This study analyzed more than 50 years of land cover and land use changes in the 260 km 2 Koga catchment in North Western Ethiopia. The data used includes 1:50,000 scale aerial photographs, Landsat MSS, TM and ETM images, and ASTER images together with ground truth data collected through field surveys and community elders' interviews. Aerial photographs have high spatial resolution but provide lower spectral resolution than satellite data. While most land use/cover change studies compare changes from different spatial scales, this study applied land use/cover classification techniques to bring the data to a relatively similar scale. The data revealed that woody vegetation decreased from 5,576 ha to 3,012 ha from the 1950s to 2010. Most of the deforestation took place between the 1970s and 1980s, but there is an increasing trend since then. No significant changes were observed in the area used for agriculture that comprises the pastures and crop fields since the 1950s, while there is an enormous increase in the area used for settlement, due to a tremendous increase in population from one point in time to another. The bare lands that used to exist in previous years were found to be totally covered with other land cover/use classes and no bare lands were observed in the study area in the year 2010. Population pressure and land use policies were found to be reasons for the changes in land use/cover while soil degradation,

Event sediment transport and yield were studied for 45 events in the upstream part of the 260 km 2 agricultural Koga catchment that drains to an irrigation reservoir. Discharge and turbidity data were collected over a period of more than a year, accompanied by grab sampling. Turbidity was very well correlated with the sediment concentrations from the samples (r = 0.99), which allowed us to estimate the temporal patterns of sediment concentrations within events. The hysteresis patterns between discharge and sediment concentrations were analysed to provide insight into the different sediment sources. Anticlockwise patterns are the dominant hysteresis patterns in the area, suggesting smaller contributions of suspended sediment from the river channels than from the hillslopes and agricultural areas. Complicated types of hysteresis patterns were mostly observed for long events with multiple peaks. For a given discharge, sediment yields in August and September, when the catchment was almost completely covered with vegetation, were much smaller than during the rest of the rainy season. The hysteresis patterns and timing suggest that the sediment availability from the agricultural areas and hillslopes affects sediment yields more strongly than does peak discharge. Two distinct types of sediment rating curves were observed for the season when the agricultural land was covered with vegetation and when it was not, indicating the dominating contribution of land use/cover to sediment yields in the catchment. The rate of suspended sediment transport in the area was estimated as 25.6 t year À1 ha À1 .

This study used the physically based distributed Annualized Agricultural Non‐Point Source model to simulate decadal trends in soil loss and runoff with changes in land use/cover in the 98·4 km2 upper part of the Koga catchment. The study indicated that soil loss in the study area has increased from 17 Mg ha−1 y−1 in 1957 to 25 Mg ha−1 y−1 in 2010 because of a decrease in the amount of woody vegetation cover. We found that over the past 50 years, high risk erosion areas with soil loss greater than 35 Mg ha−1 y−1 covered, on average, 41% of the study area, while these same areas contributed, on average, 78% of the soil loss from the study area. Twenty one percent of the study area has been in the high risk erosion category throughout all the years since 1957. The total amount of soil loss from the study area varied with changes in the amount of woody vegetation cover. Other factors such as landforms and soil type also affected the amount of soil erosion. Whereas most of the high erosion areas were found in the upper part of the catchment, most of the high runoff areas were found in the lower part of the catchment, indicating that the amount of soil erosion is not directly related to runoff volume in the study area. Crop fields contributed to most of the soil loss in the study area. Copyright © 2015 John Wiley & Sons, Ltd.