2.1 Study area
The Tebo and Geldia catchments (Figure 1) stretch from the northwestern Rift margin at ~2200 m asl to the southern lowland of the Rift Valley floor at ~1500 m asl. Most of the catchments are underlain by Quaternary lacustrine deposits intercalated with pyroclastic rocks, except the Rift margin sub-area and the Merko hillside where Tertiary sediments and Pliocene pyroclastic flow deposits prevail, respectively (Abebe et al., 2005). The catchments and permanent gullies in the catchments can be divided into two geographical sub-categories: the Rift margin sub-area (gullies) and the Valley floor sub-area (gullies; Billi & Dramis, 2003). The mean annual rainfall at the nearest rainfall gauge to the Rift margin sub-area is 881 mm (Ejere; 1976−2013) and that to the Valley floor sub-area is 874 mm (Welenchiti; 1992−2013). Rift margin gullies originate from either the Rift margin plateau or the southeastern cliff (Figure 2), whereas valley floor gullies originate from the vicinity of the Merko hillside (1500−1600 m asl, Figures 1 and 2). Soil and water conservation activities, such as soil and stone bund construction on farmland and trench construction on a hillside, have been occasionally implemented in minor parts of the catchments. No gully rehabilitation activities, e.g., check-dam construction, were implemented in both the sub-area before 2005. The soils in the Rift margin plateau are Cambisols or Vertisols (FAO, 1998), whereas those at the foot of the cliffs are Phaeozems or Kastanozems. Similarly, the Valley floor sub-area was divided into four sections: the main soil components of the Merko hill are Regosols and Leptosols, whereas Calcisols, Cambisols, and Vertisols dominate the downslope farmlands (Mukai, 2017).
Five catchments (Boruamba, Telilo, Adare, Gebruamba, and Koka) from the Rift margin sub-area and seven catchments (Hadaware, Merko, Goro, Abharo, Kawa adami, Aware, and Odalega) from the Valley floor sub-area were selected as study catchments (Figure 1).