Evapotranspiration Estimation From Unmanned Aerial Vehicles: Multispectral/Thermal Imagery For High Spatial Resolution Metric-Et Maps And Relative Humidity-Air Temperature Spatial Profile In Alfalfa And Pasture In The Ssj Delta-California

Abstract: In this Appendix, we present mapped estimates of crop evapotranspiration (ET) employing high resolution multispectral images from a drone, identified henceforth as an Unmanned Aerial Vehicle (UAV), based on the METRICtm model (Mapping Evapotranspiration at High Resolution using Internalized Calibration) for estimating consumptive use in alfalfa and pasture in the Sacramento-San Joaquin Delta. METRIC is commonly applied to estimate actual evapotranspiration (ETa) using the alfalfa-based reference evapotranspiration (ETr). The original METRIC approach employs Landsat images and produces evapotranspiration maps at a 30-meter resolution. This study presents maps at a 1-meter resolution from UAVs to evaluate in-field evapotranspiration differences otherwise not captured by the coarser resolution Landsat images.
Imagery from the UAV was acquired using a multispectral (multiple spectral bands) and thermal camera. Some flights included sensors to measure relative humidity and air temperatures. The acquired data were then interpolated and mapped, providing additional information for comparison purposes with METRIC maps.

The major objectives of the study using UAV include:

• Building reliable high spatial resolution ET maps (1-m pixel) to be compared with satellite Landsat 8 images (30-m pixel) processed by METRIC and evaluate the spatial characterization and variability of both sources at field scale.
• Sensing air temperature and relative humidity from the aerial platform to characterize a horizontal and vertical profile as well as to evaluate the feasibility of mapping ET through alternative methods such as surface renewal.

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Authors: Jorge Andrés Morandé, Ricardo Trezza, Andreas Anderson, Kyaw Tha Paw, Yufang Jin, Yangquan Chen, Josh Viers And Josué Medellín-Azuara

Associations: University of California, Davis