Using field observations and geochemistry to understand the coupled socio-hydrologic system in the Sierra de La Giganta, Baja California Sur

Physical hydrology affects self-organization and cooperation in societies persistently living in arid areas. Anthropogenic changes from global climate change to household-scale groundwater pumping may affect springwater quality, availability, and how social systems manage hydrologic resources. Traditional ranching communities living in Baja California Sur, Mexico's Sierra de La Giganta mountain range, self-identified as Choyeros, have adapted to an extremely arid climate by subsisting almost entirely off aridland springs for up to 10 generations. The absence of modern ³H (0.3 ± 0.084 TU) in springs here suggest there is no modern recharge to springs, thus total water availability is limited. Anthropological interviews were conducted from 2015-2018, and describe unique patterns of water sharing. Although spring water is considered an infinite resource with utilitarian functions, access requires social relationship maintenance. Springs are often shared between 2-5 households and are commonly located within 300m of each other. A diversity of water-use cooperation patterns have developed depending on use density by households, livestock, and gardens, as well as clear legal concession status.

Spring location is determined by geologic parameters that have shaped the social architecture. Field characterization of stratigraphy, structure, and diagenetic features indicative of past fluid flow show controls on spring location, morphology, and chemistry. Stable isotopes, major ions, and radiogenic tracers (¹⁴C, ³H, δ⁸⁷Sr) analyzed at a transect of springs in a variety of geologic settings constrain flow paths and rates. Compositions show chemical distinctions between proximal springs, suggesting that aquifers may be compartmentalized by structural barriers that cause hydrologic separation between proximal springs. Water quality issues have prompted installation of wells, inducing transience in the hydrologic system. Choyeros may need to develop new cooperation strategies across spring systems to deal with a system no longer in steady state, where local groundwater pumping may affect the water availability of hydrologically connected areas. Understanding water use, management norms, and the physical hydrologic system can inform data driven management and legal water concessions.