For each management unit you are assessing,
determine the maximum daily irrigation water applied (MDWA) in gallons.
This can be done in several ways, and is usually recorded on a very hot day in summer.
1. A water meter should
be part of all nursery or greenhouse irrigation operations because many operations
are large enough to be required to record and report water usage information
per their state permit.
A water meter on the well or in the pumping station can help to establish the total amount
of water used in total and for management unit:

For total water usage, either read the meter daily at the same time of day over a few days
in summer and compute the daily average;

For the water usage of a particular management area, read the meter before and after an irrigation event. If the area is watered twice
or more times a day, then the meter must obviously be read before and after
each watering.
Water usage, in gallons
= final meter reading  initial meter reading, in gallons
2. The nozzle discharge
rate can also be used to calculate the maximum daily application amount.
Follow the procedure for measuring nozzle discharge rate given earlier. The
average of several nozzles gives a better figure. The equation for determining
the water usage in each growing area (management unit) is given below:
This procedure can be used
for one or for several houses or growing areas (management unit) as long as
the nozzle discharge rates are correct for those areas. Watch the units! Water
usage in total gallons discharged equals the average discharge rate of the nozzle,
in gallons per minute (gpm), times the number of nozzles times the number of
minutes (60 times number of hours) of irrigation.
The "time" to be used here
should be the length of time used for onedaily irrigation or the total of individual
irrigations throughout the day. Document what the "time" means here for the
operation as the information may be needed later, i.e.  to evaluate how and
when the operation irrigates the management unit.
A representative growing
bed or irrigation zone should be used to make this calculation. The results
will be more accurate than taking the gross total water applied to the whole
nursery or greenhouse.
3. Alternatively, a
series of quality rain gauges or straightsided cans can be used to determine
the average application depth during an irrigation event.
Total the depths of
water measured and average the total. Divide the total by the number of measurements.
This depth figure times the square feet of the growing area gives cubic feet
of water. Convert to gallons by multiplying by 7.48. The Total
Applied Water diagram describes this procedure. The concept of Application Depth is covered in more detail in the next section.
4. A less refined method (if a water meter is not present) is to record information on the
pump capacity in gallons per minute and daily operating hours, as a way to define the total amount of water pumped and applied daily for the operation.
Pumping
rates may not be known accurately but the ballpark figures, in gallons per minute,
are useful in the total evaluation process. Try to get a clear understanding
of the irrigation process in the operation.