Modelling erosion on a daily basis, an adaptation of the MMF approach

Effect of soil erosion causing negative impact on ecosystem services and food security is well known. On the other hand there can be yearly variation of total precipitation received in an area, with the presence of extreme rains. To assess annual erosion rates various empirical models have been extensively used in all the climatic regions. While these models are simple to operate and do not require lot of input data, the effect of extreme rain is not taken into account. Although physically based models are available to simulate erosion processes including particle detachment, transportation and deposition of sediments during a storm they are not applicable for assessing annual soil loss rates. Moreover storm event data may not be available everywhere prohibiting their extensive use. In this paper we describe a method by adapting the revised MMF model to assesserosion on daily basis so that the effects of extreme rains are taken intoaccount. We couple it to a simplesurface soil moisture balance and include estimation of daily vegetation coverchanges for calculating rain interception and for estimating effective rain. The runoff fraction isbased on the available daily storage of the effective hydrological depth.Unlike the original MMF model which accumulates annual runoff, the daily modelaccumulates daily runoff from upstream contributing area in a predefined flownetwork according to steepest slope direction. Annual soil loss is calculated by adding daily erosionrates. We compare the obtained results with that obtained from applying therevised MMF model in two locations: (i) in sub-humid tropics in centralThailand which is affected by deforestation and land cover changes resulting inexcessive soil losses, and (ii) in semi-arid environment in Morocco which isaffected by severe gully formation. Since the model results are daily estimatesit is possible to see the effects of exceptional rain. In Morocco, the effectsof extreme rains are clearly shown which were absent in the results obtained byusing the annual model. The results also show that erosion rates can bemoderate when rainfall pattern is normal without extreme rains in a yearalthough total rain may be similar. This is clearly shown in the erosionassessment in Thailand for the years 2005 (1390 mm) and 2006 (1366 mm, and withthe presence of extreme rainy days).