The measurement of soil moisture by neutron method has the advantages of no sampling, fast measurement, high precision, and unlimited depth of measurement. It has a unique role in the measurement and dynamic observation of unsaturated water. Drying method is the most basic and commonly used measurement method, and it is also the reference standard for other measurement methods. This experiment analyzed and compared the accuracy of the two measurement methods, and its conclusion has certain reference effect on the users of the neutron soil moisture meter.
Soil moisture meter error experiment instrument and method
1.1 Instrument parameters The instrument uses the I, 520 intelligent neutron soil moisture analyzer developed and produced by the Institute of Atomic Energy, Jiangsu Academy of Agricultural Sciences. The neutron emissivity of the source is 6.6×104 neutrons/second, and the standard count in water is 950. Pulse / second, calibration using polynomial regression equation, the accuracy of the measurement in the standard bucket is better than 1% (volume water content).
1.2 Experimental site The experimental site is far away in Datian of the West Institute of Atomic Energy Research Institute, Jiangsu Academy of Agricultural Sciences. It has an area of ​​400 O, yellow loam soil, and the soil texture is basically the same within 1 meter. The average dry weight is 1.389/m3.
1.3 Measurement method Before the measurement, use a protective container to make a standard count as a reference for the measurement. Then put the instrument on a catheter and place the probe at the depth to be measured. The measurement time is generally from 30 seconds to 1 minute. The four corners of the field are buried with four thin-walled stainless steel conduits 1.1 meters apart at a distance of 10 meters. As the neutron method, the four measurement points for the amount of cesium are measured, and the drying method is applied to each of the four corners and the middle of the experiment. The measurement and drying methods are sampled in the same layer, the depth error is controlled within 1 cm, and the layer is selected from 30 cm to lm below the surface.
Result and Analysis of Error Test Result of Soil Moisture Meter
2.1 Error Analysis and Comparison of Two Measurement Methods The soil water content is a random variable in spatial distribution. It is assumed that the soil moisture content is normally distributed on the horizontal plane. The purpose of the measurement is to estimate the whole block by extracting several samples or The total moisture content of the entire field is different because of the different measurement mechanisms of the neutron method and the drying method. The accuracy of the estimation results is also different. We use the four catheter measurements as the neutron method to measure the total sample, taking 5 samples at a time. Soil sample. As an estimate of the overall sample of the drying method, the error of the two results was compared. The results are shown in Tables 1 and 2. The mean standard deviation of neutron measurements was 0.66, and the drying method was 1.60. The difference between the two is relatively large. We believe that this is mainly due to the influence of sample size. The drying method can only remove soil samples less than 120cm3 in volume at a time. The neutron method measures a radius of about 15cm and a volume of 1. The weighted average of the 4x 104cm3 spheres has a much larger sample size, which is inevitably small because the sample size is inversely proportional to the error.
According to the principle of mathematical statistics, the true value of moisture in the experimental field should be in the range of (X±into S2/n). Where x is the sample mean, n is the number of samples, S is the standard deviation of the sample, and entry is the critical value of the t distribution. According to the formula, the smaller the standard deviation of the sample, the smaller the range of true value dispersion and the higher the precision of the measurement result. Therefore, under the condition that the number of samples is basically the same, the precision of the neutron method measurement is higher than that of the drying method. More than twice as high, because the measurement accuracy depends mainly on the calibration, not discussed here.
2.2 Reproducibility Experiments and Results The purpose of the reproducibility experiment is to observe the accuracy of the measurement reproducibility. The neutron method of the experiment selected 4 different measurement layers, and each time the machine restarts, it presses the measurement program for 3 times. The drying rule is The soil is taken from the corresponding layer and the errors of the two measurements are compared. The results are shown in Table 3, Table 4:
From the results of repetitive experiments, the sample standard deviation measured by the neutron method is much smaller. This is mainly because it does not require sampling, the soil condition and measurement position can be completely reproduced, and the measurement error mainly comes from the statistical fluctuation of radioactive count. However, it can be reduced by prolonging the measurement time, and the measurement error of the drying method is very large. This is mainly because the position of each soil picking cannot be repeated. Therefore, the soil structure and texture must be considered. The error caused by the sampling depth will cause the upper layer of soil particles to fall on the soil sample and the drying method to undergo a series of processes such as box weight, wet soil weight, drying, dry soil weight, and calculation. The introduction of some errors or even gross errors, the sources of errors are numerous and complex, and it is difficult to eliminate. From the results of this experiment, the sample standard deviation measured by the neutron method is more than 2 times smaller than that of the drying method, so the neutron method measurement repeats Sex is much better than the drying method, so it is more advantageous to use neutron method for relative measurement or for observing the dynamic change of soil moisture.
Soil moisture meter measurement result error experiment conclusion
3.1 Due to the variability in the spatial distribution of soil water content, no matter which method is used to measure, it can only be performed at a certain level of accuracy. From the point of view of estimating the true value of water content in a layer of a whole field, such as measuring The number of points is basically the same, and the measurement accuracy of the neutron method is higher than that of the drying method by more than one time. Although the drying method can use the high-precision balance to measure the moisture value very accurately, the overall estimation has little benefit. When selecting measurement methods, attention should be paid to convenience and speed, and measurement data can be acquired at multiple points to facilitate accurate soil moisture estimation.
3.2 The neutron method has its own unique features in relative measurement or observing the dynamic changes of water. Because it does not require sampling, it can measure a small change in water content at a certain point, and because the neutron slowing process receives salt, the temperature The impact of such factors is relatively small, it can work stably for a long time, and it is suitable for more than ten years of ultra-long-term fixed-point observations and data accumulation.
3.3 The neutron measurement is convenient and rapid, especially for the soil layer whose depth is more than 1.5m or in the permafrost layer. It is an irreplaceable advantage to have other measurement methods.
Related Instruments: Soil Moisture Temperature Tester Fast Soil Moisture Temperature Tester Soil Temperature and Humidity Recorder
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