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- Author or Editor: William P. Kustas x
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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.
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.
Abstract
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 (z− d0 )/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.
Abstract
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 (z− d0 )/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.
Abstract
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 0ν appeared to be close to that for momentum, i.e., about 50 m; on the other hand, the water vapor roughness height z 0ν 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.
Abstract
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 0ν appeared to be close to that for momentum, i.e., about 50 m; on the other hand, the water vapor roughness height z 0ν 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.
Abstract
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, (
Abstract
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, (
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.
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.
Abstract
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.
Abstract
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.
Abstract
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.
Abstract
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.
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.
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.
Abstract
This paper investigates the spatial relationships between surface fluxes and near-surface atmospheric properties (AP), and the potential errors in flux estimation due to homogeneous atmospheric inputs over heterogeneous landscapes. A large-eddy simulation (LES) model is coupled to a surface energy balance scheme with remotely sensed surface temperature Ts as a key boundary condition. Simulations were performed for different agricultural regions having major contrasts in Ts , canopy cover, and surface roughness z 0 between vegetated/irrigated and bare soil areas. If AP from a single weather station in a nonrepresentative location within the landscape are applied uniformly over the domain, significant differences in surface flux estimation with respect to the LES output are observed. The spatial correlations of AP with the fluxes, the land cover properties, and surface states were examined and the spatial scaling of these fields is analyzed using a two-dimensional wavelet technique. The results indicate a significant local correlation of the spatial distributions of the air temperature Ta with the sensible heat flux H, the specific humidity q with the latent heat flux LE, and the wind speed U with z 0. These relationships can be described by a general linear form, suggesting that a simple regression relation may be applicable for most agricultural landscapes to estimate spatially variable AP fields. A simple yet practical method is proposed using remotely sensed observations and the land surface scheme, based on general linear expressions derived between Ta and H, q and LE, and U and z 0. The method is shown to reproduce the main spatial patterns of AP and to reduce potential errors in local and regionally averaged heat flux estimation. This approach is recommended when only local weather station observations are available.
Abstract
This paper investigates the spatial relationships between surface fluxes and near-surface atmospheric properties (AP), and the potential errors in flux estimation due to homogeneous atmospheric inputs over heterogeneous landscapes. A large-eddy simulation (LES) model is coupled to a surface energy balance scheme with remotely sensed surface temperature Ts as a key boundary condition. Simulations were performed for different agricultural regions having major contrasts in Ts , canopy cover, and surface roughness z 0 between vegetated/irrigated and bare soil areas. If AP from a single weather station in a nonrepresentative location within the landscape are applied uniformly over the domain, significant differences in surface flux estimation with respect to the LES output are observed. The spatial correlations of AP with the fluxes, the land cover properties, and surface states were examined and the spatial scaling of these fields is analyzed using a two-dimensional wavelet technique. The results indicate a significant local correlation of the spatial distributions of the air temperature Ta with the sensible heat flux H, the specific humidity q with the latent heat flux LE, and the wind speed U with z 0. These relationships can be described by a general linear form, suggesting that a simple regression relation may be applicable for most agricultural landscapes to estimate spatially variable AP fields. A simple yet practical method is proposed using remotely sensed observations and the land surface scheme, based on general linear expressions derived between Ta and H, q and LE, and U and z 0. The method is shown to reproduce the main spatial patterns of AP and to reduce potential errors in local and regionally averaged heat flux estimation. This approach is recommended when only local weather station observations are available.
