In the experiment, Water Erosion Prediction Project (WEPP), software for determining the simulation model based on fundamentals such as topography, soil, management and climate was used. Four input files (climate file, soil file, management file and slope file) were used. The climate file consists of the observed data named “Virgina Plains” and simulated data named “Dwepp” as shown in appendix 1.
The WEPP is a program whose aim is to address water erosion prediction technology for soil and assess water planning and conservation (Nearing et al., 1989). The WEPP model includes an irrigation estimation component for soil loss emanating from furrow and sprinkler irrigated fields. Simulated rainfall conditions that define soil parameters and functional relationships for the WEPP model differ from furrow irrigation erosion systematics (ENGDA, T.A. 2009).
How WEPP is used to predict runoff and soil loss
The WEPP models hydrological component is vital to soil erosion forecasts since rill erosion can be solved as a function of hydraulic shear (LAFLEN, 2011). As described in the WEPP technical documentation (Flanagan and Nearing, 1995) and presented by Fok and Chiang (1984), the WEPP furrow irrigation component calculates infiltration using the Green-Ampt infiltration equation two-dimensional approximation.
Kinematic wave hydrology component is used to calculate the PEAKRO and runoff volume. Effective runoff duration is derived from the division of runoff volume by peak runoff rate. The peak rate, runoff volume and the duration parameters are used in the component of steady-state erosion that predicts deposition, sediment detachment and transport. The interrill and rill processes of soil erosion are the two major categories in the WEPP model. Interrill erosion includes transport and soil detachment by shallow sheet flow and also by raindrops. Rill erosion processes involves transport and soil detachment and deposition in rill channels (Flanagan and Nearing, 1995). The WEPP model assumes that furrow erosion is the same as fill erosion under rainfall conditions. In rills detachment occurs when the soil critical shears are exceeded by the hydraulic shears and the rill transport capacity exceed the sediment load. A sediment deposition occurs when the sediment load exceeds the transport capacity.
It is important to understand the basic relationships between runoff, rainfall and soil loss as they are necessary for efficient management of soil conservation planning and exploitation of water resources. Sedimentation and erosion are main issues that disrupt the ecosystem. Watershed erosions are usually affected by gross runoff responses that occur as a result of multifaceted interactions between physiographic and climatological factors (Rai and Mathur, 2007). According to Herweg and Stillhardt (1999), a major threat posed to the long-term productivity of agriculture all over the world is soil erosion by water. This erosion affects quite a significant percent of the agricultural areas and the total population.
Soil and water conservation area closures and forestation are some of the land rehabilitation activities that have been used to reclaim degraded lands by governmental and non-governmental organization. Household level water harvesting systems, river diversions and dams have also been used to tackle problems associated with low agricultural production to water shortages. Many projects are not as successful as expected because information regarding rainfall and runoff soil loss processes is scarce (Derib 2005). Runoff is affected by rainfall intensity which is an important parameter in modeling rainfall runoff relationships (Beven, 2004; Amore et al., 2004).
Rainfall influences watershed hydrological responses that in turn influence soil erosion (Grunwald and Norton, 2000). It is therefore important to understand the WEPP hydrological processes since it helps in identifying runoff source areas, water resource potentials and erosion danger zones. This aids in the estimation of sediments yield and runoff a basis for the development of watershed plan management that involves water and soil conservation measures (Pandey et al, 2008; Steenhuis et al, 2008). The WEPP model helps simulate and represent real hydrological processes so that there’s prioritization for areas that are in greater need of water and soil conservation measures which is a major step to better targeting of finite resources to enhance soil conservation measures.
ENGDA, T. A. (2009). Modeling rainfall, runoff and soil loss relationships in the northeastern highlands of Ethiopia, Andit Tid watershed. Project report (M.P.S.(Int. Agr. & R. D.))-- Cornell University, August 2009.
FOK, Y., & CHIANG, S. (1984). 2???D Infiltration Equations for Furrow Irrigation. Journal of Irrigation and Drainage Engineering. 110, 208-217.
LAFLEN, J. M. (2011). Application of WEPP to Sustainable Management of a SmallCatchment in South west Missouri, US, Under Present Land Use and with Climatic Change. 168- 185.
M. A. NEARING, L., G. R. FOSTER, L., L. J. LANE, L., & S. C. FINKNER, L. (1989). A Process-Based Soil Erosion Model for USDA-Water Erosion Prediction Project Technology. Transactions of the ASAE. 32, 1587-1593.
ASCOUGH, J. C., FLANAGAN, D. C., & LIVINGSTON, S. J. (1995). WEPP user summary: USDA, Water Erosion Prediction Project. West Lafayette, Ind, National Soil Erosion Research Laboratory, USDA-ARS-MWA.
SAGHAFIAN, B., MEGHDADI, A. R., & SIMA, S. (2015). Application of the WEPP model to determine sources of run-off and sediment in a forested watershed. Hydrological Processes. 29, 481-497.
RAI, R. K., & MATHUR, B. S. (2007). Event-Based Soil Erosion Modeling of Small Watersheds. Journal of Hydrologic Engineering. 12, 559-572.
HERWEG, K., & STILLHARDT, B. (1999). The variability of soil erosion in the highlands of Ethiopia and Eritrea: average and extreme erosion patterns. Berne, Centre for Development and Environment, Univ. of Berne in association with the Ministry of Agriculture Ethiopia.
BAYABIL, H. K., TILAHUN, S. A., COLLICK, A. S., YITAFERU, B., & STEENHUIS, T. S. (2010). Are runoff processes ecologically or topographically driven in the (sub) humid Ethiopian highlands? The case of the Maybar watershed. Ecohydrology. 3, 457-466.
BEVEN, K. (2004). Infiltration excess at the Horton Hydrology Laboratory (or not?).Journal of Hydrology. 293, 219-234.