Water balance in a banded vegetation pattern - A case study of tiger bush in western Niger

The tiger bush is a patterned woodland with alternating bare area and veget ated stripes. In Niger, it covers one third of the Sahelian zone. These nat ural forests are of considerable economical interest since they are the mai n source of livestock forage and domestic energy. Its sustainable exploitat ion needs improved understanding of its dynamics. The redistribution of wat er between thicket and intervening bare areas is decisive for the water sup ply of the vegetation. Tiger bush patterning replicates an elementary unit composed of a bare area, the upslope border, the core and the downslope mar gin of the thicker. (Each zone of tiger bush is characterised by specific s oil crusting associated with vegetation). Both water storage and runoff hav e been monitored after each rain, over a period of 4 yr, including contrast ing rainy seasons, on the different zones composing the tiger bush. On the three crusted zones, runoff has a piecewise linear relationship with rain: on closed plots, runoff yield vs. annual rainfall ratio reaches 54% on bare soil, 2% on upslope border and 18% on downslope border. The measured infil tration confirms these rates on independent plots. In the core of the thick et, measured infiltration corresponds with the sum of the contributions of upslope zones, weighted by their relative lengths. This model predicts that bare area contributes up to 62% of the thicket supply, while direct rain i s 27%, the senescence zone is 10% and the upslope border contribution is ne gligible (1%). The average water infiltration in the thicket is equal to 4 X the incident rainfall, but water redistribution is not homogeneous within the core of the thicket. By the most favourable location, infiltration dep th is measured to be about 8 X the rainfall. The important runoff, mainly g enerated on the impervious bare area crosses the upslope border of the thic ket without infiltrating, and entirely benefit to the core. Nothing is left to the downslope border, only rainfed. The upslope border, often described as favourable location for young plants is only rainfed most part of the y ear. By the end of the season, its increasing porosity, due to vegetation a nd termite activity let it benefit of the last rains. The simple water bala nce model based on runoff measurement is satisfactorily validated by indepe ndent observed infiltration. (C) 1999 Elsevier Science B.V. All rights rese rved.