Charter 5 Soil Water

Section 1 The types of soil water and measuring soil moisture content

The importance of soil water：

• Effect on soil formation,erosion, and structure stability

• It is the major constituent of plant protoplasm.

• It is essential for photosynthesis and conversion of starches to sugar .

•It is the solvent in which nutrients move into and through plant parts.

一、 The types and available of soil moisture 1、 Classification of soil water

• Adsorbed water• Membranous water• Capillary water• Gravitational water

•Numerical method

（ 1 ） Soil adsorbed water ：

•held by strong electrical forces - low energy

•little movement- held tight by soil

•exists as a film

•unavailable to plants

•removed from soil by drying in an oven

（ 2） Soil Membranous water ：• held by hydrogen bonding

• liquid state in water film

• major source of water for plants

• greater energy than adhesion water

Sketch map of membranous water

（ 3 ） Soil capillary water ：

Capillary water-The water held in the “capillary” or small pores of a soil, usually with a tension >60 cm

of water.

Capillary water includes capillary hanging water and capillary rise water.

Capillarity：

0.1-1mm

Capillarity obvious

0.05-0.1mm

Capillarity strong

0.05-0.005mm

Capillarity very strong

〈 0.001mm

Capillarity disappears

Water is drawn up into the capillary tube

Soil particle

Capillary hanging water sketch map

Field capacity-The amount of water remaining in a soil after the free water has been allowed to drain away (a day or two) after the root zone had been previously saturated; expressed as a percentage.

Field capacity：

Soil particle

Capillary water rise sketch map

Groundwater table

The height of capillary water rise :

h（ cm): the hight of capillary water rise ， d: the diameter of the capillary tube（mm）

（ 4 ） Gravitational water

Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.

二、 The express methods of soil water content

（二） The volume water content （ v ） Volume water content = volume of water/bulk volume of soil=(weight of water/ρW)/(weight of dry soil/ ρb)

V=m·

（一） The mass water content（ m)

Percntage water = {[(wet soil weight)-(oven dry soil weight)]/

(oven dry soil weight)} ×100

(三） Relative water content（%）

Relative water content= soil water content/ field capacity

（三） Soil water-storage capcity

2、Water fang(方 ) （ m3）

whaFang DV 10/

VFang/mu( 亩 )＝ 2/3Dw

1、Water

deepth（ DW） DW=V·h or D hw

i

n

i,100 1

1

mm

三、 Estimating water contents

• Gravimetric method: The soil sample is dried in an oven at 105°C and the mass of dry soil recorded.

• Neutron scattering method• Time Domain Reflectometry (TDR)

Section 2 Soil Water Potential 一、 Total soil water potential and

individual potentials

SoilA

Sand Soil10%

SoilB

Clay Soil15%

Where does water flow?

Water moves from areas of high potential (wet soil : -2 or -4) to areas of lowpotential (dry soil -8)

SoilSoilRoot

-7

-2

-3-.4

-8

Soil water potential- The amount of work that must be done per unit of a specified quantity of pure water in order to transport reversibly and isothermally an infinitesimal quantity of water from a specified source to a specified destination.

（一） The matric potential（ m）

This work is less than zero or negative work, thus reported in negative values.

（二） The pressure potential

（ p）

In saturated soil, the pressure potential is always positive. In an unsatrated field soil the pressure potential is always zero.

p=wgh

（三） The solute (osmotic) potential

（ S）

The amount of work an infinitesmal quantity of water will do in moving from a pool of free water the same composition as the soil water to a pool of pure water at the same location. The solute potential is usually very small and negative values.

（四） The gravitational potential

（ g）

The amount of work an infinitesmal amount of pure free water can do at the site of the soil solution as a result of the force of gravity.

g =±MgZ

Total soil water potential ： t=m+p+s+g

Since soil matric and osmotic potentials are always negative they are often onsidered as ‘suction’ or ‘tensions’. Suction and tensions are however always expressed as positive values.

T＝ -m

How do you use suction and potentials to decide the direction of soil water movement?

Absolute positive value二、 Soil moisture suction

三、 Soil water potential measurement

The popular unite of the soil water potential is : Pa

1 Pa=0.0102-cm column of water

1 atmospheres=1033-cm column of

water=1.0133bar

1 bar=0.9896atm=1020-cm column of water

1 bar=105 Pa

Measuremets of matric potential above about -80 kPa

Tensionmeter method

四、 Soil water characteristic curve

The relationship between the soil-water content (by mass or volume) and the soil-water matric potential. S=ab

S=a(/s)b

S=A(s-)n/m

S: suction, Pa; θ:water content; a,b,A,n,m: experience constant.

0 10 20 30 40 50 60 70

Clay

SiltSand

Soil moisture content%

Affect the factors

•Texture

•Structure

•Temperature

•Phenomenon of hysteresis

Soil m

oisture su

ction

机理：墨水瓶效应

沙土比粘土明显

水分特征曲线的用途：

第四，应用数学物理方法对土壤中的水运动进行定量分析时，水分特征曲线是必不可少的重要参数。

首先，可利用它进行土壤水吸力 S和含水率之间的换算 (图3.7)。

其次，土壤水分特征曲线可以间接地反映出土壤孔隙大小的分布。

第三，水分特征曲线可用来分析不同质地土壤的持水性和土壤水分的有效性。

Section 3 Water Movement Soils

•The principle of water movement in soil•Evaporation•Infiltration•Water redistrbution

一、 Saturated Soil Water Flow

The rate of water flow through soil can be described by Darcy’s Law which states that the flux of water q is proportional to the hydraulic gradient (the gravitational potential and the pressure potential )multiplied by the conductivity or permeability of the soil.

q KH

Ls

Saturated flow-The movement of water though a soil that is temporarily saturated. Most of the water moves downwards, and some move more slowly laterally.

The rate of flow through a given amount of soil in a given time equals the water quantity collected (Qw) divided by both the cross-

sectional area of soil used (A) and the time (t) of measurement.

Saturated hydraulic conductivity (Ks)

The characteristics of the saturated hydraulic conductivity:

①The saturated hydraulic conductivity is a constant

② It is maximum in hydraulic conductivity

③ It is decided by the soil texture and the soil structure

The factors of affect the saturated hydraulic conductivity:

•The soil texture

•The soil structure

•The amount of organic matter

•The clay mineral

At

QK w

s

二、 Unsaturted soil water flow

The movement of water in soil in which the pores are not filled to capacity with water.

The unsaturted soil water flow is decided by the matric potential and the gravitational potential.

q Kd

dxm ( )

Darcy’s Law can be extended to describe unsaturated flow:

Unsaturated hydraulic conductivityThe relation between soil moisture suction and

hydraulic conductivity

K(m) :unsaturated hydraulic conductivity

d/dx: water potential gradient

The unsaturated hydraulic conductivity is a function of soil matric potential.

三、 Vapor flow in soil

The form of movement of vaporous water in soil:

1.Water vapor diffusion

2. Water vapor coagulation

Vapor flow can be considered as a diffusion mechanism in which the driving force is the vapor pressure gradient.

1 、 Phenomenon of ‘night wet’

四、 Infiltration 、 water redistribution and evaporation of soil surface （一） Soil water infiltration

The entry of water into soil.

Affect the factors：

一 , Velocity of Supply water

二 , Infiltration rate

The stable infiltration rate in several different texture soils ( millimeter/ hour)

Soil SandSandy loam Silt Clay Alkalized

clay

Final infiltration

rate>20 10-20 5-10 1-5 <1

（二） Redistribution of soil water

Redistribution of soil water- The process of soil-water movement to achieve an equilibrium energy state of water throughout the soil.

Soil water redistribution is unsaturated flow of soil water.

(三 ) Evaporation in soil surface

•Evaporation- Water lose as vapor from a soil or open water surface.

•1、 The keeping the stable stage of evaporation in soil surface

•2 、 The stage of evaporate change with moisture content in soil surface

•3 、 The stage of water vapor diffusion

五、 Soil water balance in the field

Soil water balance in the field can be written as: W=P+I+U-E-T-R-In-D

P: precipitation; I: irrigation;

E: evaporation; T: transpiration; R: runoff

Soil-plant-atmosphere continuum (SPAC) Water moves from a relatively high potential energy level in the

soil (-100 kPa) and flows down a potential gradient into the plant root (-500 kPa), plant stem (-800 kPa), and leaves (-1500 kPa), where it is eventually evaporated into the atmosphere (-10000 kPa).

Desert plants can live in -2×106 to -8×106 Pa.

Section 4、 Control of soil water

Available soil water-The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of -1.5 MPa).

一、 Availability of soil water

二、 Control of soil water

1、 Cultivation measure : plow depth, intertillage, roll etc.

2、Mulches: straw, plastic sheeting etc.

3、 Irrigation: drip irrigation, sprinkler irrigation etc.

4、 Biological save water

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