Ecohydrological changes at multiple scales
The El Morro catchment underwent intense and interconnected
hydrological, ecological and geomorphological transformations over the
last 45 years. In the longer term (1975-present) we documented a
widespread increase in groundwater storage, accompanied by the expansion
of wetlands and the extension of a network of permanent streams opened
by episodic sapping events (Figure 2b, 3 and 4). Raising groundwater
levels and increased flooding have been reported for the same period in
different locations across the sedimentary plains of Argentina during
periods of higher than normal precipitation likely in connection with
long term shifts from native forests, grasslands and planted pastures to
continuous annual crop cultivation (Nosetto et al., 2015; Kuppel,
Houspanossian, Nosetto, & Jobbágy, 2015; Gimenez et al., 2016, Giménez
et al. 2020; Alsina, Nosetto, & Jobbágy, 2020). Similar transitions
have been documented in dry farmlands of North America, Australia and
Africa (George, McFarlane, & Nulsen, 1997; Scanlon, Reedy, Stonestrom,
Pruderic, & Dennehy, 2005; Leblanc et al., 2008). The peculiar context
of the El Morro basin, with higher slopes and closer depths to the rock
basement than most of the central plains of Argentina (Jobbágy et al.,
2008), is that this hydrological change has triggered an unusually rapid
and pronounced sapping process that seems to have no precedents at the
site for the whole Holocene (as suggested by sediment dating by Tripaldi
and Forman et al 2016). Under a similar context of deforested farmlands,
sapping and piping processes have been recently documented in a piedmont
loessic plains 600 km north of our site (Pereyra, Fernández, Marcial, &
Puchulu, 2020), and massive sapping episodes may have accompanied the
long-dated expansion of cultivation in the loessic plains of China (Zhu,
2012).
In the mid-term (2000-present) the process of stream formation expanded
exponentially with episodes of increasing magnitude in 2001, 2008, 2009
and 2015 (Figure 2b and 4, Supplementary material). Meanwhile, two
opposite and coexisting trends of vegetation activity were observed. On
one hand, currently cultivated areas decreased their greenness, likely
in response to the replacement of pastures by continuous agriculture
(Contreras et al., 2013), but more so to the adoption of no-till
cultivation and the narrowing and delaying of the active growing season,
which are part of a general drought avoidance strategy (Gimenez et al.,
2020)(see Figure 7, top panel). On the other hand, remaining
uncultivated areas including forests and grasslands increased their
greenness, likely in response to rising water table levels favoring the
expansion of wetlands and the growth of forests (see Gonzalez Roglich,
Swenson, Villarreal, Jobbágy, & Jackson, 2015 for similar forest trends
in other areas of the caldén belt). In the short-term (last erosion
episode to the present) our study shows that despite the relative low
precipitation of the last three years (Figure 9), wetlands kept
expanding and greening and streams flowing (Figure 8). Groundwater
uptake by plants appeared to be widespread in forests but limited to
shallow water table depths (< 2 m) in croplands (Figures 5, 6
and 7 and Supplementary material). Mean water yields for the three years
of periodic observations approached 20 mm y-1 (3.3%
of long-term average rainfall inputs) with higher yields in the most
deeply incised sub catchment sections of Río Nuevo (30-50 mm
y-1). This output was considerably stable for a dry
region, with a seasonality governed by water demand rather than
rainfall, since lowest flows occurred in summer when rainfall was
highest but water uptake too (Figures 5,6, 7 and 8, Table 1). The
observed water yields at the outlet of the catchment are still lower
than those predicted by the most widely used global empirical model
(Zhang, Dawes, Walker, 2001), which under pure grassland or forest cover
predicts yields of 83 and 33 mm y-1, respectively,
based on the mean climatic conditions of our site.