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Charter 5 Soil Water

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.

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Charter 5 Soil Water

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  1. Charter 5 Soil Water

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

  3. 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.

  4. 一、The types and available of soil moisture 1、Classification of soil water • Adsorbed water • Membranous water • Capillary water • Gravitational water Numerical method

  5. （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

  6. （2）Soil Membranous water： held by hydrogen bonding liquid state in water film major source of water for plants greater energy than adhesion water

  7. Sketch map of membranous water

  8. （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.

  9. 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

  10. Capillary hanging water sketch map Soil particle

  11. 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.

  12. Capillary waterrisesketch map Soil particle Groundwater table

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

  14. （4） Gravitational water Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.

  15. 二、 The express methods of soil water content （一）The mass water content（m) Percntage water = {[(wet soil weight)-(oven dry soil weight)]/ (oven dry soil weight)} ×100 （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· (三）Relative water content（%） Relative water content= soil water content/ field capacity

  16. 1、Water deepth（DW） DW=V·h or mm 2、Water fang(方)（ m3） VFang/mu(亩)＝2/3Dw （三）Soil water-storage capcity

  17. 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)

  18. Section 2 Soil Water Potential 一、 Total soil water potential and individual potentials Soil A Sand Soil 10% Soil B Clay Soil 15% Where does water flow?

  19. Water moves from areas of high potential (wet soil : -2 or -4) to areas of low potential (dry soil -8) -.4 -3 -7 -8 -2 Root Soil Soil

  20. 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.

  21. （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

  22. （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

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

  24. Absolute positive value 二、Soil moisture suction 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?

  25. 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

  26. Measuremets of matric potential above about -80 kPa Tensionmeter method

  27. 四、Soil water characteristic curve The relationship between the soil-water content (by mass or volume) and the soil-water matric potential. S=ab S=a(/s)b S=A(s-)n/m S: suction, Pa; θ:water content; a,b,A,n,m: experience constant.

  28. Affect the factors • Texture • Structure • Temperature • Phenomenon of hysteresis Soil moisture suction Clay Silt Sand 0 10 20 30 40 50 60 70 Soil moisture content%

  29. 机理：墨水瓶效应 沙土比粘土明显

  30. 水分特征曲线的用途： 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。 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。

  31. Section3 Water Movement Soils The principle of water movement in soil Evaporation Infiltration Water redistrbution

  32. 一、Saturated Soil Water Flow 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 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.

  33. The characteristics of the saturated hydraulic conductivity: ①The saturated hydraulic conductivity is a constant • It is maximum in hydraulic conductivity • 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 Saturated hydraulic conductivity (Ks) 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.

  34. 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 bythe matric potential and the gravitational potential. Darcy’s Law can be extended to describe unsaturated flow:

  35. Unsaturated hydraulic conductivity The relation between soil moisture suction and hydraulic conductivity

  36. K(m) :unsaturated hydraulic conductivity d/dx: water potential gradient The unsaturated hydraulic conductivity is a function of soil matric potential.

  37. 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.

  38. 1、 Phenomenon of ‘night wet’

  39. Affect the factors： 一, Velocity of Supply water 二, Infiltration rate 四、 Infiltration、water redistribution and evaporation of soil surface （一） Soil water infiltration The entry of water into soil.

  40. Soil Sand Sandy loam Silt Clay Alkalized clay Final infiltration rate >20 10-20 5-10 1-5 <1 The stable infiltration rate in several different texture soils( millimeter/ hour)

  41. （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.

  42. (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 stageof evaporate change with moisture content in soil surface 3 、The stageof water vapor diffusion

  43. P: precipitation; I: irrigation; E: evaporation; T: transpiration; R: runoff 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

  44. 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.

  45. Section 4、Control of soil water 一、Availability 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).

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