Effects of farming terraces on hydrological and geomorphological processes. A review

Resumen

Agricultural terraces are common in mountain regions. They are built to provide a larger surface area for cultivation on hillslopes and to aid farming production in the long term. This paper reviews the hydrological and geomorphological behaviour of constructing, working on and abandoning farming terraces based on recent scientific papers (1982-2014).Terraces increase infiltration and reduce runoff. In Mediterranean areas, the runoff coefficient on abandoned terraces is between 20% and 40%, depending on the percentage of plant cover or the amount of grazing. On cultivated terraces in warm, humid climates, the coefficients are lower (10%-25%). The internal and external sections of terraces have hydrological differences. Soil saturation is reached faster in the inner parts of terraces with shallower soil, causing a rapid hydrological response (saturation runoff); high infiltration rates in external sections can generate subsurface runoff. Furthermore, a reduction in hydrological connectivity, which affects the contributing areas and peak flows, is to be expected in terraced catchments. Collapsed stone walls, small mass movements, sheet wash erosion, piping, rills, gullies and debris flows are all found in terraced landscapes. The erosion of abandoned terraces is directly related to the amount of plant cover, soil characteristics, environmental conditions and the abandonment age. On this type of terrace, the rates of erosion due to overland flow are less than 3tha-1yr-1. The rate increases if the terrace is affected by mass movements, pipes or gullies (more than 100tha-1yr-1). On cultivated terraces, erosion rates less than 1tha-1yr-1 have been recorded for rice crops, and erosion rates greater than 80tha-1yr-1 have been recorded for cassava or terraces with bare soil.The scientific literature notes that the soil loss from cultivated slopes is greater if conservation measures, such as terraces, are not included. However, this general result may vary significantly due to factors related to the geo-environmental characteristics of the region and, in particular, the management and use of the terraced slopes.