Spatial Variation of Net Radiation, Albedo and Surface Temperature of Forests

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  • 1 Northeastern Forest Experiment Station, U.S. Department of Agriculture, Durham, N.H.
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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.

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