Uncertainty in neutron probe measurements of soil moisture for irrigation scheduling Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6t053k202

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  • Soil moisture in a crop's root zone must be regularly monitored to accurately predict crop water requirements. Most irrigation scheduling services currently use the neutron probe to monitor soil moisture. Unfortunately, there is a degree of variability associated with use of the neutron probe creating uncertainty in measurements of soil moisture. A review of the literature reveals there are three major types of error in estimates of average field moisture by the probe: instrument error, calibration error, and location error. Instrument error accounts for random neutron emission by the radioactive probe source. Calibration error is introduced by inaccuracies in the calibration curve. Location error is a result of estimating average soil moisture throughout a field using only point estimates. This study attempts to provide the neutron probe user with a quick and easy means to evaluate estimates of soil moisture by the probe and to identify sources of error. A computer program is developed that quickly and easily analyzes field calibration data to evaluate the error in neutron probe measurements. Field data from Colorado are analyzed as a numerical example of applying the mechanics of the computer model. Similar data from several California soils are analyzed using the model to show how the range of moisture content over which the probe is calibrated restricts accurate neutron probe measurements to the range of calibration data. The Colorado data are further analyzed to determine to what extent measurement errors occuring in the calibration process will contribute to calibration error in general. In most cases, neutron probe access tubes are placed at biased locations within a field and not randomly as is assumed in the model. A method is developed to correct estimates of average field moisture by taking this discrepancy into account. Given the variability of field moisture content, the number of access tubes that must be sampled to achieve a desired level of accuracy may be calculated using the definition of the student's "t" parameter and referring to specific confidence intervals.
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