Introduction
Although the critical role of water inputs supporting the growth of
plants and the flow of streams is indisputable, the way in which these
two major fates of precipitation may compete with each other on a given
territory is not always obvious and represents, in fact, a very active
front of scientific inquiry. Moreover, this quest is perhaps one of the
main attractors of the convergence of Ecology and Hydrology over the
last decades. How any given piece of land, and more operatively, each
catchment, partitions the water that it receives from precipitation
between evaporative losses (among which plant transpiration is usually
dominant) and liquid losses (among which stream flow is generally the
main protagonist), is something that has been quantified for more than a
century using multiple approaches and that has sparked key integrative
theoretical developments such as the Budyko curve (Budyko, 1974; Gentine
et al., 2012). It is notable, however, that our knowledge about the
mechanisms dictating this partition is still very fragmented (McDonnell
et al., 2007). In which ways plants can ”capture” water that could
eventually reach streams or, conversely, what are the mechanisms that
allow streams to “capture” water that may be potentially usable by
plants? Particularly relevant but disconnected are our representations
of the role of saturated flows dictating a “push and pull” dialog
between plants and groundwater (Fan, Miguez-Macho, Jobbágy, Jackson, &
Otero-Casal, 2017) and eventually, streams. In this contribution we
explore conceptually how the competition for water between plants and
streams may operate in a dry basin, proposing a list of key mechanisms
that involve groundwater-mediated interactions. Plant-stream competition
should be particularly evident in dry regions (PET > PPT),
where potentially all water inputs can get evaporated (no energy
limitation) and the outcomes of the “push and pull” process should
leave a strong imprint both on plants, which would increase their growth
as their capture of limiting water resources becomes more exhaustive,
and on streams, which can potentially have a total suppression of their
flow. In this contribution, we illustrate the proposed mechanisms and
evaluate their relevance both for land ecosystems and hydrology with
data from a semi-arid sedimentary basin subject to strong vegetation
changes and, mostly as their consequence, to an extraordinary rapid
process of novel stream formation through sapping and surface erosion.
A simplified view of the atmosphere-soil-plant/stream
continuum can assume that all the water consumed by vegetation comes
from the unsaturated zone and that any by-pass or displacement flow
escaping root access becomes free to rich the saturated zone and is no
longer accessible to plants. This is still the prevalent representation
of many models that can successfully describe the partition of
evapotranspiration and streamflow in many catchments (Rahtjens, Oppelt,
Bosch, Arnold, & Volk, 2015) based on what we can define as the single
most relevant mechanism that plants have to take away water from streams
(i.e. unsaturated uptake, mechanism 1, figure 1). In dry regions where
evapotranspiration is water limited, flat and loose sedimentary
substrates occupied by deep-rooted vegetation often display an
exhaustive exploration of the wettable soil volume, leading to the total
suppression of groundwater recharge through “mechanism 1”. In these
settings any streamflow is limited to storm run-off episodes and there
is no observable base flow (Scanlon et al. 2006; Santoni, Contreras, &
Jobbágy, 2010). While simple and powerful enough to explain the behavior
of many dry catchments, unsaturated uptake runs short to explain many
biological and hydrological phenomena such as the presence of
exceptionally green vegetation patches in deserts or the fluctuating
daily or seasonal reduction of stream base flow during periods of high
water demand and no rainfall inputs. Plant roots access saturated water
directly, as in the obvious case of riparian or wetland environments
were specific vegetation types adapted to the anoxic conditions of the
substrate thrive (Lowry, Loheide, Moore, & Lundquist, 2011), or
indirectly by tapping the capillary fringe fed by the saturated zone, as
seen (not as easily) with phreatophytic ecosystems (Naumburg,
Mata-González, Hunter, & Martin, 2005; Jobbágy, Nosetto, Villagra, &
Jackson, 2011). Therefore, saturated uptake needs to be included as a
“second chance” that plants have to take away water from streams with
the unique contribution of lateral connectivity allowing vegetation in
one place to use water that plants did not use in another. The most
acknowledged component of saturated uptake is the consumption of ground
or stream water by wetland and riparian communities in the small
fraction of landscapes occupied by topographic low positions and water
body shores (i.e. focalized saturated uptake, mechanism 2, figure
1)(Bond et al., 2002). Yet, saturated uptake can be significant in the
matrix of the landscape as well, when a more widespread contact between
roots and the capillary fringe takes place (i.e. distributed saturated
uptake, mechanism 3, figure 1), something that appears to be
reciprocally and dynamically modulated by the vertical location of roots
and water tables and highly sensitive to vegetation, climate and water
managements shifts (Jobbágy & Jackson, 2004; Nosetto, Jobbágy, Jackson,
& Sznaider, 2009, Fan et al., 2017). While being increasingly
considered in catchment models (Immerzeel & Droogers, 2008, Doble &
Crosbie, 2017), saturated water uptake by plants introduces a novel
possibility for streams to take away water from plants.
Far from passively receiving the hydrological left-over of the three
main water consumption mechanisms pathways highlighted above, streams,
as active geomorphological agents, can take away water from plants when
they remove and deposit sediments. By deepening their beds, streams can
increase their groundwater capture into base flow, transiently enhancing
hydraulic gradients but more permanently by impairing mechanisms 2 and 3
as water tables plunge to reach a new equilibrium and zones where plant
roots can reach it shrink or disappear (i.e. streambed deepening,
mechanism 4, figure 1) (Bravard et al., 1998; Wurster, Cooper, &
Sanford, 2003; Schilling, Zhang, & Drobney, 2004). Another way in which
streams can actively restrict plant water consumption is by burying land
ecosystems with new sediment deposits, which are more likely to affect
active riparian and wetland zones with higher water consumption than any
other landscape component (i.e. sediment deposit, mechanism 5, figure 1)
(Kui & Stella, 2016). The relative timing of ecological succession and
geomorphological changes is critical defining the resulting balance of
mechanisms 1,2 and 3 vs. 4 and 5. Ecological changes such as the
establishment of riparian plants with deeper roots where stream
incisions have gone deeper or the recovery of buried trees where streams
have deposited fresh sediments (Rivaes et al., 2013; Kui & Stella,
2016), may outpace the slower work of streams, yet landscapes in which
their erosive power get rapidly unleashed as result of climate or land
use changes can match their forces offering an ideal observatory to
explore their interaction.