Calibration for the Watermark 200SS Soil Water Potential Sensor to fit the 7-19-96 “Calibration #3” Table from Irrometer

Rick Allen
Univ. of Idaho
October 13, 2000

The calibration data from calibration table “#3” of Irrometer[1] utilize a linear relationship between resistance (R) and Potential (P) for P > -10 kPa. A curvilinear relationship with much flatter initial slope is used for greater (more negative) potentials. A similar, steep relationship between P and R was observed experimentally by Shock et al. (1998).

The best procedure for reproducing the Irrometer data table, which represents a sensor temperature of 24°C (75°F) was to divide the resistance range into 0 £ R £1 kOhm, 1 < R £ 8 kOhm, and 8 kOhm < R. A linear function is used for 0 £ R £ 1 kOhm, an equation (no. 8) by Shock et al., (1998) [2] is used for 1 < R £ 8 kOhm, and a quadratic equation developed here is used for resistance greater than 8 kOhms.

The equation 8 by Shock fit the 1 < R £ 8 kOhm range better than any other equations developed and tested. However, the Shock equation, developed using experimental data in the range of –10 ³ P ³ -75 kPa, deviated from the Irrometer table (the Irrometer table included data in the range of 0 ³ P ³ -200 kPa) at values of P less than about –100 kPa.


Equation 1  [eqn. 1]

P is soil water potential in kPa
R is measured resistance in kiloOhms
T is sensor (soil) temperature in Celsius

The 0.018*(T-24) term represents the 1.8% shift in resistance reading per one degree C change in temperature from the 24°C base that is recommended by Irrometer. The 1.8% per C is equivalent to 1% per degree F.

For 1 kOhm £ R £ kOhm, equation 8 of Shock et al. (1998) is used:


Equation 2  [eqn. 2]

For measured resistance > 8 kOhm, the following quadratic equation is used:

 [eqn. 3]

where, again, the 1+0.018*(T-24)) adjusts for sensor temperature. Equation 3 was determined by least squares regression from the Irrometer table for the range of –10 kPa > P > -200 kPa. The equation had a coefficient of determination r² = 0.9996 and standard error of estimate of 1.07 kPa.


[1] From Irrometer Company, Inc., Riverside, CA via M.K. Hansen Company, E. Wanatchee, WA, October, 2000.

[2] Shock, C.C., J.M. Barnum, and M. Seddigh. 1998. Calibration of Watermark soil moisture sensors for irrigation management. Proceedings of the 1998 Annual Meeting of the Irrigation Association. p. 139-146.


Figures showing the fit of the various calibration equations for different graphical scales (these are all of the same plot), where the "segment" curve is the combination of equations 1-3.