Lesson 2 - Stationary Sensors (Part 2)

Transcription

1 Lesson 2 - Stationary Sensors (Part 2) Water pressure, hydraulics, and other water storages (soil, snow ) The meteorology sensors (precipitation) are important because they tell us how much water our climate is providing In this section, we will discuss sensors which help us to determine the storage and fluxes of water Key sensors: Water depth or pressure to determine storage volumes or flow rates Velocity sensors to determine fluxes, shearing (sediments), mixing, Other important sensors Soil moisture sensors to determine storage and availability of soil water, and infiltration to groundwater Snow depth sensors to determine storage in the form of snow pack 1968 Today

2 Water depth sensors Water pressure is used to determine the potential for flow in pipes, and the water depth in open system: P r i gz i where P i = pressure at location i, z i = depth below water surface at location i [L], r = water density [M/L 3 ], g = gravitational acceleration [L/T 2 ] Pressure transducers are available for a variety of depths, but are not always optimal of very shallow conditions For shallow conditions, sometimes ultrasonic sensors are better (measure distance to the water surface from above) groundwater level sensing Flow in a channel, based on depth above a weir

3 Application: Flow measurement using a sharp-crested weir Hydraulic engineers place weirs (and other structures) in channels to facilitate flow measurement Weirs control flow and allow us to relate flow to water depth with a relatively simple equation (based on analysis with the Bernoulli equation) b Typical weir shapes; V-notch is best for measuring low flows or small changes in flow

4 Application: Flow measurement using a sharp-crested weir (V-notch type) For a V-notch weir (assuming upstream velocity is << weir velocity): 8 Q Cwt tan 15 2 Here, C w is the empirical weir coefficient (correction for real-world effects) 2gH 5/ 2 b

5 Application: Groundwater levels and fluxes Monitoring the depth to groundwater is a way to monitor groundwater storage Monitoring in multiple locations can reveal the flux gradient Pressure/depth response to various pumping rates can reveal information about the geology in the subsurface (aquifer pump test) This pressure transducer is made to fit in a narrow well casing

6 Water velocity sensors Mechanical sensors (propellers) are often used for measuring local velocities, but are subject to jamming/breaking Acoustic Doppler Velocimeters (ADVs) are more common for measuring local velocity vectors These sensors are relatively expensive, containing Compass, tilt & heave sensors Extensive filtering and signal processing algorithms ADV provides a 3D vector at a single location Mechanical current meters (1D velocity)

7 Traditional stream gauging (measuring flow with a velocimeter) Break cross-section into rectangles of known size Measure average velocity in each rectangle* Sum these to arrive at total flow Q Av i i *If depth, d < 0.75 m, then measure at 0.6d below the surface If depth, d > 0.75 m, then measure at 0.2d and 0.8d below the surface, and take the average

8 Less labor-intensive (and safer) ADP measurements If channel is small, then stationary ADP will capture the velocity field and integrate for flow (Q) If the channel is large, then ADP is slowly mobilized across the channel to capture the flow field, and integrated to give Q

9 Soil moisture sensors are quite advanced because they are applicable to agriculture There are 2 main types: Water content sensors These simply provide an estimate of the water fraction in the soil Water potential sensors (more complex) Water potential is a measure of the difference in potential energy between the water in a sample and the water in a reference pool of pure, free water. Since soil water generally has an energy state lower than that of pure free water, its potential usually has a negative value. Soil is a very complicated media, with spatially variable properties; therefore, assessing soil water content and potential is challenging Soil moisture sensors

10 Soil moisture sensors (water potential) Soil water potential (also known as matric potential) Essentially, the sensor measures the negative pressure (suction) holding water in place in the soil Most water potential sensors place a well-define porous material in contact with the soil In one type (shown here) the soil pulls water out of the tube (through a porous ceramic interface), causing a vacuum to register In another type, a fine material encases a conductivity cell, and when the fine material draws in moisture, we see a change in conductivity (e.g., gypsum block sensors) This device is called a lysimeter

11 Soil moisture sensors (water potential) Sensor (top) is an electronic lysimeter (provides digital value for water potential) Sensors (bottom) are gypsum block type sensors for estimating soil water potential

12 Soil moisture sensors (water content) Working principle: Fast neutrons are emitted from a decaying radioactive source ( 241 Am/ 9 Be) and when they collide with particles having the same mass as a neutron (i.e., protons, H + --in soils, these are primarily associated with water), they slow down dramatically, building a "cloud" of "thermalized" (slowed-down) neutrons. Range: 0 to 0.6 v/v Advantages Robust and accurate (±0.005 ft 3 ft -3 ) Inexpensive per location (i.e., a large number of measurements can be made at different points with the same instrument) One probe allows for measuring at different soil depths Large soil sensing volume (sphere of influence with 4-16 in. (10.2 cm cm) radius, depending on moisture content) Not affected by salinity or air gaps Stable soil-specific calibration Drawbacks Safety hazard, since it implies working with radiation. Even at 16 in (40.6 cm). depth, radiation losses through soil surface have been detected Requires certified personnel, not automated (due to safety concerns) Requires soil-specific calibration Heavy, cumbersome instrument, requires access tube (not buried) Readings close to the soil surface are difficult and not accurate Expensive to buy The sphere of influence may vary according to the following reasons: it increases as the soil dries, because the hydrogen concentration reduces, so that the probability of collision is smaller and thereby fast neutrons can travel further from the source it is smaller in fine texture soils, because they can hold more water, thus the probability of collision is higher if there are layers with large differences in water content due to changes in soil physical properties, the sphere of influence can have a distorted shape

13 Soil Moisture Sensors (water content) Dielectric methods These estimate volumetric water content by measuring the soil bulk permittivity (or dielectric constant) that determines the velocity of an electromagnetic wave or pulse through the soil. Since the dielectric constant of liquid water (Ka w = 81) is much larger than that of the other soil constituents (e.g. Ka s = 2-5 for soil minerals and 1 for air), the total permittivity of the soil or bulk permittivity is mainly governed by the presence of liquid water. Water content sensors have an effective volume associated with their measurement Several types commercially available: Time Domain Reflectrometry-based (TDR) common Capacitance or Frequency Domain Reflectrometrybased (FDR) common Amplitude domain reflectrometry-based (ADR) less common, but available Phase transition (PT) less common, but good if larger effective volume needed and others

14 Dielectric: TDR-based Time domain reflectrometry (TDR) - TDR sensors propagate a pulse down a line into the soil, which is terminated at the end by a probe with wave guides. TDR systems measure the water content of the soil by measuring how long it takes the pulse to come back. Range: v/v (or 0 to saturation if calibrated for specific soils) Advantages: Accurate, continuous, soil-specific calibration often not necessary (if ± 0.03 v/v is adequate), unaffected by salts at low salinity (within limits), easily multiplexed, minimal soil disturbance, can simultaneously measure soil salinity levels Disadvantages: Relatively expensive; insufficient signal return if salinity is too high; soilspecific calibration required for soils having large amounts of bound water (i.e., those with high organic matter high clay, or volcanic soils), relatively small sensing volume (about 1.2 inch (3.05 cm) radius around length of waveguides) Mesa Systems PICO 64 Campbell CS 616 ESI Gro-point

15 Dielectric: Capacitance or FDR-based Frequency Domain Reflectrometry (FDR) - In FDR the oscillator frequency is swept under control within a certain frequency range to find the resonant frequency (at which the amplitude is greatest), which is a measure of water content in the soil. Capacitance In capacitance versions the dielectric permittivity of a medium is determined by measuring the charge time of a capacitor made with that medium. Advantages - High accuracy given soil-specific calibration (±0.01 v/v); can read in high salinity levels, where TDR fails (note: may require corrective calculations); better resolution than TDR (avoids the noise that is implied in the waveform analysis performed by TDRs); flexibility in probe design (more than TDR), which helps with installation sometimes; some devices are relatively inexpensive compared to TDR due to use of low frequency standard circuitry Drawbacks - The sensing sphere of influence is relatively small (radius about 4.1 cm); for reliable measurements, it is extremely critical to have good contact between the sensor (or access tube) and soil; careful installation is necessary to avoid air gaps; tends to have larger sensitivity to temperature, bulk density, clay content and air gaps than TDR; often needs soilspecific calibration Stevens Hydra-Probe Decagon Devices ECH 2 O EC-5

16 What do soil moisture data look like? Note: red dashed line designates irrigation event Notice that different sensors record events differently Soil water potential decreases (A, B, C) as moisture content increases (D) Response attenuates with depth in all cases (moisture dispersing as in infiltrates) Note: this type of domain knowledge helps with fault detection

17 Application: Agriculture (irrigation management) Where water is scarce, we need to avoid overirrigation (transceiver) This can be accomplished using sensor-based feedbackcontrol algorithms see Park et al irrigation paper provided Notice that the soil properties are heterogeneous; therefore, we should use sensors at several locations if budget allows

18 Snow depth sensors In snow-dominated watersheds, it is critical to quantify the snow pack to plan for the year s water supply In the figure, sensor 5 is a snow depth sensor (sonic pinger ) Other sensors are shown: 6, 7 and 10 for snow melting information (1) Mote (transceiver), (2) custom data-logger to interface the sensor array, (3) on-site storage, (4) 12V battery, (5) snow-depth sensor, (6) humidity and temperature sensor, (7) solar radiation sensor, (8) 10W solar panel, (9) external 8dBi antenna, (10) four soil moisture, temperature, and matric potential sensors at varying depths.

19 Application: Flow (Q) measurement using a broad-crested weir Again using Bernoulli equation, assuming upstream velocity contribution to energy line is negligible, (V 1 ) 2 /2g << y 1 3/ 2 2 3/ 2 Q Cwbb g H 3 Empirically corrected with rectangular weir coefficient Pressure (water depth) sensor in here C wb 1 H / P w H / Pw 1/ 2 b

20 Summary: Stationary sensors for water pressure, hydraulics, and other water storages (soil, snow ) The meteorology sensors (precipitation) are important because they tell us how much water our climate is providing In this section, we will discuss sensors which help us to determine the storage and fluxes of water Key sensors: Water depth or pressure to determine storage volumes or flow rates Velocity sensors to determine fluxes, shearing (sediments), mixing, Other important sensors Soil moisture sensors to determine storage and availability of soil water, and infiltration to groundwater Snow depth sensors to determine storage in the form of snow pack