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ARS Home » Research » Publications at this Location » Publication #77405
Title: AN INTERCOMPARISON OF NET RADIOMETERS

Author
item Kustas, William - Bill
item Prueger, John
item HIPPS, LAWRENCE - UTAH STATE UNIVERSITY
item Hatfield, Jerry

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/1998
Publication Date: N/A
Citation: N/A

Interpretive Summary: The surface radiation balance or net radiation (Rn) is a critical quantity needed for both long and short term monitoring of water use by natural ecosystems. A project quantifying both short term and seasonal water use of shrub and grass vegetation in the Jornada Experimental Range in New Mexico prompted a study to compare net radiation measurements using several types of net radiometers currently being used in research. For both daytime and nighttime conditions, differences among net radiometers typically ranged between 5% and 10%, but several had larger differences ranging between 10 and 20%. Differences among net radiometer types and model varied with surface moisture conditions, suggesting that there is not a consistent response among different net radiometers to changes in surface conditions. When considering the impact of this uncertainty on daily evapotranspiration (ET), it was estimated that differences in Rn typically result in variations of ET of less than 10%, except for a few cases. On the other hand, a 10% error or bias in Rn is likely to cause a significant cumulative error in ET and, thus, make it difficult to estimate the water balance at monthly and longer time scales.

Technical Abstract: Net radiation (Rn) is a critical quantity needed for both long and short term monitoring of water use by natural ecosystems. Partitioning of Rn by the soil and vegetation into ground heat flux and the turbulent fluxes of latent and sensible heat is essential for understanding land surface-climate interactions and water use by different ecosystems. A project quantifying both short term and seasonal water use of shrub and grass vegetation in the Jornada Experimental Range in New Mexico prompted a study to compare net radiation measurements using several types of net radiometers currently being used in research. Thirteen net radiometers were compared using a bare soil plot under both dry and wet soil conditions. For both daytime and nighttime conditions, differences among net radiometers typically ranged between 5% and 10%, but several had larger differences ranging between 10 and 20%. Differences among net radiometer types and model varied with surface moisture conditions, suggesting that there is not a consistent response of different net radiometers to changes in shortwave surface albedo and emitted longwave. Thus, when using different net radiometers over contrasting surfaces differences probably need to be greater than 15% to be considered significant. The impact of this uncertainty on daily evapotranspiration (ET) typically results in variations of ET of less than 10%, except for a few cases, under dry and wet conditions. However, a 10% error or bias in Rn is likely to cause a significant cumulative error in ET.