Erodibility and infiltration characteristics of five major soils of southwest Spain

Equations used to calculate erodibility in the Water Erosion Prediction Project soil erosion model (WEPP) are based on erodibility studies in the USA and may not function well in another region. This study was conducted to: (i) analyze erodibility and infiltration characteristics of some predominant soils of southern Spain, and (ii) test equations used to calculate interrill erodibility in the WEPP model on these soils. The five soils chosen for this study in Andalusia, southwest Spain, were: two terrace soils (referred to as ‘Red and Yellow Alfisols’), an alluvial soil (‘Fluvent’), a shallow hillside soil (‘Inceptisol’), and a cracking clay soil (‘Vertisol’). A static, solenoid operated rainfall simulator was operated at an intensity of approximately 60 mm h⁻¹ during a 60-min dry run followed by a 30-min wet run the next day on 0.75 m² plots with 30% ridge slopes. Infiltration rates were high (always exceeding 50% except for the wet run of the Fluvent). The Fluvent had the lowest infiltration rate (0.00 mm min⁻¹ at the end of the wet run) and highest soil loss (985 g m⁻² h⁻¹ in the dry run and 1557 g m⁻² h⁻¹ in the wet run). The Vertisol, Inceptisol and Red Alfisol had low soil loss (415, 605, and 527 g m⁻² h⁻¹ in the dry run and 824, 762 and 629 g m⁻² h⁻¹ in the wet run, respectively). Soil loss of the Vertisol doubled between dry and wet run and infiltration rate did not stabilize, suggesting that erodibility of Vertisols increases when they are wet. The Yellow Alfisol had lower final infiltration rate in the dry run (0.33 mm min⁻¹) than in the wet run (0.58 mm min⁻¹) and higher soil loss in dry run (1203 g m⁻² h⁻¹) than in wet run (961 g m⁻² h⁻¹), the reason still being unclear. Soil loss was significantly correlated to silt+very fine sand content (r=0.96), indicating that erodibility of these soils is determined by similar properties as soils in these soil orders in the USA. However, the equation for WEPP-interrill erodibility overestimated erodibility significantly (two to four times), indicating the need to develop new erodibility equations for the Mediterranean region. Infiltration rates were generally high and soil loss rates low compared to reports from the USA, suggesting that limited runoff generation is a primary reason for low erodibility of these soils.