Runoff and erosion in volcanic soils of Azores: simulation with OPUS

The islands of Azores are of very recent volcanic origin and their soils have special physical and chemical characteristics because of the specific circumstances of their formation from the volcanic materials, their young age and the presence of allophane. To assess and understand the hydrologic behavior of these soils under agricultural practices, the deterministic model OPUS was tested and calibrated using field data collected from 1996 to 1998 in two small basins of Terceira Island. The model uses a solution of the Richardsʹ equation to simulate water movement in the soil profile, which requires modified Brooks and Corey equations to describe the soil hydraulic characteristics. For runoff, the model uses either a mechanistic simulation with the Parlange and Smith infiltration equation or the Soil Conservation Service (SCS) Curve Number (CN) approach. In the first, a mechanistic detachment and transport model is used to simulate erosion, and for the second the modified universal soil loss equation (MUSLE) is utilized. Results of the model simulations for runoff and erosion match the observed data well. Runoff significantly increased from 1% to 17% of rainfall from pasture to bare soil or partially covered soil during establishment of a new pasture. Sediment yields sharply increased from <5 kg ha−1 year−1 under pasture to nearly 15 ton ha−1 during the 8-month period when the soil was unprotected.