Effect of water harvesting techniques on hydrological processes and sediment yield in Northern Ethiopia
Woldegiorgis, Berhane Grum
The study was conducted in the semi-arid northern Ethiopia aimed at selecting appropriate water harvesting techniques (WHTs) for implementation. A plot-scale experiment was set up, in the Gule catchment, on a farmland to monitor the effect of in-situ WHTs such as tied ridges and straw mulch mainly on event-based runoff, soil-moisture, and soil and nutrient losses. The off-site effect of WHTs such as check dams and percolation ponds on catchment-scale event-based runoff and sediment yield was also monitored in the Gule catchment (~12 km2) and Misbar sub-catchment (~2.4 km2), northern Ethiopia. First, a decision support approach was developed to aid the selection of WHTs in arid and semi-arid areas. The decision support approach was validated with a case study for WHTs in the upper Geba watershed in northern Ethiopia. Using the decision support methodology, eight potential WHTs were pre-selected for implementation in the watershed. Next, using suitability indicators for WHTs and a GIS-based multi-criteria analysis, suitable areas were identified for three of these WHTs, namely check dams, percolation ponds and bench terraces and suitability maps were generated. The multi-criteria analysis was validated by comparing the predicted suitable areas with the already existing locations of WHTs in the watershed. The result was that 90% of the existing check dams and 93% of the percolation ponds in the upper Geba watershed were correctly identified by the approach. The field study showed runoff reduction by WHTs from farmland between 40 to 88% and soil loss between 60 to 90%. Nutrient loss reduction from farmland by WHTs also ranged between 52 and 86%. Soil-moisture also improved due to the use of the in-situ WHTs. Model-based simulation at the Gule and Misbar outlets using LISEM showed that the current WHTs applied in the catchment are able to decrease event-based runoff by 41 and 45%, respectively. Similarly, sediment yield was reduced at both the Gule outlet and Misbar sub-outlet, by 67 and 55%, respectively. This study has verified that in semi-arid areas, such as the northern Ethiopian highlands, in-situ and catchment-scale WHTs can be used to improve the efficiency of rainwater harvesting and water availability for agricultural uses. Furthermore, these WHTs help to mitigate land degradation by decreasing soil and nutrient losses from farmland and sediment yield from catchments.