2. The University of Iowa. Copyright© 2005 A. Kruger 1 Introduction to Wireless Sensor Networks Soil Moisture Sensors 07 March 2005

3. The University of Iowa. Copyright© 2005 A. Kruger 2 Organizational Monday 4:30-5:20Room 4511 SC Thursday12:30-1:20Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Monday5:20-6:20Room 1126 SC Thursday1:30-2:30Room 1126 SC OtherBy appointmentRoom 523C SHL Class Website www.engineering.uiowa.edu/~ece195/2005/ Class Time Office Hours

4. The University of Iowa. Copyright© 2005 A. Kruger 3 Objectives What is soil moisture? Why is it important? How can we measure it? Hands-on experiment

5. The University of Iowa. Copyright© 2005 A. Kruger 4 What is soil moisture? According to the USGS soil moisture (or soil water) is defined as “the water diffused in the soil, the upper part of the zone of aeration from which water is discharged by the transpiration of plants or by soil evaporation” …nice definition, but what does it really mean???

6. The University of Iowa. Copyright© 2005 A. Kruger 5 …some more definitions… Field Capacity: as much water as the soil can hold. Permanent Wilting Point: the amount of water remaining in the soil when the plant wilts in a humid atmosphere. The water remaining in the soil is held tightly by soil particles and plant roots cannot absorb it. Available Water: the amount of water in the soil between field capacity and permanent wilting point (rule of thumb: start irrigation before soil reaches 50% of available water). What is soil moisture? (2)

7. The University of Iowa. Copyright© 2005 A. Kruger 6 Why is it important to measure it? For example to answer the following questions: –How much water do I have to provide? Do crop have enough water? Do they have too much of it? –When do I have to irrigate?

8. The University of Iowa. Copyright© 2005 A. Kruger 7 How much water do I have to provide? It depends on crop needs and on how much water the soil can store Crop water needs mostly depends on: –Temperature and humidity –Solar radiation –Crop growth stage –Depth of the roots Amount of water the soil can store mostly depends on: –Soil texture (relative amount of sand, silt or clay) –% organic matter

9. The University of Iowa. Copyright© 2005 A. Kruger 8 When do I have to irrigate? Soil moisture levels should determine timing of irrigation –Right timing and amount for higher yields –Excessive water reduces yields by carrying nitrates below depth of root penetration and by displacing soil air for too long, causing a lack of oxygen to the roots. Highly variable soil and climatic conditions require different irrigation strategies –Not difficult to be measured in a single point in space and time; however, it exhibits very large spatial and temporal variability. –Place enough sensors in the field to represent varying conditions. –For deeper rooting species, one sensor should be in the middle of the root zone and one at its bottom.

10. The University of Iowa. Copyright© 2005 A. Kruger 9 Crop production Pest Management and disease vectors –Many insects and plant and animal diseases are dependent upon some optimal levels of soil moisture. Maps of soil moisture levels would enable monitoring of potential insect infestation and outbreaks of diseases Forecasting –It effects temperature forecast (high evaporation when soil moisture is high, especially for increasingly warmer temperatures; cooling effect); –precipitation forecast (high soil moisture increases the likelihood of moisture convergence); Component of the water budget Winemakers know that soil moisture is key to quality wine grapes (Berkeley studies) …soil moisture is even measured on the moon… from http://nssdc.gsfc.nasa.gov from http://www.unc.edu Why is it important to measure it? (2)

11. The University of Iowa. Copyright© 2005 A. Kruger 10 How can we measure it? Gravimetric Technique Hygrometric Technique Tensiometric Technique Nuclear Technique Electromagnetic Technique Remote Sensing

12. The University of Iowa. Copyright© 2005 A. Kruger 11 Gravimetric Techniques The oven-drying technique Measured Parameter: –Mass water content (% of dry vs. wet soil weight) –Moisture content (%) = ((wet wt. - dry wt.)/(wet wt.)) X 100 Response Time ~ 24 hours Advantages: –Direct measurement –Ensures accurate measurements –Calibration of all other soil moisture determination techniques –Not dependent on salinity and soil type –Easy to calculate

13. The University of Iowa. Copyright© 2005 A. Kruger 12 Gravimetric Techniques The oven-drying technique Disadvantages: –Destructive test –Time consuming –Inapplicable to automatic control –Dry bulk density is required to transform data to volume moisture content Manufacturers –Lab ovens, lab scales and soil sampling equipment by many scientific instrument companies.

14. The University of Iowa. Copyright© 2005 A. Kruger 13 HYGROMETRIC TECHNIQUES Measured Parameter: soil water potential Response time: < 3 min. Advantages: –Wide soil matrix potential range –Low maintenance –Well suited for automated measurements and control of irrigation systems Disadvantages: –Sensing element deteriorates through the interaction with soil components –Each material to be tested requires special calibration Manufacturers: –Campbell Pacific –Nuclear International –Troxler Electronics –Geoquip.

15. The University of Iowa. Copyright© 2005 A. Kruger 14 TENSIOMETRIC TECHNIQUES Measured Parameter: Soil water potential (capillary potential) Response Time: 2 to 3 hours Advantages: –Irrigation policy recommendations are made with tensiometers –Inexpensive and easily constructed –Works well in saturated range –Easy to install and maintain –Operates for long periods if properly maintained –Can be adapted to automatic measurement with pressure transducers –Can be operated in frozen soil with ethylene glycol –Can be used with positive or negative gauge to read water table elevation and/or soil water tension

16. The University of Iowa. Copyright© 2005 A. Kruger 15 TENSIOMETRIC TECHNIQUES Disadvantages: –Limit range of 0 to -0.8 bar not adequate for sandy soil –Difficult to translate data to volume water content –Requires regular (weekly or daily) maintenance, depending on range of measurements –Subject to breakage during installation and cultural practice –Automated systems costly and not electronically stable –Disturbs soil above measurement point and can allow infiltration of irrigation water or rainfall along its stem. Manufacturers –Soil Moisture Equipment Corp.

17. The University of Iowa. Copyright© 2005 A. Kruger 16 Nuclear Techniques Neutron Scattering Advantages: –Nondestructive –Possible to obtain profile of water content in soil –Water can be measured in any phase –Can be automated for one site to monitor spatial and temporal soil water –Measurement directly related to soil water content Measured Parameter: Volumetric water content (percentage of volume) Response Time ~1 to 2 min. Widely used for estimating volumetric water content. Measures the slowdown of fast neutrons emitted into the soil

18. The University of Iowa. Copyright© 2005 A. Kruger 17 Nuclear Techniques Neutron Scattering Disadvantages: –Costly –Dependent on dry bulk density and salinity –Radiation hazard –Must calibrate for different types of soils –Access tubes must be installed and removed –Depth resolution questionable –Measurement partially dependent on physical and chemical soil properties –Depth probe cannot measure soil water near soil surface –Subject to electrical drift and failure Manufacturers: –Campbell Pacific –Nuclear International –Troxler Electronics –Geoquip.

19. The University of Iowa. Copyright© 2005 A. Kruger 18 Nuclear Techniques Gamma Attenuation Radioactive technique. Changes in wet density are measured Moisture content is determined from this density change. Advantages: –Can determine mean water content with depth –Can be automated –Can measure temporal changes in soil water –Nondestructive measurement Measured Parameter: Volumetric water content Response Time: < 1 min. Equipment for multiple soil physical parameters determination. Soil particles samples dispersed in water and undisturbed samples in steel cylinders

20. The University of Iowa. Copyright© 2005 A. Kruger 19 Nuclear Techniques Gamma Attenuation Disadvantages: –Restricted to soil thickness of 1 inch or less, but with high resolution –Affected by soil bulk density changes –Costly and difficult to use –Large errors possible when used in highly stratified soils Manufacturers: –Campbell Pacific –Nuclear International –Troxler Electronics

21. The University of Iowa. Copyright© 2005 A. Kruger 20 Nuclear Techniques Nuclear Magnetic Resonance Static and an oscillating magnetic field at right angles to each other. Sensors measure the spin echo and free induction decays. Measured Parameter: Volumetric water content Response Time: < 1 min. Advantages: –Nondestructive –Possible to obtain profile of water content in soil –Water can be measured in any phase –Can be automated for one site to monitor spatial and temporal soil water –Measurement directly related to soil water content

22. The University of Iowa. Copyright© 2005 A. Kruger 21 Nuclear Techniques Nuclear Magnetic Resonance Disadvantages: –Radiation hazard –Must calibrate for different types of soils –Access tubes must be installed and removed –Depth resolution questionable –Measurement partially dependent on physical and chemical soil properties –Depth probe cannot measure soil water near soil surface –Subject to electrical drift and failure

23. The University of Iowa. Copyright© 2005 A. Kruger 22 Electromagnetic Techniques Resistive Sensor (General) Dependence of resistivity of soil on the moisture content. Measured Parameter: –Soil water potential aided by electrical resistance measurements Response Time: Instantaneous Advantages: –Theoretically, can provide absolute soil water content –Can determine water content at any depth –Relatively high level of precision –Can be read by remote methods Disadvantages: –Calibration not stable with time and affected by ionic concentration –Costly

24. The University of Iowa. Copyright© 2005 A. Kruger 23 Electromagnetic Techniques Resistive Sensor (Gypsum) Porous block (Gypsum or fiberglass) containing two electrodes connected to a wire lead. Electrical conductivity Vs Matrix potential Measured Parameter: Soil moisture tension Response Time: 2 to 3 hours Advantages: Inexpensive Disadvantages: –Each block requires individual calibration –Calibration changes with time –Life of device limited –Provides inaccurate measurements

25. The University of Iowa. Copyright© 2005 A. Kruger 24 Electromagnetic Techniques Capacitive Sensor Effect of soil moisture on dielectric constant Measured Parameter: Volumetric soil water content Response Time: Instantaneous Advantages: –Provides absolute soil water content. –Water content can be determined at any depth –Relatively high level of precision –Can be read by remote methods Disadvantages: –Long-term stability questionable –Costly

26. The University of Iowa. Copyright© 2005 A. Kruger 25 Electromagnetic Techniques Time-Domain Reflectometry (TDR) Measured Parameter: –Volumetric water content aided by propagation of electromagnetic wave measurements Response Time ~ 28 sec. Advantages: –Independent of soil texture, temperature and salt content –Possible to perform long-term in situ measurements –Can be automated Disadvantages: –Costly Effect of soil water content on propogation constant. Manufacturer: –Soil Moisture Equipment Corp.

27. The University of Iowa. Copyright© 2005 A. Kruger 26 Remote Sensing Techniques Active microwave, Passive microwave and other non-contact techniques. Measurement of electromagnetic energy that has been either reflected or emitted from the soil surface. Measured Parameter: Soil surface moisture, through the measurement of electromagnetic energy Response Time: Instantaneous

28. The University of Iowa. Copyright© 2005 A. Kruger 27 Remote Sensing Techniques Active Microwave Remote Sensing Record phase and amplitude of the return signal. Compute Backscatter coefficient. Compute di-electric constant. Soil moisture is related to the di-electric constant.

29. The University of Iowa. Copyright© 2005 A. Kruger 28 Remote Sensing Techniques Passive Microwave Remote Sensing TB = (1) surface + (2) direct atmospheric + (3) reflected atmos. + (4) reflected space Normalize TB using deep soil temperature Remove vegetation effects using estimate of vegetation water content (Wv) derived from land use and NDVI Remove surface roughness effects using estimate of rms height (h) - minor effect Compute soil dielectric constant (k) Compute soil moisture using a dielectric mixing model and soil texture

30. The University of Iowa. Copyright© 2005 A. Kruger 29 Remote Sensing Techniques Advantages: –Spatial information –Method allows remote measurements to be taken –Enables measurements to be taken over a large area Disadvantages: –System large and complex –Costly –Usually used for surface soil moisture

31. The University of Iowa. Copyright© 2005 A. Kruger 30 ECH 2 O Soil Moisture Probes Probe Type –Dielectric constant measurement Measurement Time –10 ms Resolution –0.002 m 3 /m 3 (0.1%) Power –Requirements: 2.0 VDC @ 2mA to 5 VDC @7mA –Output: 10-40% of excitation voltage (250-1000 mV @ 2500 mV excitation)

32. The University of Iowa. Copyright© 2005 A. Kruger 31 ECH 2 O Data Logger and Transmitter Input Channels –5, with 12-bit A/D resolution and 2.5V excitation on each channel Data Storage: –49 kB (145 days @ 1 scan/hour) flash memory

33. The University of Iowa. Copyright© 2005 A. Kruger 32 ECH 2 O Data Logger and Transmitter Communication –900 MHz with up to 5 mile LOS range (optional 2.4 GHz spread spectrum RF, up to 1.5 mile LOS range) –RS-232 capability on Port 1 Power Requirements –4 AAA alkaline batteries Operating Conditions –-10 o C to 50 o C, up to 100% RH, weatherproof and impact resistant enclosure

34. The University of Iowa. Copyright© 2005 A. Kruger 33 Rm1 Receiver Communication –Frequency-hopping Spread-spectrum Radio: 902-928 MHz ISM (USA) or 2.4GHz ISM (Worldwide) –RS-232 Power Requirements –7-18 VDC –70-210 mA

35. The University of Iowa. Copyright© 2005 A. Kruger 34 Measurement Setup ECH 2 0 Logger Rm1 RS232 900 MHz (2.4 GHz) Soil

36. The University of Iowa. Copyright© 2005 A. Kruger 35 Graphic User Interface Basic Features –Time and measurement interval settings –Radio settings (channel, subchannel, radio mode) –Connect/disconnect, download data –Terminal

37. The University of Iowa. Copyright© 2005 A. Kruger 36 Graphic User Interface Data Features –Raw data in mV –Standard calibration to volumetric water content

38. The University of Iowa. Copyright© 2005 A. Kruger 37 Graphic User Interface Passive mode terminal –Data logger saves data into memory after each mesurement and transmits it to receiver