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calculation can be much less straightforward, particularly when soil moisture is measured as a 
tension. 

4.3.2  Soil Moisture Measurement Considerations 

Soil moisture measurements are important for a number of applications and for a number of 
different reasons. Some applications include; land slide studies, erosion, water shed studies, 
climate studies, predicting weather, flood warning, crop quality and yield optimization, 
irrigation, and soil remediation to name a few. 
 
Soil moisture values are particularly important for irrigation and the health of the crop. Equations 
[4.1], [4.2] and [4.3] can help determine when to irrigate. The following are terms commonly 
used in soil hydrology: 

!

 

Soil Saturation, refers to the situation where all the soil pores are filled with water. This 
occurs below the water table and in the unsaturated zone above the water table after a 
heavy rain or irrigation event. After the rain event, the soil moisture (above the water 
table) will decrease from saturation to field capacity.   

!

 

Field Capacity (FC in equations below) refers to the amount of water left behind in soil 
after gravity drains saturated soil. 

!

 

Permanent Wilting Point (PWP in equations below) refers to the amount of water in soil 
that is unavailable to the plant. 

!

 

The Allowable Depletion (AD in the equations below) is calculated by equation [4.1]. 
The allowable depletion represents the amount of soil moisture that can be removed by 
the crop from the soil before the crop begins to stress. 

!

 

Lower soil moisture Limit (LL from [4.3]) is the soil moisture value below which the 
crop will become stressed because it will have insufficient water. When the lower limit is 
reached, it is time to irrigate.  

!

 

The Maximum Allowable Depletion (MAD) is the fraction of the available water that is 
100% available to the crop. 

!

 

Available Water Capacity (AWC) is the amount of water in the soil that is available to 
the plant. 

The lower soil moisture limit is a very important value because dropping below this value will 
affect the health of the crops. Equations 4.1, 4.2, and 4.3 and the example show how to calculate 
the lower soil moisture limit. 

 

AD     = (FC – PWP ) x MAD   

[4.1] 

AWC =   FC – PWP 

 

 

[4.2] 

LL     =   FC – AD  

 

 

[4.3] 

 

 

 

28 

Содержание Hydra Probe II Series

Страница 1: ...Stevens Water Monitoring System Inc The Hydra Probe Soil Sensor Comprehensive Stevens Hydra Probe Users Manual 92915 July 2007 1...

Страница 2: ...UTION Be aware of protective measures against environmentally caused electric current surges Read the Stevens Engineering Applications Note Surge Protection of Electronic Circuits part number 42147 In...

Страница 3: ...lowing statement is required by the Federal Communications Commission WARNING This equipment generates uses and can radiate radio frequency energy and if not installed in accordance with the instructi...

Страница 4: ...the Analog Hydra Probe 13 2 3 Other Applications and Alternative Calibrations 13 2 3 1 Frozen Soil 14 3 Installation 15 3 1 Avoid Damage to the Hydra Probe 15 3 1 1 Lightning 15 3 2 Wire Connections 1...

Страница 5: ...oil Electrical Conductivity 33 5 3 1 Electrical Conductivity Pathways in Soil 33 5 3 2 Solution Chemistry 34 5 3 3 Cation Exchange and Agriculture Reclaiming Salt Infested Land 35 6 Maintenance and Tr...

Страница 6: ...Hydra Probe bases it measurements on the physics and behavior of a reflected electromagnetic radio wave in soil to determine the dielectric permittivity From the complex dielectric permittivity the Hy...

Страница 7: ...se plate is 25 mm in diameter Electromagnetic waves at a radio frequency are transmitted and received by the center tine The head or body of the probe contains the circuit boards microprocessors and a...

Страница 8: ...of the Hydra Probe Feature Attribute Probe Length 12 4 cm 4 9 inches Diameter 4 2 cm 1 6 inches Sensing Volume Cylindrical measurement region Length 5 7 cm 2 2 inches Diameter 3 0 cm 1 2 inches Weigh...

Страница 9: ...e no splices to fail The disadvantage is that the cable requirements and associated costs will be higher 2 1 3 SDI 12 Hydra Probe II The SDI 12 version is digital and can be used with Data Loggers tha...

Страница 10: ...olts Power Input Black Wire Ground White Wire Data Signal A non inverting signal Green Wire Data Signal B inverting signal Baud Rate 9600 8N1 Power Consumption 10 mA Idle 30 mA Active Table 2 3 Digita...

Страница 11: ...g Hydra Probe Output The output of the Analog Hydra Probe is 4 voltages These four voltages are the raw signal response of the measurement and directly represent the behavior of the reflected electrom...

Страница 12: ...truction procedure for HYD_FILE EXE can be downloaded from the Stevens website at http www stevenswater com HYD_FILE EXE is the program used for processing tables of raw voltages collected over time H...

Страница 13: ...connected to a computer Most of the technical support questions Stevens receives are not due to malfunctioning probes but rather an incorrect data logger setup A good way to verify if the probe is wo...

Страница 14: ...and experimentally solve equation 2 1 or equation 2 2 Also The user should review the matrix compatibility requirements of the probe in section 3 1 2 3 1 Frozen Soil The Hydra Probe can also be used...

Страница 15: ...arm equipment The Hydra Probe may be sturdy enough to survive getting run over by a tractor if it is buried however the compaction of the soil column from the weight of the vehicle will affect the hyd...

Страница 16: ...er on a multiplexer with a single data wire going into the single SDI 12 data port For more information refer to the data loggers operation manual For more information about the Stevens DOT Logger ref...

Страница 17: ...to the different textures If you are unsure of your soil texture determining which soil setting that is best for your soil is easy and there are a number of different ways to make the determination Bi...

Страница 18: ...eans the Hydra Probe is preset to the loam soil type setting when it leaves the factory Figure 3 2 shows the textures where the loam setting works best Figure 3 2 The shaded circle in the soil texture...

Страница 19: ...d Mollisol Grass land Soil Higher pH Loam or Silt Spodosol Needle Leaf Forest Soil Low pH Loam or Silt Alfisol Forest Soil Low pH Loam or Silt Ultisol Old Forest Soil Low pH Loam Oxisol Ancient Forest...

Страница 20: ...in the water table The Hydra Probe data is most meaningful in the unsaturated zone where soil moisture values will fluctuate If the water table rises to the depth of the Hydra Probe the Hydra Probe s...

Страница 21: ...cause if there is a gap the Hydra Probe signal will average the gap into the soil measurement and create errors Figure 3 5 Hydra Probe Installed in undisturbed soil Push the tines of the Hydra Probe i...

Страница 22: ...Soil scientist and groundwater hydrologist are often interested in studying soil horizons The Steven Hydra Probe is an excellent instrument for this application because of the accuracy and precision...

Страница 23: ...ucture pH and the visible appearance of clay films More information about soil horizons is provided by the USDA National Resource Conservation Service at http soils usda gov education resources k_12 l...

Страница 24: ...e pit needs to be backfilled with the soil that came out it It is impossible to put the horizons back the way they have formed naturally but the original bulk density can be approximated by compacting...

Страница 25: ...turn to a steady state bulk density The Hydra Probe will effectively be residing in two soil columns The tines will be in the undisturbed soil column and the head cable and conduit will be in the back...

Страница 26: ...s and makes the Hydra Probe unique among other electromagnetic based sensors Campbell 1990 Seyfried and Murdock 2004 4 1 2 V4 V4 is the raw signal response of the diode thermistor The diode thermistor...

Страница 27: ...s used to measure soil moisture They include water by weight field capacity available to a crop inches of water to inches of soil and tension or pressure They are all interrelated in the sense that fo...

Страница 28: ...ation to field capacity Field Capacity FC in equations below refers to the amount of water left behind in soil after gravity drains saturated soil Permanent Wilting Point PWP in equations below refers...

Страница 29: ...a sandy loam if the soil moisture drops below 19 or Hydra Probe reading of 0 19 the crop may become stressed and it is time to irrigate Never let the soil moisture drop below the permanent wilting poi...

Страница 30: ...its of Siemens per meter Soil electrical conductivity is indicative of dissolved salts dissolved solids and fertilizers McBride 1994 It may also be indicative of very high pH conditions The soil elect...

Страница 31: ...dia The dielectric constant is related to capacitance by the following equation oE Cx 5 1 o is the capacitance in a vacuum E is the dielectric constant and Cx is the measured capacitance of the media...

Страница 32: ...m the higher the imaginary dielectric permittivity the higher the conductivity Assuming the molecular relaxation is negligible the imaginary dielectric constant is a function of frequency and electric...

Страница 33: ...tant The temperature corrections are based on the small incremental changes of Ei and Er with temperature 5 3 Soil Electrical Conductivity The Hydra Probe measures the in situ electrical conductivity...

Страница 34: ...of pathway 3 in influenced by a number of factors that include bulk density soil type oxidation reduction reactions and translocation of ions The electric conductivity measurements provided by the Hyd...

Страница 35: ...larger spheres are chloride anions Water will dissolve the sodium chloride crystal lattice and physically separate the two ions Once in solution the sodium ion and the chloride ion will float around...

Страница 36: ...horizons of the soil the ion exchange begins The calcium ion has a 2 charge where the sodium ion has a 1 charge Because the calcium ion has a greater electric charge the soil will have a stronger affi...

Страница 37: ...submerged All configurations of the Hydra Probe should have a real dielectric permittivity of about 80 in water For special instructions for trouble shooting Analog probes see Section 2 2 3 If the Hy...

Страница 38: ...optical isolator in order to be connected to a computer Most of the technical support questions Stevens receives are not due to malfunctioning probes but rather an incorrect data logger setup DOTSet...

Страница 39: ...plants Factors such as wind temperature humidity solar radiation and soil type play a role in the rate of ET If ET exceeds precipitation there will likely be a net upward movement of water in the soil...

Страница 40: ...osed of smectite clays These clays have a large ion exchange capacity and will shrink and swell seasonally with water content The seasonal expansion and contraction homogenizes the top soil and the su...

Страница 41: ...electrical conductivity beyond 1 5 S m In general if the electrical conductivity reaches 1 S m the soil moisture measurements will be significantly affected The imaginary dielectric constant will hav...

Страница 42: ...ties The temperature correction for the imagery dielectric permittivities are used for the temperature corrected soil electrical conductivity It follows the trend that the increased molecular vibratio...

Страница 43: ...rily be set before the probes are all connected The probes reply with information only when they have received a valid command that is addressed to them It is important to note that each SDI 12 sensor...

Страница 44: ...rature Celsius G Soil Temperature Fahrenheit H Soil Moisture Water fraction by Volume wfv I Dielectric Loss Tangent J Soil Conductivity temp corrected Siemens Meter S m K Real Dielectric Permittivity...

Страница 45: ...ended Commands Command Description aXS Get soil type aXS soil Set soil type aXP0 Turn on the probe circuitry aXP1 Turn off the probe circuitry aXM Get the default measure set aXM 0 Set the default mea...

Страница 46: ...pecific data In this case the specific data is the soil type and serial number Example 012STEVENSW0000012 7ST4SN00000000 SDI 12 Extended Command Descriptions aXS Get soil type Returns the current soil...

Страница 47: ...w default measurement set Response a set CR LF aXR Reset probe to defaults Resets all probe settings to default Response a id string CR LF aXY constant Get water constant Returns the water constant sp...

Страница 48: ...ack Wire Ground White Wire Data Signal A non inverting signal Green Wire Data Signal B inverting signal Baud Rate 9600 8N1 Power Consumption 10 mA Idle 30 mA Active Table 2 3 Digital RS 485 Hydra Prob...

Страница 49: ...umber Description Returns the factory serial number Access Level Read only Read Addresses Broadcast Exact Read Command addr SN CR LF Read Response addr serial CR LF Firmware Version Description Return...

Страница 50: ...e Access Level Read Write Read Addresses Broadcast Exact Read Command addr ST CR LF addr CR LF Read Response Write Addresses Broadcast Exact Wildcard Write Command addr ST soil CR LF Write Response ad...

Страница 51: ...LF Write Addresses Exact Write Command addr DS text CR LF Write Response addr text CR LF Custom Water Constants Description Gets sets the custom probe water constants Access Level Read Write Read Addr...

Страница 52: ...roadcast Exact Read Command addr PE CR LF addr bool CR LF Read Response Write Addresses Exact Write Command addr PE bool CR LF Write Response addr bool CR LF Measurement Commands Take Reading Descript...

Страница 53: ...gent T2 Transmit Set 2 F Temp C G Temp F H Moisture O Electrical Conductivity J Electrical Conductivity tc I Loss Tangent T3 Transmit Set 3 F Temp C G Temp F H Moisture I Loss Tangent J Soil Electrica...

Страница 54: ...the Hydra Probes to the Terminal assembly Connect one of the Probes to a terminal assembly and connect the terminal assembly data wire into the SDI 12 Port on the DOT Logger The 12 volt and ground co...

Страница 55: ...moisture temperature in Celsius and soil electrical conductivity Leave Transparent Mode For this example we will assume that the second probe has an address of 0 and has the same default parameter set...

Страница 56: ...nel The user can switch channels ID The user can name the channel Logging Enabled Check this box to enable logging Reporting Enable Check this box if you have telemetry Top of Casing Offset for water...

Страница 57: ...the logger when Welcome 1 N appears in the window Step 9 Click the D box at the top of the screen This opens the Data Dump Window 1 N 12 0 VDC DOT Logger Commlink Found should appear in the window St...

Страница 58: ...recision of the Hydra Probes Parameters TUC Temperature uncorrected full scale TC Temperature corrected from 0 to 35o C The Hydra Probe offers better precision and accuracy than TDR and other FDR type...

Страница 59: ...er the R2 value is to 1 the more correlated the data the better the accuracy In the above definition The average The Standard deviation represents precision and reproducibility Precision verses accura...

Страница 60: ...permanently or semi permanently buried in soil and the portable version allows the user to collect the same quality of data with hand held devices Part Stevens Part number Hydra Probe II Digital 25 ft...

Страница 61: ...partment of Agriculture NRCS Soil Climate Analyses Network SCAN http www wcc nrcs usda gov scan The US Department of Agriculture NRCS Snotel Network http www wcc nrcs usda gov snow The US Geological S...

Страница 62: ...dizing Charaterization of Electromagnetic Water Content Sensors Part 1 Methodology Vadose Zone J 4 1028 1058 2005 Lee J H M H Oh J Park S H Lee K H Ahn Dielectric dispersion characteristics of sand co...

Страница 63: ...uite 106 Portland OR 97220 800 452 5272 In any correspondence with us or if you send us part of the product but not all please include both the model and the serial number of the product WHAT THIS WAR...

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