QUANTITATIVE TRENDS OF ACTUAL EVAPOTRANSPIRATION IN THE UPPER SAN PEDRO RIVER RIPARIAN CORRIDOR

Protected natural areas, such as the riparian corridor of the San Pedro Riparian National Conservation Area (SPRNCA) in southeastern Arizona, face ongoing challenges due to regional and global environmental changes. The SPRNCA, established in 1988, protects one of the last undammed rivers in the southwestern U.S., preserving the health of native riparian species and the hydrology that influences riparian vegetation, particularly the native phreatophytes that provide much of the plant structure along the river. The study examines riparian vegetation water usage by analyzing digitized riparian areas along the river within the defined boundaries of SPRNCA.

Actual evapotranspiration (ETa) is one of the most difficult components of the water cycle to measure, but our remote sensing estimates of ETa have been validated for dryland riparian corridor species using ground-based sensors (e.g., sap flow, tower). This study quantifies water loss from leaves and soil as actual evapotranspiration (ETa) using remote sensing and ground-based sensors. Increases in ETa indicate improving vegetation cover and, thus, positive trends in riparian ecosystem health, while decreasing ETa may signal declining riparian cover due to various factors such as drought, beetle defoliation, or increasing salinity.

Accurate estimates of riparian vegetation water use, particularly in arid environments, are crucial. We utilize Landsat-derived Enhanced Vegetation Index (EVI2) and daily Daymet Blaney-Criddle potential ET values to estimate ETa trends from 2000 to 2023. The results show that ETa over the peak growing season (May-Oct) ranged between 2.6-4.4 mmd^-1, with annual ETa ranging from 1.7-2.8 mmd^-1. These findings are valuable for decision-makers seeking to mitigate and understand the decline of riparian ecosystems, potentially offering insights into slowing or reversing these trends. This research provides important insights into the dynamics of riparian vegetation water use in the Upper San Pedro River, offering valuable data for conservation efforts and environmental management.