The Weather module inputs daily weather data and also can generate long term (e.g., 30-year) series of daily weather data that reproduce the statistical properties of a shorter series of weather years. The minimum daily weather data required are solar radiation, minimum and maximum air temperatures, and precipitation. However, for more accurate estimation of evapotranspiration, humidity (e.g., as daily mean dewpoint temperature) and wind speed are required.

The minimum weather data include the metadata for the weather station, especially latitude, longitude, elevation, and sensor height, and daily maximum and minimum temperature, rainfall, and solar radiation. Although solar radiation is not commonly measured at many remote locations, it is a required input for the accurate simulation of photosynthesis and of potential transpiration using the Priestley-Taylor equation (Priestley and Taylor, 1972).

Additional optional daily inputs include dewpoint temperature, windspeed, photosynthetically active radiation, minimum relative humidity, and vapor pressure. Depending on the user selection of method for estimating reference evapotranspiration, the model will use these values if supplied, but provide default values if the data are not provided.

Hourly variables are synthesized by the module for solar radiation, relative humidity, temperature, and windspeed for use by those routines which simulate processes on an hourly time scale.

In some cases, the period of record of historical daily weather data is insufficient, and it is desirable to have the capability to generate synthetic weather data with the same statistical characteristics as the actual weather data at the location.  The WGEN (Richardson and Wright, 1984) routines generate daily weather values for precipitation, maximum temperature, minimum temperature and solar radiation for use by the CROPGRO model.  A first-order Markov chain model is used to describe the occurrence of rainfall on a given day and a gamma distribution function is applied to fit the amount of rainfall.  The occurrence of rain on a given day will influence the temperature and solar radiation for that day and the probability of rain on the following day. 

The environmental modification capability allows a user to modify weather values during a simulation to model the effects of climate change, growth chambers, solar shades, rain shelters, or other types of artificial manipulation of environment.

Atmospheric CO₂ can be specified in several ways in the model:

1. The default method is to read measured values from a file. These values represent smoothed bi-monthly values from the Mauna Loa observatory. Daily values are linearly interpolated.
2. A default value can be used which allows a static CO₂ value of 380 ppm (corresponding to 2005 values) to be used for an entire simulation.
3. Annual or daily CO₂ values can be specified in the weather input file.
4. And the final method is to specify a CO₂ value in the Environmental Modifications section of an experiment file.

References

Priestley, C. H. B., and R. J. Taylor. 1972. On the assessment of surface heat and evaporation using large scale parameters. Monthly Weather Review 100:81-92.

Richardson, C. W., and D. A. Wright. 1984. WGEN: A Model for Generating Daily Weather Variables. U. S. Department of Agriculture, Agricultural Research Service, ARS-8, 83 p.