Fire reduces dry season low flows in a subtropical highland of central Argentina

摘要

Wildfires are recurrent in seasonally-dry ecosystems; however, their effects on streamflows at the end of the dry season (low flows) are not well understood. The infiltration-evapotranspiration trade-off hypothesis postulates that when vegetation cover is lost or degraded, low flows in the following dry season are reduced due to impaired water infiltration and storage in soils during the wet season, which surpass gains due to diminished evapotranspiration. We tested this hypothesis in the context of vegetation loss produced by fire. For this purpose,we seized a wildfire in a subtropical highland of central Argentina and selected 12 burnt and 12 unburnt catchments of 15-60 ha. We measured low flows at the outlet point of each catchment in two opportunities: one year after the fire (post-fire dry season) and in a dry season not affected by previous fire (control dry season). Using the 12 unburnt catchments, we built linear regression models to estimate the expected low flows for the post-fire dry season under a no-fire scenario. The predictor variables were low flows in the control dry season. We applied the models to burnt catchments and compared the expected with the observed low flows. We also evaluated vegetation activity through satellite images. Burnt catchments showed significant low flow reductions of 31-48 compared with expectations under a no-fire scenario, supporting the infiltration-evapotranspiration trade-off hypothesis. Vegetation activity was lower in burnt than in unburnt catchments only for the first three months after the fire, while it was more active since then for one year. Together our results suggest that post-fire reduced low flows can be explained by a reduced infiltration, as well as by higher evapotranspiration due to increased vegetation activity. Given the extent of seasonally dry ecosystems worldwide, we highlight the importance of controlling wildfires to improve streamflow in the dry-season.