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Net radiation ([R.sub.n])=key variable in hydrological studies. Measured net radiation data are rarely available and are often subject to error due to equipment calibration or failure. In addition, point measurements of net radiation do not represent the diversity of the regional net radiation values which are needed for large scale evapotranspiration mapping. A procedure has been developed to estimate daily net radiation using canopy temperature, albedo, short wave radiation and air temperature. This procedure makes it possible to estimate [R.sub.n] by combining information from satellite and local weather stations. Three different methodologies are presented to estimate net radiation. Comparisons between net radiation using the three methods resulted in average error ranging from 1 to 30% and standard error of estimate ranging from 1.06 to 5.34 MJ/[m.sup.2]/day. DOI: 10.1061/(ASCE)0733-9437(2007)133:4(291) CE Database subject headings: Evapotranspiration; Net radiation; Satellite; Solar radiation; Weather station; Classification description; Irrigation and drainage.

Net radiation is a key variable in hydrological studies. However, measured net radiation data are rarely available and are often subject to error because of equipment calibration or failure. Additionally, point measurements of net radiation do not represent the diversity of the regional net radiation values, which are needed for many physical and biological processes such as climate monitoring and evapotranspiration mapping. The authors present a methodology to estimate daytime net radiation using a combination of remote sensing and climatic data. This paper expands on previous original research by extending the estimation of net radiation to 24 h under all sky conditions. The procedure estimates daytime and nighttime net radiation and combines the results to calculate the 24-h net radiation values. The methodology combines information from satellite and local weather stations to estimate net radiation values. The procedure can estimate net radiation under all sky conditions using measured or estimated solar radiation. Two different methods are presented to estimate net radiation. Comparisons between measured and predicted daytime and 24-h net radiation using the two methods resulted in an average ratio ranging from 0.98-1.0 and a standard error of estimate ranging from . Satellite data were used for the calculation of leaf area index, albedo, and ground temperature. Although satellite data are scarce, the methodology is not limited to satellite imagery. One can easily use ground level measurements of leaf area index, albedo, and temperature to estimate daytime and 24-h net radiation using the proposed methodology. [ABSTRACT FROM AUTHOR]

A procedure is introduced to estimate daily solar radiation from maximum and minimum temperatures for areas where solar radiation is either unavailable or unreliable. The procedure uses historical temperature data for spatial and temporal self-calibration. The results from the self-calibrated procedure are compared with measured solar radiation data in one coastal and two interior regions. The self-calibrated procedure is also compared with an equation presented by Thornton and Running that also uses historical maximum and minimum temperature to estimate solar radiation. The comparison between measured and predicted solar radiation values for several years shows that the self-calibrated method can predict solar radiation with error ranging from 0-3%. The error from the Thornton and Running equation ranges from 7-11%. In general, the Thornton and Running equation tends to overestimate solar radiation especially for overcast sky conditions such as monsoon season. [ABSTRACT FROM AUTHOR]