Catchment based hydrology under post farmland abandonment scenarios

Abstract

Vegetation expansion following farmland abandonment is a complex process that depends on multiple natural and human-induced factors, resulting in differences in the evolution of land cover on former cultivated fields, with various environmental implications. To assess the complexity of the hydrogeomorphological consequences of farmland abandonment, the Instituto Pirenaico de Ecología (CSIC) and the University of La Rioja monitored three small catchments, representative of different post land abandonment scenarios, in the Pyrenees and Iberian Range respectively. In the Pyrenees, a fourth small catchment, covered by natural forest, was monitored as a reference for an undisturbed environment. This study describes the evolution of land use in the abandoned catchments and examines its implications on catchment hydrological connectivity. It also analyses the hydrological responses of the three abandoned scenarios to similar rainfall events, and compares them to that of natural forested areas. .Vegetation tended to increase in the three catchments, but there were important differences in the characteristics of the current land cover. Arnás, the catchment left to a process of natural revegetation, contained a mosaic of shrubs (64%) and forest (27%) at different stages of succession, largely conditioned by the topography and soil properties. Araguás_afforestation was extensively afforested in the 1960s, with 75% of this catchment currently covered by forest, most of it planted artificially. In Munilla, occupied by terraced fields, vegetation recovery was partly restrained by the introduction of cattle, and 80% of the catchment was covered by sparse shrubs. Land abandonment resulted in a general reduction in computed hydrological connectivity in the three studied catchments, except in localized areas close to the main channel, new forest roads and trails, and upstream of terrace wall collapses, all areas of increased hydrological connectivity. The decrease in hydrological connectivity was much lower in Munilla, characterized by an absence of dense vegetation and still dominated by a terraced topography. The hydrological responses of the catchments to similar rainfall events differed significantly, showing the influence of not only vegetation cover but of the properties of soil remaining after previous agricultural activities. Significant storm-flow discharge was observed in Arnás, even under dry conditions, with high peakflows and fast responses. Lower streamflow response was observed in Araguás_Afforestation under dry conditions; however, once the soils were wet the hydrological response was notable and was characterized by high peakflow. The response under afforested trees differed greatly from that of a catchment covered by natural forest, with the latter characterized by gentler hydrographs. The hydrological response in Munilla was the lowest, with long response times and recessions, associated with the thick soils of the terraced fields. These results demonstrated the large variability of post land abandonment scenarios and associated hydrological implications, and highlighted the need to consider these differences to reduce future uncertainties in forecasting water resources and soil conservation.