The interpretation of water use measurements appears to be dependent on the procedures used to
obtain the measurements. Because of this dependency, the methods used in this study are described in
detail below. The field operations of this study were conducted in a consistent and careful manner. The
description of the field operations was originally prepared by Tom Crump. What follows is an
abbreviated version of his original description.
Start-up and Shut-down Procedures
The date of lysimeter start-up was dependent on two factors. These were frost in the soil and
snowfall conditions. The frost had to be out of the soil to enable the voids to be saturated with water
during start-up and it was desirable not to start-up until after the last snowfall because it was difficult to
measure the amount of precipitation that was caught, especially in the presence of wind. In particular, it
was desirable to avoid dryer light snow that could easily be blown. The allowable starting times, thus,
were around the last week in May for the lower elevations and about the first week in June for the higher
elevations. The start-up procedure was as follows:
- Water was added to the lysimeter via the 4 inch PVC corner access pipe until the water
table reached the soil surface.
- The lysimeter was allowed to stabilize for at least 24 hours.
- After 24 hours, water was added if necessary to assure that the water tabl.e was at the
- Neutron probe readings at 6 inch increments were taken for the full depth of the
lysimeter with the soil in the saturated condition.
- Water was removed via the corner access tube using a hand pump. The amount was
measured and recorded.
- Again the lysimeter was allowed to set for 24 hours.
- After 24 hours, the water table depth was carefully recorded. This permitted
calculation of specific yield for each lysimeter. Neutron probe readings were then
taken at 6 inch intervals.
These readings were used to determine field capacity above the water table.
The shut-down procedure was similar to the start-up procedure. Weather conditions again affected
the date on which the lysimeters were shut-down. This was usually near the end of October. The
procedure was as follows:
- The crop was first harvested.
- Neutron probe readings were taken at 6 inch intervals and the water table depth
- Measured amounts of water were added until the water table was at the soil surface.
- After 24 hours, additional water was added if necessary to bring the water table to the
- All excess water was pumped from the lysimeter for winter.
Weekly Operation Procedure
Two types of operation were maintained during the season. To simplify the terminology, these
will be referred to as maximum and actual water use measurements. The maximum water use
- Regular surface irrigation for the entire season. An attempt was made to maintain soil
moisture depletion levels at no greater than 50 percent.
- Water table depths were maintained as close as possible to outside levels, or at 3 to 4
ft. when outside water levels dropped for the season. Water table depths were recorded
each week before any water was added or removed.
- Soil moisture measurements were taken each week at 6 inch increments using a neutron
probe. Probe readings were taken using a Campbell Pacific Nuclear neutron moisture
probe (Model 5031, Serial Number H38122580).
- All water, including precipitation, added to or removed from the lysimeters was
measured. Water was added or removed to maintain the water table at the desired
level. If water was removed, then some water was pumped through the soil profile to
assure that the profile was near field capacity. If water was added, it was added
through surface irrigation which accomplished the same objective.
- Average crop height was measured and recorded. Percent crop coverage was
- The alta fescue grass was maintained clipped to a height of 3 to 6 inches. The areas
surrounding the alta fescue and alfalfa sites was also maintained clipped to a distance
of 10 to 15 ft.
- The alfalfa and mountain meadow lysimeters were harvested at the time that the
surrounding fields were harvested and at the end of the season.
The actual water measurement included the above with the exception that irrigation was
discontinued when irrigation outside of the lysimeter was discontinued for the season.
Evaporation Pan Operation
Three Class A evaporation pans were operated during this study. The procedures were
standard for Class A pans. Reservoirs were used to maintain the water level at approximately 2 inches
below the lip of the pan. The surrounding areas were maintained clipped and debris etc., were removed
from the pans weekly.
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