**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.

NET RADIATION

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
wavelengths. The

**ACCURACY**

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.

REFERENCES

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

http://wwwdpla.water.ca.gov/cgi-bin/cimis/cimis/hq/main.pl

Provides some guidelines for water requirements for growing
landscape plants in

http://wwwdpla.water.ca.gov/urban/conservation/landscape/wucols/index.html