Soil and Water Conservation Practices (SWCPs) are seen by many as the panacea for improving productivity of land and water quality, and decreasing reservoir siltation. The few scientific studies and the continuing increase in sediment concentration in the Ethiopian rivers, despite a long history of SWCPs, suggest that current soil and water conservation (SWC) approaches should be fine-tuned. The objective of this dissertation was to examine the effectiveness of large-scale SWCPs on catchment hydrology and erosion dynamics and to suggest practices that could be more effective. The Debre Mawi watershed was selected and five years of hydro-geomorphic data were collected for five nested catchments before and after SWCPs. In this study, the researcher: (1) investigated the effect of large-scale SWC interventions on catchment hydrology, (2) examined how SWCPs affect suspended sediment concentration and catchment sediment yield, (3) examined erosion dynamics and sediment loss patterns, and (4) modeled the effects of SWCPs on discharge and sediment transport using a Parameter-Efficient Distributed (PED) model and evaluated the applicability of PED model for conservation effects assessment in the sub-humid Ethiopian highlands. The evidences obtained from this research indicated that: (1) The infiltration ditches of soil/stone faced soil bunds are effective in collecting runoff and trapping sediment from upslope fields. However, ditches are filled-up with sediment in one/two years, and bunds collapse shortly. This makes maintenance of SWC structures a critical factor for sustaining sediment trap efficiency and reducing storm runoff. (2) Large scale SWC interventions are effective in reducing storm runoff and sediment loads in the short term. However, targeting runoff sources and the proper structures thereof can enhance the effectiveness of SWC interventions i.e. placing drainage structures on saturated bottomlands rather than infiltration structures (which are suited for well-drained hillsides) should be prioritized to make the structures effective. (3) Erosion hotspots such as gullies, which are found to be the major sources of sediment should be given priority in the government led implementation of large scale SWCPs for sustained reductions in soil loss. (4) Sediment loads during peak rainfall-runoff events contribute substantial proportions of annual sediment loads. The large sediment magnitude during peak events should guide the dimensioning of hydraulic structures such as SWCPs and frequency of maintenance of structures. (5) Models such as the PED that require few parameters and properly represent the underlying hydrological processes can be used as an effective tool for assessing conservation effect in data scarce regions such as the Ethiopian highlands.