DESCRIPTION OF EVAPOTRANSPIRATION (ET), REFERENCE ET, AND THE CROP COEFFICIENT
Evapotranspiration (ET) is the amount of water that moves
from the ground (and plants on the ground) to the atmosphere through both
evaporation and transpiration. It is primarily important to people who are
monitoring plant growth and associated water usage. Measuring actual ET for a given location
requires the measurement of weather variables at different heights at the same
location and is beyond the capabilities of the current
DATA SAMPLING AND VARIABLES REQUIRED FOR CALCULATION
The console calculated ETo using samples of Temperature, Humidity, Wind Speed, Solar Radiation over a one hour period. This sampling is independent of sampling undertaken for the creation of archived data records. At the end of the hour, the arithmetic mean is calculated for each value by dividing the sum of the sampled data values by the number of samples taken. The number of samples is tracked for each sensor independently in case some sensors are not connected for some part of the period. The value of the saturation vapor pressure and actual water vapor pressure are calculated from the current values of temperature and humidity and sampled. These vapor pressure values (in kPa) are used to compute the average saturation vapor pressure and the average water vapor pressure for the hour.In addition, the raw Barometer value (i.e. not corrected for altitude) at the end of the hour is read. The temperature is calculated in tenths of a degree F, the humidity is calculated in tenths of a percent, wind speed is calculated in miles per hour, solar radiation is calculated in watts per square meter, and atmospheric pressure is read in thousandths of an inch of mercury. All arithmetic is in integers. Values that use fractions are represented by multiplying by an appropriate value. The formulas given below that use functions more complicated than addition, subtraction, multiplication, and division are calculated with table lookups with linear interpolation where appropriate.
Solar radiation is the primary source of energy that drives evapotranspiration, but what is important is the net
radiation, incoming radiation minus outgoing radiation, at all
These equations were modeled after the ones used by the California Irrigation Management Information System (CIMIS), a program run by the California Department of Water Resources. Therefore, the accuracy of the Davis ETo calculations are made against the ETo calculations made by CIMIS. Some of the differences between Davis and CIMIS ETo calculated values are due to differences in resolution, rather than accuracy.
There are two major factors that cause differences between Davis and CIMIS ETo calculations: differences in sensor measurements, and differences in net radiation values.
The Vantage Pro measures wind speed in one mile per hour increments, but maintains a higher resolution for hourly averages. As explained above, there are several different ways to calculate a hourly average vapor pressure and saturation vapor pressure values. The CIMIS method is to calculate and sample the vapor pressure value as described for the Vantage Pro. However, the saturation vapor pressure is calculated from the average temperature. This method will produce a saturation vapor pressure that is equal or lower than the average of the sampled saturation pressures.
General reference on ET
Jensen, M .E., Burman, R. D., Allen, R. G., Editors (1990) “Evapotranspiration and irrigation water requirements.” ASCE Manuals and Reports on Engineering Practice No 70.
Paper describing CIMIS’ equations and methodology:
Snyder, R. L., Pruitt, W. O. (1992). “Evapotranspiration
Data Management in
Irrigation & Drainage Session Proceedings/Water Forum ’92 EE, HY, IR, WR, div/ASCE
Paper describing net radiation:
Dong, A, Grattan, S. R., Carroll, J. J., Prashar, C. R. K. (1992). “Estimation of net radiation over well-watered grass.” J. of Irrigation and Drainage Engineering, Vol. 118, No. 3 ASCE
Web sites with useful information
CIMIS home page
Provides some guidelines for water requirements for growing
landscape plants in