Nonpoint source pollution has become a significant environmental and water quality problem worldwide. It is threatening ecosystem services. In developing countries, managing the increasing pollution load to the water bodies is problematic due to the lack of studies on the fate and transport of nutrients and pesticides in watersheds with a monsoon climate and unique geology. This Ph.D. research aims to fill the knowledge gap and investigate the fate and transport of water contaminants under varying hydrogeologic conditions. The study was conducted in two contrasting rural watersheds of the 57 km² Dangishta and the 9 km² Robit-Bata located in the humid and sub-humid volcanic Ethiopian highlands respectively. For 2 years study period, stream discharge, sediment, and phosphorus (total, dissolved, and bioavailable particulate) concentrations were determined at watershed outlet. In the sloping aquifer, groundwater water table, nitrate, and other hydrochemical constituents were monitored in 35 hand-dug wells in Robit Bata and 32 in Dangishta. In Robit Bata, pesticides were analyzed in 8 wells and in 3 storm events and baseflow in the stream. The hydrologic characteristics of the two watersheds are contrasting. Stream hydrograph separation in Dangishta indicated that subsurface flow accounted for 90% of the total flow. Due to volcanic dikes in the valley bottom, subsurface flow is blocked in this area and surfaces as spring, forming a seasonal floodplain. In Robit Bata, subsurface flow contribution is less than in Dangishta. The volcanic dikes are absent and subsurface flow direct joins the mainstream. As a result, river banks slump. The groundwater table is quicker to rise and fall in Robit Bata because the aquifer is steep with a slope of 8% compared to 5% in Dangishta. Moreover, the Thornthwaite Mather water balance model was evaluated to predict baseflow and groundwater table height for aquifers. The predicted groundwater table height fitted well with the observed for both watersheds. In Robit Bata, baseflow was predicted satisfactorily. In Dangishta, the baseflow prediction was poor but improved to excellent by considering the evapotranspiration from the saturated areas. Floodplain covers 30% of the watershed and remains saturated for a long time in a dry period (October to January). Evaporated water from this area is part of baseflow that discharged to the floodplain as spring. In this period, atual evapotranspiration was assumed to equal reference (ETO). Subtracting this value from the predicted baseflow in those months resulted in the best fit and increased the Nash Sutcliff efficiencies to 0.90. The hydrology affected the soil and phosphorus (P) transport. On average, sediment concentration in the rain monsoon phase in Robit Bata was 10.5 g L⁻¹, 11 times that in Dangishta which indicates the contribution of sediments from slipping banks and gullies. The total, P concentration was 2 mg L⁻¹ which was four times that in Dangishta. Bioavailable particulate P (BAPP) concentration in Robit Bata was only twice the concentration in the runoff in Dangishta. The low P content of the subsoil slipping in Robit Bata moderated BAPP at the outlet. The BAPP was related to available P in the top 10 cm soil layer. Average dissolved P concentrations for both watersheds were around 0.1 mg L⁻¹. Low-range DP reflects less P-enriched soil by the applied fertilizer. v In groundwater, annual average nitrate concentrations were between 4 and 5 mg N-NO3 L⁻¹ in both watersheds, but the monthly high and low concentrations occurred at different times. In Robit Bata, peak concentrations were observed in the month after fertilizer applications and then decreased rapidly due to transport out of the watershed. In Dangishta, volcanic dikes blocked the subsurface flow, and nitrate remained near or above 10 mg N-NO3 L⁻¹ for several months after fertilizers were applied. The concentrations decreased slowly due to denitrification. Regression analysis of processes that control nitrate indicated that the denitrification process contribute up to 50% in controlling nitrate fate in Dangishta while less than 10% in Robit Bata. Pesticide concentrations recorded different for detected active ingredients.The strongly adsorbed but less degraded pesticides such as chlorpyrifos and endosulfan were found in nearly all ground and surface water samples. Maximum concentrations in surface water were 8 µg L⁻¹ for chlorpyrifos and 3 µg L⁻¹ endosulfan. These concentrations is at level chronically harmfull to fish in surface water. The weakly adsorbed and fast degrading dimethoate and profenofos pesticides were detected only in the rain phase in lower concentrations during spraying to crops. Non equilibrium adsorption and preferential flow were likely main transport mode of pesticides to groundwater. Based on the equilibrium convective dispersive transport theory, estimated annual distance displaced down in the soil was less than 1m for all pesticides. Our research concludes that the hydrogeologic condition affected soil loss, nutrient (N and P), and pesticide fate and transport. Greater sediment and particulate P concentrations in runoff were related to banks slipping and gullies, thus, implementing common practices such as strengthening river banks and stabilizing gully reduce concentrations. However, the portion of P which is bioavailable particulate might not be improved by implementing those practices. In watershed with highly slopping aquifer without a volcanic dike, nitrate faster flash out from the groundwater. This is undesirable in the downstream surface water ecosystem. In the other watershed, nitrate was lost slowly. This is important in keeping nitrogen in the upland but harmful to humans if it is above 10 mg N-NO3 L⁻¹ in drinking water. Some persistent pesticides in surface water were found at levels chronically harmful to aquatic life. Detection of this pesticides in groundwater was surprisingly high as those pesticides are mobile in non-equilibrium adsorption and preferential transport in volcanic highlands. Thus, ending using long-lasting pesticides in this area is advisable. Key words: Nutrient, Pesticides, Sediment, Hydrogeology, Volcanic Highlands, Sub-Saharan Africa