ABSTRACT:

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.

ABSTRACT:

Deforestation from artisanal and small-scale gold mining is transforming large regions of the tropics, from lush rainforest to barren collections of tailings and ponds. Natural forest regeneration is slow due to dramatic soil changes and existing reforestation strategies are failing. Here we combine remote sensing, electrical resistivity imaging, and measurements of soil properties to characterize post-mining areas in the Madre de Dios region, Peru. We find that the post-mining landscape has dramatically changed water infiltration dynamics, driving decreases in subsurface water availability and presenting a major barrier to revegetation. Mining tailings are extremely hydraulically conductive, allowing for 14.7 m/day infiltration relative to 0.074 m/day in primary forest soils, leading to lower average soil moisture and extreme temperatures (<60C). Electrical resistivity imaging reveals a highly resistive, 1.5-2m deep layer of dry sand across the mining landscape. Areas close to the water table (e.g. pond edges) show higher soil moisture, lower temperatures, and greater natural regeneration compared to elevated tailings. Our results suggest that access to water should be prioritized when targeting reforestation sites, potentially requiring large-scale geomorphological reconfiguration. As gold mining is expected to expand, responsible practices and remediation strategies must account for the critical yet often overlooked role of water.