Hydrological and Erosion Processes in Terraced Fields: Observations from a Humid Mediterranean Region in Northern Portugal

Terraces are a common Mediterranean feature influencing soils, slopes and subsurface hydrology; however, little is known about their impact on hydrological processes, especially in humid regions. This work studied hydrological and erosion processes in the "aguas de lima" terrace system common in northwestern Iberia, characterized by wet season irrigation to keep soils saturated and avoid frost on winter pastures. Soil moisture, vegetation height, runoff and sediment yield were monitored for a terraced field in northern Portugal during 19 months. Relationships between rainfall, soil, vegetation, runoff and erosion were analyzed for 49 rainfall events, and within-storm patterns of soil moisture and runoff were further evaluated for the 12 largest events. Monitoring included two wet seasons with 1264 and 951-mm rainfall. Runoff followed rainfall with 20.5 and 3.8 mm, and was mostly related with event and pre-event rainfall. Combined with hydrograph analysis, this indicated a dominance of saturation-excess runoff generation, probably related with the presence of a shallow water table caused by limited drainage and constant irrigation during winter. Rill erosion was only observed as a result of run-on from the irrigation network. Sediment yield was low, 0.01 and 0.02 Mg ha(-1) in the first and second year, and related with runoff, but vegetation cover was found to limit sediment concentration. This work indicates that "aguas de lima" terraces promote saturation, runoff generation and a small amount of sediment yield which does not appear relevant for soil conservation. Further work is needed to better understand and conceptualize these processes. Copyright (C) 2016 John Wiley & Sons, Ltd.