Surface runoff modelling related to seasonal vegetation cycles (millet, groundnut and fallow) in central Senegal
The Sudanese climate is characterized by a rainy season and a dry season (mean annual rainfall between 400 and 900 mm). At the end of the dryseason (June in the northern hemisphere), the landscape is completely bare under the effect of animal grazing or soil tillage. During the first rainfalls this leads to high runoff coefficients. These runoff coefficients decrease gradually as the amount of vegetation increases during the growing season (RODIER (1984-1985); ALBERGEL (1988)). This is particularly true in the Groundnut basin of central Senegal where millet and groundnut are cultivated every other year. As the vegetative cover increases, a system of macropores develops in the soil and preferentially induces water infiltration through mesofauna burrows and along root systems. Hence, many authors have distinguished matrix infiltration governed by the generalized Darcy's law, from preferential infiltration through macropores characterized by a strongly heterogeneous spatial distribution (GERMAN, 1990). These macropores are thought to be responsible for the proportional increase in infiltration with increase in rainfall intensity observed on several experimental plots (BOUCHARDEAU and RODIER, 1960; VALENTIN, 1985; COLLINET,1985; ALBERGEL, 1988). A more complete surface ponding or a differential distribution of the macroporosity in relation with the microtopography can contribute to this phenomenon. A conceptual runoff model accounting for surface storage, which views infiltration as a function of water depth on the ground surface, is proposed to describe the aforementioned phenomenon under three characteristic vegetative canopies of central Senegal (millet, groundnut and fallow). The model (BADER, 1994) is a distributed, three parameter model that accounts for transfer between spatial elements (parameter n), runoff (parameter Hl) and infiltration (parameter S). The model solves the equation of continuity according to an explicit scheme (forward time). The discharge exiting a spatial element is defined by a power function based on the water depth on the element. The value of the transfer parameter n (dimensionless) depends on the roughness and slope of the soil surface. Parameter Hl (meters) is equivalent to the water depth from which runoff occurs and is found in the discharge expression. Infiltration is defined as the product of the squareroot of the depth of ponded water of a plot and a S parameter (dimensionless) representing surface porosity. The experimental work took place on 4 rectangular 50m2 plots (10 m by 5 m) that were initially bare and weedy. At the beginning of the rainy season, two plots were cultivated in millet and groundnut, one left fallow and the fourth stripped by a powerful herbicide. The runoff was measured by a capacitive gauging system with each tank being equipped with a pressure transducer connected to a datalogger. A tipping bucket raingauge was also connected to the datalogger and rainfall and runoff were recorded simultaneously. The measurements were made to a precision of 4 mm in the tanks (0.16 mm uncertainty for surface runoff depth). With a total seasonal rainfall of 711 mm in 1994, the cumulative surface runoff varied between 40mm for the fallow plot to 150 mm for the bare soil plot. The cultivated groundnut and millet plots had cumulative runoff depths of 55 and 60 mm, respectively. The fallow plot would have had less runoff if it had been more than one-year old. The microtopography of each plot was evaluated using a profile meter. The surface roughness was estimated by the standard-error of measured relative elevations (GUILLOBEZ and ZOUGMORE, 1994). Measurements were taken after each significant rainfall and following tillage operations. The index of roughness varied following vigorous weeding of the groundnut plot to 5 mm for the fallow plot whose microtopography remained constant throughout the season. The development of the vegetative cover was indirectly followed by the calculation of a vegetation index (NDVI) derived from red and near infrared reflectances measured with a field radiometer. Although this index tends to saturate with full ground cover, it nevertheless remains a good indicator at the start of vegetative growth. The proposed model was used to reproduce measured runoff during several storm events. Calculations were undertaken with a 10-s time step on a 1m-long spatial element with a uniform set of parameters for each plot. A sensitivity analysis was performed for all runoff events on the bare plot. Hydrograph characteristics (runoff volume, peak discharge and time-to-peak) were particularly sensitive to variations in the transfer parameter (n) and to a lesser extent to changes in the infiltration (S) and runoff (Hl) parameters. For the 42 measured runoff hydrographs for all fourplots, the results were excellent: 70% of the simulated hydrographs had a Nash's coefficient greater than or equal to 0.90. For each plot, the seasonal chronicle of each parameter is coherent with the plot cover. The parameters for the bare plot were invariant throughout the rainy season. However, for the other plots, they varied with the vegetative cover. At the beginning of the growing season, they were similar to those obtained on bare soil and, as the vegetative cover increased, they varied until the NDVI exceeded 0.35 (approximately 20 days after seeding). The evolution of the n and S parameters for the cultivated plots was linearly extrapolated from past events (seeding for the cultivated plots and chemical weeding for the fallow plot) and for S to an antecedent precipitation index. Farming practices that modified surface roughness needed to be accounted for as well. For the transfer parameter (n) of the groundnut plot, an increase of approximately 0.4 was observed when a rainfall event followed weeding. No significant increase was seen for the millet plot. A linear relationship between the index of roughness and the roughness parameter (Hl) was also derived.