Shrub removal effects on runoff and sediment transport in a mediterranean experimental catchment (Vernegà River, NE Spain)

Abstract

Shrub removal works in the Gavarres massif (NE Spain) have impacted runoff and sediment transport in a first-order experimental catchment (2.5 km2), named Vernegà. These works have consisted in removing the shrub layer that grows under a typical mixture of cork oak tree and Mediterranean pine forest, and have been undertaken regularly since 2002 to reduce risk of forest fire and increase cork oak productivity. In 2005, the works affected the catchment headwaters, resulting in significant changes in runoff and sediment transport rates. This study aimed to assess short-term changes in runoff and sediment transport rates in the experimental catchment by comparing them to rates prior to 2005. Measurements were made at two monitoring stations within the basin: i) Bosc (1.6 km2), where the main land use is forest; and ii) Campàs (2.5 km2), the catchment outlet, where the main land use is agriculture.Mean annual runoff increased from 11.7 mm before 2005 to 33.1 mm after 2005 at the Bosc station. At the Campàs station, runoff increased from 81.9 mm to 92.2 mm. These changes can be observed in the flow duration curves of both stations. An increase on the frequency when low flows (<5 l/s) take place has been observed, while the occurrence of high flows remains unchanged. The low flow increase period had a concomitant reduction of the no flow period, which reduced about 30 days in Bosc station.Suspended sediment yield increased from 250 kg·km−2·yr−1 at Bosc and 430 kg·km−2·yr−1 at Campàs to 940 kg·km−2·yr−1 and 4,350 kg·km−2·yr−1 respectively after forest works. Dissolved sediment yield also increased but did so solely as a result of the increase in runoff rates rather than of the increase in dissolved concentrations.