development of a laser-based water level sensor for fine-scale ecohydrological measurements

Engineering School of Sustainable Infrastructure and Environment

Development of a Laser-Based Water Level Sensor for Fine-Scale Ecohydrological MeasurementsJoshua Benjamin, Dr. David Kaplan

November 14th, 2017


http://water.usgs.gov/edu/graphics/evapotranspiration.gif

Evapotranspiration (ET) is a

critical component of the global

water balance

ET accounts for 70-95% of

incoming precipitation in

Florida1

ET is difficult to pinpoint due to

a lack of accurate and

affordable sensor technology

Different types of

measurement techniques http://water.usgs.gov/edu/graphics/wctranspirationwatertable.gif

Background


White Method6

Developed by Walter White in 1932

Sy = Specific Yield

∆𝑠 = is the daily change in storage

R = net inflow/outflow rate [L/T]

Major assumptions6:

Diurnal water table fluctuations are a

product of plant water use

Groundwater consumption negligible

b/w midnight and 5 AM

Constant inflow/outflow rate

Specific yield is constant over time

𝐸𝑇 = 𝑆𝑦 ×∆𝑠

𝑡+ 𝑅


Current Technology Onset HOBO U20L Water Level Data

Logger

Accuracy = 0.4 cm

Precision/Resolution = 0.14 cm

Cost = $300.00

Subject to errors (>1cm) from2:

Moisture accumulation

Differential heating across the system

Errors based on3:

Installation location and media

Differences in atmospheric and water

temperature

Variations in solar radiation

Long equilibrium times

𝑝 = 𝜌𝑔ℎ

http://www.onsetcomp.com/files/styles/image_widget_large/public/product-images/HOBO-Water-Level-Logger-

U20L-01-apart_1.jpg?itok=KNSuKT0i


New Technology

http://www.solinst.com/products/data/images/Levelogger-Edge-Barologger-Edge.jpg

Leica DISTO E7100i4

Accuracy = ±0.15 cm

Resolution = 0.01 cm

IP54 Certified

Costs $150.00

Measurement Principle:

The Phase-Shift Method

c = the speed of light [L/T]

f = the modulation frequency

[1/T]

Δφ is the phase shift between the

measurement signal and the

reference signal [L]

𝐷 =𝑐

2𝑓∙∆𝜑

2𝜋


Phase-Shift vs Time-of-Flight Time of Flight Method

c = the speed of light [L/T]

T = time [L]

More for long range distance

measurements

Light is fast (300,000,000 m/s)

Phase-shift Method

f = the modulation frequency [1/T]

Δφ is the phase shift between the

measurement signal and the reference

signal [L]

Process signals w/ heterodyne method

mm-range resolution of 0.0015 to 60.96 m

with non-cooperative targets

𝐷 =𝑐

2𝑓∙∆𝜑

2𝜋

https://upload.wikimedia.org/wikipedia/commons/4/40/Earth%27s_Moon_from_NOAA.jpg

𝐷 =𝑐𝑡

2


The Phase-Shift Method

http://spot-on.net/images/Leica%20Disto-Measuring-Principles.pdf

𝐷 =𝑐

2𝑓∙∆𝜑

2𝜋


Sensor Components

http://lasers.leica-

geosystems.com/sites/default/files/product_images/e7100i_left

_top.jpg_c636653a1m.jpg

Leica DISTO E7100i5

Floating Target Platform

Next Thing Co. C.H.I.P.

http://images.bit-tech.net/content_images/2016/10/next-thing-co-

chip-and-pocketchip-review/chip-2b.jpg


Laser Mods Modified Laser to be USB

powered

Convert 5V to 3V – USB to

AAA

Started out with a custom

board


Laser Mod – Buck

Converter

Modified Laser to be USB

powered

Converts 5V to 3V – USB to

AAA

Started out with a custom

board, transitioned to a Buck

converter mounted in a

custom casing inside of the

Disto battery casing


https://upload.wikimedia.org/wikipedia/commons/thumb/0/0a/Python.svg/2000px-Python.svg.pnghttps://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/Bluetooth-Logo.svg/1280px-Bluetooth-Logo.svg.png

Written in Python

Interfaces between the computer

and the laser

Has custom start and end time

configurations

Exports data in a .txt format

https://eltechs.com/wp-content/uploads/2013/03/eltechs-sq-512-300x300.png

Laser.py


𝐹𝑏 = 𝛾𝑓𝑙𝑢𝑖𝑑∀𝑏𝑜𝑑𝑦Floating Target

Platform


Floating

Target

Platform𝐹𝑏 = 𝛾𝑓𝑙𝑢𝑖𝑑∀𝑏𝑜𝑑𝑦

Left is MK-IV

Right is MK-V


RSE Calculation 𝑅𝑆𝐸 = 𝑦2


Results – Early Lab Trial Residual

Noise:

(Abs) (in mm)

TPT-BPT

Avg : 2.698

mm

LB-WLS

Avg: 0.083

mm

LB-WLS 32.5x

LESS

residual

noise


Results – Later Lab Trial Residual

Noise:

(Abs) (in mm)

TPT-BPT

Avg : 2.303

mm

LB-WLS

Avg: 0.163

mm

LB-WLS 14.1x

LESS

residual

noise


Longleaf

Flatwoods

Preserve Owned by the St.

John’s Water

Management District

2850 acres

Study site is Mesic

Flatwoods w/ Pomona

soil5 – sandy clay loam,

established in 20036

http://www.worldatlas.com/img/locator/city/039/20039-gainesville-locator-map.jpg


Results – July Trial

Almost 60 cm difference Laser shifted in

casing


Well Case Redesign


December

Experimental

Setup1. Leica DISTO E7100i4

Located inside of front well cap

2. Floating Target Platform

Inside Back well

3. Next Thing Co. C.H.I.P.

Located inside of gray box

4. AGM Car Battery

Inside of Black Box

5. Onset HOBO Logger

Located Inside back well

1

3

2

4

5


Results – December Trial


December Trial

Root Square Error

𝑅𝑆𝐸 = 𝑦2

LB-WLS Average = 0.05±0.04 cm/day

HOBO Logger Average = 0.19±0.16

cm/day


ET GraphLB-WLS Average = 0.19±0.06 cm/day

HOBO Logger Average = 0.23±0.10 cm/day

PET Average = 0.14±0.02 cm/day


ET 1:1 ComparisonLB-WLS Average = 0.19±0.06 cm/day

HOBO Logger Average = 0.23±0.10 cm/day

PET Average = 0.14±0.02 cm/day


ET:PET RatiosLB-WLS Average = 1.43±0.52

HOBO Logger Average = 1.80±0.82


Power Analysis

LB-WLS Component Power Consumption

DeviceMinimum

(Watt)

Maximum

(Watt)

Disto E7100i 0.10 0.35

Raspberry Pi

2B 1.25 1.40

NTC CHIP 2.75 3.25

Note: Each device operates under 5V. Note that

while the CHIP uses more power than the Pi 2B,

this is with an attached touchscreen and

keyboard, which allows for easier data entry and

retrieval in the field.

𝑡 = 𝐻𝐶

𝐼𝐻

𝑘 t is time [hr]

H is rated discharge time [hr]

C is capacity [Amp*hr]

I is the actual current [Amp]

k is the Peukert constant,

which is 1.1 for an AGM

battery.


Power Analysis 𝑡 = 𝐻𝐶

𝐼𝐻

𝑘

LB-WLS Component Power Consumption relative to the central buck converter

Parameter Maximum Load Minimum Load With LoPya

Current (out) [Amp]0.72 0.62

0.17

Power (out) [W] 3.60 3.10 0.85

Current (in) [Amp]0.40 0.34

0.09

Power (in) [W] 4.77 4.10 1.13

Est. Run Time [hr]301 355

1473

Est Run Time

[days]12.5 14.8 61.4

Note: These values assume that the efficiency (η) of the buck converter is 76%, Vout is 5V, Vin is 12V,

and that Pin*η = Pout, with P=VI, where P is power (W), V is Voltage (V), and I is current (Amp).

aLoPy measurements are potential values based on lab-scale readings and have not yet been

experimentally verified.


Power Analysis𝐼𝐹 𝐸𝑛−1 − 𝐿 + 𝐶 ∗ 𝑆 > 𝐸𝑜

𝑇𝐻𝐸𝑁 𝐸𝑛 = 𝐸0𝐸𝐿𝑆𝐸 𝐸𝑛 = 𝐸𝑛−1 − 𝐿 + 𝐶 ∗ 𝑆

E0 = the Initial capacity of the battery

(W*hr)

En-1 = the capacity in the previous

step

En = the capacity in the calculated

step

L = the load (W*hr)

C = a conditional statement, that is

equal to 1 during peak sun and 0

when it is not peak sun

S = the solar cell energy generated

per hour (W*hr).


Error Analysis

February Incorrect

Installation

Dry well trial

May 2017

Periodic noise during

the daytime


Error Analysis

Noise consistent across all

dry trials

Possible Causes:

Temperature causes

variation in air’s refractive

index9

Optical Interference

Overall does not affect

measurement accuracy

Important measurements

during the night


Future Work Transition to a lower-power

computer

Reduce daytime

measurement error

Connect to solar charging

system for continuous

usage

Implement remote access

through LoRa


UF Watershed

Ecology Group

People :

John Loeffler – 3D

Printing

Jackson Benfer -

Programming

Kevin Henson –

Field Work

Acknowledgements


Thank you!Questions?


References1. McLaughlin, D., & Cohen, M. (2013). Realizing ecosystem services: wetland hydrologic function along a

gradient of ecosystem condition. Ecological Applications, 23(7), 1619–1631. doi:10.1890/12-1489.1

2. McLaughlin, D., and Cohen, M. (2011). “Thermal artifacts in measurements of fine‐scale water level

variation.” Water Resources Research, 47(9), n/a–n/a.

3. Cain III, S. F., Davis, G. A., Loheide II, S. P., and Butler Jr., J. R. (2004). “Noise in Pressure Transducer

Readings Produced by Variations in Solar Radiation.” Groundwater, Groundwater, 42(6), 939–944.

4. Leica Geosystems. (2016). “Leica DISTO E7100i.”

5. SJWMD, 2009. Longleaf Flatwoods Reserve Land Management Plan. St Johns Water Management District.

6. White, WN, 1932. A Method of Estimating Ground-Water Supplies Based on Discharge by Plants and

Evaporation from Soil: Results of Investigations in Escalante Valley, Utah.

7. Loheide, S., J. Butler, and S. Gorelick, 2005. Estimation of Groundwater Consumption by Phreatophytes

Using Diurnal Water Table Fluctuations: A Saturated‐unsaturated Flow Assessment. Water Resources

Research 41:n/a–n/a.

8. McLaughlin, D., D. Kaplan, and M. Cohen, 2013. Managing Forests for Increased Regional Water Yield in

the Southeastern U.S. Coastal Plain. JAWRA Journal of the American Water Resources Association

9. Shim, Y., O.-J. Kwon, H.-Y. Choi, and Y.-G. Han, 2015. Influence of Diverse Atmospheric Conditions on

Optical Properties of a Pulse Laser in a Time-of-Flight Laser Range Finder. Journal of the Optical Society of

Korea 19:1–6.


Current Technology

http://www.solinst.com/products/data/images/Levelogger-Edge-Barologger-Edge.jpg

Solinst Levelogger®-Barologger®

Combo

Subject to errors (>1cm) from2:

Moisture accumulation

Differential heating across the system

Errors based on3:

Installation location and media

Differences in atmospheric and water

temperature

Variations in solar radiation

Long equilibration times

𝑝 = 𝜌𝑔ℎ

Levelogger®

Accuracy = 0.3 cm

Precision/Resolutio

n = 0.05 cm

Barologger ®

Accuracy = 0.1 cm

Resolution = 0.03

cm


SPECIFIC YIELD7𝑆𝑦𝑑−𝑐𝑜𝑚𝑝

𝑑 = 𝜃𝑠 − 𝜃𝑅 +𝜃𝑠 − 𝜃𝑅

1 + 𝛼 𝑑 𝑛 𝑚

Sy = specific yield

θs = the water content at

saturation

θR = the residual water content

d = the depth to the water table

[L]

α = van Genuchten coefficient

[1/L].

N = van Genuchten coefficient

m = van Genuchten coefficient


Specific yield effect


Residual Noise (in mm)

Laser Based Water Level SensorTotal Pressure Transducer-Barometric

Pressure Transducer

Trial #Minimum

(abs)

Maximum

(abs)Mean (abs)

Minimum

(abs)

Maximum

(abs)Mean (abs)

1 0.002 0.279 0.099 0.085 9.153 2.865

2 0.001 0.234 0.067 0.070 6.287 2.532

3 0.005 0.666 0.163 0.184 7.519 2.303

Mean (#1-2)0.001 0.256 0.083 0.077 7.720 2.698

Mean (All) 0.003 0.393 0.110 0.113 7.653 2.566Note. All values are absolute. Trial 3 data is treated separately due to the difference in

experimental conditions.

Lab Trials Overall


Buffered TPT had lower overall temperature difference

Lab Trials Temperature

Trends


December trial Residual Noise𝑅𝑀𝑆 = 𝑦2

Residual Noise(cm)

Laser Based Water Level SensorTotal Pressure Transducer-Barometric Pressure

Transducer

Dates Minimum Maximum Mean Minimum Maximum Mean

12/7-12/13 0.0046 0.1205 0.0506 0.0204 0.5865 0.2272

12/15-

12/18 0.0023 0.1143 0.0405 0.0104 0.3751 0.1371

12/20-

12/24 0.0037 0.1022 0.0446 0.0155 0.5089 0.1887

12/26-

12/28 0.0005 0.1273 0.0581 0.0158 0.3883 0.1471

Mean 0.0028 0.1161 0.0484 0.0155 0.4647 0.1750

Mean w/

rain 0.0032 0.1127 0.0465 0.0161 0.5064 0.1886

Overall 0.0003 0.1949 0.0465 0.0009 0.9429 0.1886

Note. All values are absolute.