Riparian ecosystems of the arid and semiarid Southwest are linear corridors that are highly valued for their ecosystem services. The quality of these services hinges on sustaining flow regimes that are sensitive to even small changes in the riparian water balance, with sharp changes as streams become intermittent and as groundwater declines below vegetation survivorship thresholds. Despite much disciplinary work on individual rivers, a regionally comprehensive and integrated understanding of how the riparian water balance, associated aquatic-terrestrial ecotones, and hence the ecosystem services they provide, respond to hydrologic change awaits development.
To determine the region-wide sensitivity of riparian vegetation to changes in riparian water balance, particularly as a result of climate change, we established study sites on three rivers that span a precipitation gradient from winter-rain dominated (Santa Maria-Bill Williams, Mojave Desert of western Arizona), equal winter and summer rain (Hassayampa, Sonoran Desert of central Arizona), and summer-rain dominant (San Pedro River, Sonoran/Chihuahuan Desert of southern Arizona). On each river, sites have been identified on hydrologically gaining reaches that have perennial flow and shallow groundwater and losing reaches with intermittent flow and deeper, more variable groundwater levels. On each river, we are using isotopic methods to identify the water sources that sustain riparian water flows, and relating hydrologic patterns to riparian vegetation response. Here, we present results for one study river.
Riparian groundwater composition along the San Pedro River varied between gaining and losing reaches. Locally recharged monsoon floodwaters comprise 60 to 85% of riparian groundwater in losing reaches (with regional groundwater inflow providing the remainder) whereas gaining reaches contain only 10% to 40%. Baseflow was also dominated by monsoon floodwater ranging from 80% on the upstream end and decreasing to 55% after passing though several gaining reaches. These results were in agreement with an independent riparian ecosystem condition class model, with gaining reaches having higher riparian scores than losing reaches.
These results imply that hydrologic status and the relative importance of water sources determine not only the sensitivity to climate change but also the ecosystem services valued by society. The importance of seasonal floods to the riparian water balance provides a first order hydrologic control when considering transfer of benefits to other riparian systems. Ultimately this research aims to improve understanding of the linkages between climate, hydrologic variability, riparian vegetation structure, and ecosystem services, and the regional variability in these relationships.