soil water topics –soils –soil water properties –soil water balance reading: applied hydrology...
TRANSCRIPT
![Page 1: Soil Water Topics –Soils –Soil water properties –Soil water balance Reading: Applied Hydrology Sections 4.3 and 4.4 (Green-Ampt method)](https://reader036.vdocument.in/reader036/viewer/2022062516/56649da15503460f94a8db27/html5/thumbnails/1.jpg)
Soil Water
• Topics– Soils– Soil water properties– Soil water balance
• Reading: Applied Hydrology Sections 4.3 and 4.4 (Green-Ampt method)
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Soil Map of the United States
Developed from SSURGO – Soil Survey Geographic Database
USDA Soil Classes
http://www.arcgis.com/home/webmap/viewer.html?webmap=0edea1c7bbb84ba5842d20483af11679
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Soil Map of Texas
Developed from SSURGO – Soil Survey Geographic Database
![Page 4: Soil Water Topics –Soils –Soil water properties –Soil water balance Reading: Applied Hydrology Sections 4.3 and 4.4 (Green-Ampt method)](https://reader036.vdocument.in/reader036/viewer/2022062516/56649da15503460f94a8db27/html5/thumbnails/4.jpg)
Soil Map of Central Texas
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Soils in the Brushy Creek Watershed
Brushy Bend
Dr
Dam 7
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Mollisols
• Mollisols have deep, high organic matter, nutrient-enriched surface soil (A horizon), typically between 60–80 cm in depth
• Formed under grasslands (the liveoak savannah that originally covered much of south and central Texas)
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Vertisols
• Soils that have a high content of expanding clay and that have at some time of the year deep wide cracks. They shrink when drying and swell when they become wetter
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Soil Sieves
http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg
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http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf
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Soil Texture Triangle
Source: USDA SoilSurvey Manual Chapter 3
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Soil Texture is defined by % of silt, sand, clay
Silty Clay Loam ~ 55% silt, 10% sand, 35% clay
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Soil Water Content
TotalVol
VolWater
Soil Water Content
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Soil Water Flux, q
q = Q/A
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Soil Water Tension, y
• Measures the suction head of the soil water
• Like p/g in fluid mechanics but its always a suction (negative head)
• Three key variables in soil water movement– Flux, q– Water content, q– Tension, y
02
2
zg
vz
ph
Total energy head = h
111 zh
222 zh
z=0
z1
z2
12
1212 zz
hhKq
q12
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• Tensiometers– Measure soil water potential (tension)– Practical operating range is about 0 to 0.75
bar of tension (this can be a limitation on medium- and fine-textured soils)
• Electrical resistance blocks– Measure soil water potential (tension)– Tend to work better at higher tensions (lower
water contents)• Thermal dissipation blocks
– Measure soil water potential (tension)– Require individual calibration
Soil Water Measurement
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Tensiometer for Measuring Soil Water Potential, ψ
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
Water Reservoir
Variable Tube Length (12 in- 48 in) Based on Root Zone Depth
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Electrical Resistance Blocks & Meters
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Soil Water Tension, y
• Measures the suction head of the soil water
• Like p/g in fluid mechanics but its always a suction (negative head)
• Three key variables in soil water movement– Flux, q– Water content, q– Tension, y
02
2
zg
vz
ph
Total energy head = h
111 zh
222 zh
z=0
z1
z2
12
1212 zz
hhKq
q12
![Page 19: Soil Water Topics –Soils –Soil water properties –Soil water balance Reading: Applied Hydrology Sections 4.3 and 4.4 (Green-Ampt method)](https://reader036.vdocument.in/reader036/viewer/2022062516/56649da15503460f94a8db27/html5/thumbnails/19.jpg)
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40 cm
80 cm
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Darcy’s Law
• K = hydraulic conductivity• q = specific discharge • V = q/n = average velocity through the
area
L
hKAQ
z
hKqz
L
hhK
A
updown
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Definitions
solid
Pore withair
Pore withwater
Element of soil, V(Saturated)
Element of soil, V(Unsaturated)
n0content;moisturenSV
V
S0;saturationV
VS
porosityV
Vn
waterofvolumeV
solidsofvolumeV
poresofvolumeV
elementofvolumegrossV
w
v
w
v
w
s
v
1
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Continuity Equation
CS
wCV
wddt
ddAV0
waterofvolume
elementofvolume
dxdydzV
dxdydzV
w
y
z
x
dy
dz
dx
dzz
qq z
z
z
hK
A
Qqz
dt
ddxdydzdxdydz
dt
dd
dt
dww
CVw
z
qdzdxdyqdxdydxdydz
z
qq www
CSw
dAV
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Continuity (Cont.)
CS
wCV
wddt
ddAV0
dt
ddxdydz
dxdydzdt
dd
dt
d
w
wCV
w
z
qdzdxdy
qdxdydxdydzz
w
wwCS
w
dAV
z
qdzdxdy
dt
ddxdydz ww
0 0
z
q
t
ContinuityEquation
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Richard’s Equation
• Recall – Darcy’s Law– Total head
• So Darcy becomes
• Richard’s eqn is:
z
hKqz
zh
Kz
D
Kz
K
z
zKqz
KD
Soil water diffusivity
Kz
Dzz
q
t
Kz
Kqz