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Evaporation and Salt Precipitation in Soil Cracks: Numeric Analysis
Christopher Brian Graham1 Maria Ines Dragila1, Noam Weisbrod2
(1) Department of Crop & Soil Sciences, Oregon State University, USA; (2) Department of Environmental Hydrology & Microbiology, Institute for Water Sciences & Technologies, BIDR, Ben-Gurion University of the Negev, Israel
Physical Model
Effect of fracture on evaporation (no salt)
A conceptual model of rock fracture and soil crack air
convection has been developed by Dragila and
Weisbrod.
The conceptual model describes increased evaporation
from crack surfaces due to flushing of “wet” crack air
with “dry” surface air. Increased evaporation leads to
increased salinization of the near crack soil matrix.
The conceptual model is being tested with laboratory
and field experiments and measurements. Further
testing is being done using numeric methods.
Based on soil crack and lysimeter
described in Ritchie and Adams (1972).
Lysimeter is filled with Houston Black Clay
Soil lysimeter has cross sectional area of
1.83 m x 1.83 m and depth of 1.2 m.
Crack is 50 cm deep, with surface width of
5 cm, running from corner to corner of
lysimeter.
The fracture evaporative front is seen to increase cumulative evaporation by 20% over 100 day simulation (figure x).
Effect of fracture is greatest at early time, with evaporation rates converging towards end of simulations.
Fracture lowers soil water content at all depths after 100 days. (figure x)
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0
5
10
15
Motivation
Conceptual model describes increased water loss due to
crack evaporation, with effects on water balance
calculations, plant water stress and other concerns.
Increased salinization due to crack evaporation can lead
to salinization issues at the surface and subsurface,
caused by crack preferential flow to aquifer.
-0.1
-0.15
-0.15
Depth
(m)
Depth (m)
Figure 1: Crack air convection due to thermal instability (left) and
schematic of convection enhanced evaporation (right)
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-0.2
-0.25
-0.3
-0.35
-0.2
-0.35
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-0.45
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0.15
0.2
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20
-0.5
0
0.25
20
-0.3
-0.4
0.05
15
-0.25
-0.45
-0.5
0
10
-0.05
-0.1
5
-0.05
Figure 1: Large scale representation of evaporation
from a cracked (left) and uncracked system.
Depth
(m)
Depth (m)
Picture of air
convection
0.05
Distance from fracture (m)
Distance
from crack (m)
0.1
0.15
0.2
0.25
Distance from fracture (m)
Distance
from crack (m)
Figure 1: Water content profile
in uncracked system
Cumulative
Evaporation (kg)
Cumulative
Evaporation
(kg)
Conceptual Model
2.5
Surface Only
Crack and Surface
2
1.5
1
0.5
0
0
20
40
60
80
T ime (days)
Time
(days)
Figure 1: Water content profile
in cracked system
Figure 1: Cumulative evaporation for
cracked and uncracked systems
Objectives
Develop numeric model of evaporation from soil cracks
using TOUGH2, an multiphase multicomponent porous
media simulator.
Use model to determine extent and effect of evaporation
in soil cracks on water balance and salt redistribution.
Experimental Water Mass
Simulated Water Mass
380
360
340
320
300
280
260
240
220
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10
20
30
40
50
60
70
80
Time (days)
Time (days)
Figure 1: Experimental Setup
Figure 1: Numeric grid of representative
slice. Element size is reduced near
upper and fracture boundary to increase
evaporation modeling precision
Numeric Model
Figure 1: Simulation and experimental results
Entire lysimeter is modeled using a representative
5 cm slice, 60 wide and 120 cm deep.
Hydraulic properties of Houston Black Clay are
modeled using the van Genuchten curves using
parameters for clay from Carsel and Parrish.
Evaporation rate is set at 1.4 mm/day, matching
average value from Ritchie and Adams (1972).
Lower boundary is constant head, allowing for
gravity drainage, right and left boundaries are no
flow, for symmetry. Upper boundary and fracture
are constant head, representing a well mixed
atmosphere.
4
4
Total Salt
Aqueous Salt
Solid Salt
3.5
Normalized salt content
Figure 1: Physical model of weighing lysimeter. 5 cm slice represents
entire lysimeter due to symmetry. Full depth is modeled.
Evaporation From Sand Column, 80 Days
400
Mass water (kg)
Water
mass (kg)
While EWASG module of TOUGH2 has been
shown to accurately model saturated and
unsaturated water and salt flux, and solid
phase salt precipitation, its capabilities
regarding evaporation are untested.
A small scale evaporation experiment was
constructed and simulated using the EWASG
module of TOUGH2.
A hydrometer cylinder was filled with
saturated sand and allowed to evaporate for
80 days.
Simulated results closely matched
experimental data with minor modifications to
the standard modeling procedures.
Evaporation from soil crack is seen to increase salt content near fracture by 3 to 4 times.
Salt content increases to the point of salt precipitation along fracture surface (figure x).
3
2.5
Simulations
4 Simulations are run using geometric
framework, with and without salt
1. 100 day simulation without fracture, no salt
2. 100 day simulation with fracture, no salt
3. 100 day simulation with fracture and low
salt content (15 g salt/kg solution)
4. 100 day simulation with fracture and high
salt content (100 g salt/kg solution)
2
1.5
1
0.5
0
0
2
4
6
8
Total Salt
Aqueous Salt
Solid Salt
3.5
3
2.5
2
1.5
0
20
1
40
60
0.5
80
0
0
10
2
Distance from crack (cm)
4
6
8
10
Depth below surface (cm)
100
Figure 1: Salt concentration normalized by
initial salt concentration, high initial salt
concentration simulation.
Figure 1: Salt concentration normalized by
initial salt concentration, low initial salt
concentration simulation.
Figure 1: Solid phase salt precipitation
distribution in soil matrix after 100 days
2.5
Case Study: Alvord Desert, Oregon
Conclusions
Implications
Evaporation is enhanced by soil cracking (an
increase of 20%) While not as great an
increase as previously reported (Ritchie and
Adams, 1972), this increase can have an
impact on water balance calculations,
irrigation applications and salinization control.
A larger impact is seen in salt redistribution.
Salt levels increase 3-4 times near crack.
Soil structure (cracks) must be taken into account
when calculating a water balance or planning
irrigation. Management tools may be able to reduce
water loss through evaporation by 20%
Experimental results suggest that the majority of salt
concentration occurs as precipitated salt in the crack
void, which leaves it susceptible to flushing and
aquifer salinization during rain events.
Shallow (< 1m) lake fills desert in winter and early spring, draining and evaporating through summer.
Lake dimensions: 12 x 7 miles or 21,760 hectares.
Without soil cracks, assuming clay soil and potential evaporation rate of 1.4 mm/day, can expect
evaporation of 0.26 mm/day 4,530 acre feet of water over course of summer.
With soil cracks, assuming a crack density of 1 crack/m2, expect an increase of 20%, or over 900 acre
feet of water.
Evaporation from cracks can be expected to increase soil salinity 3-4 times near cracks. Assuming a
low salt content of 15 g salt/kg solution, can expect accumulation of 139,000 tons of salt in crack voids.
Assuming a high concentration (100g/kg) can expect accumulation of 787,000 tons of salt in crack
voids.
This salt is expected to be flushed to aquifer during rain events
Figure 1: Alvord Desert
Figure 1: Alvord Desert
in winter
in summer
Effect of salt on
evaporation
Increased salt content
lowers evaporative flux
at all time in three
simulations.
Increasing salt content
leads to decreased
evaporative flux
Distance from crack (cm)
Distance from crack (cm)
Cumulative Evaporation (kg)
Cumulative
evaporation (kg)
Validation
Effect of fracture on salt redistribution
Normalized salt content
Figure 1: Cracked soil (left) and evaporation driven solid
phase salt precipitation on saturated sandstone (right)
References
2
1.5
Carsel and Parish
1
Pruess
0.5
No Salt
Low Salt
Lotsa Salt
0
0
20
40
60
Time (days)
Time
(days)
80
Ritchie and Adams
100
Figure 1: Cumulative evaporation for high,
low and no salt concentrations
Weisbrod and Dragila
van Genuchten
100