JP3342764B2 - Method and apparatus for measuring soil moisture - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring soil moisture.

[0002]

2. Description of the Related Art In recent years, elucidation of the mechanism of groundwater contamination by pesticides, fertilizers, etc., evaluation of water balance considering soil layers (unsaturated zones), evaluation of the impact on vegetation related to construction work, slopes including embankment, etc. In studies of the mechanism of collapse and prediction of flood occurrence, it is becoming important to treat the amount and behavior of water in unsaturated soil. To understand these issues, it is necessary to know the behavior of soil moisture throughout the year.

[0003] As a method of measuring soil moisture, a furnace drying method,
Tensiometer method, electric resistance method, dielectric method, heat conduction method,
There are many methods such as an electromagnetic wave method and a radiation method (neutron method, gamma ray method). However, many are still in the research stage,
At present, problems remain in the difficulty of calibration, continuous observation, and measurement accuracy.

[0004] Among these measurement methods, the tensiometer method has been used mainly in the fields of agriculture and slope moisture, and is often used for continuous field observation. This tensiometer method is a method in which a porous porous cup is buried in soil and the water pressure in the porous cup in equilibrium with the suction pressure (water tension) of the surrounding soil is measured.
By converting the water pressure at this time into a water column height, the water tension of the soil can be obtained. The water pressure in the porous cup is measured by a pressure sensor or a mercury manometer using water (degassed water) filled in the tensiometer as a medium. The conversion from the soil water tension to the soil water content is performed using a pF-water characteristic curve obtained by performing a pF test on a soil sample collected at the time of burying a porous cup or the like.

[0005] Here, pF is one of the display methods of soil moisture, and the strength of moisture sucked and held by soil particles is expressed by the logarithm of the height (unit: cm) of a water column corresponding to the suction pressure. It is a representation.

[0006]

The tensiometer method is suitable for continuous soil moisture observation in the field. However, conventionally, it has been difficult to measure moisture below the freezing point by the tensiometer method. Therefore, observation of soil moisture during winter was not so common. This was because the degassed water filled in the tensiometer was frozen.

Conventional measures for preventing the freezing of a tensiometer include an example in which a solution having a low freezing point is used as an antifreeze in place of degassed water, and an example in which the above-ground portion of equipment is heated and kept warm by a hotbed wire. is there. At present, most of the places where soil moisture observation is performed are areas without commercial power sources, such as mountainous basins, and it is difficult to heat and keep equipment for a long period of time. Therefore, as a measure for preventing freezing of the tensiometer, it is considered that the method using an antifreeze is highly versatile.

However, what kind of solution is practical to be used as the antifreeze solution depends on the low freezing point (melting point), the ease of conversion to the measured value using degassed water, and the measurement range. There are many things to consider, such as the size of the solution, the ease of handling of the solution, and the small effect (toxicity) on vegetation and soil, and the practical use of the tensiometer method in winter and cold regions Had been a problem.

An object of the present invention is to provide a soil moisture measuring method and apparatus which can solve the above-mentioned problems of the prior art.

[0010]

According to a first aspect of the present invention, in a method for measuring soil moisture using a tensiometer, a propylene glycol solution is used in place of the water charged in the tensiometer. The propylene glycol concentration of the propylene glycol solution used is varied according to the soil moisture condition to be measured.

[0011] According to a second aspect of the present invention, a conduit having a porous porous cup buried in soil at its tip, a medium tank connected to the conduit, and propylene filled in the medium tank. A glycol solution, and a pressure detecting means for detecting a pressure of the propylene glycol solution in the porous cup in equilibrium with soil water tension. Is changed according to the soil moisture state to be measured.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described in more detail with reference to the accompanying drawings.

FIG. 1 schematically shows an example of the structure of a tensiometer used in the present invention. As shown in FIG. 1, the tensiometer 10 includes a conduit 11 having a porous porous cup (moisture sensor) 12 buried in soil at a tip thereof, and a medium tank 13 communicating with the conduit 11. And a sensor cap 14 that houses a pressure-sensitive sensor and is attached to the upper part of the medium tank 13. The pressure-sensitive sensor in the sensor cap 14 is, for example, a semiconductor pressure conversion type sensor, and the medium tank 13
This is to detect the pressure of a medium such as degassed water to be charged in the porous cup 12 and the medium in the porous cup 12 in equilibrium with the soil water tension. The medium tank 13 has an air release valve 1
5, a sensor cap 14 is provided with a makeup water valve 16, and a sensor cord 17 is drawn out of the sensor cap 14.

In the process of variously examining what kind of solution is suitable as a highly practical antifreeze in place of degassed water in such a tensiometer, the present inventor compared with an ethylene glycol solution or the like. Various experiments were conducted from the viewpoint that the propylene glycol solution is promising because the propylene glycol solution has the following physical properties.

[0015] Propylene glycol is a substance mainly used in food additives, antifreeze, refrigerants, and the like.
It is completely miscible with water at an arbitrary ratio and has extremely low toxicity to the human body. The propylene glycol solution has a low melting point of -70 ° C or less and a specific gravity of 20 ° C.
It is a viscous liquid very close to water, from 1.036 to 1.038. Regarding the surface tension, propylene glycol is 36 dyne / cm 2 at 25 ° C.
2 size.

Propylene glycol is also a hygroscopic substance, and the concentration at which equilibrium changes depends on the state of water in the air. The relative humidity in the soil when in the measurable soil moisture conditions (0~-1000cmH ₂ O at a pressure hydrocephalus) in tensiometer is 99.9%. From this, the equilibrium value of the propylene glycol concentration in the soil is 4
It is assumed that the value is smaller than 0%.

The physical properties of the propylene glycol solution differ depending on the propylene glycol concentration. FIG. 3 shows the specific gravity, melting point, viscosity, and surface tension at the main concentrations (25, 50, 100%) of the propylene glycol solution together with the physical properties of water (propylene glycol concentration 0%). The relationship between the concentration of the propylene glycol solution and each item of specific gravity, melting point, viscosity, and surface tension will be described below.

First, the specific gravity of the propylene glycol solution is highest at a concentration of 75 to 80% at each temperature,
At 5% or less, the lower the concentration, the greater. However, the specific gravity of the propylene glycol solution is 1 to 1.05 times the specific gravity of water at any temperature, and the difference in specific gravity between the propylene glycol solution and water is extremely small. Therefore, in the soil moisture measurement using the propylene glycol solution for the tensiometer, it is considered that there is almost no influence on the measured value due to the difference in specific gravity between the propylene glycol solution and water.

Next, the melting point of the propylene glycol solution decreases as the concentration increases. For example, the melting point of a 25% solution of propylene glycol is about −10 ° C., and the melting point of a 50% solution of propylene glycol is about −32 ° C.
It is. When performing soil moisture measurement using a propylene glycol solution for the tensiometer, it is necessary to determine the concentration of the propylene glycol solution within a range where the melting point is lower than the lowest temperature at the measurement point or lower than the lowest ground temperature.

The viscosity of the propylene glycol solution increases as the concentration increases. As in the case of water, the viscosity of the propylene glycol solution increases as the temperature decreases. For example, the viscosity of a 25% solution of propylene glycol is about twice the viscosity of water at the same temperature, and the viscosity of a 50% solution of propylene glycol is about 5-6 times the viscosity of water at the same temperature. The tensiometer measures the tension (soil moisture tension) when the solution filled in the tensiometer is pulled outside through the gap of the porous cup. Therefore, when a propylene glycol solution is used, It is thought that the high viscosity results in tension resistance, which may affect the soil moisture measurement.

Furthermore, the surface tension of the propylene glycol solution decreases as the concentration increases. For example, the surface tension of a 25% propylene glycol solution is about 0.7 times the surface tension of water, and the surface tension of a 50% solution of propylene glycol is about 0.6 times the surface tension of water. Normal,
If degassed water is used for the tensiometer, the pF
When the soil is dried from 2.8 to 2.9, the porous cup draws air and reaches the measurement limit. Since the surface tension of a propylene glycol solution is lower than that of water, when a propylene glycol solution is used for a tensiometer, the porous cup may draw air with a lower pF value than that of water. That is, from the viewpoint of the surface tension, the use of the propylene glycol solution for the tensiometer may narrow the measurement range of the soil moisture.

From the above, it is considered that the difference in viscosity and surface tension between the propylene glycol solution and water causes an influence on the soil moisture measurement value and measurement range when propylene glycol is used for the tensiometer. . It is also considered that the property that the equilibrium value of the propylene glycol concentration changes depending on the moisture in the air may cause an influence on the measured value of soil moisture when the propylene glycol solution is used for the tensiometer.

Based on the above-mentioned relationship between the physical properties of a propylene glycol solution and the physical properties of water, the present inventor has proposed a method of using a propylene glycol solution instead of degassed water as a medium to be filled in a tensiometer. The following experiment was conducted to confirm the practicality of the method.

FIG. 2 shows a schematic configuration of an experimental apparatus used for the experiment. As shown in FIG. 2, this experimental apparatus is composed of two vertical one-dimensional soil columns 4 (columns A and B) 152 cm in height made by processing a vinyl chloride pipe having an inner diameter of 38.2 cm and a thickness of 12 mm. It has. The lower ends of the columns 4 are connected by pipes for water supply and drainage so that groundwater levels of the same height are formed in the two columns 4. Further, a water level adjusting tank 6 for controlling the groundwater level and a manometer 5 for reading the water level are connected to the plumbing pipe, and the height of the groundwater level in the column is easily increased by moving the water level adjusting tank up and down. Can be set and changed. The upper part of the column 4 is a cooling chamber 3 surrounded by a heat insulating material (styrene foam) 8, and a constant temperature environment is maintained by cool air sent from a freezer by a fan. In order to prevent the soil temperature in the soil column 4 from being affected by a change in the outside air temperature, the periphery of the column 4 is surrounded by a heat insulating material (plywood) 9 to block the outside air.

As the experimental soil, Toyoura standard sand was used.
By filling the soil column 4 with the Toyoura standard sand by the water-tight method, the Toyoura standard sand 7 is uniformly filled in the two columns 4 and the sealed air does not remain.

For the measurement of the pressure head, a tensiometer having a structure as described above with reference to FIG. 1 was used, and a porous ceramic (pore diameter: 0.1 to 1 μm) having a diameter of 18 mm and a length of 6 cm was used. Two tensiometers (manufactured by Sankei Rika, Model SK-5500) combining a cup and a semiconductor pressure conversion type sensor were used. The measurement range of the tensiometers 1 and 2 is 0 to ± 100
0 cmH ₂ O, and the measurement accuracy S. 0.5% (5cm
H ₂ O). The porous cup was buried 25 cm below the soil surface in each column 4, and the tensiometer 1 was S1 and the tensiometer 2 was S2. Pressure head is 1
The measurement was performed automatically every 0 minutes, and the measurement data was recorded in an IC card using a data logger (Unipulse, U-LOGGER L810B).

For the measurement of the temperature, a copper-constantan thermocouple (NT-350TUS3.2-100, manufactured by Nesshin Co.) was used.
10 mED type). The measurement range of the thermometers T1 to T6 is 0 to 350 ° C., and the measurement accuracy is ± 1 ° C. Measurements were taken at two soil temperatures 25 cm below the surface of the soil (T1, T
2), just above the soil surface (T3, T4), about 1 above the soil surface
4cm (Tensiometer pressure sensor height; T5)
And the temperature (T6) around the soil column 4 at four locations. The soil temperature and air temperature are automatically measured every 10 minutes, and the measurement data is stored in a data logger (Unipulse,
-LOGGER L822) recorded on the IC card.

For measuring the concentration of the propylene glycol solution charged in the tensiometer 2, a polarizing brine densitometer (BR-1 type, manufactured by Atago Co., Ltd.) was used. The measurement range of the densitometer is 0 to 70%, and the measurement accuracy is ± 1%. The measurement of the propylene glycol concentration was performed on the propylene glycol solution when filling the tensiometer 2 before the start of the experiment and on the propylene glycol solution collected from inside the tensiometer 2 at the end of the experiment.

The experiment was conducted using deaerated water and propylene glycol 2
A comparative experiment using a 5% solution (Exp. 1) and a comparative experiment using degassed water and a 50% propylene glycol solution (Ex.
p.2). Ex1 is the tensiometer 1 of S1
Both p.1 and Exp.2 are filled with degassed water, and a 25% solution of propylene glycol in Exp.
Filled with 0% solution. In each experiment, the height of the groundwater table was adjusted to an arbitrary moisture state of pF0 to 1.8 (pF0, 0.5, 1.
0, 1.3, 1.5, 1.6, 1.7, 1.8), and the changes in pressure head, soil temperature, and temperature during that time were measured. In addition, in the state of each pF value described above,
The water table is fixed until the pressure head reaches equilibrium. The movement of the groundwater surface starts from the state of pF0 (the rise of the groundwater surface is GL -25.0 cm), the process of lowering the groundwater surface (drainage process), and the process of rising the groundwater surface (water absorption process).
In order. Regarding the temperature environment in the cooling room, in order to eliminate the influence of the change in air temperature on the measured pressure head,
A state where the temperature was kept constant at around 15 ° C. was set.

Regarding the change in the concentration of the propylene glycol solution in the tensiometer 2, it is impossible to collect the propylene glycol solution from the tensiometer during the experiment. The concentration at the end of the experiment was measured.

Prior to the start of Exp. 1 and Exp. 2, S
As a result of conducting a comparative experiment and the like in the case where degassed water was filled in both S1 and S2, the change characteristics of the measured values of S1 and S2 were the same. Is set.

Since the inside and outside of the tensiometer are continuous due to the pores of the porous cup, the outflow of the propylene glycol solution into the soil and the outflow of propylene glycol due to the osmotic pressure are considered. If propylene glycol flows into the soil, the hydraulic characteristics of the soil may change due to the difference in the physical properties of propylene glycol and water. Therefore, in order to grasp the physical properties of the soil when the water in the soil is a propylene glycol solution, two kinds of propylene glycol solutions having a concentration of 25% and 50% are used as test water instead of water, and a water permeability test is performed.
An F test was performed. In addition, since the physical property test was performed in consideration of the state in which the propylene glycol solution was contained in the soil moisture, the terms “permeability coefficient” and “pF to moisture characteristic curve” are used as they are.

Using two types of propylene glycol solutions having a concentration of 25% and 50% as test water, a water permeability test was conducted by a variable water level method. The test uses a soil sample saturated with a propylene glycol solution. According to the results of the water permeability test, it was found that the water permeability was reduced as the concentration of the propylene glycol solution was increased.
It is considered that this is mainly because the viscosity of the propylene glycol solution increases as the concentration increases, thereby causing resistance to permeation.

Using two types of propylene glycol solutions having a concentration of 25% and 50% as test water, a pF test was performed by a sand column method and a pressure plate method, and a pF-water characteristic curve for the propylene glycol solution was determined. FIG. 4 shows the pF to moisture characteristic curves for the propylene glycol solutions of each concentration. In the pF test, a 25% propylene glycol solution,
With both the 50% propylene glycol solution and the pF of 2.0 or more, drainage from the sample to the measuring cylinder became difficult. For this reason, the end of drainage has not been fully confirmed and each p
It is possible that the volume moisture content at the F value is somewhat higher.

Looking at the pF value of each solution for the same volumetric water content in FIG.
Both the 50% propylene glycol solution has a lower pF value (higher pressure head) than the case of degassed water (propylene glycol concentration 0%) when the pF is 1.7 or less, and the pF 2.0
In the above, a higher pF value (lower pressure head) is taken than in the case of degassed water. When FIG. 4 is viewed from the viewpoint of the difference in volume water content at each pF value, in the case of degassed water having the same pF value at a pF of 1.7 or less, both the propylene glycol 25% solution and the propylene glycol 50% solution have a pF of 1.7 or less. It has a lower volumetric water content than that of degassed water having the same pF value at a pF of 2.0 or more.

From the above test results, when propylene glycol leaks into the soil, the water permeability decreases, and p
It can be seen that F to the moisture characteristic curve changes. However, the proportion of the leaked propylene glycol in the soil moisture is small, and because of the dilution and the movement of moisture in the soil, the effect of propylene glycol leaked from the porous cup in actual outdoor soil is affected by the concentration. 25
%, 50% propylene glycol solution is not considered to be as large as the result of the physical property test.

In Exp. 1, degassed water and a 25% propylene glycol solution were filled in S1 and S2 tensiometers, respectively, and a comparative experiment was performed. In the drainage process, S1 and S2 increase as the pF value increases, that is, as the pressure head decreases.
The difference between the pressure head measurement values of the two became large. In addition, pF1.8
In the water condition (pressure head ψ = -63.1 cmH ₂ O), S
The pressure head of 2 did not reach equilibrium. In the water absorption process, p
Although water absorption was started from the state of F1.8, the pressure head of S2 did not reach equilibrium from pF1.8 to pF1.6.
When the equilibrium was not reached, the pressure head of S2 changed at a value lower by at least 40 cmH ₂ O than the pressure head of S1.

According to the temperature measurement results, thermometers T1 and T2
, And that there was no difference between the thermometers T3 and T4, it was found that the column A and the column B had the same temperature environment (ground temperature, air temperature).

The concentration of the propylene glycol solution in the tensiometer was 25% at the start of the experiment and 16% at the end of the experiment.
%, And a decrease in the concentration was observed. Also, no decrease in the volume of degassed water or propylene glycol solution in the tensiometer during the experiment was observed.

In Exp. 2, detented water and a 50% propylene glycol solution were filled in the tensiometers S1 and S2, respectively, and a comparative experiment was performed. In the drainage process, as the pF value increases, that is, as the pressure head decreases, S1 and S2 increase.
The difference between the pressure head measurement values of the two is large. Also, pF
Moisture condition of 1.7 to 1.8 (pressure head ψ = −50.1 to −63.
At 1 cmH ₂ O), the pressure head of S2 did not reach equilibrium. In the water absorption process, water absorption started from the state of pF1.8, but the pressure head of S2 did not reach equilibrium from pF1.8 to pF1.5. S2 pressure head in the case that does not reach the equilibrium is changed between -40~-60cmH ₂ O,
In the state of pF1.8, the value was higher than S1.

According to the temperature measurement results, thermometers T1 and T2
, And that there is no difference between the thermometers T3 and T4, it was found that the column A and the column B had the same temperature environment (ground temperature, air temperature).

The concentration of the propylene glycol solution in the tensiometer was 46% at the start of the experiment and 3% at the end of the experiment.
2%, indicating a decrease in concentration. Also, no decrease in the volume of degassed water or propylene glycol solution in the tensiometer during the experiment was observed.

The present inventor has proposed such Exp. 1 and Exp.
Based on the experimental results of Example 2, the influence characteristics on the measured values when a propylene glycol solution was used for the tensiometer were examined. FIG. 5 shows the relationship between the pressure head measurement values of S1 (using degassed water) and S2 (using propylene glycol 25% solution) in the equilibrium state in Exp. FIG. 6 shows the relationship between the measured pressure head of S1 (using deaerated water) and S2 (using 50% propylene glycol solution) in the equilibrium state in Exp. The difference between the measured values of S1 and S2 when equilibrium is achieved in each moisture state is as shown in the table of FIG. The isolated points in FIGS. 5 and 6 and the values in parentheses in the table of FIG.
Shows the measured value of the pressure head when the pressure did not reach equilibrium.

The reason why the pressure head measured value of S2 reached equilibrium during the drainage process of Exp. 1 was as shown in the table of FIG.
It is in the range of ~ 1.7. The pressure head measurement value of S2 in the state where the measurement value did not reach the equilibrium is shown in FIG.
Represents the value of -130~100cmH ₂ O, it is not seen movement in response to changes in the measured values of S1. From this, it is considered that when the pressure head measurement value is non-equilibrium, the propylene glycol solution is in a state where the film is broken.

In Exp. 1, since the experiment of the water absorption process was started after draining to the state of pF1.8, the pF of the water absorption process was started.
Although the state is in a non-equilibrium state in the range of 1.6 to 1.8, when the drainage process is completed at a pF of 1.7 or less and the water absorption process is started, the pF 1.6 to 1.0 of the water absorption process is started. It is estimated that equilibrium will be reached even in the range of 7. Therefore, the measurement range when using propylene glycol 25% solution in tensiometer is considered in Toyoura standard sand in a range of pF0~1.7 (0~-50.1cmH ₂ O with pressure head) .

The reason why the pressure head measured value of S2 reached equilibrium during the drainage process of Exp. 2, as shown in the table of FIG.
It is in the range of 0 to 1.6. When the measured value did not reach equilibrium, the measured value of the pressure head of S2 was −60 to −40 cmH _2.
It shows the value of O, and no movement corresponding to the change in the measured value of S1 is observed. In Exp.2, as in Exp.1, pF1.
Since the water absorption process was started after draining to the state of No. 8, the pressure head measurement value of S2 was in a non-equilibrium state in the range of pF 1.5 to 1.8 during the water absorption process. To p
When the water absorption process is started by finishing the process up to F1.6 or less, the pF of the water absorption process is 1.5 to 1.6 (-31.6 to -3 in the pressure head).
It is considered that equilibrium is reached even in the range of 9.8 cmH ₂ O).

From the above results, the influence of the use of the propylene glycol solution in the tensiometer is that the range in which soil moisture can be measured is narrower than when degassed water is used.

When the soil moisture of Toyoura standard sand was measured using a propylene glycol solution as a tensiometer, the deaerated water was used based on the measurement value obtained using the propylene glycol solution. Measurements (true soil moisture) need to be obtained. Then, based on the experimental results of Exp. 1 and Exp. 2, the influence of the use of the propylene glycol solution on the measured value of the tensiometer was examined.
Note that since it is impossible to estimate the pressure head measurement value of S1 from the pressure head measurement value of S2 in the state where the propylene glycol solution has a film breakage, here, the water state in which S2 has reached equilibrium is described. Only considered.

The measured values of the pressure heads of S1 and S2 when the measured values are in the equilibrium state in each moisture state of Exp. 1 and Exp. 2 are as shown in FIG. 9, and the pressure head measurement of S2 at that time. The relationship between the value and the measured value of the pressure head of S1 is as shown in FIGS. The broken lines in FIGS. 7 and 8 indicate regression lines, respectively, and are represented by the following equations. Regression line of FIG. 7 S1 = 0.90 × S2 (1) (However, pF0 ≦ S1 ≦ 1.7) [r = 0.999 r: correlation coefficient] Regression line of FIG. 8 S1 = 0.97 × S2 + 0 .52 (2) (However, pF0 ≦ S1 ≦ 1.6) [r = 0.999] In the analysis of soil moisture actually observed in the field, it is rare to distinguish the drainage process from the water absorption process. In addition, since there is no difference in the characteristics between the drainage process and the water absorption process in FIGS. 7 and 8, the regression line is obtained for all data of the drainage process and the water absorption process.

When the pressure head is a positive value, that is, in a saturated state, the difference between S1 and S2 is within 0.6 cmH ₂ O in Exp. 1 and within 1.2 cmH ₂ O in Exp. It is believed that the use of the solution has no effect on the measurements, ie the osmotic pressure.

When the pressure head is a negative value, that is, in an unsaturated state, as can be seen from the regression lines in FIGS.
The difference in pressure head between S1 and S2 tends to increase in proportion to the decrease in pressure head. When applying the measurable range in the expression (1) when using propylene glycol 25% solution, the difference between S1 and S2 is 5.6cmH ₂ O at a maximum, PF1.65～1.7 (pressure The head is -45 to -50.1cm
H ₂ O) in the range of 5
exceeds cmH ₂ O. However, in general, in the analysis of soil moisture behavior, a unit less than cmH ₂ O of the pressure head is often rounded and used. Therefore, in most soil moisture conditions within the measurable range, the difference between S1 and S2 is less than the measurement accuracy. It can be said that the error is within the range. On the other hand, when the above formula (2) is applied to the measurable range when using a 50% propylene glycol solution, the maximum value of the difference between S1 and S2 is 0.7 cmH ₂ O, and measurement is accurate 5cmH significantly less than ₂ O.

From the above, when the soil moisture of Toyoura standard sand is measured using a propylene glycol solution in a tensiometer, the measured pressure head within the measurable range is the true pressure head (degassing). Pressure head when water is used).

Based on the results of the Toyoura standard sand described above, the influence on the measurement range and measured value when a propylene glycol solution is used for soil moisture measurement with a tensiometer in soils having different particle sizes and the like is also examined. I tried
In the soil moisture state within the measurable range, it was confirmed that the measured value of the pressure head when the propylene glycol solution was used showed a true pressure head regardless of the type of soil.

In Exp. 1 and Exp. 2, the decrease in the concentration of the propylene glycol solution in the tensiometer due to the experiment was measured. Exp.1 and Exp.2 both require approximately 240 hours from the start of the experiment to the end of the experiment.
In Exp.1, the concentration decreased from 46% to 32%, and Exp.2
Reduced the concentration from 25% to 16%. It is considered that the decrease in the concentration of the propylene glycol solution in the tensiometer is caused by the leakage of propylene glycol from the porous cup to the moisture in the soil due to the osmotic pressure of propylene glycol. The rate at which the concentration of the propylene glycol solution in the tensiometer decreases will depend on the length of the buried pipe of the tensiometer and the condition of the soil moisture.

When the concentration of the propylene glycol solution in the tensiometer decreases, the melting point of the propylene glycol solution increases, so that the ability as an antifreeze decreases. Therefore, when performing long-term continuous measurement of soil moisture with a tensiometer using a propylene glycol solution, it is necessary to pay attention to changes in the concentration of the propylene glycol solution in the tensiometer.

The results of a soil column experiment and a soil physical property test using Toyoura standard sand as described above as an experimental soil can be summarized as follows.

(1) When the soil moisture of Toyoura standard sand is measured using a propylene glycol solution for a tensiometer, the range in which soil moisture can be measured is narrower than when degassed water is used, and the concentration is determined. Is 25%, the measurement range is pF0 to 1.7 (0 to −50.1 cmH _{2 in} pressure head).
O), the measurement range for a 50% solution is pF0
11.6 (0 to -39.8 cmH ₂ O in pressure head).

(2) When the soil is saturated, it is considered that the use of the propylene glycol solution does not affect the value measured by the tensiometer, that is, the osmotic pressure.

(3) In the measurement range of the above (1), the influence of the use of the propylene glycol solution on the measured value of the tensiometer is within an error range less than the measurement accuracy, and can be ignored.

(4) When the soil moisture of various soils is measured using a propylene glycol solution in a tensiometer, at least the pressure head in the range of saturation to air penetration value in the pF of various soils to the moisture characteristic curve is It can be measured.

From these results, in the measurement of the soil moisture in winter by the tensiometer using the propylene glycol solution, although the range of the measurable soil moisture is narrower than in the case of using degassed water, the measured value of the pressure head is smaller. Can be treated as a true pressure head.

[0062]

According to the present invention, soil moisture can be measured by a tensiometer in a low-temperature environment, which was physically impossible until now. This makes it possible to measure soil moisture throughout the year.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a structural example of a tensiometer used in the present invention.

FIG. 2 is a diagram showing a schematic structure of an experimental apparatus used in the development of the present invention.

FIG. 3 is a table showing the concentration and physical properties of a propylene glycol solution used in the present invention.

FIG. 4 is a graph showing a concentration and water characteristic curve of a propylene glycol solution used in the present invention.

5 is a diagram showing a relationship between measured values of pressure heads of columns A and B in Experiment 1 in the experimental apparatus of FIG. 2;

6 is a diagram showing a relationship between measured values of pressure heads of columns A and B in Experiment 2 in the experimental apparatus of FIG. 2;

7 is a diagram showing the relationship between the measured values of the pressure heads of columns A and B and the regression line in Experiment 1 in the experimental apparatus of FIG.

8 is a diagram showing the relationship between the measured values of the pressure heads of columns A and B and the regression line in Experiment 2 using the experiment apparatus of FIG.

FIG. 9 is a table showing a difference in measured pressure head between degassed water and a propylene glycol solution at a pF value.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tensiometer 2 Tensiometer 3 Cooling room 4 Soil column 5 Manometer 6 Water level adjustment tank 7 Toyoura standard sand 8 Insulation material 9 Insulation material 10 Tensiometer 11 Pipe 12 Porous cup 13 Medium tank 14 Sensor cap 15 Air vent valve 16 Make-up water valve 17 Sensor code

──────────────────────────────────────────────────の Continuation of front page (58) Surveyed field (Int.Cl. ⁷ , DB name) G01N 33/24 E02D 1/02 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. A method for measuring soil moisture using a tensiometer, characterized in that a propylene glycol solution is used in place of the water charged in the tensiometer. 2. The method according to claim 1, wherein the propylene glycol concentration of the propylene glycol solution is changed according to a soil water condition to be measured. 3. A conduit having at its tip a porous porous cup buried in the soil, a medium tank connected to the conduit, a propylene glycol solution filled in the medium tank, and equilibrium with soil water tension. And a pressure detecting means for detecting a pressure of the propylene glycol solution in the porous cup in a state. 4. The soil moisture measuring device according to claim 3, wherein the propylene glycol concentration of the propylene glycol solution is changed according to a soil moisture condition to be measured.