ESSP Working Paper 61. "Hydrological modeling of sustainable land management interventions in the Mizewa watershed of the Blue Nile Basin" by Emily Schmidt and Birhanu Zemadim.
Abstract: According to the International Water Management Institute (IWMI), the Blue Nile basin is one of the least planned and managed sub-basins of the Nile (IWMI, 2008). Previous studies have examined the impact of investments in sustainable land and watershed management (SLWM) in the Blue Nile basin derived implicitly from economic analyses (Schmidt and Tadesse 2012; Pender and Gebremedhin 2006; Holden et al. 2009; Kassie et al. 2007). However, further examination using a hydrological model that takes into account biophysical differences in terrain, investment choice and magnitude (i.e. terraces vs. bunds implemented on only steep terrain vs. middle and steep terrain) within the watershed will provide greater insight as to how specific investments improve hydrological processes, and their explicit impact on agricultural productivity.
This analysis utilizes recent hydrological and meteorological data collected from the Mizewa watershed in order to better understand the physical impact of SLWM investments. The effectiveness of the simulated conservation practices (terraces, bunds, and residue management) are evaluated using the Soil and Water Assessment Tool (SWAT) model taking into account investment decisions on different terrain types. Simulations include: 1) terracing on steep hillsides (slopes greater than 20 degrees); 2) terracing on mid-range and steep hillsides (slopes greater than 5 degrees); 3) a mix of terracing and bunds on varying slope gradients; 4) residue management on all agricultural fields; and 5) a mix of terraces and residue management on steep and mid-range terrain where a majority of agricultural activity takes place. Simulated conservation practices are evaluated at the outlet of the Mizewa watershed by comparing model simulations that take into account the limited investments that currently exist (status quo) with simulations of increased terracing and residue management activities within the watershed.
Results suggest that the benefits of residue management practices were more important for less steep areas; while a mixed strategy of terracing on steep slopes and residue management on flat and middle slopes dramatically decreased surface runoff and erosion. A comprehensive investment of terraces and bunds throughout the watershed landscape provides the greatest reduction in surface flow and erosion; however, the type and amount of investment in SLWM have different implications with respect to labor input and utilization of agricultural land. It is important to note that although simulations suggest that a landscape-wide approach reaps the greatest long-term benefits, it is important to understand the costs of such an investment. Download the PDF