Identifying erosive periods by using RUSLE factors in mountain fields of the Central Spanish Pyrenees

摘要

The Mediterranean environment is characterized by strong temporal variationsin rainfall volume and intensity, soil moisture and vegetation cover alongthe year. These factors play a key role on soil erosion. The aim of thiswork is to identify different erosive periods in function of the temporalchanges in rainfall and runoff characteristics (erosivity, maximum intensityand number of erosive events), soil properties (soil erodibility in relationto freeze-thaw processes and soil moisture content) and current tillagepractices in a set of agricultural fields in a mountainous area of theCentral Pyrenees in NE Spain. To this purpose the rainfall and runofferosivity (R), the soil erodibility (K) and the cover-management (C) factorsof the empirical RUSLE soil loss model were used. The R, K and C factorswere calculated at monthly scale. The first erosive period extends from Julyto October and presents the highest values of erosivity (87.8 MJ mm ha−1 h−1),maximum rainfall intensity (22.3 mm h−1) andmonthly soil erosion (0.25 Mg ha−1 month−1) with the minimumvalues of duration of erosive storms, freeze-thaw cycles, soil moisturecontent and soil erodibility (0.007 Mg h MJ−1 mm−1). This periodincludes the harvesting and the plowing tillage practices. The seconderosive period has a duration of two months, from May to June, and presentsthe lowest total and monthly soil losses (0.10 Mg ha−1 month−1)that correspond to the maximum protection of the soil by the crop-cover ($C$factor = 0.05) due to the maximum stage of the growing season andintermediate values of rainfall and runoff erosivity, maximum rainfallintensity and soil erodibility. The third erosive period extends fromNovember to April and has the minimum values of rainfall erosivity(17.5 MJ mm ha−1 h−1) and maximum rainfall intensity (6.0 mm h−1) withthe highest number of freeze-thaw cycles, soil moisture content and soilerodibility (0.021 Mg h MJ−1 mm−1) that explain the high value ofmonthly soil loss (0.24 Mg ha−1 month−1). The interactions betweenthe rainfall erosivity, soil erodibility, and cover-management factorsexplain the similar predicted soil losses for the first and the thirderosive periods in spite of the strong temporal differences in the values ofthe three RUSLE factors. The estimated value of annual soil loss with theRUSLE model (3.34 Mg ha−1 yr−1) was lower than the measured valuewith 137Cs (5.38 Mg ha−1 yr−1) due to the low values ofprecipitation recorded during the studied period. To optimize agriculturalpractices and to promote sustainable strategies for the preservation offragile Mediterranean agrosystems it is necessary to delay plowing tillOctober, especially in dryland agriculture regions. Thus, the protectiverole of the crop residues will extend until September when the greatestrainfall occurs together with the highest runoff erosivity and soil losses.