4.1. About the MMF Erosion model

Note

This section was copied from the manual

In this chapter we are going to apply the Morgan, Morgan and Finney (MMF) method on the Hadocha sub-catchment of The Fincha’a study area. The work can be divided in five different parts; the first part is a virtual tour through the catchment. In the second part you will make the maps needed for the model from the raw data. The third part will direct you through all the different steps in the MMF model. The fourth step is combining all the different steps into one model. During the fifth step; implementing Land Improvement measures into the MMF method, you will show what you have learned about the MMF model and show your GIS capacities obtained during this course.

Note

Before you start with the next chapter, you have to read the article: Land management, erosion problems and soil and water conservation in Fincha’a watershed, western Ethiopia by Bezuayehu Tefera and Geert Sterk. You need the information from this article for the next exercises. The article can be found in Annex 1 of this practical guide.

4.1.1. Preparing the data for the Morgan, Morgan and Finney Method

Tip

You are now starting the most important part of the practical. At this point, we strongly recommend you to read briefly the rest of the manual to check the expected outcomes (you can skip the steps for now). It is essential that you understand what and why you are doing from the beginning. In this chapter you are going to prepare the data for the Morgan, Morgan and Finney Method (MMF see also Annex 1) to assess the erosion rates of the Hadocha catchment and come up the land management scenarios to reduce erosion in the catchment. GIS will help you assessing the impact of your measures.

Morgan et al. (1984) developed a model to predict annual soil loss from field-sized areas on hill slopes, which, while endeavouring to retain the simplicity of the USLE, encompassed some of the then recent advances in understanding of soil erosion processes. The approach was revised by Morgan (2001). The model separates the soil erosion processes into a water phase and a sediment phase. It considers soil erosion to result from the detachment of soil particles by raindrop impact and runoff and the transport of those particles by overland flow. The process of transport by rain splash is ignored. Thus, the sediment phase comprises three predictive equations, one for the rate of particle detachment by rain splash, one for the rate of particle detachment by runoff and one for transport capacity of overland flow. The inputs to these equations of rainfall energy and runoff volume respectively are obtained from the water phase. The model uses 12 operating functions for which 19 input parameters are required.

The effects of soil conservation practices can be included in the separate phases of the model. For example, agronomic measures will bring about changes in evapotranspiration, interception and crop management, which will affect respectively the volume of runoff, the rate of detachment and the transport capacity.

The model compares the predictions of total detachment by rain splash and runoff with the transport capacity of the runoff and assigns the lower of the two values as the annual rate of soil loss, thereby denoting whether detachment or transport is the limiting factor. The predictions obtained by the model are most sensitive to changes in annual rainfall and soil parameters when erosion is transport limited and to changes in rainfall interception and annual rainfall when erosion is detachment limited. Thus, good information on rainfall, soils and land use is required for successful prediction.

Like the USLE, the model cannot be used to predict soil loss from individual storms or from gully erosion. The model can be used to predict soil loss from small catchments by dividing the catchment into land units of similar soils, slopes and land cover and routing the annual runoff and sediment production over the land surface from one unit to another. The runoff for each unit is then calculated as the summation of the runoff generated on that unit and the runoff flowing into it from upslope; the transport capacity is calculated for the combined runoff. Similarly, the total sediment available for transport is the summation of that detached on that unit and the material transported into the unit from upslope, and this is compared to the transport capacity. When operated in this way, the model is able to identify the major source areas of sediment and the locations of deposition within a catchment and to provide the information for the sediment delivery ratio.