Under the project, ITC students doing fieldwork in the project area in Namchun watershed have applied various models such as RUSLE (Revised Universal Soil Loss Equation), RMMF (Revised Morgan Morgan and Finnay) and the WEPP (Water Erosion Prediction Project) models. One of the results of erosion assessment in Namchun: watershed is extracted (Suriyaprsit, 2008) . In one study RUSLE and RMMF models were applied in Lomkao area. The performance of the two models were tested and their sensitivity to various input parameters such as cover factor, slope gradient, rainfall amount, soil erodibility, soil organic matter content, etc were analyzed (Yazidhi, 2003). (Teklehaimanot, 2003) applied RMMF model and used soil erodibility value derived from his simple field tests. He showed that soil erodibility values are not significantly different from the erodibility values given in the literature. (Saengthongpinit, 2004) also applied RMMF but replaced the slope gradient in the model with sediment transport index, derived from digital terrain analysis. The sediment transport index takes into account not only topographic slope but also specific catchment area (contributing area for overland flow). Recently (Kassa,2007) applied RMMF and flow accumulation, derived from digital terrain analysis.
Process-based models claim to give better results but they can be very much data demanding as shown by MSc. work of (Endale, 2003). He applied WEPP (Water Erosion Prediction Project) model in Petchabun and because of un-availability of detail climatic data in Thailand he had to run the model using climatic data of Florida, USA. Although his results are interesting, application of such model in Thailand will be questionable.
The Universal Soil Loss Equation (USLE)
USLE was developed in the 1970s by United States Department of agriculture (USDA). This soil erosion model used widely within the United States and worldwide (Merritt et al., 2003). The equations in this model have been developed using statistical analysis of data from 10,000 plots years from natural run off plots together with 2,000 plot years of artificial rainfall simulators in USA (Wischmeier and Smith, 1978). Sheet and rill erosion are predicted by using values for indices that represent the four major factors affecting erosion: R-climatic erosivity, K-Soil erodibility, L- and Stopography, and C and P-landuse. The model has undergone a number of modifications. The model has also been upgraded to take into account additional information that has become available since the development of the USLE (Renard, 1997). There are some limitations in this model and can not identify events as long term erosion. The model can only predicts inter rill erosion, but not gully, channel or stream bank erosion. It can estimate soil particles movement but ignore deposition. The accuracy of the equations is bias when using only short-term rainfall records (Merritt et al., 2003).
Revised Universal Soil Loss Equation (RUSLE)
This model has the same factors as USLE. It updates the USLE model and incorporates new material that has been available informally or from scattered research reports and professional journals. It has been developed to replace the USLE, but it has the same limitations (Gebrekirstos, 2003; Saengthongpinit, 2004).
The Morgan Morgan Finney Model (MMF)
This model was developed to predict annual soil loss from field sized areas on hill slopes. The model has the simplicity of the universal Soil Loss Equation and yet it covers the advances in understanding of erosion process (Morgan et al., 1984). This model is a physically based empirical model (Mix model) and needs less data than most of the other erosion predictive models. This model divides soil erosion process in two phases including a water phase and a sediment phase. The MMF model can be easily applied in a raster-based geographic information system (Shrestha, 2007).
Revised Morgan Morgan Finney (RMMF)
The RMMF model separates the soil erosion process into two phases: the water phase and the sediment phase. The water phase determines the energy of the rainfall available to detach soil particles from the soil mass and the volume of runoff. In the erosion phase, rates of soil particle detachment by rainfall and runoff are determined along with the transporting capacity of runoff (Morgan, 2001). The difference from MMF model are the stimulate of soil particle detachment by rain drop that takes account of plant canopy height and leaf drainage, and a component has been added for soil particle detachment by flow (Morgan, 2001). The detail of RMMF model can describe as following (Morgan, 1995):
Water Erosion Prediction Project (WEPP)
WEPP is a physical based hydrological and erosion model designed to replace the USLE (Laflen et al., 1991). This model contains two sub-models with hill slope version and watershed version. Hill slope version can estimate soil detachment and deposition along a hill slope profile and the net total soil loss is estimated from the end of the slope without considering erosion, transportation and deposition processes in permanent channels. For watershed version that allows estimation of net soil loss and deposition over small catchments, it uses for applying to field areas that include ephemeral gullies which can be farmed over and links these surface erosion processes to the channel network. It can run for a single storm and on a continuous simulation.
References
Endale, M., 2003. Cropland soil erosion prediction using WEPP model: a case study on hill slope in Lom Kao, Thailand
Laflen, J.M., Lane, L.J. and Foster, G.R., 1991. WEPP, a new generation of erosion prediction technology. Journal of Soil and Water Conservation 46.
Morgan, R.P.C., 1995. Soil erosion and conservation. Longman, Harlow, 198 pp. Morgan, R.P.C., 2001. A simple approach to soil loss prediction: a revised Morgan-Morgan-Finney model. CATENA, 44(4): 305-322.
Renard, K.G.e., 1997. Predicting soil erosion by water : a guide to conservation planning with the Revised Universal Soil Loss Equation RUSLE. USDA Handbook;703. United States Department of Agriculture (USDA), Washington, D.C., 384 pp.
Saengthongpinit, C., 2004. Soil erosion assessment using revised MMF equations with special reference to terrain parameter(s) : a case study in Nam Chun sub watershed, Lomsak district, Thailand, ITC, Enschede, 80 pp.
Shrestha, D.P., 2007. Lecture notes on Soil degradation assessment and modelling. Department of Earth System Analysis, ITC, Enschede.
Suriyaprsit, M. 2008 Digital terrain analysis and image processing for assessing erosion prone areas: A Case Study of Nam Chun Watershed, Phetchabun, Thailand. Unpublished MSc thesis, ITC: Enschede. 97p.
Teklehaimanot, Gebrekirstos, 2003. Use of simple field test and revised MMF model for assessing soil erosion: case study Lom Kao area, Thailand, 94 p.
Wischmeier, W.H. and Smith, D.D., 1978. Predicting rainfall erosion losses : a guide to Conservation planning. Supersedes agriculture handbook no. 282, "Predicting rainfall-erosion losses from Cropland East of the Rocky Mountains". COPY. Science and Education Administration United States Department of Agriculture (USDA) ; Purdue Agricultural Experiment Station, .35 pp.
Yazidhi, Bamutaze, 2003. A comparative study of soil erosion modelling in Lom Kao-Petchbun, Thailand, 92 p.
Development of Methodologies for
Land Degradation Assessment Applied to
Land Use Planning in Thailand
Endale, M., 2003. Cropland soil erosion prediction using WEPP model: a case study on hill slope in Lom Kao, Thailand