Assessing Daily Evapotranspiration Methodologies from One-Time-of-Day sUAS and EC Information in the GRAPEX Project

Monitoring Energy Balance, Turbulent Flux Partitioning, Evapotranspiration and Biophysical Parameters of Nopalea cochenillifera (Cactaceae) in the Brazilian Semi-Arid Environment

The in-situ quantification of turbulent flux and evapotranspiration (ET) is necessary to monitor crop performance in stressful environments. Although cacti can withstand stressful conditions, plant responses and plant-environment interactions remain unclear. Hence, the objective of our study was to investigate the interannual and seasonal behaviour of components of the surface energy balance, environmental conditions, morphophysiological parameters, biomass yield and water relations in a crop of Nopalea cochenillifera in the semi-arid region of Brazil. The data were collected from a micrometeorological tower between 2015 and 2017. The results demonstrate that net radiation was significantly higher during the wet season. Latent heat flux was not significant between the wet season and dry season. During the dry-wet transition season in particular, sensible heat flux was higher than during the other seasons. We observed a large decline in soil heat flux during the wet season. There was no difference in ET during the wet or dry seasons; however, there was a 40% reduction during the dry-wet transition. The wet seasons and wet-dry transition showed the lowest Evaporative Stress Index. The plants showed high cladode water content and biomass during the evaluation period. In conclusion, these findings indicate high rates of growth, high biomass and a high cladode water content and explain the response of the cactus regarding energy partitioning and ET.

Comparison of Daily Evapotranspiration and Reference Evapotranspiration Fraction Values Calculated using METRIC Model and Google Earth Engine FLux

Accurate evapotranspiration (ET) calculation is crucial in water resources management. In recent years, various remote sensing techniques based models have been developed to determine ET. Among these models, Mapping EvapoTranspiration at High Resolution with Internalized Calibration (METRIC) is one of the most widely used. In the METRIC model, anchor pixels must be selected to represent extreme conditions where ET is at potential level and ET is zero in the internal calibration process. These choices should be made by experienced users in both remote sensing and ET. However, this limits the use of the METRIC model. In order to fill this gap, the Earth Engine Evapotranspiration Flux (EEFlux) application was developed in the Google Earth Engine platform. With this application, the METRIC model is calibrated automatically. The aim of this study was to compare the daily ET (ETd) and Reference ET fraction (ETrF) values obtained from the METRIC and EEFlux model. The study was carried out in the agricultural fields of barley, sunflower, wheat, corn, sugar beet, potato and onion cultivation in Uzunyazı, Çayırözü and Yeşilören villages in Merzifon district of Amasya province. Landsat 8 satellite images of three different dates were used in this research. According to the results obtained from this study, although the ETd values calculated with METRIC and EEFlux were compatible (R2=0.87), the EEFlux-ETd values were generally lower than the METRIC- ETd values (RMSE=2.5 mm day-1 and MAE=2.38 mm day-1). Calculated ETrF values were calculated with similar agreement to ETd values (R2=0.88, RMSE=0.11 and MAE=0.09). Accordingly, although there are certain differences between ETd and ETrF values calculated with EEFlux and METRIC, ETd values can be determined very quickly with EEFlux, without the need for local climate data and an experienced user.

Effective Method of Estimating the Daily Evapotranspiration of Greenhouse Grapes in the Cold Area of Northeast China

Evapotranspiration (ET) is an important basis and key link for guiding irrigation. One of the key problems to be solved is how to predict the dynamic change in the daily ET and estimate the total amount of ET in greenhouse through limited instantaneous data. In this paper, it is estimated that the daily scale of evapotranspiration by using four methods, including the evaporative fraction method (EF method), the reference evaporative fraction method (EF' method), the sine method, and the canopy resistance method (r _c method), is based on the measured ET data of grapes in a solar greenhouse in Northeast China. The relative root-mean-square pair error (RRMSE) and the efficiency coefficient (ε) are also used to study their applicability in terms of leaf area index, radiation degree, and scale-up time point. In the results, under the condition of different LAI, the simulation accuracies of ET scaled by the four methods ranked as follows (from highest to lowest): the reference evaporative fraction method, the evaporative fraction method, the sine method, and the canopy resistance method. The average RRMSE and ε of the evaporative fraction method with the best simulation accuracy were 7.19-16.46% and 0.61-0.75, respectively. Under different radiation conditions, the simulation accuracies of the four methods ranked as follows (from highest to lowest): the evaporative fraction method, the reference evaporative fraction method, the sine method, and the canopy resistance method. Under different radiation conditions, the RRSME of the four methods ranged from 11.55 to 46.62%, and the maximum of ε was 0.75. The evaporative fraction and reference evaporative fraction methods had the highest simulation accuracy, whereas the reference evaporative fraction method required fewer parameters. We concluded that the reference evaporative fraction method was the best for estimating the daily ET of greenhouse grapes in the cold area of Northeast China.

Using Remote Sensing to Estimate Scales of Spatial Heterogeneity to Analyze Evapotranspiration Modeling in a Natural Ecosystem

Understanding the spatial variability in highly heterogeneous natural environments such as savannas and river corridors is an important issue in characterizing and modeling energy fluxes, particularly for evapotranspiration (ET) estimates. Currently, remote-sensing-based surface energy balance (SEB) models are applied widely and routinely in agricultural settings to obtain ET information on an operational basis for use in water resources management. However, the application of these models in natural environments is challenging due to spatial heterogeneity in vegetation cover and complexity in the number of vegetation species existing within a biome. In this research effort, small unmanned aerial systems (sUAS) data were used to study the influence of land surface spatial heterogeneity on the modeling of ET using the Two-Source Energy Balance (TSEB) model. The study area is the San Rafael River corridor in Utah, which is a part of the Upper Colorado River Basin that is characterized by arid conditions and variations in soil moisture status and the type and height of vegetation. First, a spatial variability analysis was performed using a discrete wavelet transform (DWT) to identify a representative spatial resolution/model grid size for adequately solving energy balance components to derive ET. The results indicated a maximum wavelet energy between 6.4 m and 12.8 m for the river corridor area, while the non-river corridor area, which is characterized by different surface types and random vegetation, does not show a peak value. Next, to evaluate the effect of spatial resolution on latent heat flux (LE) estimation using the TSEB model, spatial scales of 6 m and 15 m instead of 6.4 m and 12.8 m, respectively, were used to simplify the derivation of model inputs. The results indicated small differences in the LE values between 6 m and 15 m resolutions, with a slight decrease in detail at 15 m due to losses in spatial variability. Lastly, the instantaneous (hourly) LE was extrapolated/upscaled to daily ET values using the incoming solar radiation (Rs) method. The results indicated that willow and cottonwood have the highest ET rates, followed by grass/shrubs and treated tamarisk. Although most of the treated tamarisk vegetation is in dead/dry condition, the green vegetation growing underneath resulted in a magnitude value of ET.