Soil Moisture and Groundwater Recharge

Hsin-yu ShanDepartment of Civil EngineeringNational Chiao Tung University

Soil PhysicsA scientific field that focus on the water in the vadose zoneMajor concern of agricultural engineeringRelationship between water content and pressureHydraulic conductivity of unsaturated soil

Soil in the Vadose Zone

Solids:Soil particles/AggregatesOrganic materials

Fluids:Water (Aqueous solution)AirOther liquid

Porosity and Water Content of Soil

WeightVolume

AirWater

Solids

Va Air

Water

Solids

waVvVw ww

Vs ws

Porosity: ratio of volume of the void to the total volume

ee

VVV

VVn

sv

vv

+=

+==

1Void ratio: volume of the voids to the volume of the solids

s

v

VVe =

Bulk density:

s

sb V

wρ =

Gravimetric water content: ratio of weight of water to the dry weight of solids

%x www

s

w 100=

Volumetric water content: ratio of pore water to the total volume

VVw=θ

Degree of saturation: ratio of the volume of pore water to the total volume of voids

%x VVS

v

wr 100=

Capillary and the Capillary Fringe

Capillary: water molecules at the water table subject to an upward attraction due to surface tension of the air-water interface and the molecular attraction of the liquid and the solid phases.

Tension

If fluid pressures are measured above the water table, they will be found to be negative with respect to local atmospheric pressure

Air Pressure

If air in the pores is connected, the air pressure is equal to the local atmospheric pressureIf air in the pores is not connected and is in the forms of air bubbles such as in the capillary fringe, the air pressure is equal to the pressure of water, which is negative.

Capillary rise in a tube

i f

s

λR

λσ

σcosλ

r

λ=0

gRh

wc ρ

λσ cos2=R=r

hc

water

Fig. 6.2 IDEALIZED pore diameter in a sediment with cubic packing. The equivalent capillary tube has a radius of 0.2 the diameter of the grains

Patm

0

0.4 (=0)hψ

ψψp g0.3

0.2

0.1

z (m)

Patm

water

y-4 -2 0 2 4 (J /kg)

-4 -2 0 2 4 yr l (kJ /cu.m = kP a)

-0.2 0 0.2 0.4 y /g (J /N = m)-0.4

z (m)

0.1

0.2

0.3

0.4ψp

ψh

ψg =gz

ψ

water

(J/Kg)-1 0 1 2 3 4

z (m)

0 0.2 0.4 0.6 0.8-0.2

-1.0

-0.2

-0.8

-0.4

h

ψg= gz

heads (m)

0

-0.4-0.6-0.8

H

-0.6

-1.2

1 2

Height of Capillary Rise

1.50.1000.50Fine gravel40.0400.20Very coarse sand150.0100.05Coarse sand250.0060.03Medium sand500.0030.0150Fine sand1000.00150.0075Very fine sand3000.00050.0025Coarse silt7500.00020.0008Fine silt

Capillary Rise (cm)

Pore Radius (cm)

Grain Diameter

(cm)

Sediments

Capillary Fringe

Capillary pores in the vadose zone can draw up water from beneath the water table below which the pores are saturated with water.The zone above the water table, in which the pores are saturated is termed capillary fringe

The liquid pressure in the capillary zone is negativeCapillary fringe is a part of vadose zoneThe zone of aeration is best defined as the zone where the soil moisture is under tension

The capillary fringe is higher in fine-grained soils than in a coarse-grained soilsSmaller pore opening creates greater tension

Pore-Water Tension in the Vadose Zone

Fluid pressures in the vadose zone are negativeThe negative pressure head is measured in the field with a tensiometer

TensiometerA tube that is closed at the top and filled with waterThe tube is connected to a pressure gaugeA ceramic cup at the bottom of the tube to provide a porous membraneThe ceramic cup should be saturated with water before use

b

d

a

c

∆ z 1

∆ z 2

Soil Surface

Tensiometer (1)

D A∆ z 1

∆ z 2

Soil Surface

l

E

C B

Tensiometer (2)

If the suction is lower than the entry pressure of the membrane, only water can go through itWhen the suction in the soil pulls water out from the tensiometer, the water in the tube is under tension and will cause the pressure gauge to indicate the magnitude of such a tension

z (m)

0.4

0.3

0.2

0.1

0-0.1

-0.2

Air Entry Pressure

Soil Water

Water in the vadose zone that is available to growing plants.This is not a very exact definition. Avoid using it.

Field CapacityThe soil moisture content of a layer at which the the force of gravity acting on the water equals the surface tension.Related to the specific retentionDepends on specific retention, evaporation depth, and the unsaturated permeability characteristic curve of the soil.

Field capacity is related to specific retention but has different unitsIt depends upon specific retention, evaporation depth, and the unsaturated permeability characteristic curve of the soil

The concept is vagueGravity drainage may take a long period to occurSome definitions:

Water content of soil after 48 hours of gravity drainageWater content of soil under a suction of 0.3 bar

Moisture content of a silt loam as a function of time since saturation

13.6156

14.760

15.930

17.57

20.21

θ (%)Time (days)

Wilting PointThe soil moisture content below which the plant roots cannot withdraw water from the soil.Some defined it to be the soil moisture content under a suction of 15 bars.The available water capacity of a soil is the difference between the field capacity and the wilting point.

Fig. 6.5 Hypothetical fluctuation of soil moisture for a sandy loam soil through an annual cycle in a region with a moderate amount of rainfall (500 to 750 mm per year) and heavy rains in the spring

Fig. 6.6 Water-holding properties of soils based on texture. The available water supply for a soil is the difference between field capacity and wilting point

Water Potential

The potential energy, or force potential of ground water consists of two parts: elevation and pressure (velocity related kinetic energy is neglected)

Suction of Water in Soils

Fluid pressures in the vadose zone are negative, owing to tension of the soil-surface-water contactThe negative pressure head is measured in the field with a tensiometer

a

c∆z2

d

b∆z1 Soil Surface

Suction – Calibrate Before Use

Suction

Matric suctionElevation headPressure head

Osmotic suction

Head (Water Potential)

Gravity potential, ZElevation head

Moisture potential, ψSuction headCan be several orders of magnitude greater than the gravity potential1 bar ≈ 10 m of water column

Soil Water Characteristics

Defines the relationship between water content and water potential (suction)

0 0.1 0.2 0.3 0.40

-0.2

-0.4

-0.6

-0.8

-1.0

hm (m)

θ

SWCC of a Coarse Sand

SWCC of various soils

Soil Water Characteristic Curve

Absorption and desorption characteristics with primary scanning curves

-1 0 6

-1 0 4

-1 0 2

-1 0 0

0 0 .1 0 .2 0 .3 0 .4

W a te r c o n te n t,θ

P re ssu re h e a d(cm )

h b

-1 0 7

-1 0 5

-1 0 3

-1 0 1

P o o r ly so r ted

W ell so r ted

Entry pressure hb

Volumetric water content,

ψMain drying curve

Main wetting curve

Drying scanning curve

Wetting scanning curve

θ

Absorption and desorption characteristics with primary scanning curves

Hysteresis

Ink bottle effectTrapping of airAdvancing and receding contact angle

Determination of SWCCLaboratory

Pressure plate apparatusFilter paperThermocouple PsychrometerCentrifuge

FieldTensiometerWater content measurement

z (m)

0 0.4-0.4

-1.0

-0.2

-0.8

-0.4

hψ g= gz

heads (m)

0

-0.8-1.2-1.6

H

-0.6

-1.2

Effluent port that can vary elevation

water

Ceramic plate

soil

Theory of Unsaturated FlowGravity potential, ZMoisture potential, ψ(θv)

Negative value – suction resulted from soil-water attraction

At moisture contents close to specific retention, the gravity potential is greaterWhen the soil is very dry, the moisture potential may be several orders of magnitude greater than gravity potential

Darcy’s law is valid for flow in the unsaturated zoneFlow in water saturated pores

Larger cross-sectional area to conduct flow

Flow in pores with air in themSmaller cross-sectional area to conduct flow

Total potential φ

Zv += )(θψφ

Fig. 6.7 The relationship between hydraulic conductivity and volumetric water content

1 432Hydraulic conductivity (10-4

0

-50

-100

-150

-200

-250

-300

ψcm water

Drying

Wetting

Relationship between hydraulic conductivity and soil-moisture head

Fig. 6.9 Idealized curves showing relationships of volumetric water content, hydraulic conductivity, and soil-moisture head

Fig. 6.10 Moisture profiles showing the downward passage of a wave of infiltrated water. The soil is saturated at a water content of 0.29 and has a field capacity water content of 0.06

Coarse vs. Fine-Grained Materials

At lower volumetric water content:Coarse material may have very few saturated poresFine-grained soils may have most of the pores still saturatedThus, the unsaturated hydraulic conductivity of a clay may be greater than that of a sand or gravel

Fig. 6.11 Typical soil-moisture-potential-hydraulic conductivity curves for a sandy soil showing the crossover effect for increasing moisture potential

Water-Table Recharge

When the front of infiltrating water reaches the capillary fringe, it displaces air in the pore spaces and cause the water table to rise.

Fig. 6.12 Hydrograph of a shallow well in a water-table aquifer in Long Island.

Rate of Recharge

The rate of water-table recharge depends on:

Thickness of the unsaturated zoneThe thinner the zone, the faster the rise of water tableMay generate a localized ground-water mound

Fluctuation of Ground-Water Table

Water table shows a seasonal fluctuationRising during periods of rechargeFalling when there is no precipitation or when evapotranspiration exceeds precipitation

Fig. 6.13 Monthly hydrograph of water levels in a water-table monitoring well on eastern Long Island, New York

Dep

th t

o w

ater

(fe

et)

Fig. 6.14 Hydrograph of a water-table monitoring well showing effect of discharge by evaporation on the water table elevation

Dep

th t

o w

ater

(fe

et)