RELATION BETWEEN INFILTRATION AND STONE COVER ON A SEMIARID HILLSLOPE, SOUTHERN ARIZONA

A knowledge of the spatial distribution of infiltration rates is essential to the application of realistic, distributed rainfall-runoff models in semiarid areas. One method of determining this distribution is to develop a predictive equation based on a readily observable surface property, such as stone cover. Previous studies of the relation between infiltration and stone cover on shrub-covered semiarid hillslopes have yielded both positive and negative correlations. It is suggested that positive correlations result where infiltration measurements are confined to areas between large shrubs. Negative correlations, on the other hand, reflect pronounced shrub-intershrub differences in infiltration and stone cover and are found where both shrub and intershrub areas are sampled. In this study of a semiarid hillslope in southern Arizona both shrub and intershrub areas are sampled, and a negative correlation is obtained between infiltration and stone cover. This correlation arises from the fact that under shrubs fine sediments have accumulated primarily as a result of differential splash, whereas between shrubs such sediments have been selectively removed by a combination of rainsplash and overland flow, leaving behind a gravel lag. Consequently, infiltration rates are higher under shrubs than between them, owing to the higher percentage of sand in the surface soil, the larger quantity of organic matter both within the soild and on its surface and the greater digging and burrowing by animals. In addition, simulated rainfall experiments imply that shrubs with moderately dense canopies are more effective than adjacent surface gravels in dissipating the kinetic energy of raindrops and thereby reducing surface sealing and promoting infiltration. Inasmuch as the field site is typical of gentle, shrub-covered piedmont hillslopes over much of the Sonoran Desert, inverse relations between infiltration rate and stone cover are probably characteristic of such hillslopes and may be used to predict the spatial distribution of infiltration rates for inclusion in rainfall-runoff models.