The specified pyranograph method uses observed shortwave radiation data collected by a pyranometer. This method applies the pyranograph to each subbasin as specified by the user. This method may be used to import observed values from a pyranometer or it may be used to import estimates produced by an external model. The pyranograph entered by the user will be at a chosen interval, for example, 15 minutes. However, the user may use control specifications with a time step different from the original data. Instead of re-entering the data manually, the program will automatically interpolate the data to the requested time step. The interpolation process does not affect the original data, it is performed "on-the-fly" during a compute so that the data agrees with the time step. Data is entered in units of Watts per square meter or Langley per minute.

A pyranometer is an instrument that can measure incoming solar shortwave radiation. They are not part of basic meteorological observation stations, but may be included at first order climatological stations. A pyranometer is a sensor that converts the global solar radiation it receives into an electrical signal that can be measured. 

There are three main types of pyranometers: thermopile, photodiode and silicon cell. Thermopile pyranometers are laboratory-grade and the most accurate. Thermopile pyranometers consist of two disks, one is made of black carbon and is illuminated by the sun and the second is a control disk is shaded from the sun. Differences in temperature rise between the two disks is then attributed solely to solar radiation. The thermopile sensor produces a proportional electric current in response to heating from the sun. The output is also related to the angle of the incoming solar radiation, with perpendicular sun rays producing the maximum output. This is known as the cosine response as the signal varies in relation to the cosine of the Sun's rays and the vertical.

The photodiode pyranometer uses a semiconductor that, similar to a thermopile, generates a current proportional to the amount of light detected by the photodiode. Photodiode pyranometers can measure only direct solar radiation. Silicon cell pyranometers use the photovoltaic effect to generate a current via a semiconductor material, silicon, when cells are exposed to sunlight. Silicon cell pyranometers can measure direct and diffuse solar radiation, but measurement accuracy can be impacted by temperature and humidity variations. Both photodiode and silicon pyranometers are less sensitive than thermopile pyranometers.

Required Parameters

At least one radiation gage with observed data must be input to use this method. The user will assign a radiation gage to each subbasin within the Meteorologic Model editor.

Note

This is the recommended choice for use with the Priestley Taylor Evapotranspiration Method, where an effective radiation is used which includes both shortwave and longwave radiation.