Estimating evaporation with thermal UAV data and two-source energy balance models

H Hoffmann,P Zarco-Tejada,H Nieto,T Friborg,R Jensen,R Guzinski

doi:10.5194/hess-20-697-2016

Abstract

Abstract. Estimating evaporation is important when managing water resources and cultivating crops. Evaporation can be estimated using land surface heat flux models and remotely sensed land surface temperatures (LST), which have recently become obtainable in very high resolution using lightweight thermal cameras and Unmanned Aerial Vehicles (UAVs). In this study a thermal camera was mounted on a UAV and applied into the field of heat fluxes and hydrology by concatenating thermal images into mosaics of LST and using these as input for the two-source energy balance (TSEB) modelling scheme. Thermal images are obtained with a fixed-wing UAV overflying a barley field in western Denmark during the growing season of 2014 and a spatial resolution of 0.20 m is obtained in final LST mosaics. Two models are used: the original TSEB model (TSEB-PT) and a dual-temperature-difference (DTD) model. In contrast to the TSEB-PT model, the DTD model accounts for the bias that is likely present in remotely sensed LST. TSEB-PT and DTD have already been well tested, however only during sunny weather conditions and with satellite images serving as thermal input. The aim of this study is to assess whether a lightweight thermal camera mounted on a UAV is able to provide data of sufficient quality to constitute as model input and thus attain accurate and high spatial and temporal resolution surface energy heat fluxes, with special focus on latent heat flux (evaporation). Furthermore, this study evaluates the performance of the TSEB scheme during cloudy and overcast weather conditions, which is feasible due to the low data retrieval altitude (due to low UAV flying altitude) compared to satellite thermal data that are only available during clear-sky conditions. TSEB-PT and DTD fluxes are compared and validated against eddy covariance measurements and the comparison shows that both TSEB-PT and DTD simulations are in good agreement with eddy covariance measurements, with DTD obtaining the best results. The DTD model provides results comparable to studies estimating evaporation with similar experimental setups, but with LST retrieved from satellites instead of a UAV. Further, systematic irrigation patterns on the barley field provide confidence in the veracity of the spatially distributed evaporation revealed by model output maps. Lastly, this study outlines and discusses the thermal UAV image processing that results in mosaics suited for model input. This study shows that the UAV platform and the lightweight thermal camera provide high spatial and temporal resolution data valid for model input and for other potential applications requiring high-resolution and consistent LST.

Highlights

Evaporation serves as a key component in both hydrological and land–surface energy processes
Land surface temperatures (LST) were obtained with a lightweight thermal camera mounted on a Unmanned Aerial Vehicles (UAVs) with the ability to cover a 400 × 400 m barley field in a single flight
Thermal images were successfully concatenated into highresolution land surface temperatures (LST) mosaics (0.2 m) that served as key boundary conditions to the two-source energy balance models: TSEBPT and DTD

Summary

Introduction

Evaporation (latent heat flux) serves as a key component in both hydrological and land–surface energy processes. It is often estimated indirectly because spatially distributed, physical measurements of evaporated water are cumbersome. The TSEB modelling scheme partitions the remotely sensed LST into two layers; a canopy temperature and a soil temperature, using the Priestley–Taylor approximation (Norman et al, 2000) This enables a partition of heat flux estimations into its components from canopy and soil respectively. This approach is hereafter referred to as TSEB-PT in order to differentiate it from other TSEB approaches, such as TSEB-LUE (Houborg et al, 2012), based on the Light Use Efficiency concept, or TSEB-2ART, which utilises dualangle LST observations for direct retrieval of soil and canopy temperatures (Guzinski et al, 2015)

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Methods

Conclusion