Daytime Evapotranspiration Research Articles

A Method for Estimating Daytime Average Evapotranspiration From Diurnal Land Surface Temperature Measurements

Evapotranspiration (ET) is an essential parameter in the water cycle and surface energy balance. Accurate estimation of daytime or daily ET is of great significance for many activities in human economic and social development. This study aims to propose a novel method for estimating daytime average ET by using temporal measurements of land surface temperature (LST) and net surface shortwave radiation (NSSR), following a previous developed elliptical relationship between diurnal cycles of LST and NSSR under cloud-free days. The method was primarily developed from the simulated data of a physics-based Atmosphere–Land Exchange (ALEX) model under different underlying surfaces and atmospheric conditions. Based on the simulated data, the proposed method showed considerable accuracy with the overall coefficient of determination ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of 0.958 and the root mean square error (RMSE) of 25.3 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . In addition, ground ET measurements at four Ameriflux sites (US-ARM, US-SRM, US-Whs, and US-Wkg) during the 2018 growing season were collected to assess the estimated daytime average ET. Results show an overall RMSE of 64.7 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> for the estimated ET at the four sites, and the US-Whs site reveals a best accuracy ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> =0.825, RMSE=44.4 Wm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ). These results indicated a potential for generating daytime ET with geostationary satellite observations at regional scales in future development.

A physical full-factorial scheme for gap-filling of eddy covariance measurements of daytime evapotranspiration

Surface evapotranspiration (ET) is a vital process that connects the water cycle, energy budget, and carbon cycle between the land and atmosphere. ET measurements using the eddy covariance (EC) technique typically encounter large gaps. In this study, a physics-based full-factorial scheme for filling gaps in ET from EC observations is proposed based on the decoupling model of the Penman-Monteith equation, which mechanically interconnects the full range of ET influential factors from the atmosphere, vegetation, and soil. The new method was tested and intercompared with four typical gap-filling methods, i.e., marginal distribution sampling, mean diurnal variation, constant reference evaporative fraction, and constant evaporative fraction, using the data collected at 136 sites from AmeriFlux, FLUXNET, and Tibetan Plateau Data Center over the crop, grass, forest, and other remaining land-cover types. The validation results showed that (1) the full-factorial scheme performed well for filling the artificially randomly generated hourly and daily gaps of the EC-based ET measurements, with a root mean square error (RMSE) of 28.8–96.1 W/m2 for the hourly gaps and an RMSE of 19.9–38.7 W/m2 for the daily gaps, (2) for filling the hourly gaps, the accuracy decreased with the increase in the gap length; for filling the daily gaps, the accuracy decreased with the increase in the gap length at first and then remained almost unchanged, (3) the full-factorial scheme outperformed the four typical methods for filling both the hourly and daily gaps, and (4) all five gap-filling methods presented the best performance at the grass sites, followed by the crop, and the worst performance at the forest sites. In conclusion, the proposed full-factorial scheme can produce reasonable gap-filled hourly and daily ET and is superior to the existing typical gap-filling methods. Therefore, it could be a good candidate for filling the ET gaps in EC measurements.

Dynamics of Nocturnal Evapotranspiration and Its Biophysical Controls over a Desert Shrubland of Northwest China

Knowledge about the dynamics and biophysical controlling mechanism of nocturnal evapotranspiration (ETN) in desert-dwelling shrub ecosystem is still lacking. Using the eddy covariance measurements of latent heat flux in a dried shrubland in northwest China, we examined the dynamics of ETN and its biophysical controls at multiple timescales during growing-seasons from 2012 to 2014. The ETN was larger in the mid-growing season (usually in mid-summer) than in spring and autumn. The maximum daily ETN was 0.21, 0.17, and 0.14 mm night−1 in years 2012–2014, respectively. At the diel scale, ETN decreased from 21:00 to 5:00, then began to increase. ETN were mainly controlled by soil volumetric water content at 30 cm depth (VWC30), by vapor pressure deficit (VPD) and normalized difference vegetation index (NDVI) at leaf expanding and expanded stage, and by air temperature (Ta) and wind speed (Ws) at the leaf coloring stage. At the seasonal scale, variations of ETN were mainly driven by Ta, VPD, and VWC10. Averaged annual ETN was 4% of daytime ET. The summer drought in 2013 and the spring drought in 2014 caused the decline of daily evapotranspiration (ET). The present results demonstrated that ETN is a significant part of the water cycle and needs to be seriously considered in ET and related studies. The findings here can help with the sustainable management of water in desert ecosystems undergoing climate change.

Comparison of Nighttime With Daytime Evapotranspiration Responses to Environmental Controls Across Temporal Scales Along a Climate Gradient

AbstractUnderstanding daytime (ETD) and nighttime (ETN) evapotranspiration is critical for accurately evaluating terrestrial water and carbon cycles. However, unlike ETD, the factors influencing ETN remain poorly understood. Here, long‐term ETD and ETN data from five FLUXNET sites along a climate gradient in Northern Australia were analyzed to compare their responses to environmental drivers at different temporal scales. We found that (a) across the sites, mean annual ETN/ETD (/) ranged between 5.1% and 11.7%, which was mainly determined by variations. Particularly, vegetation and meteorological variables mostly controlled , while was largely related to air temperature and net radiation (Rn) due to lower nighttime atmospheric water demands; (b) At individual sites, ETD and ETN exhibited higher correlations with meteorological and vegetation variables at monthly timescales than at annual timescales. Monthly ETD and ETN were also strongly coupled, especially under drier climatic conditions. At daily timescales, leaf area index and soil water content (SWC) controlled ETD with SWC being more important at drier sites; whereas, SWC was the dominant factor controlling ETN. At half‐hourly timescales, the boosted regression tree method quantitively showed that ETD and ETN were controlled by Rn and SWC, respectively. Overall, the results showed that ETN was less responsive to environmental variables, illustrating that ETD and ETN responded differently to diverse climate regimes and ecosystems at varying temporal scales. These findings provide a critical evaluation for contrasting ETD and ETN interactions in constantly changing environments, which has important implications for ecosystem water balance and land surface processes modeling.

Simulation analysis of local land atmosphere coupling in rainy season over a typical underlying surface in the Tibetan Plateau

Abstract. The local land–atmosphere coupling (LoCo) investigates the interactions between soil conditions, surface fluxes, planetary boundary layer (PBL) growth, and the formations of convective clouds and precipitation. Studying LoCo over the Tibetan Plateau (TP) is of great significance for understanding the TP's role in the Asian water tower. A series of real-case simulations, using the Weather Research and Forecasting (WRF) model with different combinations of land surface model (LSM) schemes and PBL schemes, has been carried out to investigate the LoCo characteristics over a typical underlying surface in the central TP in the rainy season. The LoCo characteristics in the study area are analyzed by applying a mixing diagram to the simulation results. The analysis indicates that the WRF simulations, using the Noah with BouLac, Mellor-Yamada Nakanishi and Niino Level-2.5 PBL (MYNN), and Yonsei University (YSU) produce closer results to the observation in terms of curves of Cp⋅θ and Lv⋅q, surface fluxes (Hsfc and LEsfc), entrainment fluxes (Hent, and LEent) at site BJ of Nagqu Station (BJ/Nagqu) than those using the Community Land Model (CLM) with BouLac, MYNN, and YSU. The frequency distributions of Hsfc, LEsfc, Hent, and LEent in the study area confirm this result. The spatial distributions of simulated Hsfc, LEsfc, Hent, and LEent, using WRF with Noah and BouLac, suggest that the spatial distributions of Hsfc and LEsfc in the study area are consistent with that of soil moisture, but the spatial distributions of Hent and LEent are quite different from that of soil moisture. A close examination of the relationship between entrainment fluxes and cloud water content (QCloud) reveals that the grids with small Hent and large LEent tend to have high QCloud and Hsfc, suggesting that high Hsfc is conducive to convective cloud formation, which leads to small Hent and large LEent. A sensitivity analysis of LoCo to the soil moisture at site BJ/Nagqu indicates that, on a sunny day, an increase in soil moisture leads to an increase in LEsfc but decreases in Hsfc, Hent, and LEent. The sensitivity of the relationship between simulated maximum daytime PBL height (PBLH) and mean daytime evapotranspiration (ET) in the study area to soil moisture indicates the rate at which the maximum daytime PBLH decreases with the mean ET increase as the initial soil moisture goes up. The analysis of simulated Hsfc, LEsfc, Hent, and LEent under different soil moisture conditions reveals that the frequency of Hent ranging from 80 to 240 W m−2 and the frequency of LEent ranging from −240 to −90 W m−2 both increase as the initial soil moisture increases. Coupled with the changes in QCloud, the changes in Hent and LEent as the initial soil moisture increases indicate that the rise in soil moisture leads to an increase in the cloud amount but a decrease in QCloud.

Comparison of energy fluxes between an undisturbed bog and an adjacent abandoned peatland pasture

This study compared the energy budget components of latent heat flux (LE), sensible heat flux (H), net radiation (Rn) and ground heat flux (G) between an undisturbed bog and an adjacent abandoned peatland pasture within the same peatland complex in western Newfoundland, Canada. The analysis was based on the period of April 2014 to August 2015 when measurements were concurrent at both sites, thus encompassing two growing seasons. The overall objective was to identify the impact of agricultural management and abandonment on peatland energy flux and partitioning. LE and H showed strong seasonal patterns for both sites. LE, which was higher at the pasture, peaked near the middle of the growing season. H at the bog was smaller than that at the pasture during the early growing season, but became larger during the middle and late growing seasons, with the maximum H occurring during the early growing season. G was the smallest flux in the energy budget at both sites, and it showed no obvious temporal pattern at either site. The variation in daytime average evapotranspiration (ET) was related to net radiation and vapor pressure deficit (VPD) at both sites. However, it was more strongly related to Rn. ET at the pasture site exhibited greater sensitivity to changes in Rn and VPD. Daytime ET increased and decreased with water table depth (WTD) when WTD was above and below a threshold of -0.24 m at the bog and -0.46 m at the pasture. The between-site differences in growing season H and LE (∆H and ∆LE) were mainly caused by their different vegetation conditions, and ∆H and ∆LE were negatively and positively related to ∆EVI (difference in enhanced vegetation index), respectively.

Biophysical regulation of evapotranspiration in semiarid croplands

Water vapor exchange is a key component in terrestrial surface water balance. However, the biophysical regulation processes that control evapotranspiration (ET) in semiarid farmland areas have not been fully resolved. In this study, seasonal and interannual variances in ET and their environmental controls from 2009 to 2011 were investigated in a semiarid, rain-fed farmland using eddy covariance. One moderately dry year (2010) and two extremely dry years (2009 and 2011) were observed; however, 2009 (an extremely dry year) had the same weather condition as that of 2010 (a moderately dry year), and 2011 set out as different from the other two years. The results indicated that the maximum daily ET in the 2009 (4.4 mm d−1) and 2010 (4.7 mm d−1) growth seasons was higher than that in 2011 (3.0 mm d−1). Cumulative annual ET values were 310.6, 318.0, and 210.2 mm in 2009 to 2011, which were 109%, 96%, and 72% of the precipitation (PPT) received, respectively. The contribution of PPT in summer to the annual ET was more than that in autumn, and the extremely dry climate during the pregrowth stage and early spring period restricted the annual ET in 2011. The lowest annual ET in 2011 was caused by low daytime ET rather than nighttime ET. The strong power relationship (R2 = 0.85) between the decoupling coefficient (Ω) and the canopy conductance (gc) indicated that ET was sensitive to gc, and gc control on ET increased as the day progressed. The physiological controls of ET were associated closely with soil moisture, and drought had a strong effect on the ET response to environmental factors, such as incoming shortwave radiation and vapor pressure deficit. These results have important potential implications for understanding and modeling water cycle feedback to the increasing drought climate in the Loess Plateau characterized by serious soil and water loss.

One-step approach for estimating maize actual water use: part II. Lysimeter evaluation of variable surface resistance models

This study evaluated maize variable bulk surface resistance (rs, s m−1) models developed using field/environmental data from non-irrigated fields in a humid climate. The different rs models were reported in the companion paper. Surface resistance values derived from the application of the new models were inserted in the 1965 Penman–Monteith ET equation to calculate actual maize evapotranspiration (ETa). This is the so-called one-step approach. The evaluation was performed with maize water use data measured with a large weighing lysimeter located in the middle of an irrigated maize field (semi-arid climate) near Bushland, Texas, USA. The evaluation was performed for different time scales (semi-hourly to daily) and the rs models bias (MBE) and root mean square errors (RMSE) were determined. Using daytime 30-min rs models in maize ETa (mm 30-min−1) estimation resulted with the lowest relative error of 15.4%. While daytime average rs models developed from daytime average explanatory variables resulted with the lowest relative error of 20.9% in 30-min maize ETa estimation. When rs models from 30-min and daylight average explanatory variables were used to obtain cumulative daytime maize ETa estimations, resulting errors were lower than for semi-hourly time step. In addition, when daytime 30-min and average rs models were applied in conjunction with a fixed rs nighttime value to obtain daily maize ETa, results were similar than those for the daytime time step. Furthermore, another evaluation incorporated rs values obtained adopted ranges of crop evaporative fraction (EF). When this EF-based rs values were used to estimate maize (ETa, mm 30-min−1) the error found was 5.9 ± 21.7%. This result seems high; however, when the EF range-based rs values were applied in the estimation of maize cumulative daytime ETa, results indicated an error of 5.9 ± 11.9%. In this last case, the relative error was much lower than for 30-min ETa estimations. Therefore, it is concluded that some maize rs models, reported in the companion paper, performed well when evaluated in a different climate and agronomic practices and are suitable for the estimation of ETa using the 1965 Penman–Monteith ET equation. In particular daytime 30-min rs models applied to 30-min intervals of ETa estimation and then accumulated over the day performed better than the others rs models and time steps studied.

Utility of the two-source energy balance (TSEB) model in vine and interrow flux partitioning over the growing season

For monitoring water use in vineyards, it becomes important to evaluate the evapotranspiration (ET) contributions from the two distinct management zones: the vines and the interrow. Often the interrow is not completely bare soil but contains a cover crop that is senescent during the main growing season (nominally May–August), which in Central California is also the dry season. Drip irrigation systems running during the growing season supply water to the vine plant and re-wet some of the surrounding bare soil. However, most of the interrow cover crop is dry stubble by the end of May. This paper analyzes the utility of the thermal-based two-source energy balance (TSEB) model for estimating daytime ET using tower-based land surface temperature (LST) estimates over two Pinot Noir (Vitis vinifera) vineyards at different levels of maturity in the Central Valley of California near Lodi, CA. The data were collected as part of the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX). Local eddy covariance (EC) flux tower measurements are used to evaluate the performance of the TSEB model output of the fluxes and the capability of partitioning the vine and cover crop transpiration (T) from the total ET or T/ET ratio. The results for the 2014–2016 growing seasons indicate that TSEB output of the energy balance components and ET, particularly, over the daytime period yield relative differences with flux tower measurements of less than 15%. However, the TSEB model in comparison with the correlation-based flux partitioning method overestimates T/ET during the winter and spring through bud break, but then underestimates during the growing season. A major factor that appears to affect this temporal behavior in T/ET is the daily LAI used as input to TSEB derived from a remote sensing product. An additional source of uncertainty is the use of local tower-based LST measurements, which are not representative of the flux tower measurement source area footprint.

Determination of crop and soil evaporation coefficients for estimating evapotranspiration in a paddy field

Accurate estimation of evapotranspiration is important in efficient water management for improving water use efficiency. In order to obtain evapotranspiration and evaporation beneath the canopy using the Food and Agriculture Organization (FAO) method, pan evaporation was used instead of reference evapotranspiration calculated by the Penman-Monteith equation with detailed meteorological data. The total crop coefficient and soil evaporation coefficient were determined using actual measured daytime evapotranspiration and evaporation by the Bowen ratio energy balance and lysimeter, respectively, in a rice paddy field in Japan. The average evapotranspiration was 5.3 mm/d, 4.4 mm/d, 7.4 mm/d and 6.3 mm/d and crop coefficient was 0.79, 1.18, 1.01 and 0.86 for the initial stage, development stage, middle-season stage and late-season stage, respectively. The evaporation was low and almost constant with an average value around 0.77 mm/d when the leaf area index (LAI) reached 3. The proposed average crop coefficients for different growing stages were applied to estimate daytime evapotranspiration and found suitable. A simple soil water evaporation coefficient model was developed using leaf area index for practical use and it was found that it could accurately estimate evaporation. Keywords: evapotranspiration, evaporation, paddy field, lysimeter, leaf area index (LAI), Bowen ratio energy balance DOI: 10.25165/j.ijabe.20171004.2290 Citation: Yan H F, Zhang C, Oue H, Peng G J, Darko R O. Determination of crop and soil evaporation coefficients for estimating evapotranspiration in a paddy field. Int J Agric & Biol Eng, 2017; 10(4): 130–139.

Climate controls over ecosystem metabolism: insights from a fifteen-year inductive artificial neural network synthesis for a subalpine forest.

Eddy covariance (EC) datasets have provided insight into climate determinants of net ecosystem productivity (NEP) and evapotranspiration (ET) in natural ecosystems for decades, but most EC studies were published in serial fashion such that one study's result became the following study's hypothesis. This approach reflects the hypothetico-deductive process by focusing on previously derived hypotheses. A synthesis of this type of sequential inference reiterates subjective biases and may amplify past assumptions about the role, and relative importance, of controls over ecosystem metabolism. Long-term EC datasets facilitate an alternative approach to synthesis: the use of inductive data-based analyses to re-examine past deductive studies of the same ecosystem. Here we examined the seasonal climate determinants of NEP and ET by analyzing a 15-year EC time-series from a subalpine forest using an ensemble of Artificial Neural Networks (ANNs) at the half-day (daytime/nighttime) time-step. We extracted relative rankings of climate drivers and driver-response relationships directly from the dataset with minimal a priori assumptions. The ANN analysis revealed temperature variables as primary climate drivers of NEP and daytime ET, when all seasons are considered, consistent with the assembly of past studies. New relations uncovered by the ANN approach include the role of soil moisture in driving daytime NEP during the snowmelt period, the nonlinear response of NEP to temperature across seasons, and the low relevance of summer rainfall for NEP or ET at the same daytime/nighttime time step. These new results offer a more complete perspective of climate-ecosystem interactions at this site than traditional deductive analyses alone.

Evaluation of land surface model simulations of evapotranspiration over a 12-year crop succession: impact of soil hydraulic and vegetation properties

Abstract. Evapotranspiration has been recognized as one of the most uncertain terms in the surface water balance simulated by land surface models. In this study, the SURFEX/ISBA-A-gs (Interaction Sol–Biosphere–Atmosphere) simulations of evapotranspiration are assessed at the field scale over a 12-year Mediterranean crop succession. The model is evaluated in its standard implementation which relies on the use of the ISBA pedotransfer estimates of the soil properties. The originality of this work consists in explicitly representing the succession of crop cycles and inter-crop bare soil periods in the simulations and assessing its impact on the dynamics of simulated and measured evapotranspiration over a long period of time. The analysis focuses on key parameters which drive the simulation of ET, namely the rooting depth, the soil moisture at saturation, the soil moisture at field capacity and the soil moisture at wilting point. A sensitivity analysis is first conducted to quantify the relative contribution of each parameter on ET simulation over 12 years. The impact of the estimation method used to retrieve the soil parameters (pedotransfer function, laboratory and field methods) on ET is then analysed. The benefit of representing the variations in time of the rooting depth and wilting point is evaluated. Finally, the propagation of uncertainties in the soil parameters on ET simulations is quantified through a Monte Carlo analysis and compared with the uncertainties triggered by the mesophyll conductance which is a key above-ground driver of the stomatal conductance. This work shows that evapotranspiration mainly results from the soil evaporation when it is continuously simulated over a Mediterranean crop succession. This results in a high sensitivity of simulated evapotranspiration to uncertainties in the soil moisture at field capacity and the soil moisture at saturation, both of which drive the simulation of soil evaporation. Field capacity was proved to be the most influencing parameter on the simulation of evapotranspiration over the crop succession. The evapotranspiration simulated with the standard surface and soil parameters of the model is largely underestimated. The deficit in cumulative evapotranspiration amounts to 24 % over 12 years. The bias in daily daytime evapotranspiration is −0.24 mm day−1. The ISBA pedotransfer estimates of the soil moisture at saturation and at wilting point are overestimated, which explains most of the evapotranspiration underestimation. The use of field capacity values retrieved from laboratory methods leads to inaccurate simulation of ET due to the lack of representativeness of the soil structure variability at the field scale. The most accurate simulation is achieved with the average values of the soil properties derived from the analysis of field measurements of soil moisture vertical profiles over each crop cycle. The representation of the variations in time of the wilting point and the maximum rooting depth over the crop succession has little impact on the simulation performances. Finally, we show that the uncertainties in the soil parameters can generate substantial uncertainties in ET simulated over 12 years (the 95 % confidence interval represents 23 % of cumulative ET over 12 years). Uncertainties in the mesophyll conductance have lower impact on ET. Measurement random errors explain a large part of the scattering between simulations and measurements at half-hourly timescale. The deficits in simulated ET reported in this work are probably larger due to likely underestimation of ET by eddy-covariance measurements. Other possible model shortcomings include the lack of representation of soil vertical heterogeneity and root profile along with inaccurate energy balance partitioning between the soil and the vegetation at low leaf area index.

A Revised Temporal Scaling Method to Yield Better ET Estimates at a Regional Scale

This study presents a revised temporal scaling method based on a detection algorithm for the temporal stability of the evaporative fraction (EF) to estimate total daytime evapotranspiration (ET) at a regional scale. The study area is located in the Heihe River Basin, which is the second largest inland river basin in China. The remote sensing data and field observations used in this study were obtained from the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) project. The half-hourly EF values (EFEC) calculated using meteorological observations from an eddy covariance (EC) system and an automatic meteorological station (AMS) represented the diurnal pattern of the EF across the majority of the study area. The remotely sensed instantaneous midday EF (EFASTER), which indicates the spatial distribution of the midday EF over the entire study area, was calculated from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image. The temporal stability of EFEC was examined using a detection algorithm. Intervals with inconsistent EFEC values were distinguished from those with consistent EFEC values; the total daytime ET (from 9:00 to 19:00) within these interval types was integrated separately. Validation of the total daytime ET at the satellite pixel scale was conducted using measurements from17 EC towers. Using the detection algorithm for the temporal stability of the EF and dynamic adjustment, the revised temporal scaling method resulted in a root-mean-square error (RMSE) of 0.54 (mm·d−1), a mean relative error (MRE) of 7.26% and a correlation coefficient (Corr.) of 0.81; all of these values were superior to those of the two other methods (i.e., the constant EF and variable EF methods). The revised method easily extends to other areas and exhibits a superior performance in flat and regularly-irrigated farmlands at the regional scale.

Comparison of ET estimations by the three-temperature model, SEBAL model and eddy covariance observations

The three-temperature (3T) model is a simple model which estimates plant transpiration from only temperature data. In-situ field experimental results have shown that 3T is a reliable evapotranspiration (ET) estimation model. Despite encouraging results from recent efforts extending the 3T model to remote sensing applications, literature shows limited comparisons of the 3T model with other remote sensing driven ET models. This research used ET obtained from eddy covariance to evaluate the 3T model and in turn compared the model-simulated ET with that of the more traditional SEBAL (Surface Energy Balance Algorithm for Land) model. A field experiment was conducted in the cotton fields of Taklamakan desert oasis in Xinjiang, Northwest China. Radiation and surface temperature were obtained from hyperspectral and thermal infrared images for clear days in 2013. The images covered the time period of 0900–1800 h at four different phenological stages of cotton. Meteorological data were automatically recorded in a station located at the center of the cotton field. Results showed that the 3T model accurately captured daily and seasonal variations in ET. As low dry soil surface temperatures induced significant errors in the 3T model, it was unsuitable for estimating ET in the early morning and late afternoon periods. The model-simulated ET was relatively more accurate for squaring, bolling and boll-opening stages than for seedling stage of cotton during when ET was generally low. Wind speed was apparently not a limiting factor of ET in the 3T model. This was attributed to the fact that surface temperature, a vital input of the model, indirectly accounted for the effect of wind speed on ET. Although the 3T model slightly overestimated ET compared with SEBAL and eddy covariance, it was generally reliable for estimating daytime ET during 0900–1600 h.

Upscaling of evapotranspiration fluxes from instantaneous to daytime scales for thermal remote sensing applications

Abstract. Four upscaling methods for estimating daytime actual evapotranspiration (ET) from single time-of-day snapshots, as commonly retrieved using remote sensing, were compared. These methods assume self-preservation of the ratio between ET and a given reference variable over the daytime hours. The analysis was performed using eddy covariance data collected at 12 AmeriFlux towers, sampling a fairly wide range in climatic and land cover conditions. The choice of energy budget closure method significantly impacted performance using different scaling methodologies. Therefore, a statistical evaluation approach was adopted to better account for the inherent uncertainty in ET fluxes using eddy covariance technique. Overall, this approach suggested that at-surface solar radiation was the most robust reference variable amongst those tested, due to high accuracy of upscaled fluxes and absence of systematic biases. Top-of-atmosphere irradiance was also tested and proved to be reliable under near clear-sky conditions, but tended to overestimate the observed daytime ET during cloudy days. Use of reference ET as a scaling flux yielded higher bias than the solar radiation method, although resulting errors showed similar lack of seasonal dependence. Finally, the commonly used evaporative fraction method yielded satisfactory results only in summer months, July and August, and tended to underestimate the observations in the fall/winter seasons from November to January at the flux sites studied. In general, the proposed methodology clearly showed the added value of an intercomparison of different upscaling methods under scenarios that account for the uncertainty in eddy covariance flux measurements due to closure errors.

Two-Source Energy Balance Model to Calculate E, T, and ET: Comparison of Priestley-Taylor and Penman-Monteith Formulations and Two Time Scaling Methods

Abstract. The two-source energy balance (TSEB) model calculates the energy balance of the soil-canopy-atmosphere continuum, where transpiration is initially determined by the Priestley-Taylor equation. The TSEB was revised recently using the Penman-Monteith equation to replace the Priestley-Taylor formulation, thus better accounting for the impact of large and varying vapor pressure deficits (VPD) typical of advective, semiarid climates. This study is a comparison of the Priestley-Taylor and Penman-Monteith versions of the TSEB (termed TSEB-PT and TSEB-PM, respectively). Evaporation (E), transpiration (T), and evapotranspiration (ET) calculated by the TSEB-PT and TSEB-PM versions were compared to measurements obtained with microlysimeters, sap flow gauges, and weighing lysimeters, respectively, for fully irrigated cotton (Gossypium hirsutum L.) at Bushland, Texas. Radiometric surface temperature (T R ) was used to calculate E, T, and ET in both TSEB versions in 15 min intervals and summed to intervals coinciding with times of measurements. In addition, a one-time-of-day T R measurement was used (9:45, 11:15, 12:45, 14:15, or 15:45 CST), and E, T, and ET were calculated for the appropriate measurement interval (i.e., daytime, nighttime, and 24 h) using the time scaling methods based on reference ET (TSC ET ) and reference temperature (TSC TEMP ). Measured average values of E, T, and ET during the study period were 0.94 mm (24 h), 6.9 mm (7:00 to 22:00 CST), and 7.2 mm (24 h), respectively. The TSEB-PT consistently overestimated E and underestimated T, with RMSE/MBE of up to 2.8/1.8 mm and 4.1/-3.9 mm, respectively. In comparison, the TSEB-PM greatly reduced discrepancies between calculations and measurements, with respective RMSE/MBE for E and T of only up to 1.5/0.79 mm and 1.3/±0.76 mm, respectively. For 24 h ET, the TSEB-PT resulted in maximum RMSE/MBE of 3.2/-1.9 mm, and the TSEB-PM had maximum RMSE/MBE of 1.7/0.95 mm. Daytime ET model agreement was very similar for both model versions (RMSE/MBE usually ET or TSC TEMP methods, and results did not greatly differ for TSC ET or TSC TEMP . Both time scaling methods were not very sensitive to the T R measurement time used, although morning (9:45 CST) T R measurement times did not perform as well as the other times.

Study of crop coefficient and the ratio of soil evaporation to evapotranspiration in an irrigated maize field in an arid area of Yellow River Basin in China

A field experiment was conducted in a maize field in 2006 in an arid area of the Yellow River Basin in China. The daytime evapotranspiration (ETc) and soil evaporation beneath the maize canopy (Eg) were measured by Bowen ratio energy balance method and micro-lysimeters, respectively. The results showed that the total ETc during maize growth season was 696 mm, and the maximum values occurred at about 90–140 days after sowing. The crop coefficient (Kc), which was calculated from the ratio of ETc to reference evapotranspiration (ET0), was quite different from the values reported by other researchers in similar climate areas, with average values of 0.34, 0.47, 1.0 and 0.9 for initial, development, mid-season and late-season stages, respectively. High correlations between leaf area index (LAI) and average Kc for every 4 days were obtained. The total Eg was 201.4 mm with average values ranged from 0.92 to 2.05 for four growth stages of maize; and accounted for around 28.9 % of ETc. The ratio Eg/ETc showed high negative relationship with LAI. These results were very important in precise management of irrigation for maize in Yellow River Basin areas.

Circulation Culture of Tomato for Efficient Nutrient Uptake and High Yield in Tropical Greenhouses

The rate of nutrient uptake and resultant yields of tomato ( Lycopersicon esculentum Mill), grown in protected culture at different rates and methods of water and fertilizer applications were tested in a tropical environment. The experiment was conducted in a naturally ventilated glasshouse under hot and humid conditions where the mean daytime temperature and evapo-transpiration rate (ET) in the greenhouse were 29±3.7°C and 65.6±4.7 mm/day, respectively. The standard irrigation method (T1) was subjected to changes by adjusting supply volume to 150 % of ET (T2 and T4) and doubling the standard volume together with drainage collection and re-circulation (T3). Standard soluble inorganic fertilizer dosage (in T1 and T2) was adjusted to T3 by doubling the dosage of essential plant nutrients, except N and Ca from the early fruit development (12 weeks after planting). Inorganic fertilizer dosage was completely replaced with phospho-compost and foliar application of plant extracts (T4). Plant and medium samples were tested for N, P and K compositions. Electrical conductivity and pH drifts in media and leacheate were also determined. Results revealed the necessity of excess irrigation from the flower initiation stage of tomato preferably in the form of circulation culture. Significantly high N and P nutrition together with adequate K improved the fruit growth and yield. Overall yields were below the recorded averages due to hot, humid and overcast conditions. Further improvements are needed for K nutrition in the circulation culture of soluble fertilizers and for N nutrition in phospho-compost supplemented plant nutrient supply. Tropical Agricultural Research Vol. 23 (3): 204-217 (2012) DOI: http://dx.doi.org/10.4038/tar.v23i3.4658

A novel method to convert daytime evapotranspiration into daily evapotranspiration based on variable canopy resistance

► A new method is proposed to upscale daily evapotranspiration (ET) from daytime ET. ► The method is based on variable canopy resistance derived from Todorovic model. ► The upscaled daily ET agreed well with daily ET measured from eddy covariance . ► The method need to be applied to regional ET model in the future work. A method to convert daytime evapotranspiration (ET) into daily ET based on variable canopy resistance was investigated in this paper. The study was carried out for maize and canola crops grown in Coleambally Irrigation Area which is located in Murray Darling Basin of Australia. Variable canopy resistance was simulated on the basis of Todorovic method. This study has shown that the method performed well to scale daily ET from daytime ET measurements for both crops. In the case of the maize crop, the coefficient of determination ( R 2 ) was 0.979 and the index of agreement ( d ) was 0.981. In the case of the canola crop, the coefficient of determination ( R 2 ) was 0.936 and the index of agreement ( d ) was 0.960. Future work will focus on the study of this method to scale daily ET map from satellite-derived daytime ET map.

Nocturnal evapotranspiration in eddy-covariance records from three co-located ecosystems in the Southeastern U.S.: Implications for annual fluxes

Nocturnal evapotranspiration ( ET N ) is often assumed to be negligible in terrestrial ecosystems, reflecting the common assumption that plant stomata close at night to prevent water loss from transpiration. However, recent evidence across a wide range of species and climate conditions suggests that significant transpiration occurs at night, frustrating efforts to estimate total annual evapotranspiration ( ET) from conventional methods such as the eddy-covariance technique. Here, the magnitude and variability of ET N is explored in multiple years of eddy-covariance measurements from three adjacent ecosystems in the Southeastern U.S.: an old grass field, a planted pine forest, and a late-successional hardwood forest. After removing unreliable data points collected during periods of insufficient turbulence, observed ET N averaged 8–9% of mean daytime evapotranspiration ( ET D ). ET N was driven primarily by wind speed and vapor pressure deficit and, in the two forested ecosystems, a qualitative analysis suggests a significant contribution from nocturnal transpiration. To gapfill missing data, we investigated several methodologies, including process-based multiple non-linear regression, relationships between daytime and nighttime ET fluxes, marginal distribution sampling, and multiple imputation. The utility of the gapfilling procedures was assessed by comparing simulated fluxes to reliably measured fluxes using randomly generated gaps in the data records, and by examining annual sums of ET from the different gapfilling techniques. The choice of gapfilling methodology had a significant impact on estimates of annual ecosystem water use and, in the most extreme cases, altered the annual estimate of ET by over 100 mm year −1, or ca. 15%. While no single gapfiling methodology appeared superior for treating data from all three sites, marginal distribution sampling generally performed well, producing flux estimates with a site average bias error of <10%, and a mean absolute error close to the random measurement error of the dataset (12.2 and 9.8 W m −2, respectively).