The birth of a catchment
In its subtropical and temperate belt, South America hosts a vast
aeolian plain covered by Quaternary loessic deposits and sandy mantles
spanning semiarid to subhumid climatic conditions along a 900 km
west-east extent (Zárate & Tripaldi, 2012). The few rivers crossing the
driest western half of this region have their sources in adjacent
mountain systems and loose water as they traverse it (Marchesini,
Nosetto, Houspanossian, & Jobbágy, 2020; Poca, Nosetto, Ballesteros,
Castellanos, & Jobbágy, 2020). A surprising exception emerged in recent
decades, however, when an extremely abrupt groundwater sapping process
started carving and connecting permanent stream segments that now
configure what has been named the “El Morro” catchment (Contreras,
Santoni, & Jobbágy, 2013). Located in the transition from the isolated
rock outcrop of the “El Morro” caldera (Sruoga, Ibañes, Japas, &
Urbina, 2017) and the adjacent aeolian plain, this topographically
well-defined basin had its first active surface water outlet opened less
than four decades ago and continues to gain and connect tributaries
after discrete but increasingly intense sapping episodes caused by
steadily rising groundwater levels. The deepest incision, where the Río
Nuevo stream flows permanently, is now 30 km long with the largest
cross-sections reaching 20 m of depth and 80 m of width. In its exposed
walls, ~5 m-deep aolian Holocene material laying over
pre-Holocene fluvial beds suggests that the emerging streams are not
only a new feature of this landscape in historical times, but throughout
the last interglacial period (Tripaldi & Forman, 2012). Where exposed,
the crystalline basement or its overlaying late Tertiary calcretes show
signs of fluvial erosion suggesting that a paleo catchment buried with
aeolian material is being reactivated in recent years.
Originally covered by grasslands and open woodlands (Oyarzabal et al.,
2018), the El Morro watershed has been progressively cultivated since
the beginning of the twentieth century and today more than half its area
is under dryland agriculture with native caldén (Prosopis
caldenia ) forests covering less than 10% of the area (Contreras et
al., 2013). Accompanying these changes, an increasing number of low
areas within croplands and to less extent native vegetation become
waterlogged by raising water tables, initiating a spontaneous succession
towards wetland communities (Contreras et al., 2013; Díaz, Jobbágy, &
Marchesini, 2018). While increasing rainfall trends, seismic activity
and land use changes have been pointed as possible converging causes for
the novel and rapid hydrological transformation of the El Morro
catchment (Contreras et al., 2013), observations in this region and
other semiarid areas of the plain point to the onset of groundwater
recharge following the conversion of native vegetation to dryland
agriculture as the most determinant factor (Amdan, Aragón, Jobbágy,
Volante, & Paruelo, 2013; Contreras et al., 2013; Giménez, Mercau,
Nosetto, Paez, & Jobbágy, 2016; Marchesini, Giménez, Nosetto, &
Jobbágy, 2017). The rapid shifts in groundwater level, the unusually
fast development of a stream network, and the semiarid climate making
both plants and streams highly sensitive to water supply, offer a unique
setting to explore the interplay of the five mechanisms described above.
Stimulated by this opportunity and the urgency to understand the causes
and possible adaptation or mitigation actions to cope with its negative
effects on people and nature, we initiated a long-term observation
program in the El Morro catchment. In this paper we present hydrological
and ecological observations derived from this program together with
previous observations and different sources of remote sensing
information to characterize groundwater level, stream flow, and
vegetation dynamics in three subcatchments of the El Morro basin at two
temporal scales (long-term: 20-50 years, short term: following the last
sapping event of 2015). We link these observations with the five
mechanisms described above to offer a preliminary quantification of
their relative weight shaping the current and projected partition of
water inputs between plants and streams in this semiarid context.