ISSN:0043-1397    

DOI:10.1029/94WR00426

年代:1994

数据来源: WILEY

摘要:

The Monsoon '90 multidisciplinary field campaign was conducted over the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona during June–September 1990. A primary objective of this combined ground, aircraft, and satellite campaign was to assess the feasibility of utilizing remotely sensed data coupled with water and energy balance modeling for large‐area estimates of fluxes in semiarid rangelands. The experimental period encompassed a variety of vegetation, soil moisture, and rainfall conditions characterized by large temporal and spatial gradients. This preface outlines experimental objectives, briefly discusses the field campaigns, summarizes initial observations, and provides an overview of articles that are a part of the Monsoon '90 special sect

ISSN:0043-1397    

DOI:10.1029/93WR03068

年代:1994

数据来源: WILEY

摘要:

A network of 9‐m‐tall surface flux measurement stations were deployed at eight sparsely vegetated sites during the Monsoon '90 experiment to measure net radiation,Q, soil heat flux,G, sensible heat flux,H(using eddy correlation), and latent heat flux, λE(using the energy balance equation). At four of these sites, 2‐m‐tall eddy correlation systems were used to measure all four fluxes directly. Also a 2‐m‐tall Bowen ratio system was deployed at one site. Magnitudes of the energy balance closure (Q+G+H+ λE) increased as the complexity of terrain increased. The daytime Bowen ratio decreased from about 10 before the monsoon season to about 0.3 during the monsoons. Source areas of the measurements are developed and compared to scales of heterogeneity arising from the sparse vegetation and the topography. There was very good agreement among simultaneous measurements ofQwith the same model sensor at different heights (representing different source areas), but poor agreement among different brands of sensors. Comparisons of simultaneous measurements ofGsuggest that because of the extremely small source area, extreme care in sensor deployment is necessary for accurate measurement in sparse canopies. A recently published model to estimate fetch is used to interpret measurements ofHat the 2 m and 9 m heights. Three sites were characterized by undulating topography, with ridgetops separated by about 200–600 m. At these sites, sensors were located on ridgetops, and the 9‐m fetch included the adjacent valley, whereas the 2‐m fetch was limited to the immediate ridgetop and hillside. Before the monsoons began, vegetation was mostly dormant, the watershed was uniformly hot and dry, and the two measurements ofHwere in close agreement. After the monsoons began and vegetation fully matured, the 2‐m measurements ofHwere significantly greater than the 9‐m measurements, presumably because the vegetation in the valleys was denser and cooler than on the ridgetops and hillsides. At one lowland site with little topographic relief, the vegetation was more uniform, and the two measurements ofHwere in close agreement during peak vegetation. Values of λEcould only be compared at two sites, but the 9‐m values were greater than the 2‐m values, suggesting λEfrom the dense vegetation in the valle

ISSN:0043-1397    

DOI:10.1029/93WR03037

年代:1994

数据来源: WILEY

摘要:

Remotely sensed data in the visible, near‐infrared, and thermal‐infrared wave bands were collected from a low‐flying aircraft during the Monsoon '90 field experiment. Monsoon '90 was a multidisciplinary experiment conducted in a semiarid watershed. It had as one of its objectives the quantification of hydrometeorological fluxes during the “monsoon” or wet season. The remote sensing observations along with micrometeprological and atmospheric boundary layer (ABL) data were used to compute the surface energy balance over a range of spatial scales. The procedure involved averaging multiple pixels along transects flown over the meteorological and flux (METFLUX) stations. Average values of the spectral reflectance and thermal‐infrared temperatures were computed for pixels of order 10−1to 101km in length and were used with atmospheric data for evaluating net radiation (Rn), soil heat flux (G), and sensible (H) and latent (LE) heat fluxes at these same length scales. The model employs a single‐layer resistance approach for estimatingHthat requires wind speed and air temperature in the ABL and a remotely sensed surface temperature. The values ofRnandGare estimated from remote sensing information together with near‐surface observations of air temperature, relative humidity, and solar radiation. Finally,LEis solved as the residual term in the surface energy balance equation. Model calculations were compared to measurements from the METFLUX network for three days having different environmental conditions. Average percent differences for the three days between model and the METFLUX estimates of the local fluxes were about 5% forRn, 20% forGandH, and 15% forLE. Larger differences occurred during partly cloudy conditions because of errors in interpreting the remote sensing data and the higher spatial and temporal variation in the energy fluxes. Minor variations in modeled energy fluxes were observed when the pixel size representing the remote sensing inputs changed from 0.2 to 2 km. Regional scale estimates of the surface energy balance using bulk ABL properties for the model parameters and input variables and the 10‐km pixel data differed from the METFLUX network averages by about 4% forRn, 10% forGandH, and 15% forLE. Model sensitivity in calculating the turbulent fluxesHandLEto possible variations in key model parameters (i.e., the roughness lengths for heat and momentum) was found to be fairly significant. Therefore the reliability of the methods for estimating key model parameters and potential errors needs further testing over different ecosystems and envir

ISSN:0043-1397    

DOI:10.1029/93WR03038

年代:1994

数据来源: WILEY

摘要:

As part of an interdisciplinary effort to describe the hydrologic and surface energy balance of a semiarid rangeland watershed, remote and ground radiometric surface temperature measurements were collected at the Walnut Gulch experimental watershed near Tombstone, Arizona. To correct for atmospheric effects on the remotely sensed data, radiosondes were launched on and off the watershed, and the LOWTRAN7 radiative transfer model was used to evaluate these observed as well as two standard atmospheric profiles. Comparison of the results for five atmospheric profiles, for each of three days, showed no correlation between radiosonde location or launch time and resulting temperature corrections. Comparison of corrected temperatures using five different atmospheric profiles on each of two days with the corresponding ground‐based radiometric temperatures demonstrated significant errors (greater than 2.0°C) as the atmospheric profiles generally yielded underestimates of ground radiometric temperatur

ISSN:0043-1397    

DOI:10.1029/93WR03056

年代:1994

数据来源: WILEY

摘要:

A ground‐ and air‐based high spectral resolution data set was collected during the summer Monsoon '90 experiment at the Walnut Gulch experimental watershed in southeastern Arizona for the purpose of (1) characterizing solar and view angle interactions on dry and wet season canopy spectra, and (2) exploring the use of multidirectional measurements to infer vegetation properties for semiarid watershed studies. Bidirectional reflectance factors were measured up to 40° off nadir with a spectroradiometer over a semidesert grassland site. High‐spectral resolution aircraft data were collected over grass and desert shrub sites in order to investigate scaling effects. In this study, solar and view angle effects and interactions on canopy spectra varied with spectral wavelength as well as between dry and wet seasons. The solar zenith angle modified the view angle behavior of the bidirectional reflectance factors. In general, view angle influences and spectral signature contrasts were greatest at the larger solar zenith angles and similarly, Sun angle influences were more apparent at the larger view zenith angle. The scale dependency of the data was relatively minor. The bidirectional measurements were sufficiently characterized by a physically based bidirectional reflectance distribution function (BRDF) model. The parameters retrieved from the inversion of the BRDF model corresponded with observed vegetation vari

ISSN:0043-1397    

DOI:10.1029/93WR03058

年代:1994

数据来源: WILEY

摘要:

Vegetation‐related information is critical for modeling hydrological processes. Remotely sensed spectral data provide powerful means to characterize vegetation status. However, the non‐Lambertian behavior of most surfaces induces a large view/Sun angle dependence. Two approaches are possible to correct such contamination. One approach consists of using directional reflectance models; the other consists of normalizing the Sun/view angle effects directly on vegetation indices that have the advantage of being less sensitive to surface anisotropy than individual reflectances. Ground‐based multiple view direction/angle measurements made over a semiarid grassland canopy at the Walnut Gulch experiment watershed (Monsoon '90 experiment) were used to develop and to validate a semiempirical model to normalize the MSAVI (modified soil‐adjusted vegetation index) response to a nadir, regardless of view/direction angle. We further advanced this model to account simultaneously for both Sun and view angle variations by introducing a shadow parameterization. The results showed that this model can be used to monitor the vegetation status using a single view/Sun configuration throughout the growing season. We therefore believe that we have taken a further step toward the development of a multidirectional vegetatio

ISSN:0043-1397    

DOI:10.1029/93WR03063

年代:1994

数据来源: WILEY

摘要:

A study was conducted to determine the relation between remotely sensed spectral data and measurements of vegetation‐related hydrologic parameters in a semiarid rangeland in southeast Arizona. Throughout the measurement periods, ranging from June to September 1990, eight sites in the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed were monitored for water and energy fluxes and other meteorological and biological parameters. Corresponding spectral data were acquired with ground‐based radiometers, low‐altitude aircraft‐mounted instruments, and Landsat thematic mapper sensors. Spectral indices were derived from measurements of surface reflectance, based on their response to variations in hydrologic parameters and sensitivity to unrelated variables, such as solar zenith angle and soil differences. A soil‐adjusted vegetation index, SAVI (derived from red and NIR reflectance factors), was found to be highly correlated with the temporal changes in vegetation cover and biomass, but less successful in discriminating spatial differences in cover and biomass across the watershed. Significant relations were found between the surface‐air temperature (Ts‐Ta) difference and measurements of soil moisture content, though the shape differed from that previously published for bare soil. The relation between daily evaporation rate and measurements of (Ts‐Ta) and daily net radiation was similar to that derived previously for irrigated pasture and dryland shortgrass in France but differed from that derived for irrigated wheat. These results emphasized the strengths and limitations of the use of spectral data for estimation of hydrologic characteristics of sparsely vegetated sites and suggested a need for reevaluation of common empirical relations between remotely sensed measurements and surface characteristics for application to

ISSN:0043-1397    

DOI:10.1029/93WR03066

年代:1994

数据来源: WILEY

摘要:

Radiometric surface temperatures obtained from remote sensing measurements are a function of both the physical surface temperature and the effective emissivity of the surface within the band pass of the radiometric measurement. For sparsely vegetated areas, however, a sensor views significant fractions of both bare soil and various vegetation types. In this case the radiometric response of a sensor is a function of the emissivities and kinetic temperatures of various surface elements, the proportion of those surface elements within the field of view of the sensor, and the interaction of radiation emitted from the various surface components. In order to effectively utilize thermal remote sensing data to quantify energy balance components for a sparsely vegetated area, it is important to examine the typical magnitude and degree of variability of emissivity and surface temperature for such surfaces. Surface emissivity measurements and ground and low‐altitude‐aircraft‐based surface temperature measurements (8–13 μm band pass) made in conjunction with the Monsoon '90 field experiment were used to evaluate the typical variability of those quantities during the summer rainy season in a semiarid watershed. The average value for thermal band emissivity of the exposed bare soil portions of the surface was found to be approximately 0.96; the average value measured for most of the varieties of desert shrubs present was approximately 0.99. Surface composite emissivity was estimated to be approximately 0.98 for both the grass‐dominated and shrub‐dominated portions of the watershed. The spatial variability of surface temperature was found to be highly dependent on the spatial scale of integration for the instantaneous field of view (IFOV) of the instrument, the spatial scale of the total area under evaluation, and the time of day. For the conditions which existed during most of the Monsoon '90 experiment, the differences in kinetic (physical) temperature between the vegetation and soil background were typically between 10° and 25°C at midday. These differences gave rise to large variations in radiometric composite surface temperatures observed with a ground‐based instrument configuration which allowed a ground IFOV of approximately 0.5 m. An evaluation of the frequency distribution for these observations indicated that the variance in surface temperature observed over an intensively sampled target area (approximately 500 m×120 m) increased significantly in the early to late morning hours of a typical diurnal heating cycle. For aircraft‐based composite radiometric temperature measurements at the watershed scale (with ground IFOV of approximately 40 m for each observation), much of the variability in surface temperature due to differences in soil and vegetation temperature was integrated into a single measurement; consequently, the variance between observations over the watershed was not significantly larger than those observed at len

ISSN:0043-1397    

DOI:10.1029/93WR03065

年代:1994

数据来源: WILEY

摘要:

Distribution of vegetation properties is fundamental for understanding vegetation patterns and characteristics, improving estimates of infiltration, evapotranspiration, and soil erosion. A laser altimeter mounted in a small airplane was used to measure surface patterns of the landscape on the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed near Tombstone, Arizona. The airborne laser altimeter is a pulsed gallium‐arsenide diode laser, transmitting and receiving 4000 pulses per second at a wavelength of 0.904 μm. The laser has a 1‐mrad field of view and is designed to have a vertical recording precision of 0.05 m on a single measurement. Aircraft altitude varied between 100 and 300 m for the flights. Digital data from the laser were collected with a portable computer and analyzed to provide information on changes in vegetation height, spatial patterns, and patchiness of vegetation cover. The laser‐measured vegetation properties of plant height and canopy cover (>0.3 m) were not significantly different than field measurements made using the line‐intercept transect method at seven of the eight sites evaluated. Although the laser measurements of canopy height were not significantly different from the ground measurements, the laser consistently overestimated canopy cover less than 0.3 m in height and underestimated canopy cover greater than 0.5 m. New techniques to discriminate the background noise in the laser return signal in sparsely populated shrub communities are necessary before this technique will be fully useful in estimating canopy cover on rangelands. These studies indicate the potential of airborne laser to measure vegetation patterns quickly and quantitatively. The laser also has the ability to separate and map distinctly different plant com

ISSN:0043-1397    

DOI:10.1029/93WR03067

年代:1994

数据来源: WILEY