Soil Water Balance

• Reading: Applied Hydrology Sections 4.3 and 4.4

• Topics– Soil water properties– Soil water flux– Soil water balance

Subsurface water

• Infiltration• Soil moisture• Subsurface

flow• Groundwater

flow

Porous Medium Flow

• Groundwater– All waters found beneath the ground surface– Occupies pores (void space not occupied by solid matter)

• Porous media – Numerous pores of small size– Pores contain fluids (e.g., water and air) – Pores act as conduits for flow of fluids

• The storage and flow through porous media is affected by– Type of rocks in a formation – Number, size, and arrangement of pores

• Pores are generally irregular in shape because of – differences in the minerals making up the rocks – geologic processes experienced by them.

Zones of Saturation• Unsaturated zone

– Zone between the land surface and water table

– Pore contains water and air– Also called as vadose zone or the zone

of aeration

• Saturated zone– pores are completely filled with water– Contains water at greater than

atmospheric pressure– Also called phreatic zone

• Water table– Surface where the pore water pressure is

atmospheric – Divide between saturated and

unsaturated zone

• Capillary fringe– Zone immediately above the water table

that gets saturated by capillary forces

Soil Water

1. Hygroscopic water– Microscopic film of water surrounding soil particles– Strong molecular attraction; water cannot be removed by natural forces– Adhesive forces (>31 bars and upto 10,000 bars!)

2. Capillary water– Water held by cohesive forces between films of hygroscopic water– Can be removed by air drying or plant absorption– Plants extract capillary water until the soil capillary force is equal to the

extractive force • Wilting point: soil capillary force > plant extractive force

3. Gravity water– Water that moves through the soil by the force of gravity

Three categories

• Field capacity– Amount of water held in the soil after excess water has drained

is called the field capacity of the soil.

Soil Sieves

http://www.rtg.wa.edu.au/loanpool/belmont/sieves.jpg

Soil Particle Sizes(USDA Soil Classification System

Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system.

Name of soil separate

Diameter limits (mm)

Very coarse sand* 2.00 - 1.00

Coarse sand 1.00 - 0.50

Medium sand 0.50 - 0.25

Fine sand 0.25 - 0.10

Very fine sand 0.10 - 0.05

Silt 0.05 - 0.002

Clay less than 0.002

0.001 mm

0.1 mm

.

0.01 mm

1 mm

http://www.uga.edu/srel/kidsdoscience/soils-planets/soil-particle-size.pdf

Soil Texture Triangle

Source: USDA SoilSurvey Manual Chapter 3

Soil Water Content

TotalVol

VolWater

Soil Water Content

Soil Water Flux, q

q = Q/A

Energy fluxes at Freeman Ranch

Evaporation (mm/day)

Average = 1.1 mm/day

Average = 3.1 mm/day

November

August

30 min Rainfall August 6, 2004

Daily RainfallAug 6 – Aug 13

Soil Water Content

Rainfallmm/30min

Soil Water Balance at Freeman Ranch

P E

0 cm

10 cm

20 cm

Net Exchange

Soil Water Tension,

• Measures the suction head of the soil water

• Like p/ in fluid mechanics but its always a suction (negative head)

• Three key variables in soil water movement– Flux, q– Water content, – Tension,

02

2

zg

vz

ph

Total energy head = h

111 zh

222 zh

z=0

z1

z2

12

1212 zz

hhKq

q12

Darcy’s Law

• K = hydraulic conductivity

• q = specific discharge

• V = q/n = average velocity through the area

L

hKAQ

z

hKqz

L

hhK

A

Qq

updown

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

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

Continuity (Cont.)

CS

wCV

wddt

ddAV0

dt

ddxdydz

dxdydzdt

dd

dt

d

w

wCV

w

z

qdzdxdy

qdxdydxdydzz

qq

w

wwCS

w

dAV

z

qdzdxdy

dt

ddxdydz ww

0 0

z

q

t

ContinuityEquation

Surface Tension

• Below surface, forces act equally in all directions

• At surface, some forces are missing, pulls molecules down and exert tension on the surface

• If interface is curved, higher pressure will exist on concave side

• Pressure increase is balanced by surface tension,

• = 0.073 N/m (@ 20oC)

water

air

No net force

Net forceinward

Interface

Capillary Action

• Capillary Pressure

• Related to saturation

• Suction Head

• Total Head

waterairc PPR

P 2

0airP

waterc PP

)(cc PP

Solid SolidWater

Air

R

airP

waterP

)(cP

Pressure Distribution in Subsurface

1dP

2dPPc

1d

2d Water tablez = 0, P=0

Clay

SandTop of capillary fringe

zwc PP

claynsandn

Richard’s Equation

• Recall – Darcy’s Law– Total head

• So Darcy becomes

• Continuity becomes

z

hKqz

Kz

D

Kz

K

z

zKqz

KD

Soil water diffusivity

Kz

Dzz

q

t

Kz

Kqz