Impacts of Land Abandonment on Flood Mitigation in Mediterranean Mountain Areas

From the mid-twentieth century, Mediterranean mountains were affected by a rapid and generalized land abandonment process. This chapter (1) summarizes the impacts of land abandonment on the hydrological dynamics in Mediterranean mountain areas; (2) evaluates post-land abandonment management practices (LMPs) for flood mitigation based on nature-based solutions (NBS); and (3) briefly discusses some examples in the Central Pyrenees. In general, land abandonment resulted in a natural colonization by shrubs and forests, which, in turn, led to a decrease in river discharges and sediment yields. NBS, as mitigation measures to control flood occurrence, have been carried out including afforestation, shrub clearing, landscape changes, and recovery of terraces and stonewalls. In addition, in the most hydrogeomorphologically active areas, a combination of NBS and grey infrastructures was used to control floods. Grey infrastructures produce immediate effects but they are short-lived and expensive solutions. LMPs based on NBS present advantages and disadvantages: (1) afforestation was the most common practice, reducing floods, hydrological connectivity, peak flows, and sediment yields, in spite of their lower impact in extreme events; (2) shrub clearing decreases the number of forest fires and maintains the occurrence of the most frequent floods; (3) the recovery of mosaic landscapes produces environmental consequences, being important sources of ecosystem services, such as the regulation of floods; and (4) the recovery of agricultural terraces and stonewalls is highly expensive, but presents social and cultural benefits, reducing hydrological connectivity, peak flows, and runoff. In the near future, NBS in abandoned lands should be based on cost-effective and long-term strategies to mitigate flood risk. NBS should be cost-effective, ensure longer lifetime than grey infrastructures, and be adapted to different local objectives and global scenarios.