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D.I. Stannard

Publications and source records attributed to D.I. Stannard.

12 recordsLinked to original sources
USGS2010

Interpretation of surface flux measurements in heterogeneous terrain during the Monsoon '90 experiment

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, λ E (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 + λ E ) 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 of Q with the same model sensor at different heights (representing different source areas), but poor agreement among different brands of sensors. Comparisons of simultaneous measurements of G suggest 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 of H at 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 of H were in close agreement. After the monsoons began and vegetation fully matured, the 2-m measurements of H were 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 of H were in close agreement during peak vegetation. Values of λ E could only be compared at two sites, but the 9-m values were greater than the 2-m values, suggesting λ E from the dense vegetation in the valleys was greater than elsewhere.

Water Environment Research↗
USGS2010

Surface energy balance estimates at local and regional scales using optical remote sensing from an aircraft platform and atmospheric data collected over semiarid rangelands

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 −1 to 10 1 km in length and were used with atmospheric data for evaluating net radiation ( R n ), 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 estimating H that requires wind speed and air temperature in the ABL and a remotely sensed surface temperature. The values of R n and G are estimated from remote sensing information together with near-surface observations of air temperature, relative humidity, and solar radiation. Finally, LE is 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% for R n , 20% for G and H , and 15% for LE . 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% for R n , 10% for G and H , and 15% for LE . Model sensitivity in calculating the turbulent fluxes H and LE to 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 environmental conditions.

Water Resources Research↗
USGS2010

Use of ground-based remotely sensed data for surface energy balance evaluation of a semiarid rangeland

An interdisciplinary field experiment was conducted to study the water and energy balance of a semiarid rangeland watershed in southeast Arizona during the summer of 1990. Two subwatersheds, one grass dominated and the other shrub dominated, were selected for intensive study with ground-based remote sensing systems and hydrometeorological instrumentation. Surface energy balance was evaluated at both sites using direct and indirect measurements of the turbulent fluxes (eddy correlation, variance, and Bowen ratio methods) and using an aerodynamic approach based on remote measurements of surface reflectance and temperature and conventional meteorological information. Estimates of net radiant flux density ( R n ), derived from measurements of air temperature, incoming solar radiation, and surface temperature and radiance compared well with values measured using a net radiometer (mean absolute difference (MAD) ≃ 50 W/m 2 over a range from 115 to 670 W/m 2 ). Soil heat flux density ( G ) was estimated using a relation between G / R n and a spectral vegetation index computed from the red and near-infrared surface reflectance. These G estimates compared well with conventional measurements of G using buried soil heat flux plates (MAD ≃ 20 W/m 2 over a range from −13 to 213 W/m 2 ). In order to account for the effects of sparse vegetation, semiempirical adjustments to the single-layer bulk aerodynamic resistance approach were required for evaluation of sensible heat flux density ( H ). This yielded differences between measurements and remote estimates of H of approximately 33 W/m 2 over a range from 13 to 303 W/m 2 . The resulting estimates of latent heat flux density, LE , were of the same magnitude and trend as measured values; however, a significant scatter was still observed: MAD ≃ 40 W/m 2 over a range from 0 to 340 W/m 2 . Because LE was solved as a residual, there was a cumulative effect of errors associated with remote estimates of R n , G , and H .

Water Resources Research↗
USGS2008

Differences in evaporation between a floating pan and class a pan on land

Research was conducted to develop a method for obtaining floating pan evaporation rates in a small (less than 10,000 m 2 ) wetland, lagoon, or pond. Floating pan and land pan evaporation data were collected from March 1 to August 31, 2005, at a small natural wetland located in the alluvium of the Canadian River near Norman, Oklahoma, at the U.S. Geological Survey Norman Landfill Toxic Substances Hydrology Research Site. Floating pan evaporation rates were compared with evaporation rates from a nearby standard Class A evaporation pan on land. Floating pan evaporation rates were significantly less than land pan evaporation rates for the entire period and on a monthly basis. Results indicated that the use of a floating evaporation pan in a small free‐water surface better simulates actual physical conditions on the water surface that control evaporation. Floating pan to land pan ratios were 0.82 for March, 0.87 for April, 0.85 for May, 0.85 for June, 0.79 for July, and 0.69 for August.

Journal of the American Water Resources Associatio↗
USGS2006

Effects of long-term water table drawdown on evapotranspiration and vegetation in an arid region phreatophyte community

Evapotranspiration rates and the ground water component of evapotranspiration at a site in Colorado's San Luis Valley that is dominated by shrubby phreatophytes (greasewood and rabbitbrush) were compared before and after a water table drawdown. Evapotranspiration (ET) rates at the site were first measured in 1985-1987 (pre-drawdown) when the mean water table depth was 0.92 m. Regional ground water pumping has since lowered the water table by 1.58 m, to a mean of 2.50 m. We measured ET at the same site in 1999-2003 (post-drawdown), and assessed physical and biological factors affecting the response of ET to water table drawdown. Vegetation changed markedly from the pre-drawdown to the post-drawdown period as phreatophytic shrubs invaded former wetland areas, and wetland grasses and grass-like species decreased. Lowering the water table reduced estimated total annual ET from a mean of 409.0 to 278.0 mm, a decrease of 32%, and the ground water component of ET (ETg), from a mean of 226.6 to 86.5 mm, a decrease of 62%. Two water table depth/ET models that have been used in the San Luis Valley overestimated the reduction in ETg due to lowering the water table by as much as 253%. While our results corroborate the generally observed negative correlation between ET rates and water table depth, they demonstrate that specific models to estimate ET as a function of water table depth, if not verified, may be prone to large errors. Both the water table drawdown and the vegetation change are continuing 20 years after the drawdown began, and it is unclear how site ET rates and processes will differ after the water table has stabilized and vegetation has adjusted to the new site hydrologic conditions. ?? 2005 Elsevier B.V. All rights reserved.

Journal of Hydrology↗
USGS1999

Estimating lake-atmosphere CO2 exchange

Lake‐atmosphere CO 2 flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake‐water CO 2 storage and with flux predictions from boundary‐layer and surface‐renewal models. Over a 3‐yr period, lake‐atmosphere exchanges of CO 2 were measured over 5 weeks in spring, summer, and fall. Observed springtime CO 2 efflux was large (2.3–2.7 umol m ‐2 s ‐1 ) immediately after lake‐thaw. That efflux decreased exponentially with time to less than 0.2 umol m ‐2 s −1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO 2 concentrations, lake CO 2 storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (−0.17 μmol m ‐2 s 1 ) to the lake's surface, while late fall flux was trendless and smaller (−0.0021 μmol m ‐2 s −1 ). Large springtime efflux afforded an opportunity to make direct measurement of lake‐atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air‐water gas‐transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake‐water storage change in CO 2 , agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary‐layer and surface‐renewal models. Comparison of measured and modeled transfer velocities for CO 2 , along with measured and modeled cumulative CO 2 flux, indicates that in most instances the surface‐renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary‐layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.

Limnology and Oceanography↗
USGS1997

A theoretically based determination of bowen-ratio fetch requirements

Determination of fetch requirements for accurate Bowen-ratio measurements of latent- and sensible-heat fluxes is more involved than for eddy-correlation measurements because Bowen-ratio sensors are located at two heights, rather than just one. A simple solution to the diffusion equation is used to derive an expression for Bowen-ratio fetch requirements, downwind of a step change in surface fluxes. These requirements are then compared to eddy-correlation fetch requirements based on the same diffusion equation solution. When the eddy-correlation and upper Bowen-ratio sensor heights are equal, and the available energy upwind and downwind of the step change is constant, the Bowen-ratio method requires less fetch than does eddy correlation. Differences in fetch requirements between the two methods are greatest over relatively smooth surfaces. Bowen-ratio fetch can be reduced significantly by lowering the lower sensor, as well as the upper sensor. The Bowen-ratio fetch model was tested using data from a field experiment where multiple Bowen-ratio systems were deployed simultaneously at various fetches and heights above a field of bermudagrass. Initial comparisons were poor, but improved greatly when the model was modified (and operated numerically) to account for the large roughness of the upwind cotton field.

Boundary-Layer Meteorology↗
USGS1996

Variability in surface energy flux partitioning during Washita '92: Resulting effects on Penman-Monteith and Priestley-Taylor parameters

During the Washita '92 field experiment, the local surface energy balance was evaluated at four locations in the USDA-ARS Little Washita River Watershed near Chickasha, OK, using the Bowen ratio-energy balance (BREB) approach. For any given day, differences in the partitioning of the available energy appeared to be mostly a function of the type of vegetation at the site, while the actual magnitude of the fluxes was mostly affected by cloud cover. The soil surface was initially wet, and gradually dried during the field experiment. However, there was not a corresponding decrease in the evaporative fraction, which would have indicated a decreasing contribution of soil evaporation to the total latent heat flux. Ground weather data indicated a large shift in the direction and magnitude of the surface winds, and a significant increase in air temperature and vapor pressure deficit. During this period, the evaporative fraction actually increased at two of the four sites. The response of the different sites to the changing near-surface atmospheric conditions was studied in more detail by evaluating the canopy resistance ( r c ) to evaporation using the Penman-Monteith equation and the Priestley-Taylor parameter (α). Midday averages of ( r c ) and (α) tended to decrease (increase) with increasing vapor pressure deficit for two of the sites while such a trend was not evident for the other two sites. Estimates of stomatal resistances indicated that significant plant physiological differences existed between the sites containing weedy vegetation versus the grasses at the pasture/rangeland sites. Even though soil moisture conditions were relatively wet, a was less than 1 at all sites and there was no trend in α as a function of surface soil moisture conditions. These findings suggest that vegetation types C in mixed agricultural /rangeland ecosystems can have significantly different responses to similar atmospheric forcing conditions.

Agricultural and Forest Meteorology↗
USGS1992

Tensiometers: Theory, construction, and use

Standard tensiometers are used to measure matric potential as low as −870 cm of water in the unsaturated zone by creating a saturated hydraulic link between the soil water and a pressure sensor. The direction and, in some cases, quantity of water flux can be determined using multiple installations. A variety of commercial and fabricated tensiometers are commonly used. Saturated porous ceramic materials, which form an interface between the soil water and the bulk water inside the instrument, are available in many shapes, sizes, and pore diameters. A gage, manometer, or electronic pressure transducer is connected to the porous material with small- or large-diameter tubing. Selection of these components allows the user to optimize one or more characteristics, such as accuracy, versatility, response time, durability, maintenance, extent of data collection, and cost. Special designs have extended the normal capabilities of tensiometers, allowing measurement in cold or remote areas, measurement of matric potential as low as −153 m of water (−15 bars), measurement at depths as deep as 6 m (recorded at land surface), and automatic measurement using as many as 22 tensiometers connected to a single pressure transducer. Continuous hydraulic connection between the porous material and soil, and minimal disturbance of the natural infiltration pattern are necessary for successful installation. Avoidance of errors caused by air invasion, nonequilibrium of the instrument, or pressure-sensor inaccuracy will produce reliable values of matric potential, a first step in characterizing unsaturated flow.

Geotechnical Testing Journal↗
USGS1991

A comparison of short-term measurements of lake evaporation using eddy correlation and energy budget methods

Concurrent short-term measurements of evaporation from a shallow lake, using eddy correlation and energy budget methods, indicate that sensible and latent heat flux between lake and atmosphere, and energy storage in the lake, may vary considerably across the lake. Measuring net radiation with a net radiometer on the lake appeared to be more accurate than measuring incoming radiation nearby and modeling outgoing radiation. Short-term agreement between the two evaporation measurements was obtained by using an energy storage term that was weighted to account for the area-of-influence of the eddy correlation sensors. Relatively short bursts of evaporation were indicated by the eddy correlation sensors shortly after midnight on two of three occasions.

Oklahoma↗
USGS1986

PHREATOPHYTE WATER USE ESTIMATED BY EDDY-CORRELATION METHODS.

Water-use was estimated for three phreatophyte communities: a saltcedar community and an alkali-Sacaton grass community in New Mexico, and a greasewood rabbit-brush-saltgrass community in Colorado. These water-use estimates were calculated from eddy-correlation measurements using three different analyses, since the direct eddy-correlation measurements did not satisfy a surface energy balance. The analysis that seems to be most accurate indicated the saltcedar community used from 58 to 87 cm (23 to 34 in. ) of water each year. The other two communities used about two-thirds this quantity.

Conference Paper↗