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Z. Su, T. Schmugge, W. P. Kustas, and W. J. Massman

Abstract

Roughness height for heat transfer is a crucial parameter in estimation of heat transfer between the land surface and the atmosphere. Although many empirical formulations have been proposed over the past few decades, the uncertainties associated with these formulations are shown to be large, especially over sparse canopies. In this contribution, a simple physically based model is derived for the estimation of the roughness height for heat transfer. This model is derived from a complex physical model based on the “localized near-field” Lagrangian theory. This model (called Massman's model) and another recently proposed model derived by fitting simulation results of a simple multisource bulk transfer model (termed Blümel's model) are evaluated using three experimental datasets. The results of the model performances are judged by using the derived roughness values to compute sensible heat fluxes with the bulk transfer formulation and comparing these computed fluxes to the observed sensible heat fluxes. It is concluded, on the basis of comparison of calculated versus observed sensible heat fluxes, that both the current model and Blümel's model provide reliable estimates of the roughness heights for heat transfer. The current model is further shown to be able to explain the diurnal variation in the roughness height for heat transfer. On the basis of a sensitivity analysis, it is suggested that, although demanding, most of the information needed for both models is amendable by satellite remote sensing such that their global incorporation into large-scale atmospheric models for both numerical weather prediction and climate research merits further investigation.

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K. S. Humes, W. P. Kustas, and D. C. Goodrich

Abstract

Spatially distributed radiometric surface temperatures over a semiarid watershed were computed using remotely sensed data acquired with an aircraft-based multispectral scanner during the Monsoon ’90 Large Scale Field Experiment. The multispectral scanner data provide watershed coverage of surface temperature at a resolution of 6.3 m and nearly daily temporal resolution. At high spatial resolution, the surface temperature values appear to be correlated strongly with surface aspect; at more coarse spatial resolution, the surface temperatures variations across the watershed appear to be correlated with background soil moisture variations caused by highly localized precipitation events. The surface temperature data were aggregated to 400-m spatial resolution for the purpose of computing spatially distributed sensible heat fluxes over the watershed. The practicality of using a spatially uniform transfer coefficient was evaluated by examining the variability of surface and meteorological factors across the watershed at the times of the aircraft overpasses. Maps of sensible heat flux over the area were computed for three aircraft overpass dates and compared to localized patterns of recent precipitation in the basin. Maps of instantaneous sensible heat flux tracked well with the spatial patterns of variable surface soil moisture that arose from the localized precipitation events.

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G. N. Flerchinger, W. P. Kustas, and M. A. Weltz

Abstract

While land–atmosphere transfer models have been pursued for over 30 years, Soil–Vegetation–Atmosphere–Transfer (SVAT) models are gaining attention only recently as the need to better represent the interaction between the soil and atmosphere in atmospheric circulation models becomes more apparent. The Simultaneous Heat and Water (SHAW) model, a detailed physical process model, simulates the effects of a multispecies plant canopy on heat and water transfer at the soil–atmosphere interface. The model was used in this study to simulate the surface energy balance and surface temperature of two vegetation communities using data collected during the Monsoon ’90 multidisciplinary field experiment. The two vegetation communities included a sparse, relatively homogeneous, grass-dominated community and a shrub-dominated site with large bare interspace areas between shrubs. The model mimicked the diurnal variation in the surface energy balance at both sites, while canopy leaf temperatures were simulated somewhat better at the relatively homogeneous grass-dominated site. The variation in surface fluxes accounted for by the model (i.e., model efficiency) ranged from 59% for latent heat flux at the shrub-dominated site to 98% for net radiation at both sites. Model efficiency for predicting latent heat flux at the grass-dominated site was 65%. Canopy leaf temperatures for the shrub-dominated site were consistently overpredicted by 1.8°C compared to measured values. Simulated soil surface temperatures at both sites had a model efficiency of 94% and a mean bias error of less of than 0.9°C. The ability of the model to simulate canopy and soil surface temperatures gives it the potential to be verified and periodically updated using remotely sensed radiometric surface temperature and soil moisture when extrapolating model-derived fluxes to other areas. A methodology is proposed whereby model predictions can be used with a combination of remotely sensed radiometric surface temperature and surface soil moisture to estimate soil water content within the rooting depth.

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D. W. Meek, J. H. Prueger, W. P. Kustas, and J. L. Hatfield

Abstract

Two eddy covariance instrument comparison studies were conducted before and after the Soil Moisture–Atmosphere Coupling Experiment (SMACEX) field campaign to 1) determine if observations from multiple sensors were equivalent for the measured variables over a uniform surface and to 2) determine a least significant difference (LSD) value for each variable to discriminate between daily and hourly differences in latent and sensible heat and carbon dioxide fluxes, friction velocity, and standard deviation of the vertical wind velocity from eddy covariance instruments placed in different locations within the study area. The studies were conducted in early June over an alfalfa field and in mid-September over a short grass field. Several statistical exploratory, graphical, and multiple-comparison procedures were used to evaluate each daily variable. Daily total or average data were used to estimate a pooled standard error and corresponding LSD values at the P = 0.05 and P = 0.01 levels using univariate procedures. There were no significant sensor differences in any of the daily measurements for either intercomparison period. Hourly averaged data were used to estimate a pooled standard error and corresponding LSD values at the P = 0.05 and P = 0.01 levels using mixed model procedures. Sensor differences for pre- and post-intercomparisons were minimal for hourly and daily values of CO2, water vapor, sensible heat, friction velocity, and standard deviation for vertical wind velocity. Computed LSD values were used to determine significant daily differences and threshold values for the variables monitored during the SMACEX campaign.

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W. P. Kustas, K. S. Humes, J. M. Norman, and M. S. Moran

Abstract

Single- and dual-source models of the surface energy transfer across the soil-vegetation-atmosphere interface were used in conjunction with remotely sensed surface temperature for computing the surface energy balance over heterogeneous surfaces. Both models are relatively simple so that only a few parameters are specified, making them potentially useful for computing surface fluxes with operational satellite observations. The models were tested with datasets collected from a semiarid rangeland environment with canopy cover generally less than 50% and a subhumid tallgrass prairie environment having canopy cover typically greater than 50%. For the semiarid site, differences between the single-source and dual-source model estimates of the sensible heat flux (H) and the observations averaged about 25%. For the tallgrass prairie, the disagreement between observations and single-source model estimates of H was significantly larger, averaging nearly 55%. The average difference between observations and the dual-source model predictions for the tallgrass prairie site increased slightly from the semiarid site to 30%. The latent heat flux (LE) was determined by residual from measurements of net radiation and model estimates of the soil heat flux. For the semiarid site, the single-source model estimates of LE differed on average with the observations by about 15%, whereas the LE values computed by the dual-source model differed by about 20%. For the tallgrass prairie site, the LE values from the single-source model differed from the observations by almost 35%, on average, whereas the dual-source model estimates produced an average difference of about 20%. Given the fact that energy flux observations by various techniques have been found to differ by at least 20%, the single-source model performed satisfactorily for the semiarid site but had difficulty reproducing the fluxes at the tallgrass prairie site. The dual-source model, however, performed reasonably well at both sites. To obtain results comparable to the dual-source model for the tallgrass prairie site, the single-source model required significant modifications to a parameter used in estimating the roughness length for heat.

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J. B. Stewart, W. P. Kustas, K. S. Humes, W. D. Nichols, M. S. Moran, and H. A. R. de Bruin

Abstract

Measurements of sensible heat flux, radiometric surface temperature, air temperature, and wind speed made at eight semiarid rangeland sites were used to investigate the sensible heat flux-aerodynamic resistance relationship. The individual sites covered a wide range of vegetation (0.1–4 m tall) and cover (3%–95% bare soil) conditions. Mean values of kB −1, a quantity related to the resistance of heat versus momentum transfer at the surface, for the individual sites varied between 3.5 and 12.5. A preliminary test of the utility of an excess resistance based on the mean value of kB −1 showed that the difference between the mean estimated and measured sensible heat fluxes varied ±60 W m−2 for the eight semiarid sites. For the eight sites the values of kB −1 were plotted against the roughness Reynolds number. The plot showed considerable scatter with values ranging between and beyond the theoretical curves for bluff rough and permeable rough surfaces.

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W. P. Kustas, T. J. Schmugge, K. S. Humes, T. J. Jackson, R. Parry, M. A. Weltz, and M. S. Moran

Abstract

Measurements of the microwave brightness temperature (TB) with the Pushbroom Microwave Radiometer (PBMR) over the Walnut Gulch Experimental Watershed were made on selected days during the MONSOON 90 field campaign. The PBMR is an L-band instrument (21-cm wavelength) that can provide estimates of near-surface soil moisture over a variety of surfaces. Aircraft observations in the visible and near-infrared wavelengths collected on selected days also were used to compute a vegetation index. Continuous micrometeorological measurements and daily soil moisture samples were obtained at eight locations during the experimental period. Two sites were instrumented with time domain reflectometry probes to monitor the soil moisture profile. The fraction of available energy used for evapotranspiration was computed by taking the ratio of latent heat flux (LE) to the sum of net radiation (Rn) and soil heat flux (G). This ratio is commonly called the evaporative fraction (EF) and normally varies between 0 and 1 under daytime convective conditions with minimal advection. A wide range of environmental conditions existed during the field campaign, resulting in average EF values for the study area varying from 0.4 to 0.8 and values of TB ranging from 220 to 280 K. Comparison between measured TB and EF for the eight locations showed an inverse relationship with a significant correlation (r 2 = 0.69). Other days were included in the analysis by estimating TB with the soil moisture data. Because transpiration from the vegetation is more strongly coupled to root zone soil moisture, significant scatter in this relationship existed at high values of TB or dry near-surface soil moisture conditions. It caused a substantial reduction in the correlation with r 2 = 0.40 or only 40% of the variation in EF being explained by TB. The variation in EF under dry near-surface soil moisture conditions was correlated to the amount of vegetation cover estimated with a remotely sensed vegetation index. These findings indicate that information obtained from optical and microwave data can be used for quantifying the energy balance of semiarid areas. The microwave data can indicate when soil evaporation is significantly contributing to EF, while the optical data is helpful for quantifying the spatial variation in EF due to the distribution of vegetation cover.

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W. E. Eichinger, H. E. Holder, R. Knight, J. Nichols, D. I. Cooper, L. E. Hipps, W. P. Kustas, and J. H. Prueger

Abstract

The Soil Moisture–Atmosphere Coupling Experiment (SMACEX) was conducted in the Walnut Creek watershed near Ames, Iowa, over the period from 15 June to 11 July 2002. A main focus of SMACEX is the investigation of the interactions between the atmospheric boundary layer, surface moisture, and canopy. A vertically staring elastic lidar was used to provide a high-time-resolution continuous record of the boundary layer height at the edge between a soybean and cornfield. The height and thickness of the entrainment zone are used to estimate the surface sensible heat flux using the Batchvarova–Gryning boundary layer model. Flux estimates made over 6 days are compared to conventional eddy correlation measurements. The calculated values of the sensible heat flux were found to be well correlated (R 2 = 0.79, with a slope of 0.95) when compared to eddy correlation measurements in the area. The standard error of the flux estimates was 21.4 W m−2 (31% rms difference between this method and surface measurements), which is somewhat higher than a predicted uncertainty of 16%. The major sources of error were from the estimates of the vertical potential temperature gradient and an assumption that the entrainment parameter A was equal to the ratio of the entrainment flux and the surface heat flux.

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J. H. Prueger, J. L. Hatfield, T. B. Parkin, W. P. Kustas, L. E. Hipps, C. M. U. Neale, J. I. MacPherson, W. E. Eichinger, and D. I. Cooper

Abstract

A network of eddy covariance (EC) and micrometeorological flux (METFLUX) stations over corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] canopies was established as part of the Soil Moisture–Atmosphere Coupling Experiment (SMACEX) in central Iowa during the summer of 2002 to measure fluxes of heat, water vapor, and carbon dioxide (CO2) during the growing season. Additionally, EC measurements of water vapor and CO2 fluxes from an aircraft platform complemented the tower-based measurements. Sensible heat, water vapor, and CO2 fluxes showed the greatest spatial and temporal variability during the early crop growth stage. Differences in all of the energy balance components were detectable between corn and soybean as well as within similar crops throughout the study period. Tower network–averaged fluxes of sensible heat, water vapor, and CO2 were observed to be in good agreement with area-averaged aircraft flux measurements.

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W. P. Kustas, D.C. Goodrich, M.S. Moran, S. A. Amer, L. B. Bach, J. H. Blanford, A. Chehbouni, H. Claassen, W. E. Clements, P. C. Doraiswamy, P. Dubois, T. R. Clarke, C. S. T. Daughtry, D. I. Gellman, T. A. Grant, L. E. Hipps, A. R. Huete, K. S. Humes, T. J. Jackson, T. O. Keefer, W. D. Nichols, R. Parry, E. M. Perry, R. T. Pinker, P. J. Pinter Jr., J. Qi, A. C. Riggs, T. J. Schmugge, A. M. Shutko, D. I. Stannard, E. Swiatek, J. D. van Leeuwen, J. van Zyl, A. Vidal, J. Washburne, and M. A. Weltz

Arid and semiarid rangelands comprise a significant portion of the earth's land surface. Yet little is known about the effects of temporal and spatial changes in surface soil moisture on the hydrologic cycle, energy balance, and the feedbacks to the atmosphere via thermal forcing over such environments. Understanding this interrelationship is crucial for evaluating the role of the hydrologic cycle in surface–atmosphere interactions.

This study focuses on the utility of remote sensing to provide measurements of surface soil moisture, surface albedo, vegetation biomass, and temperature at different spatial and temporal scales. Remote-sensing measurements may provide the only practical means of estimating some of the more important factors controlling land surface processes over large areas. Consequently, the use of remotely sensed information in biophysical and geophysical models greatly enhances their ability to compute fluxes at catchment and regional scales on a routine basis. However, model calculations for different climates and ecosystems need verification. This requires that the remotely sensed data and model computations be evaluated with ground-truth data collected at the same areal scales.

The present study (MONSOON 90) attempts to address this issue for semiarid rangelands. The experimental plan included remotely sensed data in the visible, near-infrared, thermal, and microwave wavelengths from ground and aircraft platforms and, when available, from satellites. Collected concurrently were ground measurements of soil moisture and temperature, energy and water fluxes, and profile data in the atmospheric boundary layer in a hydrologically instrumented semiarid rangeland watershed. Field experiments were conducted in 1990 during the dry and wet or “monsoon season” for the southwestern United States. A detailed description of the field campaigns, including measurements and some preliminary results are given.

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