Assessing methodologies for daily evapotranspiration estimation from sUAS over commercial vineyards in California

Authors: NASSAR, A. - Utah State University; TORRES, A. - Utah State University; Kustas, William - Bill; MCKEE, M. - Utah State University; Alfieri, Joseph; HIPPS, L. - Utah State University; Prueger, John; NIETO, H. - University Of Alcala; ALSINA, M. - E & J Gallo Winery; White, William - Alex; McKee, Lynn; COOPMANS, C. - Utah State University; SANCHEZ, L. - E & J Gallo Winery; DOKOOZLIAN, N. - E & J Gallo Winery

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2021
Publication Date: 7/23/2021
Citation: Nassar, A., Torres, A., Kustas, W.P., McKee, M., Alfieri, J.G., Hipps, L.E., Prueger, J.H., Nieto, H., Alsina, M., White, W.A., McKee, L.G., Coopmans, C., Sanchez, L., Dokoozlian, N. 2021. Assessing methodologies for daily evapotranspiration estimation from sUAS over commercial vineyards in California. Remote Sensing. 13(15):2887. https://doi.org/10.3390/rs13152887.
DOI: https://doi.org/10.3390/rs13152887

Interpretive Summary: Continuous daily evapotranspiration (ET) monitoring is essential for water resources management, drought monitoring, and for improving water use efficiency in crop irrigation. Spatial information from satellites have limitations for daily ET estimation due to frequency of field scale acquisition over the area of interest, presence of clouds at overpass time, and latency in imagery delivery for real-time irrigation scheduling. The advent of advanced small Unmanned Aerial Systems (sUAS) remote sensing technology, with lightweight sensors, have potential when used in concert with satellite data to overcome some of these limitations since sUAS can provide high spatial and temporal resolution information unobscured by clouds at any day and time. In this paper, different extrapolation methods from the literature were assessed for estimating daily ET from one-time-of-day sUAS ET estimate for several vineyard sites in California. sUAS image acquisition between 1030 and 1330 local time using the solar radiation extrapolation method resulted in less than 0.5 mm/day error with daily ET estimates. This indicates sUAS remote sensing technology has the potential to provide reliable daily ET supplementing satellite ET products for water management applications in vineyards

Technical Abstract: Daily evapotranspiration (ETd) plays a key role in irrigation water management and is particularly important in drought-stricken areas such as California and for high-value crops. Remote sensing allows for cost-effective estimation of spatial evapotranspiration, and the advent of small Unmanned Aerial Systems (sUAS) technology has made it possible to estimate instantaneous high-resolution ET at plant, row, and subfield scales. sUAS ET estimates use “instantaneous” remote sensing measurements typically with half-hourly/hourly forcing data yielding hourly fluxes in W/m2 that are then translated to daily scale (mm/day) under two assumptions (a) that evaporation (E) and transpiration (T) rates are constant over the daytime period and (b) nighttime E and T contributions are negligible. While assumption (a) may be reasonable for unstressed full cover crops (no exposed soil), for partially vegetated cover conditions, diurnal variation in soil and crop temperatures and interactions between soil and vegetation elements in agricultural environments such as vineyards and orchards E and T rates may significantly vary over the course of the day. In this study, five existing interpolation schemes that compute daily ET (nighttime ET contrition ignored) from a single remotely-sensed ET estimate from the sUAS and measurement from eddy covariance (EC) flux towers were evaluated under different weather, vine variety, and trellis design. Each extrapolation technique (evaporative fraction (EF), solar radiation (Rs), net radiation to solar radiation (Rn/Rs) ratio, Gaussian (GA), and Sine) takes advantage of clear skies and quasi-sinusoidal diurnal variation of hourly ET and other meteorological parameters. The sUAS ET estimate and EC measurement of ET were collected during multiple years and times over different vineyard sites in California as part of the USDA-Agricultural Research Service Grape Remote Sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX). Optical and thermal sUAS imagery data at 10 cm and 60 cm, respectively, were collected by the Utah State University AggieAir sUAS Program and used in the Two-Source Surface Energy Balance (TSEB) model to estimate sUAS ET at the overpass time. The hourly ET from the EC measurements were also used to evaluate the extrapolation techniques. Overall, the analysis using EC measurements indicates that Rs, EF, and GA methods presented the smallest goodness-of-fit statistics and MAPE for a window of time between 10:30 and 13:30 with the Rs ranking top among the extrapolation approaches. Similar results were found using TSEB and sUAS. The same time window for EC provides the greatest agreement between daily EC ET vs. extrapolated TSEB daily ET, with the Rs method ranking again top. The expected accuracy of the upscaled TSEB daily ET estimates is below 0.5 mm/day, (EC extrapolation accuracy was found 0.34 mm/day), making the results from TSEB at daily scale reliable and suitable for day to day water management applications.