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.