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- Author or Editor: C. A. Federer x
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Abstract
Economical radiometers were placed at 12 locations within an area of about 1 hectare:six over a hardwood forest 10 m in height, two over white pine of 6 m height, two over juniper openings, one over a ledge opening, and one over an old field. Measurements were made about every half hour on selected clear days in summer, autumn and winter. Results of 4–6 successive measurements were averaged to reduce instrument and measurement errors.
The time-averaged hardwood albedos generally had a range (spatial variation, maximum minus minimum values) of 0.03, while the albedos of all locations had a range of about 0.13, except in winter. Hardwood surface temperatures had a range varying from 1–5C, while the range for all sites was from 5–11C. These spatial variations in surface temperature and albedo were about equally important in affecting net radiation. The range of net radiation among hardwood locations was within 0.03 ly min−1, while over all sites it was within 0.11 ly min−1, except in winter. When mean net radiation for a forest area is desired, one or two measurements within each cover type should suffice if the radiometer is high enough to see the crowns of a number of trees.
Abstract
Economical radiometers were placed at 12 locations within an area of about 1 hectare:six over a hardwood forest 10 m in height, two over white pine of 6 m height, two over juniper openings, one over a ledge opening, and one over an old field. Measurements were made about every half hour on selected clear days in summer, autumn and winter. Results of 4–6 successive measurements were averaged to reduce instrument and measurement errors.
The time-averaged hardwood albedos generally had a range (spatial variation, maximum minus minimum values) of 0.03, while the albedos of all locations had a range of about 0.13, except in winter. Hardwood surface temperatures had a range varying from 1–5C, while the range for all sites was from 5–11C. These spatial variations in surface temperature and albedo were about equally important in affecting net radiation. The range of net radiation among hardwood locations was within 0.03 ly min−1, while over all sites it was within 0.11 ly min−1, except in winter. When mean net radiation for a forest area is desired, one or two measurements within each cover type should suffice if the radiometer is high enough to see the crowns of a number of trees.
Abstract
A topographic map of the upper surface of the canopy of a red pine (Pinus resinosa) plantation was drawn from 275 canopy heights measured on a grid of 0.9 × 1.5 m. The distribution of heights was approximately normal, with a mean of 11.6 m; and a standard deviation of 1.6 m; this is an improved method of designating stand height. The roughness parameter s 0 and zero-plane displacement of the stand were estimated from the canopy map data, using both Kung's logarithmic formula and Lettau's equation for obstacle size and shape. These values were compared with measured s 0 and d from wind and temperature profiles in near-neutral conditions. Lettau's formula, assuming the obstacles were uniform square-packed paraboloids, gave s 0=138cm and d10.6 m. Kung's formula gave s 0=75 cm and d=9.7 m. Measured profiles gave a median s 0=100 cm after d was fixed at its median value of 9.6 m.
Abstract
A topographic map of the upper surface of the canopy of a red pine (Pinus resinosa) plantation was drawn from 275 canopy heights measured on a grid of 0.9 × 1.5 m. The distribution of heights was approximately normal, with a mean of 11.6 m; and a standard deviation of 1.6 m; this is an improved method of designating stand height. The roughness parameter s 0 and zero-plane displacement of the stand were estimated from the canopy map data, using both Kung's logarithmic formula and Lettau's equation for obstacle size and shape. These values were compared with measured s 0 and d from wind and temperature profiles in near-neutral conditions. Lettau's formula, assuming the obstacles were uniform square-packed paraboloids, gave s 0=138cm and d10.6 m. Kung's formula gave s 0=75 cm and d=9.7 m. Measured profiles gave a median s 0=100 cm after d was fixed at its median value of 9.6 m.
Abstract
Simulations of soil water and evapotranspiration with physically based models at broad scales vary in both complexity of processes modeled and in parameterization of soil and root properties. Sensitivity of annual evaporation E ann to some of these processes and parameters was tested with both a model allowing multiple soil layers (BROOK90) and a single-layered water balance model (WBM). For nine widely scattered locations in North America E ann was controlled primarily by climate and cover type, but within a location–type combination, E ann was controlled primarily by the available water capacity W ac, which is the product of available water fraction and effective root depth. The definition of the upper limit of available water is important; it is precisely defined here as the water volume fraction at 30-cm depth after 48 h of drainage from an initially saturated, homogeneous profile with a fixed gravity potential gradient at 2-m depth. Specification of root depth was as important as specification of available water fraction in determining W ac. In climates of intermediate wetness a 100-mm change in W ac caused a similar change in E ann at low W ac, but little change in E ann at high W ac. WBM responded similarly to BROOK90. In BROOK90, texture-dependent hydraulic properties caused additional effects of less than ±50 mm in E ann for short covers and even less for tall covers. Effective root depth interacted with both distribution of infiltration and upward movement of water in the soil profile, but the effects were also on the order of only 50 mm. A multilayered model does not seem necessary for simulating E ann at the global scale if the primary objective is budget closure. Improvement in estimating E ann with WBM or similar global water budget models is not likely to result from making the model more complicated with respect to soil and root properties in the context of much larger uncertainties in atmospheric forcings.
Abstract
Simulations of soil water and evapotranspiration with physically based models at broad scales vary in both complexity of processes modeled and in parameterization of soil and root properties. Sensitivity of annual evaporation E ann to some of these processes and parameters was tested with both a model allowing multiple soil layers (BROOK90) and a single-layered water balance model (WBM). For nine widely scattered locations in North America E ann was controlled primarily by climate and cover type, but within a location–type combination, E ann was controlled primarily by the available water capacity W ac, which is the product of available water fraction and effective root depth. The definition of the upper limit of available water is important; it is precisely defined here as the water volume fraction at 30-cm depth after 48 h of drainage from an initially saturated, homogeneous profile with a fixed gravity potential gradient at 2-m depth. Specification of root depth was as important as specification of available water fraction in determining W ac. In climates of intermediate wetness a 100-mm change in W ac caused a similar change in E ann at low W ac, but little change in E ann at high W ac. WBM responded similarly to BROOK90. In BROOK90, texture-dependent hydraulic properties caused additional effects of less than ±50 mm in E ann for short covers and even less for tall covers. Effective root depth interacted with both distribution of infiltration and upward movement of water in the soil profile, but the effects were also on the order of only 50 mm. A multilayered model does not seem necessary for simulating E ann at the global scale if the primary objective is budget closure. Improvement in estimating E ann with WBM or similar global water budget models is not likely to result from making the model more complicated with respect to soil and root properties in the context of much larger uncertainties in atmospheric forcings.
