4.1 Translating people-land-water connectivity evidence into targeted mitigation actions
The modelled flow lines between different catchment areas map indicate direct hydrological connection between severely eroded pastoral land in the mid and lower slope regions of the catchment (Figure 6e) with upslope runoff generation on cultivated plots. Connectivity is enhanced through interconnection of major tracks with natural flow convergence lines, where downslope gully onset was often initiated directly below road channels entering the rangelands (exemplified again in Figure 6e). The whole catchment view of this study is critical in underpinning community-led action. These observations also have important implications for track management and planning of runoff drains that can release water and eroded sediment and nutrients back into the fields.
Within plot improvements in soil infiltration capacity to reduce overland flow generation (Kuyah et al., 2019; Nishigaki et al., 2017) is key as is improvement of and aggregate stability, e.g. with amendment by organic matter, to reduce risk of soil capping/crusting (Laker and Nortjé, 2019; Smith et al., 2019) and improve overall soil health (Belayneh et al., 2019; Mesfin et al., 2018). While such within-plot actions can be locally beneficial, analysis in this study has shown that different agricultural plot terraces on the same slope are chained to each other. At the edge of plots, implementation or augmentation of features that can attenuate or slow overland flow to retain soil within the plots are important but poor practice upslope, however, can generate significant overland flow that overwhelms the integrity of downslope plot terraces, even if the latter uses conservation agriculture practices that promote infiltration and good soil health. Herein slope length is a key factor (Gourfi et al., 2018). This is exemplified in zone B1 and B2 where slope length was high (< 800 m) and the relatively recent conversion of the land with under-developed boundaries meant that attenuation features could not cope with the amounts of overland flow generated. This allowed an extensive and well-connected rill network to develop and link all plots to each other and the main track (Figure 6b). This was augmented by a large proportion of plots cultivating crop rows in the direction of slope even though the plots themselves, and their boundaries, were aligned across the slope. Off-contour cropping is widely known to enhance rill development (Ayele et al., 2018). In contrast, visual evidence of rilling was markedly lower in the longer established plots (e.g. zone D) where farmers have used contour ploughing and the plot boundaries themselves are more closely aligned with contours. While the upper slope angle is still high, the hilltop steep land has been left in natural vegetation both reducing run-on from the upslope contributing area and effectively reducing the overall length of the slope segment. With the longer-term establishment of plot boundaries there is further resilience to development on connected rill networks due to edge-of-plot barriers that are not overwhelmed by any overland flow that does occur under extreme rainfall conditions. The strong influence of the direction of crop lines is further exemplified by comparison of the two valley sides of Zone A (Figure 6a).
Comparison of the longer-established cultivated plots and recently converted land in this study highlights a problem in the initial phase of slow-forming terraces, as not enough time has passed for the buffers to become effective in stopping flow and the slope gradients to decrease, giving almost free reign to erosive processes. The community picked up the key issue of plot boundary integrity and flow retardation by natural vegetation, wherein suggestions of planting trees and permanent grasses were made. The integration of people, land and water into one connectivity framework exposed a direct chain of cause and effect that can promote cooperative community-led mitigation to stop the downstream propagation of runoff. This is potentially facilitated by the agro-pastoral structure of the community, wherein farmers are both dependent on cropland and on the rangelands for livestock grazing, presenting an opportunity for integrated catchment management.