Campbell Scientific 253-L and 257-L (Watermark 200) Soil Matric Potential Sensors - Manual (Page 34)

253-L and 257-L Soil Matric Potential Sensors
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3. When the soil dries out to the point where you are seeing readings higher
than 80 kPa, the contact between soil and sensor can be lost because the
soil may start to shrink away from the sensor. An irrigation which only
results in a partial rewetting of the soil will not fully rewet the sensor,
which can result in continued high readings from the 257. Full rewetting
of the soil and sensor usually restores soil to sensor contact. This is most
often seen in the heavier soils and during peak crop water demand when
irrigation may not be fully adequate. The plotting of readings on a chart is
most useful in getting a good picture of this sort of behavior.
9. Reference
Thompson, S.J. and C.F. Armstrong, Calibration of the Watermark Model 200
Soil matric potential Sensor, Applied Engineering in Agriculture, Vol. 3, No. 2,
pp. 186-189, 1987.
Parts of this manual were contributed by Irrometer Company, Inc.,
manufacturer of the Watermark 200.