Ephemeral gully erosion in southern Navarra (Spain)

Ephemeral gully erosion is a common type of water erosion in the loamy soils formed on the Miocene Marls of southern Navarra. A study has been conducted to describe the different types of ephemeral gullies and to determine their 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 collecting the runoff from another upstream area, and discontinuity ephemeral gullies due to some abrupt slope change in the landscape. During 2 consecutive years of normal precipitation, ephemeral gully erosion has caused significant soil losses, locally over 2.66 kg m−2 yr−1, which is above tolerable levels. In each year, one short and intense rainfall event has been responsible for almost all soil losses from ephemeral gullies. The main cause of gully formation is the lack of any proper waterway for conveying water excess. Conventional tillage practices provoked gully occurrence, while the maintenance of stubble or vegetation cover completely prevented their formation. Although farmers refill gullies every year, they reappear at the same position if no preventive measures are taken. Early winter has been the most critical period for gully formation, when the soil is wetter and the surface cover is scarce. But a strong summer thunderstorm also caused severe gully 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.