We provide two examples of remotely sensed evapotranspiration (ET) using optical methods at different spatial scales and applied to (i) upper Colorado River basin riparian zones that have been affected by the tamarisk (saltcedar) beetle (Diorhabda carinulata) and (ii) riparian vegetation in the Colorado River delta in Mexico in response to an environmental flow. In the first example, we show the results for scaling tamarisk beetle presence/absence ground-based data to remotely sensed-based estimates of ET using Landsast8 Normalized Difference Vegetation Index (NDVI) and Terra MODIS Enhanced Vegetation Index (EVI) in the wider riparian reaches along a large number of western US rivers where tamarisk stands have been altered by the beetle. In the second example, we present the impacts of a 2014 environmental flow (Minute 319), released to the Colorado River delta in Mexico, on vegetation greenness and estimated ET using Landsat NDVI and MODIS EVI data. These two applications range in spatial scales from a few hectares to several thousand hectares. Remote sensing methods can produce accurate estimates of ET across wide temporal and spatial scales, limited mainly by the accuracy of the ground methods by which they are calibrated and validated.
Results/Conclusions
The northern population of saltcedar beetles, Diorhabda carinulata, is especially important to model and monitor because it has colonized nearly all the rivers in the upper Colorado River basin and in the past five years has moved down the Virgin River to Lake Mead. In recent work, we have collected several years of ground data on the movements of saltcedar beetles and provide ground-validated remote sensing methods to track beetle movements, and to model their spread and make projections about their future range expansion in the lower Colorado River basin.
We have analyzed the effects of this historic release of water, providing insight into the efficacy of environmental pulses as a tool for restoring delta’s riparian corridor, an ecosystem heavily affected by decades of impoundments and diversions. We found a significant increase (3%) in ET and a significant increase (17%) in NDVI from 2013 (pre-pulse) to 2014 (post-pulse) within the delta’s riparian zone (P < 0.05). While NDVI declined in 2015, it was still significantly higher than in 2013 (P < 0.05). This reverses an overall decline in NDVI and ET since the last major flood in 2000. Based on preliminary findings, pulse flows could be an effective tool for restoring the lower Colorado River’s riparian zone.