Riparian Corridors of the Sonoran Desert: New Estimates of Riparian Evapotranspiration Change Using Daymet and Landsat Vegetation Index Methods

Accurate estimates of riparian vegetation water use are import-ant to quantify, particularly in arid environments. In these narrow riparian corridors, we quantify loss of water from leaves and soil as one variable, actual evapotranspiration (ETa). 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). Increases in ETa are indicative of increasing vegetation cover and therefore increasing ‘losses’ of water through ETa represent positive trends in riparian ecosystem health; decreasing ETa may indicate dwindling riparian cover due to less available water for canopy growth due to drought, groundwater flux, beetle defoliation, fire, increasing salinity.

The objective of this study was to calculate actual annual ETa (mmyr^-1) for selected riparian areas in the Sonoran Desert in the southwestern U.S. Riparian reaches for a dozen rivers in the Lower Colorado River Basin, mostly in Arizona, were delineated and monitored using the two-band Enhanced Vegetation Index (EVI2). We acquired 30-m resolution Landsat scenes, processed and performed a pixel-wise quality assessment to remove pixels with high aerosols and clouds, and computed EVI2 every 16-days over 20 years. We then computed daily potential ET using the Blaney-Criddle formula with input temperature data from gridded weather data using Daymet (1 km). Riparian ETa was quantified using the Nagler ET(EVI2) model to produce time-series data for the period 2000-2021.

From 2000 to 2021, various rivers were studied to determine the average annual ET(EVI2) (mmyr-1) for riparian corridors, unrestored areas, and restored areas. The findings indicate that the Salt River experienced a 13.7% increase from 800 mmyr^-1 to 910 mmyr^-1, whereas the Gila River only saw a 2.7% increase from 725 mmyr^-1 to 745 mmyr^-1 during the same period, with occasional periods of decreases (e.g., 2002, 2013) followed by increases. The San Pedro increased 7.4%. The Santa Cruz River showed the most significant increase in average annual ET(EVI2) with a 24.0% increase from 770 mmyr^-1 to 955 mmyr^-1 (2000-2021). The increasing trends on these rivers could be due to riparian species composition altered by the tamarisk beetle followed by secondary or replacement species which established green canopies, restoration efforts or other changes in water or land management. This study provides valuable estimates of riparian water use that may assist with decision-making by natural resource managers tasked with allocating water and managing habitat along these riparian corridors. Our findings have continued to be used to assist managers with decision-making for ecological restoration success. These data, tools, methods, and results can be utilized by decision makers in their quest to mitigate and understand how declines of riparian ecosystems can be slowed or possibly reversed.