Differences in streamflow after farmland abandonment –a comparative study in four small mountain catchments

Resumen

Farmland abandonment usually leads to vegetation expansion with important environmentalconsequences. In this study, we assess the streamflow response at the flood event scale infive long-term monitored small headwater catchments in northern Spain. Three of them arerepresentative of different post farmland abandonment scenarios: natural revegetationdominated by shrubs (Arnás), afforestation (Araguás_afforestation) and abandoned terracedfields (Munilla). Two more cathments, covered by dense natural forest (San Salvador) andbadlands (Araguás), were used as reference for undisturbed and degraded environments,respectively.The five catchments registered a larger number of flood events in late autumn and spring,showing the influence of the Mediterranean climate. While the badlands, shrubs andafforested catchments generated events over the entire year, the terraced and forestedcatchments showed very limited response during the summer. Furthermore, the badlands,shrubs and afforested catchments recorded more than twice floods per year than the othertwo catchments. At the flood event, the mean runoff coefficient was higher in the badlandsand shrubs catchments. However, under wet conditions, both the forested and afforestedcatchments could registered high hydrological reponses, with similar runoff coefficients thanthose recorded in the badlands and shrubs. The terraced cathment showed the lowest runoffcoefficients. The highest peakflows were observed in the badlands catchment, charcaterizedby sharp flood hydrographs, suggesting the dominance of overland flow processes. The floodhydrographs of the abandoned catchments differed significantly, suggesting the influence ofboth vegetation cover and soil properties inherited from past agricultural activities. In theshrub catchment, peakflows were always greater; however, under very wet conditions and/orintense rainfalls, the afforested catchment recorded high peaks, comparable to that of theshrub catchment. The response time in both catchments were fast and recession limbs wereshort, suggesting the occurrence of ovelrand flow processes, probably over localized degradedareas. This greatly differed from the hydrographs of the terraced and forested catchments,characterized by gentler hydrographs, with long response times and recessions, indicatingsubsurface flow processes associated with thick and well developed soils. Differences in theland use legacy, which affects vegetation cover but also soil properties, may explain thedifferences in the hydrographs characteristics, suggesting contrasting dominant runoffgeneration processes in each catchment. These results highlight the need to consider thesedifferences to reduce future uncertainties in forecasting water resources and soil conservationin areas affected by farmland abandonment