Afforestation and changes in forest composition affect runoff in large river basins with pluvial regime and Mediterranean climate, Chile

• We study the water response related with forest plantation expansion in large basins.
• Annual runoff decreases with afforestation affecting over 16% of total catchment area.
• Summer runoff decreases due to afforestation should be carefully interpreted.

SummaryChile has developed an important economy based on some 2.6 million hectares of planted forests which have been questioned because of their impact on water resources. This report analyses the water response associated with forest plantation expansion and changes in tree species composition in six large river basins (from 94 to 1545 km2) of pluvial regime located between 37°30′ and 39°S in central-southern Chile, an area characterized by a Mediterranean climate. Long term discharge and precipitation records were obtained from national data bases and land uses for different time periods were generated using satellite images and digital maps. Annual water reductions were detected in three river basins where net increases in forested area affected more than the 16% of total catchment area. In the same basins and between the same periods reductions of summer runoff were also noticed although these decreases should be carefully considered as summer runoff also decreased in basins where no significant land use changes occurred. The analysis of the daily flow duration curves indicates that the highest effects occur for Q50% (daily discharge exceeded 50% of time) while Q80%, Q85% and Q90% are much less affected. Although forest plantation expansion and changes in tree species composition have the potential to reduce streamflow, since Q80% and Q90% are used to define permanent and continuous Water Rights according to the Chilean Water Code, it is possible that already existing permanent and continuous water rights could be only marginally affected by the increase of the planted area. The results presented in this document can be used to support discussions related to the development of water allocation regulations among competing economic water users.