar patterns with the entire Hawassa watershed.rnIn this study, a bathymetric map was prepared using advances in global positioningrnsystems, portable sonar sounder technology, geostatistics, remote sensing andrngeographic information system (GIS) software analysis tools with the aim of detectingrnmorphometric changes against the first extensive hydrographic map of Lake Hawassarnin 1999. Results showed that the surface area of Lake Hawassa increased by 7.5% inrn1999 and 3.2% in 2011 from that of 1985. Between 1999 and 2011, while water volumerndecreased by 17%, silt accumulated over more than 50% of the bed surface has causedrna 4% loss of the Lakeâ€™s storage capacity. The sedimentation patterns identified mayrnhave been strongly impacted by anthropogenic activities including urbanization andrnfarming practices located on the northern, eastern and western sides of the lakernwatershed. The investigation also demonstrated geostatistical modeling approach tornbe a rapid and cost-effective method for bathymetric mapping.rnFinally, a coupled surface and groundwater modeling system, MIKE SHE, suitable tornmodel lake watersheds effectively, was used to diagnose the responses of Tikur Wuharncatchment in Hawassa watershed to LULC changes and climate variability. Twornmodels based on the Bridge and Dato village stations flow data were developed torntackle the huge difference between the two data sets. The models were calibrated andrnvalidated, and were able to capture the dominant runoff processes and streamflowrndynamics of the catchment. Streamflow simulations and water balance assessmentrnindicated that, evapotranspiration accounted 85%, while the other componentsrnrepresented 22% of the total rainfall the area received. This showed that the watershedrnhad given off its reserves to satisfy the water balance of the hydrologic components.rnThe models demonstrated that climate variability was found to have impactedrnunsaturated zone storage but have smaller impact on the rest of the water balancerncomponents in the watershed during the study period. Tikur Wuha River flow andrnthe components of the catchment water balance were adversely modified by land usernland cover changes; especially evapotranspiration, overland flow, unsaturated zonesrnstorage, base flow to river and the saturated drain to river components. Thesernindicated that impacts in the watershed are reversible with the proper catchmentrnmanagement supported by sound land use policies. Annual water balance wasrnmoderately affected by the changes while streamflow was most susceptible to landrnuse change for both models of the catchment. Simulated streamflow indicated that thernBridge model deteriorated with time while the Dato model simulated well but failedrnto distinguish land use impacts. The study highlighted the importance of soil andrnwater conservation interventions in the various LULC classes particularly in thernagricultural land use systems. The study showed the gaps in streamflow data accuracyrnand emphasized on the reassessment of the stations condition.