Ground-based infrared thermometry reveals seasonal evapotranspiration patterns in semiarid rangelands

Authors: HWANG, KYOTAEK - Syracuse University; CHANDLER, DAVID - Syracuse University; Flerchinger, Gerald

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2023
Publication Date: 3/2/2023
Citation: Hwang, K., Chandler, D., Flerchinger, G.N. 2023. Ground-based infrared thermometry reveals seasonal evapotranspiration patterns in semiarid rangelands. Hydrological Processes. 37(3). Article e14827. https://doi.org/10.1002/hyp.14827.
DOI: https://doi.org/10.1002/hyp.14827

Interpretive Summary: Quantifying evapotranspiration (ET) is crucial for understanding the hydrological water balance assessing plant water stress. Directly measuring ET is difficult, time-consuming, expensive. Methods have been proposed to approximate ET from remotely-sensed surface temperature using infrared thermometry. Two such methods were tested and compared with measurements of ET. One method that assumes the plants are not water stressed was shown to seriously overpredict ET late in the growing season in semi-arid areas where precipitation is limited. Estimates from a second method that computes ET and surface heat transfer by difference between measured surface and air temperature showed good agreement with measured ET. The second proposed method offers a potential means to monitor land surface ET and vegetation health using surface temperatures measured using infrared thermometers.

Technical Abstract: Detailed assessment of small-scale heterogeneity in the local water balance is essential to the accurate estimation of evapotranspiration (ET) in semiarid climates. Meteorological approaches are often impractical to implement in spatially mixed vegetation cover, with seasonally variable leaf canopy features. Ground-based infrared thermometry provides spatially and temporally continuous resolution of surface skin temperature that can be directly related to the land surface energy balance. Here we present initial results to demonstrate the applicability of this approach in a semiarid rangeland ecosystem based on other studies demonstrating reasonable prediction over well-irrigated plant patches. Estimates of sensible (H) and latent heat flux (LE) were compared to eddy covariance measurements to disaggregate the expression of seasonal phenology of sagebrush species across wetness and elevation. We compare two different approaches to compute surface energy fluxes from thermal infrared (TIR) sensing: the surface energy balance (SEB) and the Bowen ratio (BR). SEB directly estimates sensible heat flux based on the physical derivation of heat transport of the ambient air parcels at the stand scale. BR empirically establishes the vertical heat and moisture profiles using the Bowen ratio. Intensive field data collection for two-week periods near peak foliage and near the end of the growing season were made to compare phenological stages of four sagebrush communities over a growing season. Despite relatively high error ranges in LE (bias = 5.8-88.8 for SEB and 156.4-216.6 W m-2 for BR; root mean squared error = 82.7-146.2 for SEB and 192.3-235.7 W m-2 for BR), estimations showed good agreement with ground-based eddy covariance observations for most cases. Predictability declined substantially late in the growing season as the fraction of senescent foliage increased in dry conditions. The assumption of a fully saturated leaf surface for BR may be regularly violated in semiarid conditions when plants are water stressed, as demonstrated by the consistent underestimation of H and overestimation of LE. The proposed methods suggest the potential for reliable monitoring of the land surface energy fluxes and plant health at very fine spatial scale. The ability to partition heat fluxes from various plant communities over a range of moisture availability will provide valuable information associated with the consumptive water use and phenological processes in the semiarid West.