| estimate_ea {simET} | R Documentation |
Estimating missing humidity data
Description
Where humidity data are lacking or are of questionable quality, an estimate of actual vapour pressure, ea, can be obtained by assuming that dewpoint temperature (Tdew) is near the daily minimum temperature (Tmin). This statement implicitly assumes that at sunrise, when the air temperature is close to Tmin, that the air is nearly saturated with water vapour and the relative humidity is nearly 100 %.
Usage
estimate_ea(Tmin)
Arguments
Tmin |
the minimum tem daily. |
Value
A vector for humidity
Note
The relationship Tdew near Tmin holds for locations where the cover crop of the station is well watered. However, particularly for arid regions, the air might not be saturated when its temperature is at its minimum. Hence, Tmin might be greater than Tdew and a further calibration may be required to estimate dewpoint temperatures. In these situations, "Tmin" in the above equation may be better approximated by subtracting 2-3 degrees Celsius from Tmin. Appropriate correction procedures are given in Annex 6. In humid and subhumid climates, Tmin and Tdew measured in early morning may be less than Tdew measured during the daytime because of condensation of dew during the night. After sunrise, evaporation of the dew will once again humidify the air and will increase the value measured for Tdew during the daytime. This phenomenon is demonstrated in Figure 5.4 of Annex 5. However, it is standard practice in 24-hour calculations of ETo to use Tdew measured or calculated durin early morning. The estimate for ea from Tmin should be checked. When the prediction by Equation 48 is validated for a region, it can be used for daily estimates of ea.
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.