Microrelief induced by tillage: measurement and modelling of Surface Storage Capacity

The micro-topography of a groundnut plot in Senegal has been recorded over a full cultivation cycle, using an automated device able to measure 16.2 m2 at every 5 cm with an accuracy of 1 mm. Tillage is horse drawn, perpendicular to the general slope, and generates oriented microreliefs. Surface Storage Capacity (SSC) was calculated on both raw and slope-detrended surfaces. Additionally, various boundary conditions (BC) were used: no-wall; three-wall (up, left and right); or mirror (the Digital Elevation Model (DEM) surrounded by eight alternately reversed images of itself). SSC is more affected by these variants than by the variations of microrelief itself. Whatever the calculation method, SSC (as well as random roughness), follows a decreasing exponential with cumulated rainfall, but the coefficients of the exponential differ widely to each other. This suggests that SSC values could be of little use when they are obtained on various slopes, arbitrarily detrended or not, and calculated with arbitrary BC. We suggest a simple geometric model to characterise the way microrelief empties as the slope increases. The model has two calibrated depth-ratio parameters, one in each direction. It gives a more coherent framework for calculation and use of SSC. The model was applied to one of the DEMs of the data set, sampled after the first rain following hoeing. With the mirror-BC and detrended slope, SSC was 3.6 mm. Microrelief was found to behave in the same proportions, when tilted, than a tetrahedral container 94 times wider than deeper in the tillage direction and 11 times perpendicularly. This model represents the volume of surface water that cannot flow in any direction. With three-wall-BC, SSC was 6.7 mm, 1.4 mm remaining on the plot whatever the slope angle, and 5.3 mm behaving the same as a container 69 times wider than deeper. A possible use of this model is illustrated with an attempt to upscale the sampled plot to the watershed to which it belongs.