Plants vs. Streams: Their groundwater-mediated competition at “El
Morro”, a developing catchment in the dry plains of Argentina
Abstract
Our understanding of the mechanisms routing precipitation inputs to
evapotranspiration and streamflow in catchments is still very
fragmented, particularly in the case of saturated flows. Here we explore
five mechanisms by which plants and streams compete with each other for
water, based on multiple scales of observations in a flat semiarid
sedimentary catchment of central Argentina subject to abrupt
hydrological transformations. Since the 80s, the “El Morro” catchment
(1334 km2, -33.64°, -65.36°) experienced a fast expansion of crops over
native forests and grasslands, rapid water table level rises
(~0.3 m y-1), spontaneous expansion of wetlands and
permanent streams by groundwater sapping. Based on episodic and
continuous groundwater level, stream flow, and remote sensing data we
show that plants not only take away water from streams by drying the
unsaturated zone (mechanism 1), but by tapping the saturated zone in the
expanding waterlogged environments (mechanism 2) and in the upland
environments that remain uncultivated and display increasing tree cover
(mechanism 3). Conversely, streams take away water from plants through
pulsed bed-deepening and water table depression (mechanism 4), and
riparian and wetland zones burying with fresh sediments (mechanism 5).
While earlier work established widespread support for mechanisms 1
preventing stream formation, diurnal and seasonal fluctuations of water
table levels and base streamflow records in this study proved the
importance of mechanisms 2 and 3 under the current high-water table
conditions. These data together with remotely-sensed greenness showed a
growing but localized relevance of mechanism 4 and 5 as the stream
network developed. The distinction of recharge- vs.
topography-controlled groundwater systems is useful to organize the
interplay of these concurrent mechanisms. Findings point to the
unsaturated-saturated contact zone as a crucial and dynamic hub for
water partition and for ecological, geomorphological, and hydrological
knowledge integration.