Four Riparian Corridors in the Lower Colorado River Basin: New Estimates of Riparian Evapotranspiration and Consumptive Water Use

Accurate estimates of riparian vegetation water use are important to quantify. In these narrow riparian landscapes, we quantify loss of water from leaves and soil as one variable, actual evapotranspiration (ETa). ETa is the most difficult component of the water cycle to measure, but remote sensing estimates of ETa have been validated for dryland riparian corridors using ground-based sensors (e.g., sap flow, tower data). 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, etc.

The objectives of this study were to calculate ETa daily (mmd^-1) and annually (mmyr^-1) and derive riparian vegetation annual consumptive use (CU) in acre-feet (AF) for select riparian areas of four rivers in the Lower Colorado River Basin. Select riparian reaches from the Lower Colorado, Bill Williams, San Pedro, and Virgin Rivers were delineated using digitized riparian plant area, comprised of shrubs and trees, so that we could track plant greenness using the two-band Enhanced Vegetation Index (EVI2) and ETa with Landsat for the recent decade (2014-2022). We acquired Landsat-8 OLI scenes, processed and filtered the data and computed EVI2 as a proxy for vegetation every 16-days over the study period. We then computed daily potential ET (ETo, mmd^-1) using the Blaney-Criddle formula with input temperature data from Daymet (1 km), an indirect remote sensing measurement from gridded weather data. These data were then averaged over 16-days using the 8-days before- and after- the Landsat overpass date. After fusing the delineated riparian areas with 30-m resolution Landsat data, riparian ETa was quantified using the Nagler ET(EVI2) approach to produce time-series ETa data and the first CU measurements for these riparian zones. Both a digitized-vector layer and best-approximation raster-area for each of the four riparian corridors were utilized in determining the water metrics, ETa and CU, based on these two acreage estimation methods.

The average annual ETa (mmyr^-1) for the Lower Colorado River decreased from ca. 950 to 800 mmyr^-1 (2014-2022). The average annual ETa (mmyr^-1) for the Bill Williams River decreased from ca. 925 to 600 mmyr^-1 (2014-2022). The average annual ETa (mmyr^-1) for the San Pedro River increased from ca. 975 to 1075 mmyr^-1 (2014-2022). The average annual ETa (mmyr^-1) for the Virgin River increased from ca. 675 to 825 mmyr^-1 (2014-2022). The two unaltered rivers depict positive riparian ecosystem responses. We produced four estimates of CU based on the corresponding riparian areas studied, each with a digitized vector area and best-approximation raster area. Our CU estimates for these four riparian corridors range from 30,000 AF (digitized) to 37,000 AF (best-approximation) and are in the range reported for similar arid riparian areas. 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.