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Study region: Six small agricultural watersheds in three tropical climatic regions of Ethiopia. Three regions of contrasting climate: highland, midland, and lowland regions with respectively high, middle, and low elevations and rainfalls. Study focus: Lag time (TL) and time of concentration (TC) are two measures of how quickly a stream responds to runoff-producing rainfall. These parameters are the main inputs used to estimate peak flow under flood conditions in ungauged watersheds. Many empirical methods have been proposed to estimate TL and TC, but the validity of none of them has been tested. This study compared 10 commonly used methods by using measured TL and TC. New hydrological insights: Measured median values of TL and TC for 176 rainfall–runoff events were used to evaluate the performance of empirical methods. For individual watersheds, the estimates of TL and TC differed by up to 2.6 h and 4.4 h, respectively. Most of the empirical methods tended to substantially underestimate TL and TC, which would lead to overestimation of runoff volume. TL and TC computed by two methods that consider both overland and channel flow were closest to the measured values of TL and TC, because such mixed flow is typical of tropical climate regions. Our results show the need for caution when empirical methods developed in regions with a particular climatic and geomorphological conditions are applied elsewhere.

Study region: Laguna watershed of the Upper Blue Nile basin, which is representative of drought-prone tropical sub-humid watersheds of Ethiopia. The river of the watershed drains into Lake Tana, one of the largest lakes in Ethiopia. Study focus: This study evaluated the impacts of five alternative land capability-based land use and management options in reducing runoff and sediment loss by integrating observed plot- and watershed-scale data with the Soil and Water Assessment Tool. New hydrological insights: We modified flow- and sediment-sensitive model parameters based on six best-performing plot-scale land management practices to simulate the runoff and sediment loss of the watershed under five alternative options. The results revealed that alternative land use options could reduce runoff and sediment loss by about 21–30% (from 740 to 517 mm) and 32–83% (from 101 to 18 t/ha), respectively, compared to the baseline data. The simulations of best alternative land management options also decreased runoff by 10–41% and sediment loss by 40–89%. Runoff and sediment loss reductions were estimated to be as high as 72% (from 740 to 210 mm) and 95% (from 101 to 5 t/ha), respectively, when land use and land management options are combined. These results indicate that implementing suitable land use and management practices has the potential to rehabilitate degraded watersheds in the sub-humid Ethiopian highlands and similar environments elsewhere.

Study region: Robit-Bata watershed, Upper Blue Nile basin, Ethiopia. Study focus: Stable isotopes of water (Oxygen-18 and Deuterium) were used as tracers to estimate the contribution of groundwater in shallow hillslope aquifers to streamflow in the Robit-Bata watershed. To assess the spatiotemporal variability of shallow groundwater and develop a hydrograph separation technique, we collected rainfall, shallow groundwater, and streamflow samples and analyzed their δ18O and δ2H isotopic compositions. The local meteoric water line (LMWL) and local evaporative line (LEL) of the study area were determined and compared with the global meteoric water line (GMWL). A standard unweighted two-component isotope-based hydrograph separation model was used to determine the percentage contribution of shallow groundwater to streamflow. New hydrological insights for the region: The LMWL (δ2H = 8.63·δ18O + 18.2) mostly showed heavy isotopic enrichment relative to GMWL, and the LEL (δ2H = 5.45·δ18O + 6.96) indicated isotopic enrichment compared to Ethiopian lakes. Shallow groundwater responded rapidly to rainfall, with good spatial correlation depending on topographic positions of wells. Pre-event water contributed 90% when the watershed reached maximum storage. This finding gives insight towards the predominant runoff generation process and has significant implications for sustainable dry season irrigation expansion in the area as the sub-surface flow drains out of the watershed from October onwards reducing water tables in the shallow wells.