Application of connectivity theory to model the impact of terrace failure on runoff in semi-arid catchments

Terraces are a common feature of Mediterranean landscapes. In many places they are no longer maintained so that the number of intact terraces is in prolonged decline. The aim of this paper is to examine the effect of terrace removal and failure on hydrological connectivity and peak discharge in an agricultural catchment (475 ha) in south-east Spain. The situation of 2006 is compared to that in 1956 and to a scenario without terraces (S2). The spatial distribution of concentrated flow was mapped after four storms in 2006. The degree of connectivity was quantified by means of connectivity functions and related to storm characteristics, land use and topography. For 1956, 2006 and scenario S2, connectivity functions and peak discharge to the river were determined for a storm with a return period of 8.2 years. The results show that the decrease in intact terraces has led to a strong increase in connectivity and discharge. The contributing area to the river system has increased by a factor 3.2 between 1956 and 2006. If all terraces were to be removed (scenario S2), the contributing area may further increase by a factor 6.0 compared to 2006. The spatial extent of concentrated flow and the degree of connectivity are related to storm magnitude as expressed by the erosivity index (EI30). Although a large part of the concentrated flow (25-50%) occurs on dirt roads, it appears that croplands become a major source of runoff with increasing rainfall. The results suggest that connectivity theory can be used to improve rainfall-runoff models in semi-arid areas. Copyright (c) 2009 John Wiley & Sons, Ltd.