RUNOFF AND SOIL LOSS RESPONSE TO VEGETATION REMOVAL IN A SEMIARID ENVIRONMENT

The removal of vegetation is the main cause of soil degradation in sem iarid areas. We hypothesized that the changes in soil properties induc ed by vegetation removal modified the runoff and soil erosion response in a semiarid area of Spain. To test this, two plots (15 by 5 m) were installed on a 23% slope on a Lithic: Haploxeroll. Vegetation was rem oved by clipping from one of the plots (Plot D) and the changes in sur face runoff and soil loss were compared with an undisturbed control pl ot (Plot N) from 1989 to 1993. Total runoff was significantly greater in Plot D (48.8 mm) than in Plot N (34.9 mm). Runoff ratios between th e disturbed and natural plots increased with time from 1.4:1 in 1990 t o 2.5:1 in 1993. Vegetation removal increased the soil losses in Plot D (150.2 g m(-2)) by 127% compared with those in Plot N (66.3 g m(-2)) . The annual soil loss ratio between the disturbed and natural plots i ncreased from 1.6:1 in 1989 to 4.2:1 in 1993. The observed increase in surface runoff and soil loss was attributed to a progressive deterior ation of soil physical properties in Plot D. Bulk density increased by 8.4% (from 1.55 to 1.68 Mg m(-3)) and a reduction of total organic ca rbon (from 4.0 to 2.6%) and the percentage of stable aggregates (from 81.6 to 56.3%) were observed. There was no evidence of vegetation reco very, suggesting that reduced vegetal cover might lead to irreversible soil degradation in semiarid areas.