Drier streams despite a wetter climate in woody-encroached grasslands

Abstract

Grasslands, covering 40% of ice-free Earth surface, are experiencing woody encroachment globally. The hydrological impacts of woody encroachment are highly uncertain because they are compounded by the concurrent influence of climate change. Here we ask the questions 1) How water balance (evapotranspiration versus streamflow) and streamflow partitioning (into surface runoff and subsurface flows) evolve over time in woody-encroached grasslands? 2) What is the relative influence of climate change and woody encroachment? We used the hydrology model HBV-light and decades of hydrometeorological and streamflow data from two intermittent streams draining catchments with different degrees of woody encroachment at the Konza Prairie, Kansas, US. Results indicate that both streams have become drier and have experienced more hydrological droughts over time, more so in the substantially encroached site, with increasing evapotranspiration despite a wetter climate. In contrast, a modelled hypothetical “Climate Only” scenario without woody encroachment suggests streamflow would have increased under climate change alone. Moreover, results suggest that flow paths have deepened with increasing fractions of deeper groundwater flow in the substantially encroached site. These findings raise questions about mechanisms of these changes and commonality of drier streams in a wetter climate in woody-encroached areas and beyond. Answers to these questions can have far-reaching implications for the occurrence of droughts, water availability, water quality, and ecosystem health.