Michel Esteves

Overland flow and infiltration modelling for small plots during unsteady rain: numerical results versus observed values

This paper reports the development and application of a two-dimensional model based on an explicit finite difference scheme coupling overland flow and infiltration processes for natural hillslopes represented by topographic elevation and soil hydraulics parameters. This model allows modelling of hortonian overland flow and infiltration during complex rainfall events. Original procedures have been developed in order to simulate complex rainfall events on natural slopes. The accuracy of the results is tested by comparison with experimental field data on the basis of calibrated soil and surface friction parameters. Good agreement between the calculated result and the measured data was found. The scheme proposed was found to be appropriate. The results of tests presented illustrate the effect of microtopography on the distribution of the flow depths, the magnitude and direction of flow velocities and infiltration depths.

Runoff generation processes: results and analysis of field data collected at the East Central Supersite of the HAPEX-Sahel experiment

Within the scope of the HAPEX-Sahel experiment, the hydrological functioning of two small nested catchments was studied at two different scales: the plot scale (of the order of 100 m2) and the catchment scale (0.2 km2). At local scale, four runoff plots were set-up on the typical soil surface conditions observed on the catchments (plateau bare soil, two plots on fallow grassland) and an additional one was installed on a millet field. soil moisture investigations at the plot scale have shown that infiltration was limited between 0.6 to 2 m deep on three sites, but was deeper than 3.4 m on the most pervious site (millet). The maximum water storage on all the sites was found to be reached at the maximum activity of the rainy season (late August), and not at the end of the season. During the dry months, the soil was fully dried off by evapotranspiration, resulting in the absence of inter-annual soil water storage. No influence of vegetation cover on runoff was observed on the fallow sites, but runoff generation was found to be very sensitive to tillage on the millet field. The parameter Pu, calculated from a rainfall hyetograph and defined as the rainfall depth that can actually produce runoff, is shown to be relevant to compute runoff on untilled soils, as it explains more than 87% of the variance in runoff depth. On tilled soils, it is necessary to take into account additionally the temporal evolution of the soil surface, especially the days after weeding operations. Simple linear relationships were derived to compute runoff depth from Pu on the plots for the most typical soil moisture conditions observed, and modified SCS equations have been derived for the catchments. Using the linear equations derived at the plot scale in a simple, empirical, semi-distributed model lead to formulate the assumption that the partial source area concept applied on the catchments. Analysis of discharge data at the catchment scale highlights that seepage through the bottom of a gully between two gauging stations leads to the abstraction of non negligible volumes of water. Moreover, the water totally infiltrates in a spreading zone downstream from the outlet of the largest catchment showing that discontinuities occur in the surface water transmission within a catena. Such discontinuities constitute a major problem for the concern of aggregation of hydrologic processes.

Influence of soil surface features and vegetation on runoff and erosion in the Western Sierra Madre (Durango, Northwest Mexico)

Abstract In mountainous areas, runoff and soil erosion are closely linked to soil surface features, particularly stoniness. Depending on the size of rock fragments (gravel, pebbles, stones and/or blocks) and especially the way they are integrated into the soil matrix, they may facilitate or hinder infiltration and promote soil losses. The present study examines the role of different soil surface features and their influence on runoff formation and on soil erosion in an area seriously affected by overgrazing. Based on measurements made on hillslopes for 2 years at the plot scale, the results show that grass cover, pebbles and sand content increase runoff and erosion. Inversely, slope value, tree cover percentage, structural stability and organic matter content are negatively correlated with runoff and soil losses. It is shown that the correlations can be explained by the major role played by the surface features on hydrologic behaviour of the hillslopes. Two main surface features were identified and hydraulically characterised, namely: (i) crusted surfaces with embedded gravel widespread on gentle slopes which induce high runoff and erosion rates; and (ii) stony surfaces, where free pebbles and blocks protect the top soil against raindrops and overland flow kinetic energy and lead to reduce runoff and soil losses.

A multi-scale approach of runoff generation in a Sahelian gully catchment: a case study in Niger

Abstract Runoff production conditions in a small gully catchment are studied at four different scales: the point scale (0.001 m 2 ), the local scale (1 m 2 ), the field scale (of the order of 100 m 2 ) and the catchment scale (0.2 km 2 ). At the point scale, infiltration measurements were conducted using a tension infiltrometer. At the local and the field scale, runoff plots were setup on typical soil surface conditions of the catchment (plateau bare soil, hillslope bare soil and fallow grassland). At the catchment scale, stream discharges were measured at two gauging stations. The overland flow yield is significantly nonuniform in space, due to the high spatial variability of infiltration capacities and the depressional storage of the soil surface. The runoff and the infiltration data collected confirmed the major role played by soil crusting on runoff generation in that part of Sahel. At the point scale, hydraulic conductivity measurements have shown that infiltration and runoff were driven by the hydraulic properties of the crust. At the field scale, microtopography and heterogeneity in the soil surface crusting decreased discharge volumes. The influence of vegetation growth on runoff yield was evident in the case of the fallow sites. Analysis of discharge data at the catchment scale highlights that infiltration through the bottom of the gully between two gauging stations leads to considerable runoff water transmission losses.

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

Abstract 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.

Tracing sediment sources in a tropical highland catchment of central Mexico by using conventional and alternative fingerprinting methods

Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright

Tracing sediment sources in a tropical highland catchment of central Mexico by using conventional and alternative fingerprinting methodsTracing sediment sources in a tropical

Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright

Raindrop erosion of tillage induced microrelief: possible use of the diffusion equation

The purpose of this paper is to evaluate the possibility of using the diffusion equation for raindrop erosion modelling. We wanted in particular to know if such a model could provide accurate interpolations of microrelief between two known dates. In a theoretical section, we show that the assumption that soil particles follow parabolic trajectories when splashed by raindrop impacts leads to a diffusion equation. This equation suggests a linear relation between Dz, the variation of height between two dates, and the Laplacianr 2 z (r 2 za@ 2 z/@x 2 a@ 2 z/@y 2 ). This relation is confirmed by data from a simulated rainfall experiment carried out in the sandy soils of the Senegalese groundnut belt. Four square plots of side 4 m each were used. They were hoed with a traditional horse-drawn three-tined hoe. Three rains of 70 mm h ˇ1 lasting 30 min each were applied. An automated relief meter designed and constructed by the authors was used to measure the distribution of heights for every 5 cm before the first rain, and after the first and the third rains. The mean correlation coefficient of the model was 62% for the first rain and 46% for the next two rains. Besides raindrop erosion, compaction occurred during the first rain. Adding a crude description of compaction enhanced the mean of the correlation coefficients of the model up to 70% for the first rain. Furthermore, the coefficient of variation of the four adjusted total diffusion lessens from 10 to 6%. The simulated surfaces were smoother than the real ones, which was an expected result, but the surface storage capacity was overestimated. The latter result illustrates the role of runoff in shaping the flow paths it follows and, consequently, in lessening the surface storage capacity. The main conclusion is that the diffusion equation provides a promising frame for further development of models simulating microrelief evolution during rainfall. Another conclusion is that these models should integrate existing routines for runoff erosion at small scale in order to simulate surfaces with realistic hydraulic properties. # 2000 Published by Elsevier Science B.V.

Relationship between raindrop erosion and runoff erosion under simulated rainfall in the Sudano-Sahel: consequences for the spread of nematodes by runoff

This paper presents a rainfall simulation experiment carried out on three 50 m2 plots in the Senegalese groundnut belt. One plot was not cultivated. Groundnut and millet had previously been grown in the other two. The experiment consisted of three rain events applied over 5 days at the end of the dry season. Erosion was monitored inside the plots by the use of a relief meter and, at their outlets, by sampling the discharge. The number of indigenous nematodes, and an exotic species introduced before the first rain event, was monitored in the soil and in the discharge. This experiment allows, for the first time, a set of simple hypotheses to be proposed to explain the spread of nematodes by the runoff: raindrop impacts on the soil surface set them in suspension; then, their low bulk density and their relatively large size do not allow them to settle when the raindrops shake the water surface. Thus, nematodes follow the flow path where they are as far as its velocity remains significant. The biological aspects are decisive in the mobility of nematodes, which can vary by a factor of 100 depending on the trophic groups. A very high raindrop erosion occurred during the experiment, up to 60 tons per hectare for the first rain event after hoeing. This represents more than 40 per cent of the volume of soil previously moved by soil work. The geometric properties of the plough, and their hydraulic consequences, appear very ephemeral. And yet these large movements of soil inside the plots are little related to the sediment load at the outlet, which follows its own rules. Analysis of the results indicates that the carrying capacity of the runoff at the scale of 10 m2, on gentle slopes ploughed perpendicular to the slope, could not be directly calculable from the discharge. It could depend on the history of past discharges because the shape of the flow paths, which condition their carrying capacity, permanently interacts with the discharge. These interactions could explain the great difficulties encountered by the erosion models in the case of low discharges on non-cohesive soils. Copyright

Runoff evolution due to land-use change in a small Sahelian catchment

Abstract Land-use changes have been significant these last decades in West Africa, particularly in the Sahel region; in this area, climatic and demographic factors have led to a rise in cropped areas in recent decades causing strong changes in the water cycle and in river regimes. This study compares the rainfall–runoff relationships for two periods (1991–1994 and 2004–2011) in two small and similar neighbouring Sahelian catchments (approx 0.1 km2 each). This allows identification of the different hydrological consequences of land-use/land-cover change, particularly the fallow shortening and the consequent degradation of topsoil. The main land surface change is a 75% increase in crusted soil area. Runoff increased by more than 20% on average between the two periods while flood duration decreased by 50% on average. However, runoff values remained largely constant in the lower part of the northern basin due to a strong increase in in-channel infiltration. Editor D. Koutsoyiannis; Associate editor T. Wagener