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.