Deep soil water reservoirs modulate land use and drought effects on the water budget of Amazon headwaters

The southeastern Amazon has been transformed by widespread land use and climate changes, altering the hydrologic cycle. In this seasonally dry tropical forest, the soil water reservoir plays a strong role in mediating the water balance by buffering forests during dry seasons and moderate droughts, as well as modulating runoff to streams. Few studies have examined the response of soil water reservoirs to large-scale forest loss in headwater regions, much less how these changes may influence catchment water balances during droughts. This study compares the water balance of forested and cropland watersheds in the headwaters of the Xingu River basin (Mato Grosso, Brazil). We combined measurements of stream discharge and soil moisture (from 0.3-8m depth) from the 2014-2018 water years, along with remotely sensed precipitation and evapotranspiration data, comparing normal precipitation years to an extreme drought year. Streams within agricultural catchments had four times higher discharge (29% of total precipitation) than in forested catchments (8%). During normal years, this difference was balanced by differences in evapotranspiration. We found that groundwater outflow – water that bypasses a catchment without reaching the stream – is a significant water export term under both landcovers (19% in forest; 26% in croplands). However, during drought years, this outflow disappears in forested catchments and decreases in agricultural catchments, suggesting enhanced groundwater uptake by vegetation that diminishes contributions to rivers downstream. Multiyear droughts projected under future climate changes could threaten the soil water reservoir, leaving forests without a critical resource and downstream communities vulnerable to streamflow loss.