Post-Fire hidrological and sediment delivery dynamics of two Mediterranean nested catchments

Abstract

A continuous monitoring network of nested catchments was set - up as soon as possible after the largest wildfire that affected Mallorca, which occurred in July 2013. The Sa Font de la Vila catchment is a Mediterranean afforested catchment of 4.8 km 2 , which is located in the western part of the island. Altitude s range between the 71 to 515 m with an average gradient slope of 38%. The lithology is mainly composed by Keuper clays and loams in the bottom valleys with gentle gradient slopes ca. <10%. Meanwhile, Raethian dolomites and Lias limestones predominate in t he upper parts of the catchment. The intensive agricultural use in the past is witnessed by the widespread occurrence of traditional soil conservation structures (i.e., terraces and valley bottom terraces), occupying 65% of the area. It was affected by 2 l arge wildfires in the last 20 years, i.e. in 1994 and 2013 affecting 44% and 71% of the area, respectively. The Sa Murtera is a headwater sub - catchment of 1.2 km 2 representative of lithology, land uses and fire affection. Altitudes range between the 185 to 515 m with an average gradient slope of 36%. In October 2013, the outlet of both the Sa Font de la Vila and Sa Murtera was instrumented with a gauging station, which recorded water level, average turbidity, electric conductivity and temperature at 15 minu te intervals (average values based on 1 minute readings). Within the study area, also precipitation, air temperature and soil moisture content were measured also continuously with a meteorological station and a humidity probe. The results indicate a very low sediment yield in the two post - fire hydrological years, showing important differences between the two nested catchments, although monitoring problems during the first year (2013 - 14) caused that results were only available from the Sa Font de la Vila. In this station, total runoff and sediment yield were 5 mm and 23 t km - 2 respectively, with a total sediment load of 111 t, although 99% was exported in the first event after the wildfire when 50 mm were accumulated in only 15 minutes. During the second post - fire year (October 2014 - July 2015) in Sa Font de la Vila the tota l runoff was higher with 23 mm. However, the sediment yield was lower – 0.5 t km - 2 – showing a significant decrease. At Sa Murtera, the 31 m m of runoff and 3.3 t km - 2 of sediment yield showed greater results, indicating that most of the sediment yield from t he upstream parts of the catchment must have been deposited before reaching the outlet of the Sa Font de la Vila. Despite the fire effects on erosion and delivery processes, sediment is therefore progressively trapped along the main channel during its tran sport due to the downstream gradient reduction.