EFFECTS OF SOIL AND VEGETATION ON RUNOFF ALONG A CATENA IN SEMIARID SPAIN

Runoff and infiltration were investigated on abandoned fields of patch y vegetation in semi-arid Spain during 15 months of natural rainfall a nd by rainfall simulations. The aim was to ascertain sources and sinks of runoff and the effects of soils and plant cover. Soils of the cate na developed from mica schists of the upper hillslopes, fan deposits o f the lower hillslopes, and an alluvial terrace at the bottom. Runoff from natural events were from three sets of three pairs each of 10 x 2 m runoff plots. The pairs of each set had different densities of plan t cover; the sets were vegetated with tussock grass, Stipa tenacissima , a shrub, Anthyllis cytisoides, and a bush, Retama sphaerocarpa. Nine teen natural rainfall events of intensities up to 18 mm/h produced 400 mm of rain during the study period. Because the rainfall threshold fo r runoff production was about 20 mm, only eight events produced runoff . The rainfall simulations used a sprinkler that produced 50 mm/h of r ain for 30 minutes; runoff was recorded each minute in 0.24 m(2) bound ed plots. The depth and structure of the soil mantle provide the main controls on runoff rates, which are lowest on the lower fan deposits a nd highest on the thin upslope soils. The river-bank terrace, with a s urface covered by crusts and mosses, also yields relatively high runof f. In general, vegetation density varies inversely with runoff. Nevert heless, shrub and bush litter favor runoff, as does a particular spati al distribution of individual plants on the hillslope. Settling of the upper few centimeters of soils of the alluvial fan following cessatio n of cultivation 15 to 40 years ago has produced a near-surface compac ted layer favoring shallow subsurface runoff. Apparently contradictory results between runoff plots and rainfall simulations are the result of differing processes.