Modelling the effect of grazing on infiltration, runoff and primary production in the Sahel

This paper describes a step-wise method to quantify the effect of biomass removal by grazing on infiltration, runoff and primary production and is used to explore the long-term consequences of grazing on the stability of the Sahelian rangeland ecosystem in average and dry years. A concept of Rawls et al. (1989) was used to estimate the effect of canopy cover on a single composite infiltration parameter, i.e. the sorptivity, S, for Sahelian rangelands. Dependence of this sorptivity (mm min−1/2) on canopy cover, CAN (%), for a common soil type (loamy sand) can be described by: S = 0.48 + 0.016 ∗ CAN - 0.000055 ∗ CAN2. From 30-year rainfall records in the north and south Sahel, four normal years (probability 50%) and four dry years (probability 90%) were analyzed in detail. Daily rainfall was distributed over three classes, i.e. < 10, 10–20 and > 20 mm. Average rainfall in each of these classes was 4.4, 14.6 and 32.9 mm. Combination of these classes with intensity classes (from a separate rainfall intensity analysis) resulted in 12 standard showers. For each of these standard showers, infiltration was calculated using the equation I = S ∗ √t. Using the number of showers in each class, an annual runoff can be computed that results in the relation: R (% of rainfall) = 66.02 ∗ exp(−0.549 ∗ S). The potential length of the growing season is derived from a statistical analysis of 30-year rainfall records. Actual length of the growing season, L, depends on soil evaporation and runoff. Nutrient-limited growth can be calculated as: Y(Nut) = L ∗ 35 (kg ha−1 yr−1). Crop available water, AW, in the actual growing season is calculated and water-limited production calculated as: Y(Wat) = AW ∗ 10 000TC (kg ha−1 yr−1), TC being the transpiration coefficient of Sahelian pastures. Actual primary production is the minimum value of Y(Wat) and Y(Nut). Primary production functions that were derived are: Y(normal year) = 3604 + 15.06 ∗ R − 0.89 ∗ R2 and Y(dry year) = 3104 − 17.77 ∗ R − 0.80 ∗ R2 (kg ha−1 l yr−1). The obtained equations are used to explore the consequences of grazing on the stability of the Sahelian rangeland ecosystem. For a sequence of normal years, different grazing intensities lead to different equilibrium values for the annual runoff. For a sequence of 1, 2 or 3 consecutive dry years, the initial runoff value determines the maximum grazing intensity.