PEER Awardees’ Conference 2014, Arusha, Tanzania, Aug 4-7, 2014
Reducing soil loss through effective soil and water conservation practices using hydrologic considerations and farmers’ participation in Blue Nile Basin Seifu A Tilahun1, Christian D Guzman2, Dessalegn Chanie3, Fasikaw Atanaw1,3, Getanhe Kebede3, Mamaru Ayalew1,3, Muluken Lackachew3, Assefa D Zegeye2,4, Tigist Tebebu2, Selamawit Damtew5, Rahel Seifu5, Christopher Barrett6,Tammo S Steenhuis1,2 1School of Civil & Water Resources Engineering, Bahir Dar University, Bahir Dar, Ethiopia) (
[email protected]), 2Biological & Environmental Engineering, Cornell University, Ithaca, NY, 15853,3Integrated Water Management PhD program, SCWRE, Bahir Dar, Ethiopia, 4Adet Research Center, ARARI, Bahir Dar, Ethiopia, 5 Engineering Hydrology MSc program, Bahir Dar, Ethiopia 6 Department of Agriculture Economics and Management, Cornell University, Ithaca, NY, 15853.
Project Introduction
Results and Discussion
Soil erosion and nutrients loss decreases food production and hampers poverty reduction efforts in the highlands of eastern Africa. Although intensive efforts have been underway to halt land degradation since the 1980s, erosion continues unabated and gullies are swallowing up productive cropland (Tesemma et al., 2010). Some of the lost soil fills up reservoirs in Sudan and will be a treat for the new planned reservoirs (such as the Grand Renaissance Dam) in Ethiopia. Soil nutrients carried by the water and eroded soil are causing eutrophication on natural lakes such as Lake Tana. Current measures to reduce soil loss are ineffective, and new approaches that both consider the hydrology of the whole landscape (instead of the current plot based erosion research) and use traditional farmer’s knowledge for locating erosion control practices are required. The goal of this project is therefore to propose more effective soil and water conservation practices by identifying those parts in the landscape that contributes most of the sediment and nutrients at the outlet.
Figure 2: Cultivated area expansion from 1973 to 2013 in the study areas
Figure 2 shows that the indigenous vegetation (forest and grass land) on these upland watersheds was cleared and converted to agricultural land in 40 years period. This change leads to 1) an increase of surface and subsurface runoff from the hillside to the valley bottoms that initiates gullies (Tebebu et al., 2010 ) 2) these cultivated areas become highly susceptible to erosion. These were confirmed by the farmers during the discussion and transect walks in Debre Mawi. The runoff and sediment concentration monitored in the watersheds before the current massive conservation practices showed gully dominated watersheds have greater soil loss (Figure 3). Figure 3: Sediment load in the study areas (1ton/ha/year is equivalent with 0.07mm soil depth
Hypothesis Severe erosion and nutrient loss are spatially distributed within a watershed and only by treating the most vulnerable areas, not just focusing on steep sloped areas, can erosion rates be reduced.
Table- 2: Impact of conservation on load and concentrations
Sediment Yield (t/ha/year)
2010
2011
66
50
2012
2013
9
13
Mean Sediment concentration
2010
(g/l)
2011
9.4
8.6
2012 8.9
2013 10.3
We evaluated this conservation practice on the sediment load and concentrations in Debre Mawi, and found that the sediment load decreased due to the reduction in runoff as short term impact but sediment concentration is not decreased since gullies are not targeted in this campaign. Gullies were considered by farmers as they were created by their God to punish them for acts against their will. In this project, we attempted to develop a case study on participatory gully rehabilitation for Bir watershed and total amount of sediment trapped in one rainy season was 2,400 Tons compared with 700 and 580 ton soil loss in control gullies (Figure 6).
Study Area Description Gully Erosion One of the peculiar difference between Mizewa watershed and the two watersheds (Bir and Debre Mawi) was gully erosion in the valley bottom or saturated areas (Figure 4).
Figure 6: Gullies before treatment (left) and after treatment in 2013 (right). 8.4 tons of forage was produced from this gully rehabilitation as a result, farmers in the watershed targeted to rehabilitate 5 more gullies in 2014
Soil Nutrients Upslope Mid-slope
Figure- 1: Location Map of Study area watersheds Table- 1: Watersheds characteristics
Watersheds
Area
Annual
Elevation
Slope
(km2)
Rainfall
(m.a.s.l.)
range
(mm)
(%)
Bir (Enchilala)
4.14
1200
2000-2414
3 to 63
Debre Mawi
0.95
1240
2212-2306
6 to 30
7
1200
1887-2291
1 to 40
Mizewa
Methodology PhD program in Integrated Water Management established since 2012 with the help of Cornell University. Land use & land cover change since 1970’s were investigated in the 3 watersheds using Land sat image. During the rainy phase of the 2010, 2011, 2012 and 2013 monsoons rainfall, runoff and sediment concentrations were measured by PhD students. In addition, perched groundwater tables, soil nutrients (N, P and K), and gully expansion were measured. Six age- and gender differentiated groups in were interviewed for discussions concerning agricultural practices, soil, hydrology. References:
Down slope
Figure 4: Gullies at Debre Mawi (left) and Bir (right) located at relatively gentle slopes or valley bottoms
Investigation of the gully evolution processes for developing better gully bank rehabilitation options in the Debre-Mewi watershed showed that about 96% of the sediment load measured in the outlet of gully was originated from the gully banks indicating that gully treatment has a paramount importance to reduce siltation of reservoirs.
Figure 7: Mizewa watershed with its landscape where dissolved phosphorous from piezometers and available phosphorus from soils monitored in 2013. Table- 3: Magnitude of dissolved and available phosphorus Mizewa watershed up slope Mid-slope Down slope
Dissolved Phosphorus Available phosphorus (mg/kg) (ppm) 0.04 2.9 0.08 18.3 0.21 8.5
Soil and Water Conservation
Conclusions
Figure 5: Government mobilized the community to large scale Soil and Water Conservation works on the Ethiopian highland since 2012
Tebebu, T. Y., Abiy, A. Z., Zegeye, A. D., Dahlke, H. E., Easton, Z. M., Tilahun, S. A., Collick, A. S., Kidnau, S., Moges, S., Dadgari, F., and Steenhuis, T. S.: Surface and subsurface flow effect on permanent gully formation and upland erosion near Lake Tana in the northern highlands of Ethiopia, Hydrol. Earth Syst. Sci., 14, 2207–2217, doi:10.5194/hess-14-2207- 2010, 2010.; Tesemma, Z. K., Mohamed, Y. A., and Steenhuis, T. S.: Trends in rainfall and runoff in the Blue Nile Basin: 1964–2003, Hydrol. Process., 24(25), 3747–3758, doi:10.1002/hyp.7893, 2010; Akoma A. C. : Hydrobiological Survey of the Bahir Dar Gulf of Lake Tana, Ethiopian International Multi-Disciplinary Journal, Vol. 4 (2) , 57-70, April, 2010.
This project that was instrumental in educating 5 PhD students, showed conclusively that sediment concentrations from the humid uplands in Ethiopia can be reduced but only by designing soil and water conservation practices that are in tune with the hydrology of the humid monsoon climate and that are not copied from dry land agriculture elsewhere.
SPECIAL THANKS TO:: AID-OAA-A-11-00012
