Assessing soil erosion in a Pyrenean mountain catchment using GIS and fallout 137Cs

The soil and vegetation in some areas of the central Pyrenean middle mountains are in poor condition. The main cause of degradation has been identified as soil erosion due to intensive use, including deforestation, overgrazing and extensive agriculture since the Middle Ages, followed by land abandonment starting at the beginning of the 20th century and intensifiying during the early 1950s. In this work, a raster geographical information system (GIS) combined with an expert evaluation system and fallout 137Cs were used to assess soil erosion in the Arnas catchment in the southern Pyrenees, where changes in land use were quite rapid at the beginning of the last century. A soil survey was carried out in the Arnas catchment to identify the soil types and to determine physico-chemical properties related to soil erodibility. A grid pattern was established across the study area and 77 sites were sampled. The GIS was used to integrate the information derived from an automated land evaluation system that, in turn, identified the erosion risk of areas by combining data on various soil properties and physiographic and bioclimatic factors. According to the map of erosion risks generated for the catchment, there were three distinct areas with different soil erosion features where fallout 137Cs was used to assess the soil redistribution pattern. The depth distributions of 137Cs were measured along three transects that represented the three main areas. The 137Cs content in the soils on the shrub slope transect was smallest and significantly different from the deeper and better developed soils on the forest slope, indicating different patterns of soil mobilization. The relationship of some basic soil properties was also examined, such as organic matter and texture, in terms of the patterns of radioisotope distribution and soil erosion. In the catchment, soil redistribution was affected by physiographic and landscape features such as soil properties, slope gradient as well as orientation and vegetation cover. Soil loss was highest along the shrub transect where the rate of soil erosion was much higher than soil formation. Conversely, the soils on the forest transect were mainly stable although some sediment was also deposited. Both aggradation and erosion were found in the valley bottom. The automated evaluation system and GIS used to identify areas vulnerable to erosion combined with the 137Cs technique have proven to be a suitable approach to assess soil erosion aimed to design effective strategies for soil conservation in Mediterranean mountain environments.

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