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

Abstract 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 assess erosion on daily basis so that the effects of extreme rains are taken into account. We couple it to a simple surface soil moisture balance and include estimation of daily vegetation cover changes for calculating rain interception and for estimating effective rain. The runoff fraction is based on the available daily storage of the effective hydrological depth. Unlike the original MMF model which accumulates annual runoff, the daily model accumulates daily runoff from upstream contributing area in a predefined flow network according to steepest slope direction. Annual soil loss is calculated by adding daily erosion rates. We compare the obtained results with that obtained from applying the revised MMF model in two locations: (i) in sub-humid tropics in central Thailand which is affected by deforestation and land cover changes resulting in excessive soil losses, and (ii) in semi-arid environment in Morocco which is affected by severe gully formation. Since the model results are daily estimates it is possible to see the effects of exceptional rain. In Morocco, the effects of extreme rains are clearly shown which were absent in the results obtained by using the annual model. The results also show that erosion rates can be moderate when rainfall pattern is normal without extreme rains in a year although total rain may be similar. This is clearly shown in the erosion assessment in Thailand for the years 2005 (1390 mm) and 2006 (1366 mm, and with the presence of extreme rainy days).