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William P. Kustas
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William P. Kustas

Abstract

One of the applications of remotely sensed surface temperature is to determine the latent heat flux (LE) or evapotranspiration (ET) from held to regional scales. A common approach has been to use surface-air temperature differences in a bulk resistance equation for estimating sensible beat flux, H, and to subsequently solve for LE as a residual in the one-dimensional energy balance equation. This approach has been successfully applied over uniform terrain with nearly full, actively transpiring vegetative cover; however, serious discrepancies between estimated and measured ET have been observed when there is partial canopy cover.

In an attempt to improve the estimates of H and as a result compute more accurate values of ET over partial canopy cover, one- and two-layer resistance models are developed to account for some of the factors causing the poor agreement between computed and measured ET.

The utility of these two approaches for estimating ET at the field scale is tested with remotely sensed and micrometeorological data collected in an and environment from a furrowed cotton field with 20 percent cover and a dry soil surface. The estimates of LE are compared with values measured using eddy correlation and energy balance methods. It is found that the one-layer model generally performed better than the two-layer model under thew conditions; but only when using a bluff-body correction to the resistance based on a conceptual model of beat and water vapor transfer at the surface taking place by molecular diffusion into Kolmogorov-scale eddies. The empirical adjustment to the surface resistance with the one-layer approach assumed to be applicable for a fairly wide range of conditions was found to be inappropriate. This result is attributed to the significant size of the furrows relative to the height of the vegetation.

Furthermore, a sensitivity analysis showed that the one-layer model with the empirical adjustment for the resistance was significantly affected by the changes in the surface roughness, whereas the physically based bluff- body correction was relatively insensitive to thew variations. For the two-layer model, a large change in the input variable for computing soil evaporation had a relatively small impact on the computed fuxes while a significant change in the leaf area index appeared to amplify the deviations between measured and modeled LE-values.

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Wilfried Brutsaert and William P. Kustas

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An analysis was made of vertical profiles of mean specific humidity in the hilly pre-Alpine region in Switzerland under conditions of new neutral atmospheric stability. The profile data were derived from radiosonde observations during the summer season of 1982. A logarithmic layer was found to extend from approximately 2h to 6h [where h(=95 m) is the mean height of the hills]. The zero-plane displacement height for water vapor d appeared to be close to that for momentum, i.e., about 50 m; on the other hand, the water vapor roughness height z for those ascents with a wet surface was found to be around 10 to 15 times smaller than the momentum roughness z 0(=3.8 m). The values of evaporation, deduced from the humidity profiles with the friction velocity of the corresponding velocity profiles, were very close to those measured with a lysimeter.

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Wilfried Brutsaert and William P. Kustas

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Measurements were made of the profiles of mean wind velocity, of temperature, and of specific humidity in the unstable atmospheric boundary layer over macro-rough terrain; these data were obtained from radiosonde observations in a calibrated watershed of 3.2 km2 in the hilly Pre-Alps of Switzerland during the summer of 1982. The regional evaporation was reasonably well correlated (R = 0.7) with these profile measurements through a logarithmic height dependency between roughly 2h0 (where h0 ≈ 100 m is the mean height of the roughness obstacles) and 0.6h (where h is the height of the boundary layer above the mean valley level). The shapes of the profiles appear to be essentially independent of the Monin–Obukhov parameter (zd0)/L, but they display a dependency on the wind shear aloft and on the value of (z0/h) (where z0 is the roughness height). Over this rugged surface the relative importance of mechanical turbulence, as compared to convective turbulence, is larger than over terrain with smaller (z0/h) for the same degree of instability of the atmosphere.

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William P. Kustas and Wilfried Brutsaert

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Inversion fluxes of water vapor are calculated for 11 clear days using profile data from a sequence of radio soundings over hilly terrain in the Pre-Alpine region of Switzerland. The relationships between the mixed layer and inversion layer gradients of specific humidity, and the inversion and surface fluxes of water vapor together with other turbulence parameters are investigated. The mean mixed layer gradient, ∂q/∂z, appears to be fairly well related to the inversion flux of water vapor, (wq)h, scaled with u * h or σh. The symbol u * represents the surface shear velocity, σ is a combination of u &ast and the convective velocity scale, w &ast and h the height of the mixed layer above the surface. There is also evidence that a relationship exists between the gradient in the inversion layer, (wq)h and an eddy diffusivity scaled as u *Δh, where Δh is the thickness of the inversion layer. The atmospheric water vapor budget below the inversion, with (wq)h parameterized, is tested with the present data as a means of determining surface evaporation. Transport in the mixed layer appears to be more strongly affected by mechanical-type than by convective-type turbulence.

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William P. Kustas and Karen S. Humes

Abstract

Radiometric surface temperature images from aircraft observations over the Walnut Gulch Experimental Watershed, a semiarid rangeland watershed, were used with ground-based meteorological data at a reference site for extrapolating estimates of surface sensible heat flux across the basin. Two approaches were used. One method assumed that the resistance to heat transport and other meteorological data at a reference site were constant over the watershed. This resulted in a simple scheme (constant resistance approach) for computing spatially distributed sensible heat flux since the variation in sensible heat flux was directly proportional to surface temperature differences from the reference site. The second approach (the variable resistance approach) used spatially distributed estimates of the surface roughness for momentum and heat, as well as air temperature and wind speed. The sensible heat flux values derived by both techniques were compared to measurements made at several other locations in the watershed for three different days. The environmental conditions for these days ranged from uniformly dry surface soil moisture to variably wet conditions caused by several high intensity and spatially variable rainfall events. Comparisons between these two schemes with observations indicated that the more detailed method of accounting for changes in surface roughness over the basin gave significantly better agreement than the simpler scheme. The average percentage of difference with measured values was 30% for the constant resistance approach compared to approximately 20% for the variable resistance method.

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William P. Kustas, John H. Prueger, and Lawrence E. Hipps

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A riparian corridor along the Rio Grande dominated by the Eurasian tamarisk or salt cedar (Tamarix spp.) is being studied to determine water and energy exchange rates using eddy covariance instrumentation mounted on a 12-m tower. The potential of using remotely sensed data to extrapolate these local estimates of the heat fluxes to large sections of the Rio Grande basin is under investigation. In particular, remotely sensed (radiometric) surface temperature can be used to estimate partitioning of net radiation energy into sensible and latent heat fluxes from vegetated landscapes. An important issue that has not been addressed adequately in the application of radiometric surface temperature data is the effect of using different time-averaged quantities in heat transfer formulations. This study evaluates the impact on sensible heat flux estimation of using relatively short time-averaged (1 min) canopy temperatures measured from a fixed-head infrared radiometer with 1-, 10-, and 30-min time-averaged micrometeorological input data used in estimating the resistance to heat transfer. The results indicate that, with short time-averaged radiometric surface temperatures (essentially “instantaneous” from a satellite), variations in sensible heat flux strongly correlate to fluctuations in net radiation conditions. Under near-constant net radiation input, natural perturbations in surface temperature also contribute to variations in sensible heat flux but are typically an order of magnitude smaller. The resulting implications for computed heat flux estimates using data from remotely sensing platforms and validation with flux tower measurements along riparian corridors are discussed.

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Wade T. Crow, Fuqin Li, and William P. Kustas

Abstract

The treatment of aerodynamic surface temperature in soil–vegetation–atmosphere transfer (SVAT) models can be used to classify approaches into two broad categories. The first category contains models utilizing remote sensing (RS) observations of surface radiometric temperature to estimate aerodynamic surface temperature and solve the terrestrial energy balance. The second category contains combined water and energy balance (WEB) approaches that simultaneously solve for surface temperature and energy fluxes based on observations of incoming radiation, precipitation, and micrometeorological variables. To date, few studies have focused on cross comparing model predictions from each category. Land surface and remote sensing datasets collected during the 2002 Soil Moisture–Atmosphere Coupling Experiment (SMACEX) provide an opportunity to evaluate and intercompare spatially distributed surface energy balance models. Intercomparison results presented here focus on the ability of a WEB-SVAT approach [the TOPmodel-based Land–Atmosphere Transfer Scheme (TOPLATS)] and an RS-SVAT approach [the Two-Source Energy Balance (TSEB) model] to accurately predict patterns of turbulent energy fluxes observed during SMACEX. During the experiment, TOPLATS and TSEB latent heat flux predictions match flux tower observations with root-mean-square (rms) accuracies of 67 and 63 W m−2, respectively. TSEB predictions of sensible heat flux are significantly more accurate with an rms accuracy of 22 versus 46 W m−2 for TOPLATS. The intercomparison of flux predictions from each model suggests that modeling errors for each approach are sufficiently independent and that opportunities exist for improving the performance of both models via data assimilation and model calibration techniques that integrate RS- and WEB-SVAT energy flux predictions.

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William P. Kustas, Jerry L. Hatfield, and John H. Prueger

Abstract

The Soil Moisture–Atmosphere Coupling Experiment (SMACEX) was conducted in conjunction with the Soil Moisture Experiment 2002 (SMEX02) during June and July 2002 near Ames, Iowa—a corn and soybean production region. The primary objective of SMEX02 was the validation of microwave soil moisture retrieval algorithms for existing and new prototype satellite microwave sensor systems under rapidly changing crop biomass conditions. The SMACEX study was designed to provide direct measurement/remote sensing/modeling approaches for understanding the impact of spatial and temporal variability in vegetation cover, soil moisture, and other land surface states on turbulent flux exchange with the atmosphere. The unique dataset consisting of in situ and aircraft measurements of atmospheric, vegetation, and soil properties and fluxes allows for a detailed and rigorous analysis, and the validation of surface states and fluxes being diagnosed using remote sensing methods at various scales. Research results presented in this special issue have illuminated the potential of satellite remote sensing algorithms for soil moisture retrieval, land surface flux estimation, and the assimilation of surface states and diagnostically modeled fluxes into prognostic land surface models. Ground- and aircraft-based remote sensing of the land surface and atmospheric boundary layer properties are used to quantify heat fluxes at the tower footprint and regional scales. Tower- and aircraft-based heat and momentum fluxes are used to evaluate local and regional roughness. The spatial and temporal variations in water, energy, and carbon fluxes from the tower network and aircraft under changing vegetation cover and soil moisture conditions are evaluated. An overview of the experimental site, design, data, hydrometeorological conditions, and results is presented in this introduction, and serves as a preface to this special issue highlighting the SMACEX results.

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Joseph A. Santanello Jr., Mark A. Friedl, and William P. Kustas

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Relationships among convective planetary boundary layer (PBL) evolution and land surface properties are explored using data from the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed in the southern Great Plains. Previous attempts to infer surface fluxes from observations of the PBL have been constrained by difficulties in accurately estimating and parameterizing the conservation equation and have been limited to multiday averages or small samples of daily case studies. Using radiosonde and surface flux data for June, July, and August of 1997, 1999, and 2001, a conservation approach was applied to 132 sets of daily observations. Results highlight the limitations of using this method on daily time scales caused by the diurnal variability and complexity of entrainment. A statistical investigation of the relationship among PBL and both land surface and near-surface properties that are not explicitly included in conservation methods indicates that atmospheric stability in the layer of PBL growth is the most influential variable controlling PBL development. Significant relationships between PBL height and soil moisture, 2-m potential temperature, and 2-m specific humidity are also identified through this analysis, and it is found that 76% of the variance in PBL height can be explained by observations of stability and soil water content. Using this approach, it is also possible to use limited observations of the PBL to estimate soil moisture on daily time scales without the need for detailed land surface parameterizations. In the future, the general framework that is presented may provide a means for robust estimation of near-surface soil moisture and land surface energy balance over regional scales.

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