Soil and Soil Moisture: From Measurement to Mesoscale

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Soil and Soil MoistureFrom Measurement to
Mesoscale

• Benjamin Hatchett
• Division of Atmospheric Sciences
• Desert Research Institute
• Reno, Nevada

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Overview

• Soils 101
• A Deeper View of Soil Moisture
• Surface Energy Budget and Implications from Micro
  to Mesoscale
• Measurement Methods

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An Introduction to Soils

• In the structure and functioning of landscapes,
  soils are the matrix through which energy, water,
  biomass, and nutrients flowthe interface in the
  cycling of water between the atmosphere and
  landthe location of large transformations of
  energy.

Bonan, 2002
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Soil Formation

• Two processes form soil
• Chemical Weathering Reactions!
• Physical Weathering Disintegration!
• Soil type influenced by various factors
• Climate
• Geology
• Topography
• Time

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Physical Weathering
Is the actual disintegration of rocks due to
SCOURING by wind, water, and/or ice
In simple terms
Melt/Freeze, Wet/Dry Expansion/Contraction (cra
cks in sidewalk)
time
Water and Wind in Death Valley
Plants help too!!!!
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Chemical Weathering

• Climate important Kinetic rates increase with
  temp.
• Rocks dissolve due to reactions between rock
  minerals and water, acid, or other chemicals
• Hydrolysis Mg2SiO4 4H 4OH- ? 2Mg2
  4OH- H4SiO4
• Dissolution CO2 H2O -gt H2CO3 then H2CO3
  CaCO3 -gt Ca(HCO3)2
• Oxidation 4Fe 3O2 ? 2 Fe2O3

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Soil Structure

• Soils Composed of
• Organic Matter (gt80 organic soil, lt10 mineral
  soil)
• Minerals (From parent geology, 55 in mineral
  soil)
• Air
• Water
• Type, abundance, arrangement of particles govern
  heat flow, water flow, nutrient availability

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5 General Soil Structure Profiles
Place matters!!!
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Soil Texture

• Relative abundance of sand, silt, and clay
  determines soil texture
• Irregular shapes create
• voids, called pore spaces
• Porosity Volume of soil
• occupied by air and water

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Implications of Porosity

• Close packing How much space?
• Sand Low porosity, large pore space, fast water
  movement
• Clay High Porosity, small pore space, very slow
  water movement

So, porosity has strong influence on spatial and
temporal presence and patterns of soil moisture
presence. Has implications for remote sensing and
modeling applications
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General Patterns?

• Soil Type
• Dont worry about something-sols, think
  agriculture and place
• Soil Moisture
• Green Wet
• Red/Yellow Dry

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Soil Thermodynamics

• Soils are repository of heat
• Moderates diurnal and seasonal range in Tsurf
• Gain heat during day/warm months
• Lose heat during night/cold months

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Soil Temperature Equation

• C1 Thermal Conductivity
• CV Volumetric Heat Capacity
• K Thermal Diffusivity Constant

• Thermal conductivity and heat capacity depend on
• Mineral Composition (e.g. quartz)
• Porosity (less pores higher conductivity)
• Organic Matter Content (very porous, low C1,
  insulate)
• Water Content (C1 20x air, CV 3500x air)

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Thermodynamic Responses to Soil Moisture
Warner, 2004

• Note nonlinearities
• Implications for modeling

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Soil Water

• Richards Equation
• (from Darcys Law)
• K Hydraulic conductivity
• ?  Pressure head
• ?  Water Content
• Influence of time and place

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The Surface Energy Budget
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Simple Model of the Surface Energy Budget
Rn Total Radiation H Surface Sensible Heat
Flux LE Latent Energy Heat Flux G Ground
(Soil) Heat Flux

• Role of Soil in Each Term
• H Heat from soil warms (-)/cools air ()
• LE Heat used to evaporate water/freeze water
• G Heat stored in soil (remember C1 and CV terms
  from thermodynamic equation)

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Evaporation Rates and Model Initialization
Warner, 2004

• Nonlinear evaporation rate
• Limit hydraulic diffusivity/moisture threshold
  (remember soil structure!)
• How will model initialization runs vary as a
  result?

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Linked In Evapotranspiration

• EtotEdirEtEc

Etot Total Evaportranspiration from Soil and
Vegetation Edir Direct Evaporation from Soil Et
Transpiration from Plant Canopy Ec
Evaporation from Canopy Intercepted Rainfall
Represents a moisture flux that can be
approximated by comparing resistances to
potential flux (Ohms Law FluxP/R)

• Resistances include
• Available Soil Moisture
• Canopy (Stomatal) Resistance (Vegetation type,
  Greeness)
• Atmospheric Winds, Stability

Bottom Line Many Interacting Factors in Soil
Moisture/Energy Budget !!!
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Microscale

• Effect Varies with Topography
• Slope
• Aspect
• Topographic Convergence
• Vegetation Growth
• Crops have ideal growth temperature
• Heat stress (out of LE to evaporate, increases H)
• Plant diseases due to condensation
• Local Surface Temperatures
• Moderated by Soil Moisture
• Wet soils cold, Dry soils warm (heat
  capacity)
• Diurnal and seasonal flux of sensible heat
• Latent heat use (evaporation cools, condensation
  warms)

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Influence on Mesoscale Convection

• Soil Moisture linked to Mesoscale Convection
  (e.g. Betts and Ball 1998, Sullivan et al. 2000)
• Remains open research question due to many
  feedbacks/complicating factors
• Sometimes wet soils suppress convection, dry
  soils aid propagation (Taylor and Ellis, 2006)
• Role of Evaporation
• Patchiness of wet/dry, creating gradients (Sahel,
  Central Plains US) that force surface PBL
• BUT! Not always true Findell and Eltahir 2003
  found that antecedent wet soils aided convection
  in SE US

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Soil Moisture, Soil Temperature, ABL Heat Flux

• Dry soil heats quickly with afternoon insolation,
  results in very high sensible heat flux to
  boundary layer

Soil Temperature
Soil Moisture
2m Air Temperature
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Large-eddy simulation of a coupled
land-atmosphere system
Sullivan et al. 2000

• Response of the atmospheric boundary layer to
  heterogeneous soil moisture. The dramatic changes
  in boundary layer structure result from the
  non-linear dependence of soil properties on soil
  moisture.

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Modeling the ABL

• Siquiera et.al 2008

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Bowen Ratio and ABL Heights as Functions of Soil
Moisture

• Siquiera et.al 2008

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Measurement Methods

• Passive Remote Sensing
• Aircraft
• Towers
• Field Collection

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Scales of Measurement

• Satellite Data
• 50km resolution
• Aircraft Data
• 1km resolution
• Tower Data
• 10m resolution
• Field Data
• To lt10cm resolution
• Problem with scale
• Spatial variation in SM at larger scales and
  application of same retrieval algorithms to all
  scales
• Nonlinearities, once again!

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Field Measurement Techniques

• Used to calibrate/verify Remote Sensing Data
• Neutron Depth Moisture Gauge
• Single Radium-Berillium source probe
• Number of neutrons deflected back to probe is
  proportional to H20 in soil
• Gives total water content in profile
• Gamma Meter
• Two probes, Cs 137 in one, detector in other
• Intensity of radiation received proportional to
  density of material, density in soil constant
  except for changes in water content

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Factors in Soil Reflectance

• A goal of remote sensing is to disentangle
  spectral response recorded and indentify
  proportions and influences of the characteristics
  within the instantaneous field of view of the
  sensor system (Jensen, 2007)
• Soil Texture
• Soil Moisture Content
• Organic Matter
• Fe-Ox Content
• Salinity
• Surface Roughness
• Vegetation

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Soil Response

• Note absorption bands
• Why wet soils appear darker!
• Implications of SM
• Precipitation
• Measurement timing
• Soil type!

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Porosity Revisted

• Dry Soil

Wet Soil
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Microwave Remote Sensing

• Use of RADAR
• Pulse of microwave energy that interacts with
  Earths terrain
• Measure of materials electrical characteristics
• Complex Dielectic Constant ability to conduct
  electrical energy (why microwave!)
• Dry surfaces 3-8um
• Water 80um
• Therefore, amount of moisture on surface
  influence amount of backscattered energy

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Jackson (1993) Inverse Soil Moisture Retrieval
Model

• Model is a summation of research since 1970s that
  has established and verified use of passive
  microwave emission from land surfaces

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Advanced Microwave Scanning Radiometer Earth
Observing System (AMSR-E)
West Africa, June 2006 Note Moisture Gradient,
Pattern
Gantner et. al
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Food for Thought

• Soil moisture is difficult phenomena to measure
  and model because
• Place matters! (Soil type, vegetation,
  topography)
• Time matters! (For measurement, e.g. pre/post
  precip, initial conditions)

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But Improving Our Understanding and Measurement
Capabilities Will

• Improve Land Surface Component of Coupled Models
• Increase abilities to forecast
• Convective Processes
• Seasonal Climate
• QPF

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References

• Bonan, G. 2002 Ecological Climatology. Cambridge
  Univ. Press
• Betts, A. K., and J. H. Ball, 1998 J. Atmos.
  Sci., 55, 10911108.
• Findell, K. L., and E. A. B. Eltahir, 2003 J.
  Hydrometeorology, 4, 552-569
• Findell, K. L., and E. A. B. Eltahir 2003 J.
  Hydrometeorology, 4, 570-583
• Findell, K.L. 2003 Journal of Geophysical
  Research 108(d8) 8385
• Harpstead, M.I., T.J. Sauer, W.F. Bennett. 2001
  Soil Science Simplified. Blackwell Publishing
• Jensen, J.R. 2007 Remote Sensing of the
  Environment. Prentice Hall.
• Marshall, C. 1999 COMAP Symposium 99-1
• Taylor C.M., and Ellis R.J. 2006 Geophysical
  Research Letters 33(3)
• Siqueira, M., K. Gabriel, Submitted 2008. J.
  Hydrometeorology
• Warner, T.T. 2004. Desert Meteorology. Cambridge
  Univ. Press
• https//courseware.e-education.psu.edu/simsphere/w
  orkbook/figures/7.3.gif
• http//www.nrmsc.usgs.gov/files/norock/research/so
  il_moisture.gif
• http//www.mmm.ucar.edu/modeling/les/images/les_lg
  .jpg
• http//nature.berkeley.edu/biometlab/images/olive_
  apilles.GIF
• http//grapevine.com.au/pbeirwirth/images/bagovie
  w.jpg
• http//oceanworld.tamu.edu/resources/environment-b
  ook/groundwater.html
• http//www.orcbs.msu.edu/environ/programs_guidelin
  es/wellhead/glossary_faq/capillary_fringe.jpg
• http//techalive.mtu.edu/meec/module06/Packing.htm
  

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Questions????