Runoff and erosion in volcanic soils: Testing deterministic and semi-empirical modelling

To assess the hydrologic behaviour of the volcanic soils of Azores as influenced by land use, two experimental basins were installed for. observation of the soil water regime, runoff and sediment transport. One plot has been for three years under permanent pasture for grazing, and the second was with pasture for the first year, with maize for silage during the second year, and was replanted with pasture after maize harvesting. The simulation of the hydrologic processes was performed with the model OPUS using both the deterministic and the semi-empirical versions of the model. Adopting the deterministic version, breakdown rainfall data are required. When the semi-empirical version is selected, daily rainfall data are used, infiltration and runoff are computed with the SCS curve number method, and erosion is computed with the MUSLE method. This paper shows the calibration and validation of OPUS for both approaches. Results for simulation of the soil water storage, runoff and erosion are compared with observed data, and are analysed at the event and the seasonal time scales. It is shown that pasture lands produce very small runoff (1% of rainfall) and negligible erosion (3kg/ha/year). However, runoff increases to near 20% of rainfall, and seasonal soil losses increase to near 15 ton/ha/season when the soil is not covered from planting to full establishment of a new pasture. Results from the deterministic and the semi-empirical approaches have shown to be similar, so the model can be used for prediction of impacts of land use with any of the versions depending on the available data.