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Soil, Water, and Hydrologic Cycle

Soil, Water, and Hydrologic Cycle. Introduction –Distribution of Water Hydrologic Cycle –Water balance Structure and Properties of Water Energy of Soil Water Measurements of Soil Water Soil Water Availability to Plants. 1. Global Distribution of Water.

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Soil, Water, and Hydrologic Cycle

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  1. Soil, Water, and Hydrologic Cycle Introduction –Distribution of Water Hydrologic Cycle –Water balance Structure and Properties of Water Energy of Soil Water Measurements of Soil Water Soil Water Availability to Plants

  2. 1. Global Distribution of Water • About 1400 m km3 of water scattered all over the earth. • Oceans (97.25%) • ice sheets/glaciers (2%) • groundwater (0.7%) • All other water (0.05%) • lakes, streams (0.0305%) • soil water (0.0165%) • Atmosphere (0.003%) • Surface layer of ocean, shallow groundwater, etc ?

  3. Distribution of Water

  4. 2. The Hydrologic Cycle • Describes the movement of water from the ocean, to the atmosphere, to land, and back to the ocean. • All these movements are driven by the energy of the sun that stimulates evaporation. • Soil moderates the rate of flow of precipitation into streams, lakes and other waterways.

  5. The Hydrologic Cycle

  6. Soil Water Balance • Water Input • Precipitation • Irrigation • Water Output • Plant uptake • Evaporation and Evapotranspiration • Leaching to groundwater • Water Stored in Soil • Described by changes in volumetric water content

  7. Water Stored in Soil • The Water stored in soil plays a central role in the global water use and cycling by moderating the adverse effects of excesses and deficiencies of water. • Where: Ss = Storage • Iw = water input • Et = Evapotranspiration • D = Discharge

  8. 3. Structure & Properties of Soil Water Water molecule H2O is a polar molecule with 105º angle. This gives it several important attributes

  9. Attributes of water structure(H-Bonding, Cohesion, Adhesion)

  10. Attributes of water structure (Forces of Cohesion and Adhesion) Hydrogen Bonding - A relatively low energy coupling in which a hydrogen atom of one molecule is attracted to the oxygen end of a neighboring molecule. Cohesive Forces (cohesion) - Attraction of water molecules to other water molecules. Adhesive Forces (adhesion) - Attraction of water molecules to molecules other than water.

  11. -from the forces of cohesion and adhesion come secondary water properties • Surface Tension - At liquid-air interfaces, water molecules have a greater attraction for adjacent water molecules (cohesion) than for the air molecules above them. This results in an inward force at the surface that causes the water to behave as if its surface were covered with a stretched elastic membrane.

  12. Capillary Movement - The rise of water up a small diameter tube. h=2T/rdg • Capillarity is due to two forces: • The attractive force of water for soil solids (adhesion) • The surface tension of water (attractive force of water to itself) (cohesion). where T=surface tension, r=radius of tube, d=density of liquid, g=force of gravity. For water, this equation reduces to h=0.15/r

  13. 4. Energy of Soil Water • Retention and movement of water in soils; water uptake and translocation in plants are all energy related. • Two kinds of energy that are involved with moisture dynamics are: • Kinetic Energy (KE) • Energy of motion • Potential Energy (PE) • Energy of position There is no rapid motion in soil water. Therefore KE is negligible for soil water. But PE is very important in determining water movement in soil. PE is referred to as energy for soil water. (water moves from a higher to a lower PE)

  14. Forces Affecting Potential Energy of Soil Water From the structure and properties of water, we know that 3 important forces affect the energy of soil water: adhesion provides matric force; osmosis provides osmotic force; and gravity, gravitational force • Matric force • Is due to attraction of water to soil matrix (Adhesion –attraction of water molecules to soil solids) that results in matric potential • Osmotic force • Is due to attraction of water to ions and other solutes resulting in osmotic forces (Osmosis) that results in osmotic potential • Gravitational force • Due to downward pull of water by gravity (Gravitational attraction) that results in gravitational potential

  15. Soil Water Potential • The energy status of soil water depend on the forces acting on soil water • The difference in these energy levels from place to place in soil determines the rate and direction of water movement in soils and plants (e.g wet and dry soil). • Water will move from a zone having a high soil water potential to one having a lower soil water potential.

  16. Total Soil Water Potential Where: t = total soil water potential g = gravitational potential m = matric potential o = osmotic potential

  17. Matric potential – the attractive forces between soil water and soil solids (recall adsorption and capillarity) reduces the water’s potential energy level compared to that of pure water. The degree of the reduction is termed matric potential. Matric potential is considered to be negative. • Osmotic potential – the attraction between water molecules and chemicals (salts and other solutes) reduces the potential energy of the water. The degree of the reduction is termed osmotic potential. Since matric and osmotic forces reduce the water’s potential energy level compared to that of pure water, they are considered negative. • Gravitational potential – gravitational force reduces the energy level of the soil. The degree of that reduction is termed gravitational potential. Since the soil water elevation is usually chosen to be higher than that of the reference pool, the gravitational potential is usually positive.

  18. 5. Methods of Measuring Soil Water Content/Potential a. Gravimetric techniques • Standard and direct measurement of soil moisture • A moist soil sample is weighed and placed in a oven at 110oC for approximately 24 h then cooled and weighed. The process is repeated until the sample reaches a constant weight. The weight loss represents the soil water which can be expressed on a volume or mass basis. • The gravimetric method is a destructive method • It cannot be automated and so is poorly suited to monitoring soil moisture

  19. b. Neutron moisture meter • Device that emits high energy fast neutrons into the soil. • These neutrons collide with H-atoms (in water molecules) and result in slowed and scattered neutrons. • The amount of low slowed neutrons that are reflected back to the detector is then correlated with the soil water content.

  20. c. Tensiometers (Soil Water Potential) • Devices designed to measure soil matric potential. • As water is pulled out of the porous ceramic cup in response to the matric potential of the soil, water pulls on the vacuum gauge changing the reading. • It is restricted to matric potentials in the range of 0 to -1 bar.

  21. d. Electrical Resistance Blocks • Consists of two electrodes embedded in a porous block. • When device is placed in soil, porous block absorbs water in proportion to the water content. • And the resistance to flow of electricity between the embedded electrodes correlates with soil moisture content

  22. e. Pressure Plate apparatus • Moist soil is placed in the upper chamber of a pressure-plate apparatus. Pressure is increased to the desired level and the soil is allowed to equilibrateWhen the pressure is increased to a certain value, all water held at energies less than that value will be forced through the porous plate into the lower chamber and quantified by a loss of mass or volume.

  23. 6. Qualitative Description of Soil Water Content (and Plant-Water Relations) As water saturated soil dries out, both the soil as a whole and the soil water it contains undergo a series of changes in physical behavior and relationships with plants • Saturated Water Content - immediately after rain, all pores are full. When all soil pores are filled with water, the soil is said to be saturated – Maximum retentive capacity. Matric potential is zero. No stress. • Field Capacity Water Content – Once the rain ceases, water in the largest soil pores drain downward in response to gravity. • After 1-3 days, the downward movement of the water by gravity becomes negligible and matric forces become dominant. • So soil is holding the maximum amount that it is capable of against the force of gravity - drained soil. Approx. –10 to-30 kPa pressure (1/3 bar). No stress (sometimes referred to as capillary water)

  24. Qualitative Description of Soil Water Content(and Plant-Water Relations) Contd. • Permanent Wilting Point Water Content - The point of moisture deficit at which plants wilt beyond recovery. Approx. -1500 kPa (15 bars) pressure. • Plant Available Water Content - that water that is available for plant use (between field capacity and permanent wilting point).

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