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.

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.

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.

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.

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.