The Bristow Campbell method is an empirically-based method that provides a way to estimate solar radiation based on temperature. The method relies on Bristow and Campbell's (1984) demonstration of a correlation between the difference in temperature extremes and the magnitude of incoming solar radiation. 

This concept led to a temperature-based equation for estimating daily average solar radiation, ST.

S_t = T_t*S_o

where

So is the estimated daily extraterrestial solar radiation or insolation in W/m^2

Tt is the total radiative transmittance 

 Total transmittance can be estimated as a function of minimum and maximum temperatures as:

Tt= A[1-e^{(-B*\Delta T^C)}]

where

A, B and C are empirical coefficients 

\Delta Tis the range of daily minimum and maximum temperature

A is the clear sky transmittance which is the fraction of radiation transmitted to the earth's surface on a clear day free of cloud cover. This varies based on elevation and air pollution at the site of interest and is typically estimated as approximately 0.7. Values for B and C dictate how rapidly Tt increases in relation to the range in daily temperatures. A single value of A, 0.7, and C, 2.4, are defined by the user for the Bristow Campbell method in HMS. B may vary seasonally based on average monthly temperature range as defined by:

B = 0.036 * e^{(-0.154*\Delta T)

Required Parameters

The parameters required to utilize this method within HEC-HMS are the clear sky transmittance, the exponent coefficient and the average monthly temperature range.