Lecture5 soil and infiltration dung

 Undisturbed forested area:  Surface:  Subsurface e.g., 30 cm deep:  Fine-textured clay soil:  Sandy soil:  Organic peat soils:... Types of water in soils: • Gravitational water

VFU-2016

Principles of

Watershed management

Lecture #5

Soil and Infiltration

Dr Bui Xuan Dung- Department of Environment Management

Importance of Soils

 Key principle of watershed management is

to maximize infiltration and minimize

surface runoff;

 Minimizes surface erosion;

 Lower peak flows and less flooding;

 More water for plant growth;

 Need to understand how water and soils interact (“soil physics”) for both land

management and resource protection!

Soil generation

Weathering of bedrock material

Alluvial sediment transport (vận chuyển chất lắng đọng do bồi tích)

SOILS CONTENTS

soil horizons - distinct soil layers

soil profile - sequence of layers

Soils = parent material + organics + water + air

Soil horizons and profile

USDA particle size classification

USDA soil textural triangle

What happens to rainfall as it

hits the soil surface?

First some more basic principles

Soil is a 3-phase system

Air Water

Solid

Volumetric Relations

Air Water

Air Water

v

V

V V

V

n   1 

Gravimetric (Mass) Relations

Air Water

M



r

What are some typical dry

bulk densities?

 Undisturbed forested area:

 Surface:

 Subsurface (e.g., 30 cm deep):

 Fine-textured clay soil:

 Sandy soil:

 Organic (peat) soils:

What are some typical dry

How much pore space in soils?

 For most minerals in soils the particle

density is 2.65 g cm-3;

 Porosity = 1 – (Ρdry /2.65 g cm-3)

 If Ρdry is 1.2 g cm-3, what percent of the soil is voids?

How much pore space in soils?

 Surface soils in forests typically are what percent pore space?

 Clay soils typically have more or less

porosity than sandy soils?

 Organic soils can be what percent pore space?

Porosity and pore sizes

 Sandy soils have mostly big pores, and

about 40-50% porosity;

 Clayey or fine-textured soils are dominated

by smaller pores, but tend to have higher total porosity (50-60%);

 In all soils organic matter tends to bind the particles into bigger clumps, resulting in

much larger pores;

How much pore space in soils?

 Soils are surprisingly porous given that we stand and build on them;

 If soils are commonly 50% porosity, what

is the change in bulk density from dry to saturated?

 Under what conditions would we expect

landslides to occur?

Quantifying soil water content

 Volumetric water content, θ

Quantifying soil water content

 Mass wetness or gravimetric water

content, w:

w = Mw / Ms (range of 0 to 60%)

Soil and water

Types of water in soils:

Types of water in soils:

• Gravitational water

• Capillary water

• Hygroscopic water

Soil and water

Why this happen?

Why does capillary rise occur?

 Water is a polar molecule, so it is viscous and stays

liquid at high temperatures!

In a circular capillary tube: Pc = - 2γ / r

Pc is capillary pressure; γ is surface tension (73 dynes

Surface Tension force

p = 0

Capillary Rise

Figure 8.2, Hornberger et al., 1998

Capillary rise

 Water will rise 1.5 cm in a tube that is the

diameter of coarse sand (2.0 mm);

 Water will rise 15 cm in a tube that is the

diameter of a fine sand (0.2 mm);

 Water will rise 15 m in a tube that is the size

of a particle on the clay-silt boundary (0.002 mm);

 So large pores drain more easily, and small pores hold the water more tightly (more

negative pressure);

Types of water in soils:

• Gravitational water

• Capillary water

• Hygroscopic water

Soil and water

Caused by gravity force

Water of Soil

Definitions

soil after it is allowed to freely drain for 24 hours;

 Commonly defined as -1/3 bar, or -340 cm of pressure head;

remaining in the soil that plants cannot

readily extract;

 Commonly defined as -15 bars or -153 m of pressure head;

Units of soil tension (tension is positive, pressure is negative)

bars kPa MPa m of H2O

 Plant-available water or available water capacity (AWC) is the amount of water between field capacity (FC) and wilting point (WP):

AWC = FC – WP

Complications!

 Variations in soil texture result in:

 Varying amounts of porosity;

 Different pore size distributions;

 Variations in porosity and pore size distributions result in widely varying amounts of plant-available water;

What pore sizes hold most of the water?

What soil is best for growing

trees and crops?

Moisture contents over time for different

elevations above the drainage point

43

Jan 1 Feb 1 Mar 1 Apr 1 May 1 Jun 1 Jul 1 Aug 1 Sep 1 Oct 1 Nov 1 Dec 1 0.1

0.2 0.3 0.4 0.5

Review - 1

 Water is a polar molecule with high surface tension and viscosity;

 Capillary force (negative pressure) holds

water in the soil pores against gravity;

 Water is held more tightly in smaller pores;

 Sandy soils have larger particles and larger pores;

 Fine-textured soils (silts and clays) have

more fine pores and slightly greater

porosity than sandy soils;

Review - 2

 Surface soils in forests have more pore

space than solid particles;

 Water content can be expressed as a

volume (θ), a mass, or percent of pore

space that is filled (percent saturation, S);

 Field capacity is defined as the water

remaining in the soil after it has drained for 1-3 days, or a pressure of -1/3 bar;

 Wilting point is defined as -15 bars, which

is when plants can no longer extract

enough water to survive;

INFILTRATION

Definitions

Infiltration - The downward entry of water into the immediate

surface of soil or other materials

Infiltration Capacity - The maximum rate at which water can

infiltrate into a soil under a given set of conditions

Infiltration Rate - The rate at which water penetrates the surface

of the soil and expressed in cm/hr, mm/hr, or inches/hr

Percolation -Vertical and Lateral

Movement of water through the soil

by gravity

Cover crust

Factors that affect infiltration rate

Soil Factors that Control Infiltration Rate:

- Vegetative Cover, Root Development and Organic Content

- Moisture Content

- Soil Texture and Structure

- Porosity and Permeability

- Soil Bulk Density and Compaction

- Slope, Landscape Position, Topography

Soil type effect on infiltration

• Sand soils have the highest infiltration rates

• Clay soils have the lowest infiltration rates

• High organic matter improves infiltration rates

Infiltrated depth

Soil type and infiltration rate

Land use effects

Land use types Infiltration rate (mm/h)

Effects of Compaction

Soils are full of macropores

 Animal burrows, earthworm

holes, decayed root channels,

freeze/thaw cracks, etc…

Compaction occurs from

machinery, livestock, roads,

etc

Effects of soil water repellency (Tính khó thấm

nước của đất)

Soil water repellency after forest fire

Evidence of overland flow and soil erosion

Effects of roads

Infiltration into a dry and a wet soil

Typical Infiltration Rates (mm hr-1)

Soil type Bare

Soil

Row Crops

Poor Pasture

Good Pasture

fine textured till 1 2 2 5 6

Why are forests so different ?

 Soil cover leading to sealing;

 Changes in temperature (viscosity);

Seasonal changes in infiltration

Seasonal changes in infiltration

How do we measure infiltration?

1 Ring infiltrometers (point scale):

 Maintain constant water level, and measure

water added per unit time;

 Divide by area to get infiltration rate (L T -1 );

2 Rainfall simulator (plot scale):

 Apply artificial rainfall onto a fixed plot at a

known rate;

 Measure runoff; calculate infiltration (=P – Q);

 Often do sequential simulations to measure

infiltration under wet and dry conditions;

Single Rings Infiltrometers

 Edge effects:

Flow is not vertical!

Effect of ring diameter

(5 cm of ponding, most soils)

Use of a double-ring is often assumed to minimize the edge effects

“ the infiltration from the inner cylinder is also very much affected by divergence and edge

effects Field studies also have shown that there

is little difference between the infiltration rate

from the inner cylinder and that of the larger

cylinder Increasing the size of the infiltrometer is the only way to reduce the effect of lateral

divergence of the flow below the cylinder.”

H Bouwer, Methods of Soil Analysis , Amer Soc of

Agronomy, 1986

Double Rings Infiltrometers

•Outer Rings are 6 to 24 inches in

diameter (ASTM - 12 to 24 inches)

•Mariotte bottles can be used to

maintain Constant Head

•Rings Driven - 5 cm to 6 inches in

the soil and if necessary sealed

Double Rings Infiltrometers

Ring infiltrometers: other problems

 How do you know when to stop?

 Typically stop when several successive

measurements show no change in the amount of water added;

 Reality is that you never know if the rate will

change again—almost anything can happen!

 Takes longer to reach the asymptotic infiltration rate:

 Under dry conditions;

 For finer-textured soils;

Typical infiltration curve: real data

Effect of soil type on ring infiltration accuracy (60-cm ring, 5 cm of ponding)

Soil type Infiltration/K s

Coarse sands or gravels 1.3

Most agricultural soils 1.6

Clays and silts 2.3 Compacted clay or silt 5.7

Effect of ponding depth

(60-cm ring, most soils)

Rainfall Simulations

Rainfall Simulations

Rainfall simulators

 Advantages of rainfall simulators:

 Can control amount and rate of precipitation;

 Rainfall simulates actual rainfall, in contrast to a ring infiltrometer;

 Can select locations to vary slope, cover, soil

type, or other factors;

 Rainfall simulators: Disadvantages

 Need water supply, so usually have to do the

simulations close to a road;

 Logistically difficult to do plots larger than about

1 m 2 , but need larger plots to simulate other

processes, such as rilling

Rainfall simulators aren’t simple

 Need to choose an application rate;

 Drops should mimic natural raindrops in size and velocity;

 Need to know size of natural raindrops for desired application rate;

 Drop size and velocity varies with rainfall

rate and pressure;

 Very difficult to get even coverage on a

plot

Rainfall simulator using a jet nozzle

Soil layer：thin Fault

Bedrock

Out crop

Soil layer：thick

Bedrock

Effects of measurement methods

Each methods have different sizes for target area of

infiltration measurement

Single ring infiltrometer

Plot scale rainfall

Inftriltration rate at different methods

Why so variable?