Biometeorology, ESPM 129 1 Lecture 34 Soil Physics, Moisture Transfer in Soils, Part 2 November 24, 2010 Instructor: Dennis Baldocchi Professor of Biometeorology Ecosystem Science Division Department of Environmental Science, Policy and Management 345 Hilgard Hall University of California, Berkeley Berkeley, CA 94720 Topics 1. Theory, Moisture Transfer a. Moisture transfer, Darcy’s Law and the Richard’s Equation Soil Release curve. c. Force Restore models, Bucket models d. Role of different boundary conditions e. Evaporation models f. CO2 diffusion models 2. Observations, Moisture profiles a. Seasonal patterns b. Influence of soil texture 3. Soil Evaporation a. measurements b. model calculations 34.1 INTRODUCTION The soil is the reservoir for moisture that is available to roots. How much moisture it contains and the rate that water is lost from the soil depends on its texture, physical/hydraulic capacity and the activity of plant contained within it and the extent of their root system. The presence or absence of moisture and active biology in soils affects how it weathers (Jenny 1994). To understand changes in soil moisture we must start with an understanding of the sources and losses of water.Biometeorology, ESPM 129 2Sources of water to the biosphere from the atmosphere include precipitation, snow (in the later form of melt water), condensation/dew, fog interception and stemflow. Sources of water from the soil include capillary rise from the water table, direct axis of the water table by roots and hydraulic lift of moisture by roots. Losses of water from the biosphere include losses to the atmosphere and to deep soil layers. Losses to the atmosphere include direct evaporation of surface water, evaporation from the soil matrix, sublimination of snow, plant transpiration. Losses through the soil column involve transport via perculation and saturated flow and unsaturated flow. Flow through the soil column is vertical and lateral, a distinctly three-dimensional process. Figure 1 Flows of moisture in and out of soil column. Adapted from Tindall and Kunkel (1998) Water TableSurface Watersnow/iceCapillary RiseEvaporation/SublimationWetting FrontPrecipitation/CondensationEvaporationPerculation/saturated flowunsaturatedflowEvaporation/TranspirationSnow MeltHydraulic Liftingby rootsFogInterception/StemFlowBiometeorology, ESPM 129 3A thermodynamic quantity, soil water potential, is used to describe soil hydrodynamics. The total soil water potential is the amount of work done, per unit quantity of pure water, to transport an infinitesimal amount of water from a pool of pure water. The process is isothermal and a reference pressure (Tyndall and Kunkel, 2000). The components of soil water potential for shallow soils are: Gravitational potential: The force of gravity exerted on a water column produces the gravitation potential. The gravitation potential is related to the work done to transport water from one pool to another, as when lifting a column of water up the xylem of a tree: glgz Turgor or pressure potential: Water potential exerted by the pressure or weight of water pwP Vapor potential: vvsRTee ln( ) R is the universal gas constant Matric potential: It is water potential due to attraction between water and soils. Adhesive and cohesive forces bind water to soil particles (Campbell and Norman, 1999). These interactions reduce the potential of water, giving it a negative sign. mbaw~ Osmotic potential: Osmotic potential arises from the dilution of solutes in water, eg salts, sugars etc. For the osmotic potential to drive water flow, a semi-permeable membrane must separate two bodies of water, such as cells, and pools of water. ocvRT c is the concentration,  is the osmotic coefficient and  is the number of ions per mole (Campbell and Norman 1998). The total water potential is thus:Biometeorology, ESPM 129 4pogm Which components are significant or not depends on the system we are studying (e.g. Baver et al, 1976). Water potential is expressed as energy (kg m2 s-2) per unit volume (m-3), giving it units of kg m-1 s-2, which is equivalent to pressure units, or force per unit area, kg m s-2 m-2). E/V = F x/V = F/A=P In other instances pressure potentials may be normalized by water density |masswP (kg m-1 s-2) * (m3 kg-1) We know the mass of water is 1000 kg = m3 and water is incompressible, we can substitute mass for density, effectively 1 J/kg = 1 kPa Soil-Water-Plant Relations (apolplast) om Osmotic and matric potential are important for plant-water relations and water movement in the apoplast and through the xylem. Gravimetric potential is negligible as the suction needed to raise water, typically 1 m is less than 0.1 bar. Organisms, cells (symplast): po Inside cells turgor potential (eg pressure potential) and osmotic potentials are most important Unsaturated Flow: gm Gravitational and matrix potential are dominant Saturated Flow pg Pressure and gravity are the main components driving water flow in saturated soils. The pressure term includes overburden effects. At a point a point beneath the water table, theBiometeorology, ESPM 129 5pressure potential is equal and opposite to the gravitational potential. Osomotic potential is important only if there are solutes in the water. 0 pgpg; Flow in the field pgm This terminology considers mixed flow in the saturated and unsaturated zones. The pressure term is zero above the water table. The matrix potential is zero below the water table. Physical Properties The relative humidity of the soil pores was evaluated from thermodynamic principles as:  exp( )gRTw (7 g is the acceleration due to gravity,  is the capillary potential, Rw is the gas constant for water vapor and T is absolute temperature. Many popular pedo-transfer functions exist that relate soil water potential and volumetric soil moisture content. Early relations were reported by Gardner et al and Clapp and Hornberger (Clapp and Hornberger 1978) that fitted power functions: satsb (8 for sand,  equals 0.1 m3 m-3, b is 4.05, and  is -0.121 m. .Biometeorology, ESPM 129 6 Figure 2 Soil moisture retention using equation of Gardner Table 1 Important physical