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Weather Parameters |
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Weather Parameters |
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Weather Parameters
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Weather Parameters |
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Weather Parameters |
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Weather information is entered through a screen accessed by selecting Weather from the Input Menu. The Weather input screen displayed depends on the type of weather file specified in the project file.
Potential evapotranspiration, PET, is the sum of the evaporation from the soil and plant surfaces and transpiration from the plants when there is a sufficient supply of soil water. PET demands may be satisfied with the upward movement of water from the water table (upward flux) and available soil water in the root zone. If the sum of upward flux and the water in the root zone is less than PET, the actual evapotranspiration (ET) will be less than PET. Otherwise PET and ET will be the same.
Two options are available for PET. The user can create PET data files or daily maximum and minimum air temperature data files can be used to estimate PET. If the temperature data files are used, then DRAINMOD computes PET using Thornthwaite's equation (1948). This method uses the latitude and heat index for the location along with the temperature data.
The Thornthwaite PET estimates can be adjusted on a daily or monthly basis by the user if more reliable average values for crop ET are known for a specific area. Daily or monthly factors can be used to adjust the PET. The factors must be greater than zero. Factors less than 1.0 will decrease the PET and factors greater than 1.0 will increase the PET. In order to obtain the PET factors an initial run should be done with wet conditions prevailing so that the output ET will coincide with the Thornthwaite PET. Average values of PET are calculated from the output. The multiplication or correction factors for each day or month is then computed as the ratio of the known average to that predicted by Thornthwaite using DRAINMOD. The factors should be set to 1.0 if the user is inputting PET values directly into DRAINMOD.
Potential Evapotranspiration (PET) Format
Hints and Notes on DRAINMOD Weather Files