1Quantifying water run-off in the water catchments of the tropical belt was one of Orstom’s traditional tasks. It was meant to facilitate the development of roads or sanitation systems in countries where there was still a great deal to be done, a colossal task that, in the late 1970s, researchers set out to put to statistical use. The objective was to create a model to estimate the level of 10-year floods, based only on a limited number of parameters such as the shape of the water catchment, the type of soil and, vegetation and the rainfall pattern. This original idea was a failure, as the nature of soil appeared to be entirely unrelated to the run-off level measured on the surface. Researchers struggled to understand this paradox.

Water erosion, Niger.

••• Soil surface characteristics provide information on the soil’s ability to support water run-off •••

2The turning point occurred after 1980, when researchers understood that soil, as envisioned by soil scientists, is indicative of its history and formation, but its surface properties are not. Said properties vary depending on the agricultural use, plant cover and weather conditions. This led researchers to switch from the notion of soil-based hydrological units to that of units based on “soil surface characteristics”.

Run-off study in Mexico.

3These characteristics can be measured using large portable watering cans: rainfall simulators. This field equipment helps predict the level of water run-off in a given place, and draw up a new kind of map identifying soil surface characteristics. This data provided a more realistic representation of water transfers, while allowing IRD to develop models more in tune with reality. This progress significantly improved forecast accuracy in terms of floods, low water periods and even the quality of water.

“Research in the dynamics of the layered surfaces of fine and sandy crop-growing soils in the Sahel revealed that these soils tend to undergo widespread crusting which causes substantial run-off.
More importantly, this research highlighted the fact that this phenomenon is essentially due to their strong susceptibility to the formation and extension of the erosion crust, the type of soil surface which causes most run-off, that even long-term fallow periods cannot seem to significantly counter.” 
Jean-Marie Ambouta Karimou, professor at the Abdou Moumouni University of Niamey, Niger