Soil Water and Thermal Gradients in the Vadose Zone: Assessing Evapotranspiration, Recharge Rates and Shifts in Phreatophytic Water Source
Jeremy E. Koonce1,2, Michael H. Young3, Dale Devitt4, Zhongbo Yu1, Amanda Wagner5, Lynn Fenstermaker6
1Department of Geoscience, University of Nevada Las Vegas, NV (jkoonce@unlv.nevada.edu)
2Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, NV
3Bureau of Economic Geology, University of Texas at Austin, TX
4School of Life Sciences, University of Nevada Las Vegas, NV
5Water Resources Management Program, University of Nevada Las Vegas, NV
6Division of Earth and Ecosystem Sciences, Desert Research Institute, Las Vegas, NV
With large uncertainty in precipitation rates from interannual variability and
increased demand for water resources, understanding these processes is critical
for assessing the movement of mass and energy through the vadose zone.
• Water Level: Mod / Low
R
Can we correctly estimate water flux in (infiltration/recharge) and out
(ET) of the vadose zone using soil temperature?
II.
Do changes in temperature signals follow shifts in water sources for
plants?
I
R
T – Transpiration
E – Evaporation
I – Infiltration/Percolation
R – Recharge
Experimental Location and Design
Eddy Covariance & Meteorological
Tower (ET and Meteorological Data)
UTAH
Spring
Valley
T
E
Water Table
---- Capillary Fringe
Type T
Thermocouples
(Temperature)
6
5
4
3
2
1
SV6
10
TDR
8
(Water
Content)
Focus of Study
(Upper 300 cm)
Summer
T
14
16
0.20
100 cm
200 cm
0.12
0.08
300 cm
0.12
0.08
300 cm
250
300
0.00
• Recharge: Low
• Near Surface θ: Mod
FO DTS – Fiber Optic Distributed Temperature Sensing
• Water Level: Low
Groundwater Well w/ Pressure
Transducer (Depth to Water)
Fall
• ET: Low E / Low T
Soil Temp (Deg C)
I
100
150
200
4/7
4/17
4/27
FO DTS appears to pick up
cold wetting fronts near
surface
100 cm
200 cm
Small variations between100
and 300 cm possibly due to
poor insulation of equipment
box
5/2
300 cm
6/2
6/12
6/7
6/22
6/17
7/2
6/27
7/7
24
22
20
18
16
14
12
10
8
6
Continue soil profile analysis to the water table (~550 to 600 cm)
20
15
Use HYDRUS 1D to optimize water content based on changes in soil
temperatures
10
250
300
3/28
Using high resolution FO DTS in the subsurface provides continuous
temperature measurements both spatially (depth only) and temporally
allowing for a better understanding of the processes within the vadose
zone, and subsequently a better understanding of the flux in and out of the
system
25
50
Seasonal temperature changes
through entire profile
18
16
14
12
300 cm
10
8 200 cm
6
100 cm
4
30 cm
2
0
3/23
4/2
4/12
4/22
Changes in soil temperature observed during this time period are due
mostly to atmospheric conditions, but are also influenced by changes in
water content (volumetric heat capacity) due to infiltration, percolation,
and ET
Continued Work
30 cm
• Infiltration: Low
Changes in soil water observed below 200 cm appears to be influenced
predominantly by ET
Early Spring and Summer wetting fronts do not reach lower depths
Clay soils from ~80 to 300 cm
Recharge through valley floor does not occur during this time period
Soil Temp
(Deg C)
Soil Temperature
Diurnal variations in near
surface; dampening effects
deeper in the profile
200
2
0.04
300 cm – Constant theta in
Spring; decrease of theta in
Summer
150
4
0.16
200 cm
200 cm – Constant theta in
Spring and Summer
50
0.28
0.20
30 cm
0.04
100
30
25
20
15
10
5
0
-5
-10
-15
0.24
100 cm – Increased theta in
Spring (1st wetting front);
continued increase into
Summer, followed by a
decrease, and then another
increase (2nd wetting front)
0.16
Changes in soil water observed in the upper 100 cm appears to be
influenced by infiltration and percolation from large precipitation events
(increase in theta) and ET (decrease in theta)
Loam soils from surface to ~80 cm
18
100 cm
30 cm
0.24
• Surface: Moist
FO DTS Pole
(Temperature)
HDU – Heat Dissipation Sensors
12
Water Content (theta)
30 cm – Increased theta
following precipitation events;
decrease follows
E
Site located in east-central NV:
~323 km (~200 miles) north of
Las Vegas, NV
TDR – Time Domain Reflectometry Sensors
6
10
Higher temperatures and
larger diurnal changes during
Summer
0.28
Depth (cm)
12
8
Ambient Air Temperature
Lower temperatures and
smaller diurnal changes during
Spring
30
25
20
15
10
5
0
-5
-10
-15
14
HDU
(Matric
Potential)
6
Increased ET following
precipitation events
0
Soil Temp
(Deg C)
I
4
May (29–30) precipitation
event (19.56 mm)
0.00
Ely, NV
2
ET and P
April (2–8) precipitation event
(20.07 mm)
7
R
Spring
NEVADA
Focus of Study
(Upper 300 cm)
E
Water Content (m3/m3)
I.
• ET: Low E / Low T
8
Ambient Temp (Deg C)
Winter
Goals and Questions
Overall goal of this research is to have a better understanding of the impact of
interannual variability of shallow groundwater semi-arid systems. In doing so, we
hope to answer the following questions:
Discussion and Conclusions
Data provides interannual atmospheric and physical soil variability in Spring
Valley, NV (early Spring and Summer, 2011)
Precipitation (mm/day)
• Recharge: Low
• Near Surface θ: Mod
T
Early Summer Results
(29 May through 7 July 2011)
Ambient Temp (Deg C)
I
Evapotranspiration (mm/day)
• Infiltration: Low
R
Early Spring Results
(23 March through 2 May 2011)
• Surface: Frozen/Moist
E
Water Content (m3/m3)
T
Depth (cm)
Background and Motivation
Soil water and temperature are important variables in water and energy balance
studies, particularly to processes involved in evapotranspiration (ET), which
provides a direct link between the balances and is crucial for closing the water
budget.
Abstract #
GC31A-1024
5
Soil Temp (Deg C)
Abstract #
GC31A-1024
Others (Dr. D. Devitt and A. Wagner) are looking at isotopes and sap flow
measurements to determine shifts of plant water from vadose and phreatic
zones
Acknowledgements
Funding for FO DTS supplies in Spring Valley, NV, provided by NSF Cooperative
Support Agreement (EPS-0814372). Educational opportunities and funding for
Jeremy Koonce provided by NSF EPSCoR under Cooperative Support
Agreement (EPS-0814372). Additional support from my academic committee
(Dr.’s Young,Yu, Nicholl, Jiang, and Devitt), UNLV Geoscience, and Desert
Research Institute (J. Healey, B. Lyles, and Dr.’s Berli, Jasoni, and Arnone).