Hydrological and Geomorphological Significance of Riparian Vegetation in Drylands

Drylands are regions encompassing hyperarid, arid, semiarid, or subhumid climatic conditions (see also Chap. 1). They include cold and warm subtropical deserts, savannas, and the Mediterranean environments. Our focus here is on warm drylands, which are generally characterized by the existence of a well-defined dry season dominated by subtropical high pressure (Malanson 1993) and a rainy season with an average precipitation of less than 700 mm/year. Such regions cover approximately 50% of the continents, with about 20% of the world’s population living in these areas (Le Houerou 1982; Nanson et al. 2002). This explains the growing scientific interest in the study of drylands. Here, we focus on the interactions between fluvial geomorphology and riparian vegetation. These interactions act at different spatial and temporal scales, suggesting the existence of an intrinsic and remarkable sensitivity of riparian ecosystems to hydrological and geomorphological modifications. In this respect, geomorphological resilience to disturbances of either climatic or anthropic origin has recently been questioned (Tooth 2018). Dryland riparian ecosystems are (spatially) linear oases playing the role of humid spots in dryland regions (see Tooth and McCarthy (2007) for a review) used by people and wildlife (Fig. 10.1). However, such ecosystems have been affected by heavy anthropogenic disturbances and risks associated with the encroachment of invasive riparian species, with great reductions in spatial extent (up to 80%, as in certain USA sites) with respect to presettlement times (Smith et al. 1991; Tooth 2000a, b; Salinas et al. 2000; O’Connor 2001; Pettit et al. 2001; Williams et al. 2013). This also sets the risk of reducing common property resources in drylands, e.g., water bodies and related ecological functions being benefited by a community or a group of communities (Gaur et al. 2018) and ecosystem species (McGinnes et al. 2010).