Water scarcity is a growing concern in the Mediterranean basins due to an increasingly dry and hot climate. Its effects are already being felt in different sectors of society, from agriculture to industry, energy production or domestic supply and consumption.

Of the water that rains in a watershed, some evaporates directly or is transpired by plants (“green water”), while the rest may flow surface or underground into aquifers and rivers (“blue water”). In relation to this, some studies show that the ratio of blue to green water is decreasing due to the increase in temperature and evapotranspiration. In addition, in recent decades, the abandonment of agricultural land and managed forest areas has led to reforestation and increased vegetation in many watersheds, which has contributed to further reducing this ratio and thus the availability of river water

Thinning is a common forest management practice that can be carried out, depending on the objectives, more or less intensively and selectively. Often, clearings are intended to prevent fires or facilitate the growth of large trees resistant to episodes of water stress. Now, in addition, they can be considered as a measure to increase the production of blue water in a context of increasing water scarcity.

Despite this, there are still few studies that have quantified the effectiveness of clearing in the production of blue water, and how this varies depending on its intensity and the type of forest. A study recently published by a team of researchers from the Catalan Water Research Institute (ICRA-CERCA) , led by Xavier Garcia and Vicenç Acuña , addresses these questions. In addition, the study quantifies the spatial value of the water produced by drainage in different internal basins in Catalonia, and compares these benefits with the economic costs of drainage. Thus, the study contributes with data to the discussion on the economic viability of this type of management actions.

Incorporating benefits from clearing such as wood production, reduced forest fire risk or greater resilience to climate change could ensure its economic viability

SWAT+ model simulations show increases in blue water of 5% in light thinning scenarios (10% removal of aerial forest biomass), but also increases of up to 35% in heavy thinning scenarios (removal of 50 % of aerial forest biomass). Coniferous forests would be the most sensitive to thinning actions, with 45% increases in blue water due to heavy thinning, compared to 27% for sclerophyllous forests and 2% for deciduous forests.

Despite the increase in blue water in the scenarios evaluated, in most forests the thinning costs would exceed the economic benefits of water produced with B/C ratios between 0% and 25%. Therefore, the value of the water produced could not justify, on its own, the clarification actions. Incorporating other benefits of clearing such as wood production, reduced forest fire risk or greater resilience to climate change could ensure its economic viability. The work also supports the development of systems to reward forest owners for the extra water resources generated by their management efforts.