Flow Permanence Along Ephemeral Streams in the Huachuca Sky Island, Arizona and its Consequences for Vegetation Distribution

Flow in ephemeral and intermittent streams is a major determinant on ecosystem health and functioning within dryland riparian environments, yet there is limited available information on which streams flow and when. The Sky Islands are small, isolated mountain ranges scattered across the Sonoran Desert in southern Arizona. They function as refugia for plants and animals requiring higher moisture and lower temperatures than the desert. Understanding how water is distributed in these distinctive landscapes and its effects on vegetation communities will allow us to bring relevant information to land managers and better predict the consequences of climate change in this water-limited region. We used data from 37 electrical conductivity sensors set in the stream channel to reconstruct daily flow records for ten intermittent and ephemeral streams draining the Huachuca Mountains. We classified flow permanence and variability both along each stream and between streams on the east facing slope of the Huachucas. Comparing this flow permanence data with daily precipitation and snowfall showed that some channel reaches react strongly to precipitation events while others will remain dry or flowing steadily, irrespective of rainfall, which we interpret as caused by underlying geology. We then examined whether elevation and flow permanence affect the density and distribution of vegetation along the channel around each sensor through Normalized Difference Vegetation Index (NDVI) values derived from Sentinel-2. NDVI allowed us to gather information on vegetation density and functional type (grass, deciduous or evergreen tree), to be compared against elevation and flow permanence. As expected, we found that elevation is a first-order control on vegetation distribution, with grassland, deciduous trees and evergreen trees layered along an elevation gradient, from low to high. At lower elevations, flow permanence does not seem to play an important role in vegetation distribution, maybe due to the presence of a shallow water table. At higher elevations, an increase in dryness leads to a shift toward more trees and less grass. These findings suggest that streamside vegetation in higher reaches might be more vulnerable to shifts in water distribution and climate change.