Seasonal streamflow response in a Mediterranean catchment with abandoned agricultural terraces

Résumé

Agricultural terraces have been built in mountain regions worldwide in order to provide alarger surface for cultivation, improve water availability and reduce soil erosion, as they favourinfiltration and reduce runoff. Their construction substantially alters the dynamics of thehillslope hydrology. In the Mediterranean region, many valleys were occupied by terracecultivation; however, during the XXth century, rural population declined resulted in theabandonment of agricultural practices and the maintenance of the terraced fields. The aim ofthis study is to analyze the seasonality of the hydrological response of an environmentcharacterized by abandoned agricultural terraces. For this purpose, a small catchment (1.9km2) was monitored in 2012 in the north-western Iberian ranges (Spain), largely occupied byabandoned terraces, which are currently covered with sparse grass and shrubs. Annual rainfallin the area is 450±150 mm, with two clear rainy seasons, autumn and spring. The equipmentinstalled in the catchment registers continuously meteorological data and streamflowdischarge at the outlet of the catchment. Here we present the results of the hydrologicalresponse at the flood event scale of 7 hydrological years (from 2012-13 to 2018-19). Eventrunoff coefficient (RC) was moderate (mean RC<0.10), with higher values occuring in winterand spring, which occasionally could reach >0.30. These two seasons registered the largernumber of floods. Floods were rarely recorded in autumn and summer. However, autumncould registered high RC associated to large rainfall events. Summer rainfall events with highintendity were not able to produce high RC nor high peakflows. Higher peakflows were mainlyregistered in winter and spring and were mostly related to rainfall amount. Catchmentresponse was on average relatively slow (mean time response >8h) and flood eventspresented long duration (mean duration > 2 days), except in summer when they lasted onaverage < 15h and response time was shorter (mean < 5h).