Latent Heat Flux Measurements Research Articles

Intercomparison of sensible and latent heat flux measurements from combined eddy covariance, energy balance, and Bowen ratio methods above a grassland prairie

We present a comparison of four different methods of measuring sensible (H) and latent (LE) heat fluxes for a year over a mixed grass prairie ecosystem in the Nebraska SandHills [eddy covariance (EC), energy balance/Bowen ratio (EBBR), residual energy (RES), modified Bowen ratio (MBR) methods]. Additionally, we developed a set of quality control criteria for each method and present a simplification to the traditional EBBR setup. Using EC as reference, all methods yielded similar estimates of yearly H (regression slopes (m) ~ 2% from unity; HEC > HEBBR, HRES, and HMBR). For yearly LE, EBBR and RES yielded similar estimates with EC (m ~ 2% from unity; LEEC < LEEBBR and LERES), while a larger bias was found from MBR (m ~ 8% from unity; LEEC > LEMBR). At shorter time scales (~ hourly), moderate scatter was found about linear regression fits for H between EBBR and EC (R2 = 0.81), with smaller scatter between RES and MBR, and EC (R2 = 0.91). For LE, smaller scatter was also measured between EC, and EBBR and RES (R2 = 0.89 and 0.87, respectively), with the larger scatter between EC and MBR (R2 = 0.65). This suggests methods other than EC may be well suited to longer-term applications (≥ yearly), but have larger uncertainty on individual measurements.

Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

Abstract. It is known from arid and semi-arid ecosystems that atmospheric water vapor can directly be adsorbed by the soil matrix. Soil water vapor adsorption was typically neglected and only recently received attention because of improvements in measurement techniques. One technique rarely explored for the measurement of soil water vapor adsorption is eddy covariance (EC). Soil water vapor adsorption may be detectable as downwardly directed (i.e., negative) EC latent heat (λE) flux measurements under dry conditions, but a systematic assessment of the use of negative λE fluxes from EC flux stations to characterize adsorption is missing. We propose a classification method to characterize soil water vapor adsorption, excluding conditions of dew and fog when λE derived from EC is not trustworthy due to stable atmospheric conditions. We compare downwardly directed λE fluxes from EC with measurements from weighing lysimeters for 4 years in a Mediterranean savanna ecosystem and 3 years in a temperate agricultural site. Our aim is to assess if overnight water inputs from soil water vapor adsorption differ between ecosystems and how well they are detectable by EC. At the Mediterranean site, the lysimeters measured soil water vapor adsorption each summer, whereas at the temperate site, soil water vapor adsorption was much rarer and was measured predominantly under an extreme drought event in 2018. During 30 % of nights in the 4-year measurement period at the Mediterranean site, the EC technique detected downwardly directed λE fluxes of which 88.8 % were confirmed to be soil water vapor adsorption by at least one lysimeter. At the temperate site, downwardly directed λE fluxes were only recorded during 15 % of the nights, with only 36.8 % of half hours matching simultaneous lysimeter measurement of soil water vapor adsorption. This relationship slightly improved to 61 % under bare-soil conditions and extreme droughts. This underlines that soil water vapor adsorption is likely a much more relevant process in arid ecosystems compared to temperate ones and that the EC method was able to capture this difference. The comparisons of the amounts of soil water vapor adsorption between the two methods revealed a substantial underestimation of the EC compared to the lysimeters. This underestimation was, however, comparable with the underestimation in evaporation by the eddy covariance and improved in conditions of higher turbulence. Based on a random-forest-based feature selection, we found the mismatch between the methods being dominantly related to the site's inherent variability in soil conditions, namely soil water status, and soil (surface) temperature. We further demonstrate that although the water flux is very small with mean values of 0.04 or 0.06 mm per night for EC or lysimeter, respectively, it can be a substantial fraction of the diel soil water balance under dry conditions. Although the two instruments substantially differ with regard to the measured ratio of adsorption to evaporation over 24 h with 64 % and 25 % for the lysimeter and EC methods, they are in either case substantial. Given the usefulness of EC for detecting soil water vapor adsorption as demonstrated here, there is potential for investigating adsorption in more climate regions thanks to the greater abundance of EC measurements compared to lysimeter observations.

Dynamics and biophysical controls of nocturnal water loss in a winter wheat-summer maize rotation cropland: a multi-temporal scale analysis

Nocturnal water loss (NWL), the evapotranspiration minus dew at night, has been identified to play a significant role in the surface water and energy balance. However, few studies have examined the variability in NWL in croplands with high water consumption across different timescales. Here, based on the eddy-covariance measurements of latent heat flux during nighttime, the dynamics and biophysical controls of NWL were examined in a winter wheat-summer maize rotation cropland in the North China Plain over a 5-year period (2008-2012) at nocturnal, seasonal and inter-annual scales. Besides, its relationship to drought was also revealed. Nocturnally, NWL showed a decreasing tendency from 20:00 to 05:00, with large diurnal variations in May. Seasonally, NWL was higher for wheat than that for maize. Interannually, the annual average daily NWL showed the same variation as the NWL during the wheat growth across the five years. The NWL was dominantly controlled by the difference between surface temperature and air temperature (Ts-Ta) nocturnally; Seasonally, NWL was influenced by vapor pressure deficit (VPD), wind speed (Ws) and site bulk surface conductance (gs) for wheat, and by VPD, gs and net radiation (Rn) for maize; NWL was mainly regulated by Ts-Ta and soil water content at 40 cm below the ground (VWC40) interannually. VPD and gs controlled NWL more during dry years. NDVI exhibited stronger control on NWL for wheat in wet years. The most favorable conditions for NWL during dry months were at Ta<16 ℃, VPD <0.8 kPa, and VWC40 <0.46 m3 m−3. NWL accounted for 4.0% of precipitation during the non-dry period but 19.6% during drought. Our results highlight the importance of biophysical factors in regulating NWL of the cropland ecosystem, and more attention should be paid to the influence of NWL on the water and carbon cycles in the context of climate change.

The effect of relative humidity on eddy covariance latent heat flux measurements and its implication for partitioning into transpiration and evaporation

While the eddy covariance (EC) technique is a well-established method for measuring water fluxes (i.e., evaporation or 'evapotranspiration’, ET), the measurement is susceptible to many uncertainties. One such issue is the potential underestimation of ET when relative humidity (RH) is high (>70%), due to low-pass filtering with some EC systems. Yet, this underestimation for different types of EC systems (e.g. open-path or closed-path sensors) has not been characterized for synthesis datasets such as the widely used FLUXNET2015 dataset. Here, we assess the RH-associated underestimation of latent heat fluxes (LE, or ET) from different EC systems for 163 sites in the FLUXNET2015 dataset. We found that the LE underestimation is most apparent during hours when RH is higher than 70%, predominantly observed at sites using closed-path EC systems, but the extent of the LE underestimation is highly site-specific. We then propose a machine learning based method to correct for this underestimation, and compare it to two energy balance closure based LE correction approaches (Bowen ratio correction, BRC, and attributing all errors to LE). Our correction increases LE by 189% for closed-path sites at high RH (>90%), while BRC increases LE by around 30% for all RH conditions. Additionally, we assess the influence of these corrections on ET-based transpiration (T) estimates using two different ET partitioning methods. Results show opposite responses (increasing vs. slightly decreasing T-to-ET ratios, T/ET) between the two methods when comparing T based on corrected and uncorrected LE. Overall, our results demonstrate the existence of a high RH bias in water fluxes in the FLUXNET2015 dataset and suggest that this bias is a pronounced source of uncertainty in ET measurements to be considered when estimating ecosystem T/ET and WUE.

Biophysical controls of dew formation in a typical cropland and its relationship to drought in the North China Plain

Dew, as an important contribution of non-rainfall water (NRW), is found to be an important hydrological component for ecosystem water stress alleviation and further ecosystem functions maintenance during rainless seasons. The North China Plain (NCP) is facing severe water shortage mainly due to intensive agricultural irrigation. However, few studies have examined the variations and controlling mechanism of dew in the typical croplands predisposed to drought. Here, seasonal variation in dew was examined during growing period in a wheat and maize rotation cropland in the NCP over 3 years (2008–2010), based on the eddy-covariance measurements of latent heat flux (LE) during nighttime, and the ecosystem drought was defined to occur when the relative extractable water content (REW) in the soil dropped below 0.4. Results showed that annual (from April to September) mean dew amount for this cropland was 7.61 ± 0.02 mm, accounting for 2.5 % of precipitation (PPT), and the annual mean dew amount during drought period was 3.11 mm, accounting for 3.4 % of PPT. Seasonal variation in dew was mainly regulated by relative humidity (RH) and surface stomatal conductance (gs), furthermore, dew was influenced by gs and vapor pressure deficit (VPD) during the wheat growing period, and by gs during maize growing period. 51.1 %–72.0 % of drought events occurred during wheat growing season. Compared to maize growing season with higher nocturnal evapotranspiration (ETN), the higher frequency of dew occurrence during wheat growing season had reduced the ETN amount. Drought timing, duration, and degree together influenced dew amount. Long-term drought (e.g. >1-month drought) would induce an increase of dew amount after drought period. Our findings clarified the temporal dynamics in dew across different crop growing periods (such as wheat and maize); and contribute to the sustainable utilization of non-rainfall water (especially dew) in cropland in the context of extreme climate (such as droughts) in the future.

The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

Abstract. The role of precipitation (P) variability with respect to evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E), from savannas continues to draw significant research interest given its relevance to a number of ecohydrological applications. Our study reports on 6 years of measured ET and estimated T and E from a grazed savanna grassland at Welgegund, South Africa. Annual P varied significantly with respect to amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy-covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T for the 4 wet years with frequent early wet-season rainfall was nearly constant, 326±19 mm yr−1 (T/ET=0.51), but was lower and more variable between the 2 dry years (255 and 154 mm yr−1, respectively). Annual T and T/ET were linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grasses as well as trees' ability to use water from deeper layers. During extreme drought, grasses respond to water availability with a dieback–regrowth pattern, reducing leaf area and transpiration and, thus, increasing the proportion of transpiration contributed by trees. The works suggest that the early-season P distribution explains the interannual variability in T, which should be considered when managing grazing and fodder production in these grasslands.

Estimation of Corn Latent Heat Flux from High Resolution Thermal Imagery

Crop evapotranspiration (ET), which is directly related to latent heat flux, is also a key indicator in determining the water status of crops. In order to estimate the latent heat flux, two-source energy balance (TSEB) models have been developed for thermal imagery from satellite platforms. However, because of the coarse resolution of thermal sensors on the satellite, distinguishing soil and vegetation is difficult which complicates the calculation process and introduces errors in latent heat estimates. In this research, high-resolution thermal datasets (0.05 m) and corresponding RGB datasets (0.03 m) were used for calculating crop latent heat flux using an adapted TSEB model. The RGB datasets were used for supervised classification of soil and vegetation, and the classification results were then used to filter the thermal mosaics to separate vegetation and soil temperatures. The vegetation temperature is used for calculating latent heat flux and the results are validated against the ground reference measurements of latent heat using a handheld porometer. The objective of this research is to introduce a workflow including an adapted TSEB model which is customized for high resolution thermal images from unmanned aircraft systems (UAS) to estimate the latent heat flux of row crops in agricultural fields. Nine dates of data collection in 2018 and 2020 have been evaluated and the root mean square error (RMSE) varies between 16 to 106 W/m2 depending on the days after planting (DAP) and the time of measurement for each day. The results indicate that the workflow introduced here is able to provide estimates of instantaneous latent heat flux (evapotranspiration) measurements for row crops in agricultural fields which will enable people to make reliable decisions related to irrigation scheduling.

The Effect of Relative Humidity on Eddy Covariance Latent Heat Flux Measurements and its Implication for Partitioning into Transpiration and Evaporation

The Effect of Relative Humidity on Eddy Covariance Latent Heat Flux Measurements and its Implication for Partitioning into Transpiration and Evaporation

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.

An analog period method for gap‐filling of latent heat flux measurements

AbstractPractically all records of eddy‐covariance flux measurements are affected by gaps, caused by several reasons. In this work, we propose analog period (AP) methods for gap‐filling, and test them for filling gaps of latent heat flux at five AmeriFlux sites. The essence of the methods is to look for periods in the record that bear a strong resemblance, in the variable to be filled, to the periods immediately before and after the gap. Similarity between periods is gauged by the coefficient of determination, and the search for similar periods and their ranking is straightforward. The methods are developed in a univariate version (that uses only the latent heat flux data series itself) and several multivariate ones, that incorporate sensible heat flux, ground heat flux and net radiation data. For each set of independent variables used for gap‐filling, the methods are tested against an existing gap‐filling procedure with similar data requirements. Thus, the univariate version is tested against the mean diurnal variation method, and the multivariate versions are tested against corresponding simple and multiple linear regression techniques that use energy‐budget data, and in one case the evaporative fraction as well. In our tests, the proposed univariate version performs better than the mean diurnal variation method, and the multivariate versions perform better than simple/multiple linear regression methods. An often used available computer package, REddyProc, was also tested as a basis of comparison. In general, the proposed methods (in univariate and multivariate versions) and simple/multiple linear regressions performed better than REddyProc. For the datasets analysed, gap filling via the evaporative fraction method performed poorly.

Air-Sea Interactions over Eddies in the Brazil-Malvinas Confluence

The Brazil–Malvinas Confluence (BMC) is one of the most dynamical regions of the global ocean. Its variability is dominated by the mesoscale, mainly expressed by the presence of meanders and eddies, which are understood to be local regulators of air-sea interaction processes. The objective of this work is to study the local modulation of air-sea interaction variables by the presence of either a warm (ED1) and a cold core (ED2) eddy, present in the BMC, during September to November 2013. The translation and lifespans of both eddies were determined using satellite-derived sea level anomaly (SLA) data. Time series of satellite-derived surface wind data, as well as these and other meteorological variables, retrieved from ERA5 reanalysis at the eddies’ successive positions in time, allowed us to investigate the temporal modulation of the lower atmosphere by the eddies’ presence along their translation and lifespan. The reanalysis data indicate a mean increase of 78% in sensible and 55% in latent heat fluxes along the warm eddy trajectory in comparison to the surrounding ocean of the study region. Over the cold core eddy, on the other hand, we noticed a mean reduction of 49% and 25% in sensible and latent heat fluxes, respectively, compared to the adjacent ocean. Additionally, a field campaign observed both eddies and the lower atmosphere from ship-borne observations before, during and after crossing both eddies in the study region during October 2013. The presence of the eddies was imprinted on several surface meteorological variables depending on the sea surface temperature (SST) in the eddy cores. In situ oceanographic and meteorological data, together with high frequency micrometeorological data, were also used here to demonstrate that the local, rather than the large scale forcing of the eddies on the atmosphere above, is, as expected, the principal driver of air-sea interaction when transient atmospheric systems are stable (not actively varying) in the study region. We also make use of the in situ data to show the differences (biases) between bulk heat flux estimates (used on atmospheric reanalysis products) and eddy covariance measurements (taken as “sea truth”) of both sensible and latent heat fluxes. The findings demonstrate the importance of short-term changes (minutes to hours) in both the atmosphere and the ocean in contributing to these biases. We conclude by emphasizing the importance of the mesoscale oceanographic structures in the BMC on impacting local air-sea heat fluxes and the marine atmospheric boundary layer stability, especially under large scale, high-pressure atmospheric conditions.

Comparison of sensible heat fluxes by large aperture scintillometry and eddy covariance over two contrasting−climate vineyards

Measurements of turbulent energy fluxes between land surface and the atmosphere are commonly carried out applying the eddy covariance (EC) method. Nevertheless, in some cases (e.g. surface heterogeneity, remote sensing or model validation), the use of a Large Aperture Scintillometer (LAS) is more appropriate. We compared LAS and EC measurements of sensible heat flux (H) over two contrasting vineyards of Southern (MIN site) and Northern (LIS site) Italy, mainly differentiating for climate (i.e. dry and humid), water management (i.e. irrigated and rainfed), and LAS setup. The relationship between H by LAS (HLAS) and by EC (HEC) was good at both sites, but with slightly higher HLAS. At MIN, where multiple EC stations were deployed, the best agreement was found with the station in the center of LAS path. Surface heterogeneity due to irrigation had different impact on HLAS depending on location of the source area. The canopy growth at LIS during the study period caused the approaching of LAS beam to the roughness sublayer, resulting in a reduction of HLAS compared to HEC. The closure of energy balance was higher (79%) at MIN and lower (64%) at LIS, but the agreement between HLAS and HEC was similar at both sites, suggesting that particular attention should be given to latent heat flux measurements by EC in order to better understand the energy balance closure issue.

Application and Evaluation of a Two-Wavelength Scintillometry System for Operation in a Complex Shallow Boreal-Forested Valley

Two-wavelength scintillometer systems can provide much needed measurements of area-averaged sensible and latent heat fluxes. However, these devices rarely have been deployed on canopy-covered complex terrain, and never in the circumpolar boreal biome, where large-scale fluxes are essential to hydroclimate modellers. We present a comparison of fluxes measured above a boreal-forested valley with a two-wavelength scintillometer and an eddy covariance system. Instruments were deployed in late summer 2017, and 19 days of data were retained for the analysis. The scintillometer path was 1347-m long and projected across the valley between 5 and 100 m above the ground, with an effective height of $$\approx $$88 m. The limitations of deriving surface fluxes using scintillometry in complex terrain are discussed, and the effects of atmospheric conditions on the flux comparison are quantified. Fluxes are calculated with the scintillometer only, and using a number of atmospheric variables from the eddy-covariance system; impacts of these calculation methods on the correlation between instrumental systems are assessed. Despite a weak agreement of structure parameters between instruments, the comparison of scintillometer and eddy-covariance fluxes yields good correlation ($$R^2$$ up to 0.82). Scintillometry correlates best with eddy-covariance data when the atmospheric surface-layer top is above the scintillometer effective height, but $$R^2$$ only drops slightly otherwise (average decrease of 0.11). The validity of scintillometer fluxes appears dubious during night-time and stable periods. We show that area-averaged flux measurements using two-wavelength scintillometers are possible in hilly forests, but more studies are needed to pinpoint the best methodological framework.

Fixed and variable components of evapotranspiration in a Mediterranean wild-olive - grass landscape mosaic

Dry regions are typically characterized by heterogeneous ecosystems where trees are competing with the surrounding grasses for limited amount of water. In these regions, evapotranspiration (ET) is the leading loss term in the soil water budget, and its estimate, and the dynamic contribution of each ET component (i.e. tree and grass transpiration, and dry bare soil and wet surface evaporation), are still poorly quantified. In a typical heterogeneous Mediterranean ecosystem in Sardinia, we combined eddy-covariance estimates of ET with sap flux and energy balance estimates of wild-olive tree transpiration, a common tree species of the region, and with modeled evapotranspiration from the seasonal grass. Trees located in the southern edge of clumps, thus receiving more radiation, transpired more and showed a greater sensitivity to increasing vapor pressure deficit and soil moisture than trees located in clump centers or northern edges. Transpiration of the tree clumps in the footprint (Et), summed up with the modeled evapotranspiration components of the surrounding grass (mostly transpiration during the wet season and evaporation during the dry season), matched latent heat flux measurements, lending confidence in the estimates. Proper accounting for heterogeneity of sources within the eddy covariance footprint seems to have overcome potential errors from not preserving an important assumption of the method, the land-surface homogeneity, highlighting the methods reliability in such inhomogeneous ecosystem. Compared to ET, Et of wild olives was nearly constant over the hydrologic year, insensitive to variation in soil moisture and atmospheric conditions. In contrast, under favorable spring environmental conditions (radiation, vapor pressure deficit, and soil moisture), the pasture leaf area transpires at high rates, contributing to, and dominating the high ET during that season. Conversely, in dry periods, when evapotranspiration from the grass cover is dominated by low evaporation from the, principally, bare soil, Et dominants ecosystem ET.

Surface renewal measurements of H, λE and CO2 fluxes over two different agricultural systems

The importance of understanding turbulent scalar exchange over agricultural landscapes motivated this study of the surface renewal (SR) method for deployment in place of or alongside eddy covariance (EC) instrumentation. High-frequency (20 Hz) scalar data were used with turbulence and similarity parameters for SR measurements of turbulent sensible heat (H), latent heat (λE), and CO2 (Fc) flux. The eddy covariance method was used to provide a reference data set to compare the performance of SR to EC flux measurements when the SR input requirements were determined with either: (1) fast-response (SR1) or (2) slow-response (SR2) wind velocities. We test SR1 and SR2 over two agricultural crops, cotton and rice, that are suitable for adaptive (“climate-smart”) management solutions which rely on decisions informed by extensive micrometeorological measurements. One advantage that the SR method provides is its simplicity of deployment and maintenance, requiring less energy and resources than typical EC networks. Regardless of the crop and scalar eddy flux, both SR flux methods agreed well with the EC flux measurements; coefficients of determination (R2) and slopes (s) of linear regression analysis ranges were [0.88, 0.98] and [1.01, 1.22], respectively. It was concluded that when EC measurements are unavailable, SR may be used as an alternative for additional climate-smart agriculture research efforts and this method may allow for higher spatial monitoring resolution because of the possibility to avoid the use of sonic anemometry.

Towards estimating root‐zone soil moisture using surface multispectral and thermal sensing: A spectral and hydrometeorological dataset from the Dookie experiment site, Victoria, Australia

AbstractThis paper describes surface hydrometeorological and spectral datasets collected from two tower sites located in the University of Melbourne's Dookie experimental farm, Victoria, Australia. The datasets were collected from different vegetation types including wheat, canola, and grazed pasture during the 2012, 2013, and 2014 cropping seasons. The dataset includes 30‐min frequency latent and sensible heat flux measurements and layer‐average soil moisture data at profile depths of 0–5, 0–30, 30–60, 60–90, and 90–120 cm. Air temperature, wind speed, wind direction, relative humidity, precipitation, and incoming and outgoing longwave and shortwave radiation data were also collected from two locations in the study area. The dataset described in this paper is available online.

Effects of variable fetch and footprint on surface renewal measurements of sensible and latent heat fluxes in cotton

Understanding crop evapotranspiration (ET) is important for efficient irrigation management. The eddy covariance (EC) technique is useful in measuring whole canopy latent heat flux from field crops. However, it requires expensive equipment and complex data analysis; hence, is relevant for research only. With the aim of developing a low-cost and simple method, we investigated here the surface-renewal (SR) method. This method estimates sensible heat flux from high frequency temperature measurements by using a fine wire thermocouple (TC). Next, latent heat flux is derived from the energy balance closure. Since fine wire thermocouple measurements of air temperature can be performed near the canopy top, it was hypothesized (Castellvi, 2012) that fetch requirements can be relaxed relatively to those for EC, and a relatively small fetch is sufficient for reliable flux measurements by SR. In the present study, the SR technique was examined in a cotton field in southern Israel. Seven fine wire thermocouples were installed at various distances from field edges and at different heights above the ground, providing variable fetch from 50 to 200 m, depending on field geometry and wind direction. An EC system was installed within the field at a position that provided sufficient fetch (400 m) for reliable reference values of sensible and latent heat fluxes. Excellent energy balance closure of 0.96 (R2 = 0.95) was obtained from a non-continuous period of 31 days of measurements. SR data from fine-wire thermocouples at all heights were classified by either available fetch or by 90% flux footprint. Only cases in which footprint was smaller than the available fetch were included in the analysis. Two footprint models were examined, KJ (Kljun et al., 2015) and HS (Hsieh et al., 2000). Results of fetch classification showed that the SR weighting factor varied between 0.67 and 1.34 and was independent of fetch. Footprint classification provided weighting factors between 0.46 and 1.18 for the KJ model and between 0.8 and 1.26 for the HS model. LE derived from SR sensible heat flux and energy balance closure, was regressed against LE reference values measured by the eddy covariance. Results showed deviations of up to 15% and 30% between SR and EC, for the KJ and HS model data, respectively. We conclude that in the cotton field under study the SR technique was reliable in estimating sensible and latent heat fluxes and the weighting factor was essentially independent of the geometrical fetch and the flux footprint of the sensors.

Attributing the energy imbalance by concurrent lysimeter and eddy covariance evapotranspiration measurements

The commonly observed lack of energy balance closure at eddy covariance flux tower sites represents an outstanding problem in micrometeorology and significantly compromises the value of eddy covariance latent and sensible heat flux measurements. Here we used concurrent lysimeter and eddy covariance evapotranspiration measurements to correct for the energy imbalance attributable to the eddy covariance latent heat flux measurements (32%) and then, by assuming that the Bowen ratio is correctly quantified by the eddy covariance method, attributed the remainder of the energy balance to the sensible heat flux (10%) and the available energy (58%). We discuss our findings with respect to the ongoing discussion on the causes of the energy imbalance and approaches to force energy balance closure.

Field-scale intercomparison analysis of ecosystems in partitioning surface energy balance components in a semi-arid environment

In situ land surface flux monitoring studies can be useful to extrapolate canopy scales processes to weather and climate analysis. This paper evaluated the interaction between ecosystems and environmental forces in partitioning available energy from multi-sensor platform in the semi-arid region of the Snake River basin in Idaho, USA. Field measurements of latent and sensible heat fluxes using scintillometers and the eddy covariance flux data during the growing season in 2011–2012 were able to identify spatial and seasonal variability in partitioning of surface energy components, including net radiation, latent, sensible, ground heat fluxes. Available energy measured from sagebrush, cheatgrass and lodgepole pine ecosystems indicate that 79%, 58%, and 62% partitioned into latent heat fluxes of 24%, 20%, and 35%, respectively. Role of precipitation and soil moisture, which in turn influenced the latent and sensible heat flux more profoundly were evident in sagebrush and cheatgrass as compared to lodgepole pine with a higher vapor pressure deficit and decreased relative humidity especially in the summertime between June and September. The Budyko analysis revealed that aridity index ratio was found to vary between 3 and 5 suggesting a degree of aridity in these ecosystems. Evapotranspiration (ET) was severely constrained by lack of soil moisture for cheatgrass and sagebrush when compared to the lodgepole pine ecosystem. In addition, it has been concluded that the sagebrush ecosystem regions can serve as recharge zones for enhancing groundwater storage in the Snake River Plains as they exhibit lower evapotranspiration rates in comparison to other ecosystems. This study emphasizes that use of field data can provide a better understanding of boundary layer fluxes, which in turn can help validate the fluxes simulated by land surface models. Implications of these results include the need for sustained monitoring and land–atmosphere interaction studies that are beneficial for effective water resource assessment and management.

Evaluating four gap-filling methods for eddy covariance measurements of evapotranspiration over hilly crop fields

Abstract. Estimating evapotranspiration in hilly watersheds is paramount for managing water resources, especially in semiarid/subhumid regions. The eddy covariance (EC) technique allows continuous measurements of latent heat flux (LE). However, time series of EC measurements often experience large portions of missing data because of instrumental malfunctions or quality filtering. Existing gap-filling methods are questionable over hilly crop fields because of changes in airflow inclination and subsequent aerodynamic properties. We evaluated the performances of different gap-filling methods before and after tailoring to conditions of hilly crop fields. The tailoring consisted of splitting the LE time series beforehand on the basis of upslope and downslope winds. The experiment was setup within an agricultural hilly watershed in northeastern Tunisia. EC measurements were collected throughout the growth cycle of three wheat crops, two of them located in adjacent fields on opposite hillslopes, and the third one located in a flat field. We considered four gap-filling methods: the REddyProc method, the linear regression between LE and net radiation (Rn), the multi-linear regression of LE against the other energy fluxes, and the use of evaporative fraction (EF). Regardless of the method, the splitting of the LE time series did not impact the gap-filling rate, and it might improve the accuracies on LE retrievals in some cases. Regardless of the method, the obtained accuracies on LE estimates after gap filling were close to instrumental accuracies, and they were comparable to those reported in previous studies over flat and mountainous terrains. Overall, REddyProc was the most appropriate method, for both gap-filling rate and retrieval accuracy. Thus, it seems possible to conduct gap filling for LE time series collected over hilly crop fields, provided the LE time series are split beforehand on the basis of upslope–downslope winds. Future works should address consecutive vegetation growth cycles for a larger panel of conditions in terms of climate, vegetation, and water status.