Study catchments
The study area, at the western dry edge of the sedimentary plains of
central Argentina, sits in the ecotone of Espinal forests and Pampas
grasslands (Figure 2a). Its temperate semiarid climate shows a mean
annual temperature of 15.7 C with the coldest (July) and warmest
(January) months, having a mean minimum temperature of 1.0 C° and a mean
maximum temperature of 28.6 C°, respectively. Mean annual precipitation
(1903–2019, Villa Mercedes; -33.65, -65.42; 525 m) was 601 mm
y-1. Rainfall is concentrated in the warm season (70%
between November and March) originating mainly in convective storms with
high spatial variability. A-type tank evaporation is 1640 mm
y-1 (2006-2009), reaching maximum values of 11 mm
d-1 during summer.
We run periodic observations in three adjacent sub-catchments of the El
Morro Basin that cover together 1334 km-2 and range
from 1100 to 500 m of elevation along 50 km in a NNW-SSE direction
(Figure 2a). These sub catchments show an east-west gradient of
developing age with La Guardia having completed its connection to the
main pre-existing collector (Quinto river) in 1986, followed by Río
Nuevo, connected in 2009 but still developing tributaries, and
Quebrachal, which is currently an isolated segment that infiltrates
without reaching the collector (Figure 2b). Río Nuevo has the deepest
and widest incision (20 x 80 m in 2020) and has generated the largest
sediment deposit in its lower section (8 km-2deposited in 2015), while La Guardia displays the shallowest incision
and the largest wetland area (Table 1). The most recent incision
episodes took place in September 2001, January-February 2008, December
2009 and February 2015, following periods of several months of high
rainfall and affecting mainly the Río Nuevo and Quebrachal sub
catchments (Contreras et al., 2013; Buono, Menéndez, Cáceres, Jobbágy,
& Nosetto, 2018). The landscape of these sub catchments includes a
higher section (1100-700 m of elevation, 2% slope) with isolated rock
outcrops and dissected surfaces in its high end and the initiation of
most incisions in wetlands on its low end, an intermediate section
(700-550 m of elevation, 0.7% slope) with a transverse dune surface and
the greatest depth to the crystalline basement (>50 m),
associated to a local tectonic depression (Barbeito, 2008), and the
largest incisions; and the lowest section (550-500 m of elevation 0.5%
slope) with a flat sandy mantle that has hosted most of the recent
fluvial deposits (Ríos 2020). Soils are Entic Haplustolls in the
higher catchment and Typic Ustipsamments and Typic
Ustorthents in the intermediate and lower catchment (Galván & Collado,
2009).
Dryland agriculture is the dominant activity followed by cattle raising
on pastures and native vegetation. Dominant crops are soybeans and
maize, growing in a spring-summer season that starts slightly after the
onset of rains between October and December and ends in April, similarly
to what is seen throughout the western plains of Argentina (Gimenez et
al., 2020). No till prevails and soils remain free of weeds during the
fallow periods. These agricultural systems have progressively replaced
not only native vegetation but perennial pastures of the exotic grassEragrostis curvula , which where the basis of the livestock
production systems that prevailed until the end of the twentieth century
(Viglizzo, Roberto, Lértora, Lopez Gay, & Bernardos, 1997), and are
still present as minor component of the landscape. Native vegetation
includes perennial tussock grasslands and forests dominated by the
leguminous trees Prosopis caldenia (caldén) and Geoffrea
decorticans (chañar) (Oyarzabal et al., 2018, Cabido et al., 2018).
Wetlands have expanded in the area as a result of raising water table
levels (Contreras et al., 2013). Two types of spontaneous communities
are found, one related to freshwater environments in the
higher-intermediate belt of catchments, dominated by Cortaderia
selloana tussocks and Typha sp., and the other associated to
saltier environments in the lower belt of catchments, dominated byTamarix ramosissima (Natale, Zalba, Oggero, & Reinoso, 2010;
Diaz et al., 2018).