Assessing deficiencies in remotely sensed actual evapotranspiration (AET): introducing AET signatures

Abstract. Hydrological signatures are statistical metrics useful to quantify and infer behaviours of hydrological processes, but there has been limited use of signatures for non-streamflow variables, such as actual evapotranspiration (AET). AET signatures can assist in tasks such as evaluating remotely sensed products, diagnosing deficiencies in hydrological models, and improving understanding of hydrological processes, such as the role of AET in driving hydrological drought. This study focuses on first of these three applications. To achieve this, the study proposes eight AET signatures defined at temporal scales from daily to annual. Two remotely sensed AET (AETRS) products are assessed against flux tower AET (AETFluxtower) at seventeen FluxNET sites in Australia. The two AETRS products are Moderate Resolution Imaging Spectroradiometer (MODIS,16A2GFv06.1), and CSIRO MODIS Reflectance-based Scaling Evapotranspiration (CMRSET). Annually, median AETRS closely matches AETFluxtower, except in less-arid regions. However, signatures reveal AETRS largely underestimates the variability of flux tower data at both annual and monthly scales. Other monthly indices are better matched, such as indices of water stress and AET asynchronicity with potential evapotranspiration. However, some metrics are better matched in one product than the other, such as the strength and timing of seasonal fluctuations, with MODIS exhibiting a phase shift. Overall, the signatures reveal that regionally-developed CMRSET outperformed globally-developed MOD16A2GFv061. This study, the first to systematically define AET signatures, offers a way of assessing various aspects of AET dynamics across temporal scales. Furthermore, the case study highlights specific deficiencies in AETRS and may assist in selecting appropriate AETRS, including for modelling studies.