Introduction to Wireless Sensor Networks Soil Moisture Sensors

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Organizational Class Website www. engineering. uiowa. edu/~ece 195/2005/ Class Time Monday 4: 30 -5: 20 Room 4511 SC Thursday 12: 30 -1: 20 Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Office Hours Monday 5: 20 -6: 20 Room 1126 SC Thursday 1: 30 -2: 30 Room 1126 SC Other By appointment Room 523 C SHL The University of Iowa. Copyright© 2005 2 A. Kruger

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? ? ? The University of Iowa. Copyright© 2005 4 A. Kruger

What is soil moisture? (2) …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). The University of Iowa. Copyright© 2005 5 A. Kruger

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? The University of Iowa. Copyright© 2005 6 A. Kruger

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 The University of Iowa. Copyright© 2005 7 A. Kruger

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. The University of Iowa. Copyright© 2005 8 A. Kruger

Why is it important to measure it? (2) from http: //nssdc. gsfc. nasa. gov • 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) • from http: //www. unc. edu …soil moisture is even measured on the moon… The University of Iowa. Copyright© 2005 9 A. Kruger

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

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 The University of Iowa. Copyright© 2005 11 A. Kruger

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. The University of Iowa. Copyright© 2005 12 A. Kruger

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. The University of Iowa. Copyright© 2005 13 A. Kruger

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 The University of Iowa. Copyright© 2005 14 A. Kruger

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. The University of Iowa. Copyright© 2005 15 A. Kruger

Nuclear Techniques Neutron Scattering • Widely used for estimating volumetric water content. • Measures the slowdown of fast neutrons emitted into the soil • • • The University of Iowa. Copyright© 2005 Measured Parameter: Volumetric water content (percentage of volume) Response Time ~1 to 2 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 16 A. Kruger

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. The University of Iowa. Copyright© 2005 17 A. Kruger

Nuclear Techniques Gamma Attenuation • Radioactive technique. • Changes in wet density are measured • Moisture content is determined from this density change. • 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 • Advantages: – Can determine mean water content with depth – Can be automated – Can measure temporal changes in soil water – Nondestructive measurement The University of Iowa. Copyright© 2005 18 A. Kruger

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 The University of Iowa. Copyright© 2005 19 A. Kruger

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 The University of Iowa. Copyright© 2005 20 A. Kruger

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 The University of Iowa. Copyright© 2005 21 A. Kruger

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 The University of Iowa. Copyright© 2005 22 A. Kruger

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 The University of Iowa. Copyright© 2005 23 A. Kruger

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 The University of Iowa. Copyright© 2005 24 A. Kruger

Electromagnetic Techniques Time-Domain Reflectometry (TDR) • Effect of soil water content on propogation constant. • 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 • • • Manufacturer: – Soil Moisture Equipment Corp. The University of Iowa. Copyright© 2005 • 25 A. Kruger

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 The University of Iowa. Copyright© 2005 26 A. Kruger

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. The University of Iowa. Copyright© 2005 27 A. Kruger

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 The University of Iowa. Copyright© 2005 28 A. Kruger

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 The University of Iowa. Copyright© 2005 29 A. Kruger

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 @ 2 m. A to 5 VDC @7 m. A – Output: 10 -40% of excitation voltage (250 -1000 m. V @ 2500 m. V excitation) The University of Iowa. Copyright© 2005 30 A. Kruger

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

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 The University of Iowa. Copyright© 2005 32 A. Kruger

Rm 1 Receiver • Communication – Frequency-hopping Spread-spectrum Radio: 902 -928 MHz ISM (USA) or 2. 4 GHz ISM (Worldwide) – RS-232 • Power Requirements – 7 -18 VDC – 70 -210 m. A The University of Iowa. Copyright© 2005 33 A. Kruger

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