A comparative analysis of OpenET for evaluating evapotranspiration in California almond orchards

Authors: Knipper, Kyle; Anderson, Martha; BAMBACH, NICOLAS - University Of California, Davis; MELTON, FOREST - California State University; ELLIS, ZAC - Olam; YANG, YUN - Mississippi State University; VOLK, JOHN - Desert Research Institute; McElrone, Andrew; Kustas, William - Bill; Roby, Matthew; CARRARA, WILLIAM - California State University; CASTRO, SEBASTIAN - University Of California, Davis; ALLEN, RICK - University Of Idaho; FISHER, JOSHUA - Chapman University; RUHOFF, ANDERSON - Universidade Federal Do Rio Grande Do Norte

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2024
Publication Date: 7/3/2024
Citation: Knipper, K.R., Anderson, M.C., Bambach, N., Melton, F., Ellis, Z., Yang, Y., Volk, J., Mcelrone, A.J., Kustas, W.P., Roby, M.C., Carrara, W., Castro, S., Allen, R., Fisher, J., Ruhoff, A. 2024. A comparative analysis of OpenET for evaluating evapotranspiration in California almond orchards. Agricultural and Forest Meteorology. 355. https://doi.org/10.1016/j.agrformet.2024.110146.
DOI: https://doi.org/10.1016/j.agrformet.2024.110146

Interpretive Summary: In California's almond orchards, managing water use is becoming increasingly crucial due to ongoing droughts, the effects of climate change, and new water laws. The T-REX project is using advanced technology to measure water loss from plants (evapotranspiration) and improve water use strategies in these orchards. The project uses a tool called OpenET, which gathers detailed satellite data to estimate water use across the Central Valley's almond farms. This study tested OpenET in six diverse almond orchards and compared its data with actual water use records from 148 orchards. The findings reveal that OpenET provides useful estimates of water use, although it sometimes struggles to keep up with rapid changes, like those during the almond harvest. The study also found that the tool's accuracy might be affected by local conditions, such as neighboring dry areas that can lead to differences between actual and estimated water use. Despite these challenges, OpenET closely matched the actual water used in the orchards, missing the mark by only 13% on average. This study emphasizes that while tools like OpenET are valuable for managing water in farming, understanding the specific conditions of each orchard is crucial for their effective use.

Technical Abstract: The almond industry in California faces water management challenges that are being exacerbated by droughts, climate change, and groundwater sustainability legislation. The Tree-crop Remote sensing of Evapotranspiration eXperiment (T-REX) aims to explore opportunities to improve precision irrigation management for woody perennial cropping systems. Almond orchards in the California Central Valley were equipped with eddy covariance flux measurements to evaluate satellite remote sensing-based evapotranspiration (RSET) models. OpenET provides high-resolution (30-m spatial and daily temporal) RSET data, synthesizing decades of research for practical water management purposes. This study provides an evaluation of OpenET performance at six almond sites covering a significant range in soils, age, and variety. It also compares OpenET ensemble ET data with applied irrigation and precipitation records over an additional 148 almond orchards located in the Central Valley of California. Results show OpenET models, including the ensemble ET value, produced reasonable and actionable ET estimates, with overall R2 and mean absolute error values of 0.73- and 0.95-mm d-1 at the daily time step, respectively. However, given the temporal sampling of Landsat (8-day revisit) and the interpolation methods used, the assessed ET models had difficulty in capturing short-term variability in almond ET; for example, the rapid decline in measured ET observed as a response to lack of irrigation preceding and during almond harvest. The study also drew attention to the spatial complexity in scenarios where irrigated orchards are surrounded by hot/dry areas, causing discrepancies between measured and modeled ET values. In comparison with irrigation records, OpenET ensemble ET was capable of quantifying water input (applied irrigation + precipitation) in almond orchards to within 13% when evaluating monthly data. Initial results presented here reinforce the idea that RSET models, such as in OpenET, are powerful tools, yet their application requires nuanced understanding and careful consideration of local conditions.