Plants vs. streams: Many aces under their sleeves
Far from following a simple predictable partition of rainfall inputs
into evapotranspiration and streamflow, the El Morro catchment has shown
over the last decades a rapidly switching balance between them. After an
early stage in which mechanism 1 has kept the system free of streamflow
(and streams), away from the expected water yield suggested by global
empirical models (Zhang et al., 2001), the expansion of cultivation,
aided by a relatively rainy period, has brought the system to a new
stage of higher groundwater storage. Saturated water consumption
(Mechanisms 2 and 3) in this new stage had increased evapotranspiration
locally but was unable to prevent stream formation (Mechanism 4) and the
onset of a stable water yield that approaches empirical global
predictions. Sapping erosion is still taking away water from plants in
the catchment, yet its long-term outcome is hard to predict. Climate
fluctuations, but more importantly, land use changes and spontaneous
vegetations shifts can twist it. Acknowledging the non-additive and
non-linear effects of vegetation and streams on groundwater is crucial
to develop more successful hydrological models for dry sedimentary
catchments. The peculiarities of the El Morro catchment may cast light
on overlooked (and not so peculiar) hydrological mechanisms.