ESTIMATES OF EVAPOTRANSPIRATION AND CONSUMPTIVE WATER USE FOR THE RIPARIAN AREAS OF THE LITTLE COLORADO RIVER IN THE DRYLANDS OF NORTHEAST ARIZONA, USA

Riparian areas of the Little Colorado River are of critical importance to the Navajo Nation. Select reaches were delineated using digitized riparian vegetation so that we could track plant health and its evapotranspiration (ET) with Landsat for the recent six years (2015-2020). We acquired six Landsat scenes, processed and filtered the data and computed the two-band Enhanced Vegetation Index (EVI2) as a proxy for vegetation at a 16-day interval. We then computed daily potential ET (ETo, mmd^-1) using Blaney-Criddle with input temperature data from two sources, Daymet (1 km) and PRISM (4 km) data. ETo from Blaney-Criddle was then averaged over 16-days using the 8-days before- and after- the Landsat overpass date. Next, we created a 10 m buffer to aggregate individual shrubs and trees and then rasterized the buffered area resulting in a mask over the digitized shrub and tree layer. We report the rasterized area for the riparian area to be 25,615 ha (63,296 acres), whereas the digitized area included only a fraction of the total vegetative area, was only 4,974 ha (12,291 acres). We utilized ETo to estimate actual ET (ETa) using EVI2 (mmd^-1). Average annual ETa (mmyr^-1) increased from 361.3 to 489.2 mmyr^-1 or 127.9 mmyr^-1 (30%) over the recent six years, 2015-2020. Precipitation decreased 106% from 380.9 mmyr^-1 (2015) to 117.7 mmyr^-1 (2020). The water use difference (WD), like ETa, shows an increasing trend from 48.2 mmyr^-1 (2015) to 373.5 mmyr^-1 (2020); this is an increase in WD of 325.3 mmyr^-1 (154%). We produced three estimates of consumptive water use (CU) based on riparian area using a best-approximation and conservative-estimate from the rasterized area, and vector area. Our CU estimates for the riparian corridor range from 31,648 (conservative) to 36,983 (best; Daymet) to 41,585 (PRISM). These findings refine predictions in the range between 25,387 and 46,397 acre-feet using only literature for similar areas. Better estimates of water use are valuable to the Navajo Nation in the adjudication of water rights.