Key Points:• ECOSTRESS is a state-of-the-art combination of thermal bands, spatial and temporal resolutions, and measurement accuracy and precision • Data from 82 eddy covariance sites were coalesced concurrently with the first year of ECOSTRESS for Stage 1 validation • Clear-sky ET from ECOSTRESS compared well against a wide range of eddy Abstract The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was launched to the International Space Station on 29 June 2018 by the National Aeronautics and Space Administration (NASA). The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as Level-3 (L3) latent heat flux (LE) data products. These data are generated from the Level-2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear-sky ET product (L3_ET_PT-JPL, Version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear-sky instantaneous/time of overpass: r 2 = 0.88; overall bias = 8%; normalized root-mean-square error, RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are overrepresented. The 70-m-high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1-km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data.

No abstract

No abstract

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights

1 2 The recent paper by Morillas et al. [Morillas, L. et al. Using radiometric surface temperature for 3 surface energy flux estimation in Mediterranean drylands from a two-source perspective, Remote 4Sens. Environ. 136, 234-246, 2013] evaluates the two-source model (TSM) of Norman et al. 5(1995) with revisions by Kustas and Norman (1999) over a semiarid tussock grassland site in 6 southeastern Spain. The TSM -in its current incarnation, the two-source energy balance model 7 (TSEB) -was applied to this landscape using ground-based infrared radiometer sensors to 8 estimate both the composite surface radiometric temperature and component soil and canopy 9 temperatures. Morillas et al. (2013) found the TSEB model substantially underestimated the 10 sensible H (and overestimated the latent heat LE) fluxes. Using the same data set from Morillas 11 et al. (2013), we were able to confirm their results. We also found energy transport and 12 exchange behavior derived from primarily the observations themselves to differ significantly 13 from a number of prior studies using land surface temperature for estimating heat fluxes with 14 one-source modeling approaches in semi-arid landscapes. However, revisions to key vegetation 15 inputs to TSEB and the soil resistance formulation resulted in a significant reduction in the bias 16 and root mean square error (RMSE) between model output of H and LE and the measurements 17 compared to the prior results from Morillas et al (2013). These included more representative 18 ground-based vegetation greenness and local leaf area index values as well as modifications to 19 the coefficients of the soil resistance formulation to account for the very rough (rocky) soil 20 surface conditions with a clumped canopy. This indicates that both limitations in remote 21 estimates of biophysical indicators of the canopy at the site and the lack of adjustment in soil 22 resistance formulation to account for site specific characteristics, contributed to the earlier 23 findings of Morillas et al. (2013). This suggests further studies need to be conducted to reduce 24 the uncertainties in the vegetation and land surface temperature input data in order to more 25 accurately assess the effects of the transport exchange processes of this Mediterranean landscape 26 on TSEB formulations. 27 28 29 Reporting errors in the modeled latent heat flux (LE) of approximately 90% mostly due to 30 a significant underestimate of the sensible heat flux (H) (70 Wm -2 ), the recent study by Morillas 31 et al. (2013) suggests that the two-source energy balance (TSEB) model, which has been 32 successfully applied to a wide variety of landscapes and climates (Kustas and Anderson, 2009), 33 could not produce reliable estimates of LE in a semiarid Mediterranean tussock grassland site in 34 southeast Spain (Balsa Blanca). The Balsa Blanca site is representative of arid regions which 35 cover ~25 % of the Earth's land surface (Fensholt et al., 2012) and are characterized by having 36 low LE fluxes resulting in H being the domi...

[1] An adaptation of a simple model for evapotranspiration (E) estimations in drylands based on remotely sensed leaf area index and the Penman-Monteith equation (PML model) ) is presented. Three methods for improving the consideration of soil evaporation influence in total evapotranspiration estimates for these ecosystems are proposed. The original PML model considered evaporation as a constant fraction (f) of soil equilibrium evaporation. We propose an adaptation that considers f as a variable primarily related to soil water availability. In order to estimate daily f values, the first proposed method (f SWC ) uses rescaled soil water content measurements, the second (f Zhang ) uses the ratio of 16 days antecedent precipitation and soil equilibrium evaporation, and the third (f drying ), includes a soil drying simulation factor for periods after a rainfall event. E estimates were validated using E measurements from eddy covariance systems located in two functionally different sparsely vegetated drylands sites: a littoral Mediterranean semiarid steppe and a dry-subhumid Mediterranean montane site. The method providing the best results in both areas was f drying (mean absolute error of 0.17 mm day À1 ) which was capable of reproducing the pulse-behavior characteristic of soil evaporation in drylands strongly linked to water availability. This proposed model adaptation, f drying , improved the PML model performance in sparsely vegetated drylands where a more accurate consideration of soil evaporation is necessary.

Climate‐driven tree mortality has increased globally in response to warmer temperature and more severe drought. To examine how tree mortality in semiarid biomes impacts surface water balance, we experimentally manipulated a piñon‐juniper (PJ) woodland by girdling all adult piñon trees in a 4 ha area, decreasing piñon basal area by ~65%. Over 3.5 years (2009–2013), we compared water flux measurements from this girdled site with those from a nearby intact PJ woodland. Before and after girdling, the ratio of evapotranspiration (ET) to incoming precipitation was similar between the two sites. Girdling altered the partitioning of ET such that the contribution of canopy transpiration to ET decreased 9–14% over the study period, relative to the intact control, while noncanopy ET increased. We attributed the elevated noncanopy ET in the girdled site each year to winter increases in sublimation and summer increases in both soil evaporation and below‐canopy transpiration. Although we expected that mortality of a canopy dominant would increase the availability of water and other resources to surviving vegetation, we observed a decrease in both soil volumetric water content and sap flow rates in the remaining trees at the girdled site, relative to the control. This postgirdling decrease in the performance of the remaining trees occurred during the severe 2011–2012 drought, suggesting that piñon mortality may trigger feedback mechanisms that leave PJ woodlands drier relative to undisturbed sites and potentially more vulnerable to drought.

No abstract