NRES 201 Lectures 16 17 Fall 2014 Soil Water NRES 201 Soil Water 1 Soil Water Today s lecture topics Properties of water Capillarity and soil water Energetics of soil water Classification of soil water 2 1 NRES 201 Lectures 16 17 Fall 2014 Soil Water Properties of water Natural occurrence The only chemical compound that occurs naturally on Earth as a solid liquid and gas The liquid phase is by far the most abundant The only inorganic compound that occurs naturally on Earth as a liquid Yellowstone National Park Source http earthobservatory nasa gov 3 Polarity H2O forms by covalent bonding V shaped configuration due to greater electron affinity by O than H The result is a polar molecule consisting of An electronegative O atom Two electropositive H atoms Source Brady and Weil 2008 4 2 NRES 201 Lectures 16 17 Fall 2014 Soil Water Hydrogen bonding Arises when a H atom of one water molecule is attracted to the O end of another The reason why water polymerizes and is unusually high in Boiling point Specific heat Viscosity Source http apbrwww5 apsu edu 5 Hydration Polar water molecules are attracted to ions or clay particles Charge structured orientation H end toward negative ions or clay particles O end toward positive ions Due to polarity water is the universal solvent Better than any other liquid Source http www terragis bees unsw edu au Source https www ionways com 6 3 NRES 201 Lectures 16 17 Fall 2014 Soil Water Cohesion versus adhesion Two forces are responsible for water retention and movement in soils Cohesion the attraction of water molecules for each other Adhesion the attraction of water molecules for Source Brady and Weil 2008 solid surfaces Both are due to H bonding Also causes flickering H2O clusters to form and dissociate in liquid water 7 Surface tension A property of liquids in contact with air The liquid molecules are attracted more to themselves cohesion than to air adhesion Unusually high for water due to H bonding Important for capillarity in soils Common examples Water strider Source http anniesbiozone blogspot com Source http www krassycandoit com 8 4 NRES 201 Lectures 16 17 Fall 2014 Soil Water Capillarity and soil water Capillary action Water rises in a tube when Some of the molecules climb up the tube wall by adhesion And they pull others up by Source http springtolife weebly com cohesion A curved surface meniscus forms where water meets air Simple example Dye movement up a celery stalk Source http discovermagazine com 9 Height of capillary rise Determined by the upwardacting force of attraction to the tube wall Which effectively reduces the downward acting force of gravity for water in the tube relative to free water Free water outside the tube Source Increases with decrease in http www daviddarlling info tube radius Because the wall is in contact with a greater proportion of water in the tube 10 5 NRES 201 Lectures 16 17 Fall 2014 Soil Water Also increases With lower liquid density And with Greater surface tension And greater adhesive attraction to the tube surface Calculated as 0 15 cm2 h cm r cm r Source Brady and Weil 2008 11 Capillary rise in soils Occurs in all moist soils But limited by Tortuous nature of soil pores Source http www groasis com Entrapment of air Height of rise clays loams sands Rate of rise sands loams clays Source Brady and Weil 2008 12 6 NRES 201 Lectures 16 17 Fall 2014 Soil Water Energetics of soil water Some fundamentals Starved Rock State Park Water runs downhill High potential From a place of higher energy potential energy To a place of lower potential energy The difference in energy levels determines the direction and rate of Low potential energy water flow Soil water varies in energy Source http trekearth com Low energy at high water content High energy at low water content 13 Soil water potentials Matric potential m Water is attracted to soil solids soil matrix This attraction adhesion lowers the free energy of the water m represents the amount of work that must be expended to extract water that has bonded to soil solids Source http www tankonyvtar hu 14 7 NRES 201 Lectures 16 17 Fall 2014 Soil Water m ranges From 0 kPa for water in thick films To 106 kPa for thin films What is kPa A unit of pressure force per unit area kPa stands for kilopascal Analogous to pounds per square inch psi Standard atmospheric pressure is 101 325 kPa So 1 kPa is approximately 1 of atmospheric pressure 15 Osmotic potential o Due to its polarity water is attracted to ions and other solutes These attractions lower the free energy of Source http www advancedaquarist com water o represents the amount of work needed to remove water from a solution with a given concentration of solutes o ranges From 0 kPa for pure water To become increasingly negative for higher solute concentrations Can be ignored if water and solutes move 16 together 8 NRES 201 Lectures 16 17 Fall 2014 Soil Water Gravitational potential g The amount of work required to move a given mass of water against gravity Positive value when above the reference elevation Usually chosen within or at the lower boundary of the soil profile Important for removing excess water from the upper profile Source http www bbc co uk 17 Hydrostatic potential h Positive pressure of water in the saturated zone below the water table Causes wet basements h ranges From 0 kPa for unsaturated soil above the water table Source http shayshomeimprovements com To positive values that increase with depth below the water table 18 9 NRES 201 Lectures 16 17 Fall 2014 Soil Water Total soil water potential t Represents the work that must be expended to remove water from the soil The sum of all the individual potentials t m o g h Simplifies to t m when Water and solutes move together in nonsaline soil o 0 kPa Elevation differences are small for water uptake by crops g 0 kPa The soil is unsaturated h 0 kPa 19 Classification of soil water Dynamics of soil drying As an initially saturated soil is dried water is increasingly confined to smaller pores and thinner films The decrease in water content brings a corresponding increase in the energy of water retention Four stages are characterized along the continuum of soil wetness 20 10 NRES 201 Lectures 16 17 Fall 2014 Soil Water Saturation Also known as maximum retentive capacity All soil pores filled with water Macro and micropores Volumetric water content v total porosity Typically 0 45 0 55 m3 m3 at saturation m 0 kPa Saturated soil Free water on the soil surface Water will drain by