Consumptive Water Use for the Riparian Areas of the Little Colorado River within Navajo Nation

Accurate estimates of natural plant area water use or evapotranspiration (ET, mm/day) are important to quantify so that in-stream use can be partitioned for human and natural environments. The natural grasses, shrubs and trees that grow alongside rivers and streams are collectively called riparian vegetation and their leaves transpire water that is considered a loss to the ecosystem. Bare soil also loses water through evaporation. In the landscape, we quantify both losses as one variable, actual evapotranspiration (ETa). ETa is the most difficult component of the water cycle to measure. Furthermore, estimates of ETa in uncultivated lands are a fraction of the estimates studied compared with cropped, agricultural lands. Riparian areas of the Little Colorado River are of critical importance to the Navajo Nation. Select riparian reaches were delineated using digitized shrubs and trees so that we could track plant health and its evapotranspiration (ET) with Landsat for the recent seven years (2014-2020). 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. The riparian corridor’s high-definition digitized shrubs and trees were aggregated using a 10 m buffer in ArcGIS and then rasterized to match the Landsat 30 m grid pixels. Two raster masks were created; the first used a 50% threshold majority option to include/exclude the grid pixels resulting in a ‘conservative’ estimate of the riparian area, and the second considered all pixels that intersected the vegetation buffered outline, resulting in the ‘best-approximation’ estimate of the riparian acreage. The best-approximation raster-area for the riparian corridor was 25,615 ha (63,296 acres) and the conservative raster-area estimate was 19,362 ha (47,846 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). Including seven years, 2014 through 2020, the average annual ETa (mmyr^-1) increased from 423.9 to 489.2 mmyr^-1 or 65.3 mmyr^-1 (15%) over the recent seven years, 2014-2020. Precipitation decreased by 73.1 mmyr^-1 (38%) from 190.8 mmyr^-1 (2014) to 117.7 mmyr^-1 (2020). The water deficit (WD), like ETa, shows an increasing trend from 235.6 mmyr^-1 (2014) to 373.5 mmyr^-1 (2020); this is an increase in WD of 137.9 mmyr^-1 (59%). 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) acre-feet. 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.