Soil degradation in central Spain due to sheet water erosion by low‐intensity rainfall events

Abstract Runoff and sediment lost due to water erosion were recorded for 36 (1 m 2 ) plots with varying types of vegetative cover located on sloping gypsiferous fields in the South of Madrid. 75% of the events had maximum 30‐minute intensity ( I 30 ) less than 10 mm h −1 in the period studied (1994–2005). As for the vegetative cover, maximum correlation between runoff and soil loss was found in the least protected plots (0–40% cover) during the most intense rainfall events; however, a significant positive correlation was also observed in plots with greater coverage (40–60%). If coverage exceeded 60%, rainfall erosivity declined. The average amount of sediment produced in high‐intensity events was significantly greater (approximately 7 g m −2 per I 30 event >10 mm h −1 ) than that produced in the rest of the moderate‐intensity events (approximately 3 g m −2 per I 30 event <10 mm h −1 ), but due to the high rate of occurrence of the latter throughout the year sediment loss during the period studied totaled 128 g m −2 . By comparison, only 40 g m −2 was produced by the I 30 events greater than 10 mm h −1 . Even though the amount of soil lost is relatively insignificant from a quantitative standpoint, the organic matter content lost in the sediment (six times more than in the soil) is a permanent loss that threatens the development of the surface of the soil in this area when the vegetative cover is less than 40%. The soil here experiences a chronic loss of 0·02 mm annually as a consequence of frequent, moderate events, in addition to any loss produced by extraordinary events, which, though less frequent, are much more erosive. If moderate events are ignored, an important part of soil loss will be lost in the long run. Copyright © 2007 John Wiley & Sons, Ltd.