Application of the water erosion prediction project (WEPP) in olive orchards on Vertic soil with different management conditions

Vertic soils are common in Mediterranean areas, such as in some olive-growing areas in Andalusia insouthern Spain. Model calibration can be especially complex in orchards due to the higher spatial variability induced bythe tree/lane distribution and the impact of management, and even more in Vertic soils because of the complexityintroduced by the cracking/swelling cycles of the soil, whose impact can remain for many months. The WEPP model wasevaluated using monthly runoff and erosion data collected from an experiment in an olive orchard on a Vertic soil duringfour hydrological years, comparing three different soil management systems: no tillage with bare soil (NT), conventionaltillage (CT), and cover crop (CC). These three systems differed greatly in average annual runoff and soil losses.Satisfactory calibration of the model required characterization of the main hydrological features of the study period(distinguishing “dry” and “wet” years) due to their large impact on the infiltration properties of the soil. The bestpredictions were obtained for the wet years and NT management. High values of r2 and Nash-Sutcliffe model efficiency(NSE) were obtained at a monthly scale under wet conditions. Annual erosion rates were not well predicted, probably dueto the difficulties in simulating the spatial complexity of olive orchards and the limitations of WEPP for modeling soilmoisture dynamics at different depths derived from the cracking/swelling processes of Vertic soils as well as their impacton erodibility. The presented parameterization constitutes a basic guideline for the application of the WEPP model inolive-growing areas and on Vertic soils.