Daily Latent Heat Flux Research Articles

Evapotranspiration controls across spatial scales of heterogeneity

AbstractIn semi‐arid regions where irrigation causes thermal surface heterogeneities, evapotranspiration is controlled not only by the land surface but also by the interaction between the surface and the atmospheric boundary layer. The spatial scale of heterogeneity impacts the processes that drive the diurnal variability of evapotranspiration. In this study, we combine data with a conceptual, coupled land–atmosphere model to study the drivers of evapotranspiration across spatial scales of heterogeneity for the Land Interactions with the Atmosphere in the Iberian Semi‐arid Environment (LIAISE) field campaign. We use a latent heat tendency equation as a diagnostic tool to quantify the contributions of various surface‐ and boundary‐layer driven processes on surface latent heat flux. We define the following spatial scales based on surface characteristics: regional, landscape, and local. We find that, at the regional and landscape scales (1 km), the feedback mechanisms between the surface fluxes and the resulting boundary‐layer dynamics enhance the daily latent heat flux by 64% to 77%. Conversely, at the local scale (100 m), surface‐driven processes are most important for governing evapotranspiration. At the local scale, we find that the energy stored at the surface enhances evaporation in the late afternoon, which could explain the observed time lag between net radiation and latent heat flux. Furthermore, the combined effects of entrainment at the boundary‐layer top and advection of heat and moisture enhance daily latent heat by 27% at the regional scale to 43% at the local scale. This analysis also has implications for the land‐surface modelling community. It suggests that properly reproducing evapotranspiration to capture the relevant processes depends on capacity of the model to resolve the interaction of scales in surface heterogeneous conditions.

English

This study analyzed latent heat flux from Amazon deforestation runoff above the Central/North American Boundary currents from 1988-2015. The purpose is to propose Atlantic hurricane intensification  from the heat flux via condensation. The author divided those currents into ten  areas of evaporation considering water budget data and regional and water-vapor-transport. A spreadsheet program consisting of two models had three inputs. Evaporation in each of the ten areas became the first input. For simplicity, each area’s evaporation decremented incoming runoff one time, and passed through less runoff, considering the currents’ average velocities and ocean condensation residency. Recent high-flow runoff data, limited from June 1 through November 30, a typical hurricane season, became the other inputs: all Amazon runoff (Model-A), only Amazon deforestation runoff (Model-O). The spreadsheet converted the condensation heat flux from each season (km3) into 10^17 J/day. This study compared those values to the 10^17 J/day wind energy of Category-1 or Category-3 hurricanes, finding order of magnitude similarity for such a crude comparison. The author then correlated hurricane Emily’s July 2005 daily path interface with the daily latent heat flux from the deforestation runoff. The analysis indicated that daily heat flux interfacing with Emily’s path measured 5.82% of a Category-3’s 10^17 J/day. When considering reuse runoff in deforested areas aggregate from 1970 to 2004, that 5.82% increases to  possibly 12.85%. This study’s simple analysis is by like terms (J/day) and similar order of magnitude (10^17) only, necessitating a more complex analysis.    Key words: Deforestation, hurricanes, latent heat flux, modelling, runoff.

СЕЗОННАЯ ИЗМЕНЧИВОСТЬ ПОТОКОВ ДИОКСИДА УГЛЕРОДА, ЯВНОГО И СКРЫТОГО ТЕПЛА В СЕВЕРОТАЕЖНОМ ЛИСТВЕННИЧНОМ ЛЕСУ СРЕДНЕЙ СИБИРИ ПО ДАННЫМ ПУЛЬСАЦИОННЫХ ИЗМЕРЕНИЙ

The seasonal and daily variability of sensible and latent heat and carbon dioxide (CO2) fluxes, as well as the sensitivity of the fluxes to changing environmental conditions were derived by the eddy covariance flux measurements in an undisturbed northern-taiga larch forest on continuous permafrost in Central Siberia from the end of May to the end of September in 2013 and 2015. The measurements showed a high seasonal and daily variability of CO2, sensible and latent heat fluxes. It was found that whereas dry and sunny weather conditions of June and July in 2013 and 2015 resulted in lower daily latent heat fluxes as compared to sensible ones, mostly cloudy and rainy weather in the second half of the summer led to the reduced net radiation and increased latent heat fluxes. The daily values of gross primary production from the beginning of June to mid-August in 2013 and 2015 exceeded the values of the ecosystem respiration, and the forest ecosystem was the sink of CO2 from the atmosphere. The maximum CO2 uptake was observed in late June and early July, primarily due to high photosynthesis rates influenced by high incoming solar radiation, optimal air temperature, and soil moisture conditions. Since mid-August, the balance between the CO2 uptake and release was close to zero. The total net ecosystem CO2 exchange during the growing season of 2013 was -76.5 g C/m2, while it was slightly higher (-103.6 g C/m2) during the growing season of 2015, primarily due to higher air temperatures in June and greater precipitation in the summer months of 2015.

High-frequency variability of latent-heat flux in the South China Sea

Synoptic-scale disturbances of latent-heat flux in the South China Sea are examined using Xisha automatic weather station measurements and satellite-derived daily latent heat-flux datasets. The power spectra of observations suggest a synoptic feature with a pronounced energy peak at a period of 4–8 days, comparable with the timescales of the atmospheric synoptic variabilities in the region. The characteristics of the synoptic latent-heat flux variations in summer are remarkably different from those in winter. The amplitudes of significant synoptic oscillations are about 20 and 40 W·m−2 during summer and winter monsoons, respectively. An active monsoon is clearly correlated with positive and negative phases of the synoptic latent-heat flux oscillations in the South China Sea. Using a composite analysis, some fundamental features of the synoptic latent-heat flux variabilities were revealed. In summer, the synoptic-scale latent-heat flux fluctuations are highly correlated with wind speed. While the synoptic variations of winds were removed during summer, the latent-heat anomalies precede about 1 day. In winter, however, they are primarily associated with winds and near-surface air humidity. When the synoptic oscillations of near-surface air humidity are removed, the amplitude of the composite latent-heat flux is reduced by 70%.

Evapotranspiration based on equilibrated relative humidity (ETRHEQ): Evaluation over the continental U.S.

AbstractA novel method of estimating evapotranspiration (ET), referred to as the ETRHEQ method, is further developed, validated, and applied across the U.S. from 1961 to 2010. The ETRHEQ method estimates the surface conductance to water vapor transport, which is the key rate‐limiting parameter of typical ET models, by choosing the surface conductance that minimizes the vertical variance of the calculated relative humidity profile averaged over the day. The ETRHEQ method, which was previously tested at five AmeriFlux sites, is modified for use at common weather stations and further validated at 20 AmeriFlux sites that span a wide range of climates and limiting factors. Averaged across all sites, the daily latent heat flux RMSE is ∼26 W·m−2 (or 15%). The method is applied across the U.S. at 305 weather stations and spatially interpolated using ANUSPLIN software. Gridded annual mean ETRHEQ ET estimates are compared with four data sets, including water balance‐derived ET, machine‐learning ET estimates based on FLUXNET data, North American Land Data Assimilation System project phase 2 ET, and a benchmark product that integrates 14 global ET data sets, with RMSEs ranging from 8.7 to 12.5 cm·yr−1. The ETRHEQ method relies only on data measured at weather stations, an estimate of vegetation height derived from land cover maps, and an estimate of soil thermal inertia. These data requirements allow it to have greater spatial coverage than direct measurements, greater historical coverage than satellite methods, significantly less parameter specification than most land surface models, and no requirement for calibration.

The surface energy budget on the debris-covered Koxkar Glacier in China

Energy fluxes were measured by using the eddy covariance system plus an automatic weather station at the debris-covered area on Koxkar Glacier from March to August, 2009. The coldest month of the glacier was January, and air temperature reached a maximum in July and August. Wind velocity at 2.0 m was higher in summer and lower in winter as a whole. Precipitation was concentrated from May to September and accounted for about 80 % of the total. Daily latent heat fluxes were higher than daily sensible heat fluxes during the observation period. The main reason for higher latent heat fluxes from March to April was snow cover. From June to August, latent heat fluxes during the daytime were limited by surface water content, and were lower than sensible heat fluxes, but latent heat fluxes were higher than sensible heat fluxes during summer nights because of air convection in the debris layer. Summer evaporation was higher than in the spring, and evaporation was 53.7 % of the precipitation from 19 June to 23 August. The Bowen ratio ranged from −2.0 to 2.0 at the site.

Evaluation of a MODIS triangle-based evapotranspiration algorithm for semi-arid regions

The current study investigates a MODIS-based remote-sensing-based algorithm for the assessment of daily latent heat flux (LE), or evapotranspiration (ET), with a focus on semi-arid regions. The approach uses the triangle method, relying on remotely sensed inputs and a previously developed net radiation model. A major difference from previous studies is that we utilize only MODIS products for estimation of ET at the daily timestep for clear and cloudy days. The algorithm is evaluated at four flux tower locations in the San Pedro River basin in Arizona. The mean daily LE varies significantly between the sites, ranging from 144 to 179 W/m 2 in the riparian areas to 36 to 76 W/m 2 at the rangeland sites. Comparison of the flux towers shows good correlation and low root mean square error at the riparian sites (0.81 to 0.82 and 37 to 51 W/m 2 , respectively) with slightly larger errors at the upland sites, where ET is strongly correlated to precipitation events. The model assumption of a linear variation in evaporative fraction across the triangular domain (LST/EVI space) results in more uncertainty under water-stressed conditions such as those found at the upland sites. Overall, the proposed MODIS-based algorithm provides reasonable estimates of riparian and upland plant water use and unique spatial and temporal information.

Temporal upscaling of instantaneous evapotranspiration: An intercomparison of four methods using eddy covariance measurements and MODIS data

The quantification of land surface evapotranspiration (ET) at daily or longer time scales is of great significance in modeling the global hydrological cycle, studying climate change, and managing water resources. However, current remote sensing-based ET models can generally only provide snapshots of ET at the time of a satellite overpass and do not satisfy the expectations of hydrologists, irrigation engineers, or water resource managers concerned with practical applications. Four commonly used ET upscaling schemes, namely, the constant reference evaporative fraction (EFr) method, the constant evaporative fraction (EF) method, the constant extraterrestrial solar radiation ratio (R-p) method, and the constant observed global solar radiation ratio (R-g) method, were evaluated in this study using ground-based eddy covariance (EC) measurements and the Moderate Resolution Imaging Spectroradiometer (MODIS) derived estimates from a two-source energy balance model. Analysis was made of both closed and un-closed surface fluxes and of different assumed satellite overpass times from mid-morning to mid-afternoon. Data for the analysis were collected at the Yucheng comprehensive experimental station in Northern China, spanning the period from late April 2009 to late October 2011. The results show that all four upscaling factors from noon to mid-afternoon had a better agreement with their corresponding daily averages. Overall, the EFr (R-g) method had the best (second best) performance of the four upscaling methods. The EF method was found to significantly underestimate the daily latent heat flux (LE) and performed the worst of the four upscaling methods. With the correction of the energy imbalance of EC measurements, the R-g method's performance was improved and outperformed the EF, method in the morning. The presence of clouds either increased or decreased the BIAS but generally increased the root mean square error (RMSE) for all four upscaling methods. When the upscaling methods were applied to convert the instantaneous MODIS remote sensing estimates of ET to daily values, the accuracy of the extrapolated daily ET was controlled by the accuracy of both the remote sensing ET estimates and the upscaling methods themselves. With the insignificantly biased estimates of the instantaneous LE from the N95 model at 101 MODIS overpass times in this study, using the EF method underestimated the daily ET by 11%, while all the remaining three factors overestimated the daily ET by a range of 5%-18%. This study was conducted to provide a scientific basis for developing an operational and more accurate ET-upscaling method with easy access to data in future studies. (C) 2013 Elsevier Inc. All rights reserved.

Validation of Satellite-Derived Daily Latent Heat Flux over the South China Sea, Compared with Observations and Five Products

Abstract This study describes the development of the South China Sea (SCS) daily satellite-derived latent heat flux (SCSSLH) for the period of 1998–2011 at 0.25° × 0.25° resolution using data mainly from the Tropical Rain Measuring Mission (TRMM) Microwave Imager (TMI). Flux-related variables of daily TMI data smoothed with 3-day running mean were finally chosen because of the best fit with the 1727 high-quality observations from seven moored stations and 24 ship surveys. Near-surface air specific humidity was computed using the global relationship based on satellite precipitable water. Verification against 1016 high-resolution radiosonde profiles from 1998 to 2012 and the time series from the Xisha automatic weather station during 2008–10 indicate that this satellite-derived air specific humidity can reasonably capture observed mean condition and temporal variability. They are therefore used to derive SCSSLH based on the Coupled Ocean–Atmosphere Response Experiment version 3.0 (COARE 3.0) algorithm. Compared with five other latent heat flux products—the Goddard Satellite-Based Surface Turbulent Fluxes version 2 (GSSTF2), the objectively analyzed air–sea heat fluxes (OAFlux), the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data version 3 (HOAPS3), the National Centers for Environmental Prediction/Department of Energy Global Reanalysis 2 (NCEP-2), and the European Centre for Medium-Range Weather Forecasts (ECMWF)—the daily SCSSLH shows the highest spatial resolution and realistic values in the SCS, with an exception along the northern continental shelf. More importantly, the other five products seem to overestimate the latent heat flux systematically. The flux representation in this study comes not only with a better flux algorithm but also with the improved estimates of bulk variables based on in situ measurements, which further highlights the unique role of high-quality meteorological measurements and atmospheric weather stations in evaluating the air–sea interaction in the SCS.

Verification of land surface evapotranspiration estimation from remote sensing spatial contextual information

AbstractEstimation of evapotranspiration (ET) is of great significance in modeling the water and energy interactions between land and atmosphere. Negative correlation of surface temperature (Ts) versus vegetation index (VI) from remote sensing data provides diagnosis on the spatial pattern of surface soil moisture and ET. This study further examined the applicability of Ts–VI triangle method with a newly developed edges determination technique in estimating regional evaporative fraction (EF) and ET at MODIS pixel scale through comparison with large aperture scintillometer (LAS) and high‐level eddy covariance measurements collected at Changwu agro‐ecological experiment station from late June to late October, 2009. An algorithm with merely land and atmosphere products from MODIS onboard Terra satellite was used to estimate the surface net radiation (Rn) and soil heat flux. In most cases, the estimated instantaneous Rn was in good agreement with surface measurement with slight overestimation by 12 W/m2. Validation results from LAS measurement showed that the root mean square error is 0.097 for instantaneous EF, 48 W/m2 for instantaneous sensible heat flux, and 30 W/m2 for daily latent heat flux. This paper successfully presents a miniature of the overall capability of Ts–VI triangle in estimating regional EF and ET from limited number of data. For a thorough interpretation, further comprehensive investigation needs to be done with more integration of remote sensing data and in‐situ surface measurements. Copyright © 2011 John Wiley & Sons, Ltd.

Evapotranspiration in the Pampean Region using field measurements and satellite data

Evapotranspiration (LE) is an important factor for monitoring crops, water requirements, and water consumption at local and regional scale. In this paper, we applied the semi-empirical model to estimate the daily latent heat flux (LEd=Rnd+A−B(Ts−Ta)). LEd has been estimated using satellite images (Thematic Mapper sensor) and a local dataset (incoming and outgoing short- and long-wave radiation) measured during three years. We first estimated the daily net Radiation (Rnd) from a linear equation derived from the instantaneous net Radiation (Rnd=CRni+D). Subsequently, coefficients A and B have been estimated for two different cover vegetations (pasture and soybean). For each vegetation cover, an error analysis combining Rnd, A, B, and surface and air temperatures has been calculated. Results showed that Rnd had good performance (nonbias and low RMSE). LEd errors for pasture and soybean were ±28Wm−2 and ±40Wm−2 respectively.

Energy balance and its closure over a young larch forest in eastern Siberia

To elucidate the energy balance over a forest, seasonal changes in the water and energy exchanges over a young larch forest (E-YL) in eastern Siberia were investigated and compared with the reported characteristics of a mature larch forest (W-ML) in the same region. Observations of the energy fluxes were conducted during the growing season from 15 April through 14 September 2000. The sum of the turbulent heat fluxes H and λE, ground heat flux G and heat storage change rate Δ S was generally in good agreement with the net radiation flux R NET, except when the friction velocity u * was <0.2 m s −1. Atmospheric instability also reduced the energy balance ratio EBR in the daytime. The EBR was defined as the ratio of the turbulent heat flux H + λE toward the available energy input R NET − G − Δ S for the reference horizontal plane, whereas the energy closure fraction CF was defined as the ratio of the net ecosystem heat exchange H + λE + Δ S atmosphere to the available energy input R NET − G − Δ S biomass for the biomass surface. EBR and CF had different features in their relationship to u * at night. The threshold to reject nighttime data at low u * values was estimated to be 0.2 m s −1 by the EBR, but >0.3 m s −1 by the CF. The averaged features of the CF relationships with u * and atmospheric stability ζ were similar to those reported for Canadian boreal forests. In contrast, hourly CF and hourly EBR were widely scattered. The probability functions of EBR for different wind velocities were consistent with previous results from large eddy simulation. To determine the seasonal course of daily flux values, the Bowen ratio energy budget method was used to estimate the turbulent heat fluxes. The daily latent heat flux maintained a value of approximately 50 W m −2 during the summer, whereas the daily sensible heat flux varied with changes in the available energy. The Bowen ratio had a clear “U” shape over the course of the growing season. The minimum values of approximately 1.0 occurred in the summer from June to August. The general features of seasonal changes at E-YL in 2000 were similar to those at W-ML in 1998, except for the lower Bowen ratio in August at E-YL.

Validation of the Canadian land surface scheme (class) for a subarctic open woodland

This study reports on testing of the peatland version of the Canadian Land Surface Scheme (CLASS) for simulating the energy balance of subarctic open woodland terrain. Model results are compared against several years of measured data from a site near Churchill, Manitoba. In contrast to most forest environments, the floor of the open forest plays a large role in total ecosystem energy exchange. This behaviour presents a significant challenge for land surface models like CLASS and their simplified treatment of vegetation canopies. Simulations of summer energy balance for seven years encompassing a wide range of meteorological conditions produced consistent results. Root mean square errors for sensible and latent heat fluxes fell between 11 and 28 W m−2. CLASS consistently underestimated slightly the daily latent heat flux and overestimated the sensible heat flux, average mean bias errors being ‐7.6 and 9.1 W m−2, respectively. The soil heat flux was less well represented. In general, CLASS was able to capture the diurnal and seasonal behaviour of the measured fluxes under a range of conditions with reasonable accuracy. In a full year simulation, CLASS reproduced the annual variations in energy balance with some discrepancies associated with snow accumulation and ablation periods. The model performance was sensitive to both snow density and specification of the surface cover. Recommendations for improving the model for subarctic woodlands and terrain types with similar features are discussed.

Productivity and water use of wheat under free‐air CO2 enrichment

AbstractA free‐air CO2 enrichment (FACE) experiment was conducted at Maricopa, Arizona, on wheat from December 1992 through May 1993. The FACE apparatus maintained the CO2 concentration, [CO2], at 550 μmol mol−1 across four replicate 25‐m‐diameter circular plots under natural conditions in an open field. Four matching Control plots at ambient [CO2] (about 370 μmol mol−1) were also installed in the field. In addition to the two levels of [CO2], there were ample (Wet) and limiting (Dry) levels of water supplied through a subsurface drip irrigation system in a strip, split‐plot design.Measurements were made of net radiation, Rn; soil heat flux, Go; soil temperature; foliage or surface temperature; air dry and wet bulb temperatures; and wind speed. Sensible heat flux, H, was calculated from the wind and temperature measurements. Latent heat flux, λET, and evapotranspiration, ET, were determined as the residual in the energy balance. The FACE treatment reduced daily total Rn by an average 4%. Daily FACE sensible heat flux, H, was higher in the FACE plots. Daily latent heat flux, λET, and evapotranspiration, ET, were consistently lower in the FACE plots than in the Control plots for most of the growing season, about 8% on the average.Net canopy photosynthesis was stimulated by an average 19 and 44% in the Wet and Dry plots, respectively, by elevated [CO2] for most of the growing season. No significant acclimation or down regulation was observed. There was little above‐ground growth response to elevated [CO2] early in the season when temperatures were cool. Then, as temperatures warmed into spring, the FACE plants grew about 20% more than the Control plants at ambient [CO2], as shown by above‐ground biomass accumulation. Root biomass accumulation was also stimulated about 20%. In May the FACE plants matured and senesced about a week earlier than the Controls in the Wet plots. The FACE plants averaged 0.6 °C warmer than the Controls from February through April in the well‐watered plots, and we speculate that this temperature rise contributed to the earlier maturity. Because of the acceleration of senescence, there was a shortening of the duration of grain filling, and consequently, there was a narrowing of the final biomass and yield differences. The 20% mid‐season growth advantage of FACE shrunk to about an 8% yield advantage in the Wet plots, while the yield differences between FACE and Control remained at about 20% in the Dry plots.

Estimates of Surface Humidity and Latent Heat Fluxes over Oceans from SSM/I Data

Monthly averages of daily latent heat fluxes over the oceans for February and August 1988 are estimated using a stability-dependent bulk scheme. Daily fluxes are computed from daily SSM/I (Special Sensor Microwave/Imager) wind speeds and EOF-retrieved SSM/I surface humidity, National Meteorological Center sea surface temperatures, and the European Centre for Medium-Range Weather Forecasts analyzed 2-m temperatures. Daily surface specific humidity (Q) is estimated from SSM/I precipitable water of total (W) and a 500-m bottom layer (WB) using an EOF (empirical orthogonal function) method. This method has six W-based categories of EOFs (independent of geographical locations) and is developed using 23 177 FGGE IIb humidity soundings over the global oceans. For 1200 FGGE IIb humidity soundings, the accuracy of EOF-retrieved Q is 0.75 g kg−1 for the case without errors in W and WB, and increases to 1.16 g kg−1 for the case with errors in W and WB. Compared to 342 collocated radiosonde observations, the EOF-retrieved SSM/I Q has an accuracy of 1.7 g kg−1. The method improves upon the humidity retrieval of Liu and is competitive with that of Schulz et al. The SSM/I surface humidity and latent heat fluxes of these two months agree reasonably well with those of COADS (Comprehensive Ocean–Atmosphere Data Set). Compared to the COADS, the sea–air humidity difference of SSM/I has a positive bias of approximately 1–3 g kg−1 (an overestimation of flux) over the wintertime trade wind belts and wintertime extratropical oceans. In the summertime extratropical Pacific and summertime eastern equatorial Pacific Ocean, it has a negative bias of about 1–2 g kg−1 (an underestimation of flux). The results further suggest that the two monthly flux estimates, computed from daily and monthly mean data, do not differ significantly over the oceans.

High‐frequency variability of Special Sensor Microwave/Imager derived wind speed and moisture during an intraseasonal oscillation

Special Sensor Microwave/Imager data are used to examine the day‐to‐day variability of wind speed and precipitable water during the passage of an intraseasonal oscillation. First, SSM/I precipitable water and wind speed algorithms are evaluated for the tropical western Pacific Ocean. The algorithm by Alishouse for retrieving precipitable water vapor retrieves precipitable water values that compare well with island radiosonde observations, but the wind speeds retrieved using the algorithm by Goodberlet et al. compare with buoy wind speeds only marginally. A revised wind speed algorithm, based on regression with buoy wind speeds, is derived for use in this region. During the intraseasonal oscillation, anomalously high values of precipitable water, with maximum amplitude at 10°S, accompany the movement of convection eastward. Anomalously high wind speeds are found to the west of the convection and accompany its movement east. Differences of up to 20% are found along 10°S between the means of daily latent heat flux and U*3 values and the 60‐day mean of these quantities. This implies that high frequency variability must be explicitly accounted for in diagnostics and modeling of air‐sea interaction in the western equatorial Pacific Ocean.

Is Evaporation an Important Component in High Alpine Hydrology?

The knowledge of evaporation in the high mountain areas of the European Alps is still rather poor. It is generally regarded as a component of secondary importance in the water balance. The available mean areal evaporation data are based on conventional water balance estimations and suffer from inaccuracies in the determination of precipitation. This is also obvious from the rate of decrease in mean annual evaporation with altitude indicated by different authors; these values range from 71 mm to 356 mm pro 1,000 m of altitude. From heat balance studies on glaciers it is evident that evaporation/condensation as a process of high specific energy exchange can be a determinative factor in the shortterm variations of melt rates. The scale width of the daily latent heat fluxes reaches at magnitudes equal to or larger than those of net radiation and sensible heat flux.

Turbulent Transport and Energy Balance as Affected by a Windbreak in an Irrigated Sugar Beet (Beta vulgaris) Field1

AbstractTurbulent exchange coefficients for sugar beets sheltered by corn windbreaks and unsheltered were calculated by means of an energy balance. Occasionally measurements were made within the boundary layer characteristic of the underlying surfaces at four locations in shelter and in the open field. At such times the coefficients were 25% lower on the average in the sheltered area. These results agree with the few available reports on this subject.Decreased exchange coefficient in the sheltered area acts to suppress the transport of sensible heat. The suppression of upward sensible heat transport in a Great Plains irrigated valley during the morning results in an increase in energy available for consumption as latent heat. During the afternoon, however, when sensible heat is advected to the crop, latent heat flux is lower in the sheltered crop than in the open field. Thus the total daily latent heat flux may be the same in both plots. The sheltered crop is subjected to a reduced evaporative demand, however, during periods when water stress is most likely.