5.土壤水E

Capillary hanging water sketch map
Soil particle
Field capacity：
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.
membranous water
Schematic diagram 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.
Percentage water = {[(wet soil weight)-(oven dry soil weight)]/ (oven dry soil weight)} ×100
（二） The volume water content （ θv）
Volume water content = volume of water/bulk volume of soil=(weight of water/ρW)/(weight of dry soil/ ρb)
（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 （一）The mass water content（θm) （
Clay Sand Silt
•Temperature •Phenomenon of hysteresis
（四） The gravitational potential （ψg）
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.
Capillary water rise sketch map
Soil particle
Groundwater table
The height of capillary water rise :
h（cm): the height of capillary water rise ，d: the （ diameter of the capillary tube（mm） （ ）
Section 2 Soil Water Potential
一、 Total soil water potential and individual potentials
Soil A Sand Soil 10%
Where does water flow?
Soil B Clay Soil 15%
Water moves from areas of high potential (wet soil : -2 or -4) to areas of low potential (dry soil -8)
How do you use suction and potentials to decide the direction of soil water movement?
三、Soil water potential measurement
The popular unite of the soil water potential is : Pa 1 Pa=0.0102-cm column of water 1 atmosphere pressure=1033-cm column of water=1.0133bar 1 bar=0.9896atm=1020-cm column of water 1 bar=105 Pa
S=aθb S=a(θ/θs)b S=A(θs-θ)n/θm
S: suction, Pa; θ:water content; a,b,A,n,m: experience constant.
Affect the factors •Texture •Structure
Soil moisture suction Soil moisture suction
ψg =±MgZ ±
Total soil water potential ：
ψt=ψm+ψp+ψs+ψg ψ ψ ψ ψ
二、Soil moisture suction
Absolute positive value
Since soil matric and osmotic potentials are always negative they are often considered as ‘suction’ or ‘tensions’. Suction and tensions are however always expressed as positive values. T＝|-ψm |
一、The types and available
of soil moisture 1、Classification of soil water
• • • •
Adsorbed water Membranous water Capillary water Gravitational water
•Numerical method
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
（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
（三） The solute (osmotic) potential （ψS)
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.
DW=θV·h or Dw,100 = ∑θ 1 • h θ
i =1 i
n
mm 2、Water fang(方)（ m3） 、 方（
V Fang / ha = 10 D w
VFang/mu(亩)＝2/3Dw 亩
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)
-4
-3 -7
-8 Root
-2 Soil Soil
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（ψm） （ This work is less than zero or negative work, thus reported in negative values. （二） The pressure potential （ψp） In saturated soil, the pressure potential is always positive. In an unsaturated field soil the pressure potential is always zero. ψp=ρwgh ρ
Chapter 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.
θV=θm·ρb θ ρ
(三）Relative water content（%） 三 （ ）
Relative water content= soil water content/ field capacity
（三）Soil water-storage capacity
1、Water depth（DW） 、 （
（2）Soil Membranous water ： ）
• held by hydrogen bonding • liquid state in water film • major source of water for plants • greater energy than adhesion water
1、Tensionmeter met matric potential above about -80 kPa
2、Pressure membrane method 、
四、Soil water characteristic curve (SWC)
The relationship between the soil-water content (by mass or volume) and the soil-water matric potential.