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Remote Sens. 2013, 5(10), 4735-4752; doi:10.3390/rs5104735

Evaluation of Clear-Sky Incoming Radiation Estimating Equations Typically Used in Remote Sensing Evapotranspiration Algorithms

2,* , 1
1 National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan 2 Civil & Environmental Engineering, University of Connecticut, 261 Glenbrook Road, Storrs, CT 06269, USA 3 Illinois State Water Survey, UIUC, 2204 Griffith, Champaign, IL 61820, USA 4 Department of Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM 88003, USA 5 Ecosystem Processes and Dynamics, CSIR, PO Box 395, Pretoria 0001, South Africa
* Author to whom correspondence should be addressed.
Received: 23 July 2013 / Revised: 17 September 2013 / Accepted: 18 September 2013 / Published: 25 September 2013
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Net radiation is a key component of the energy balance, whose estimation accuracy has an impact on energy flux estimates from satellite data. In typical remote sensing evapotranspiration (ET) algorithms, the outgoing shortwave and longwave components of net radiation are obtained from remote sensing data, while the incoming shortwave (RS) and longwave (RL) components are typically estimated from weather data using empirical equations. This study evaluates the accuracy of empirical equations commonly used in remote sensing ET algorithms for estimating RS and RL radiation. Evaluation is carried out through comparison of estimates and observations at five sites that represent different climatic regions from humid to arid. Results reveal (1) both RS and RL estimates from all evaluated equations well correlate with observations (R2 ≥ 0.92), (2) RS estimating equations tend to overestimate, especially at higher values, (3) RL estimating equations tend to give more biased values in arid and semi-arid regions, (4) a model that parameterizes the diffuse component of radiation using two clearness indices and a simple model that assumes a linear increase of atmospheric transmissivity with elevation give better RS estimates, and (5) mean relative absolute errors in the net radiation (Rn) estimates caused by the use of RS and RL estimating equations varies from 10% to 22%. This study suggests that Rn estimates using recommended incoming radiation estimating equations could improve ET estimates.
Keywords: net radiation; incoming shortwave radiation; incoming longwave radiation net radiation; incoming shortwave radiation; incoming longwave radiation
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Sun, Z.; Gebremichael, M.; Wang, Q.; Wang, J.; Sammis, T.W.; Nickless, A. Evaluation of Clear-Sky Incoming Radiation Estimating Equations Typically Used in Remote Sensing Evapotranspiration Algorithms. Remote Sens. 2013, 5, 4735-4752.

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