Ephemeral gully erosion in southern Navarra (Spain)

Ephemeral gully erosion is a common type of water erosion in the loamy soil s formed on the Miocene Marls of southern Navarra. A study has been conduct ed to describe the different types of ephemeral gullies and to determine th eir origin, evolution and importance as sediment sources. Three main types were identified according to their origin: classic ephemeral gullies formed within the same field where runoff starts, drainage ephemeral gullies coll ecting the runoff from another upstream area, and discontinuity ephemeral g ullies due to some abrupt slope change in the landscape. During 2 consecuti ve years of normal precipitation, ephemeral gully erosion has caused signif icant soil losses, locally over 2.66 kg m(-2) yr(-1), which is above tolera ble levels. In each year, one short and intense rainfall event has been res ponsible for almost all soil losses from ephemeral gullies. The main cause of gully formation is the lack of any proper waterway for conveying water e xcess. Conventional tillage practices provoked gully occurrence, while the maintenance of stubble or vegetation cover completely prevented their forma tion. Although farmers refill gullies every year, they reappear at the same position if no preventive measures are taken. Early winter has been the mo st critical period for gully formation, when the soil is wetter and the sur face cover is scarce. But a strong summer thunderstorm also caused severe g ully erosion in unprotected soils. Two simple topographic indices based on the area and the slope of the watersheds are useful to describe total soil losses. (C) 1999 Elsevier Science B.V. All rights reserved.