Soil and Soil Moisture: From Measurement to Mesoscale

1. Soil and Soil Moisture:From Measurement to Mesoscale Benjamin Hatchett Division of Atmospheric Sciences Desert Research Institute Reno, Nevada

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

3. An Introduction to Soils “In the structure and functioning of landscapes, soils are the matrix through which energy, water, biomass, and nutrients flow…the interface in the cycling of water between the atmosphere and land…the location of large transformations of energy.” Bonan, 2002

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

5. 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 (cracks in sidewalk) time Water and Wind in Death Valley Plants help too!!!!

6. 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 -> H2CO3 then H2CO3 + CaCO3 -> Ca(HCO3)2 • Oxidation 4Fe + 3O2 → 2 Fe2O3

7. Soil Structure • Soils Composed of: • Organic Matter (>80% organic soil, <10% mineral soil) • Minerals (From parent geology, ~55% in mineral soil) • Air • Water • Type, abundance, arrangement of particles govern heat flow, water flow, nutrient availability

8. 5 General Soil Structure Profiles Place matters!!!

9. 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

10. 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

11. General Patterns? • Soil Type • Don’t worry about something-sols, think agriculture and place… • Soil Moisture • Green = Wet • Red/Yellow = Dry

12. 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

13. 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)

14. Thermodynamic Responses to Soil Moisture • Note nonlinearities… • Implications for modeling Warner, 2004

15. Soil Water • Richards Equation (from Darcy’s Law): K = Hydraulic conductivity ψ = Pressure head θ = Water Content • Influence of time and place…

16. The Surface Energy Budget

17. 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)

18. Evaporation Rates and Model Initialization • Nonlinear evaporation rate • Limit = hydraulic diffusivity/moisture threshold (remember soil structure!) • How will model initialization runs vary as a result? Warner, 2004

19. Linked In: Evapotranspiration Etot=Edir+Et+Ec 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 (Ohm’s Law: Flux=P/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 !!!

20. 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)

21. 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 Eltahir2003 found that antecedent wet soils aided convection in SE US

22. 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 2m Air Temperature Soil Moisture

23. Large-eddy simulation of a coupled land-atmosphere system 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. Sullivan et al. 2000

24. Modeling the ABL Siquiera et.al 2008

25. Bowen Ratio and ABL Heights as Functions of Soil Moisture Siquiera et.al 2008

26. Measurement Methods • Passive Remote Sensing • Aircraft • Towers • Field Collection

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

28. 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

29. 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

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

31. Porosity Revisted Wet Soil Dry Soil

32. Use of RADAR • Pulse of microwave energy that interacts with Earth’s terrain • Measure of material’s 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 Microwave Remote Sensing

33. 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

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

35. 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)

36. But Improving Our Understanding and Measurement Capabilities Will… • Improve Land Surface Component of Coupled Models • Increase abilities to forecast: • Convective Processes • Seasonal Climate • QPF

37. References • Bonan, G. 2002 Ecological Climatology. Cambridge Univ. Press • Betts, A. K., and J. H. Ball, 1998 J. Atmos. Sci., 55, 1091–1108. • 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/workbook/figures/7.3.gif • http://www.nrmsc.usgs.gov/files/norock/research/soil_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/bagoview.jpg • http://oceanworld.tamu.edu/resources/environment-book/groundwater.html • http://www.orcbs.msu.edu/environ/programs_guidelines/wellhead/glossary_faq/capillary_fringe.jpg • http://techalive.mtu.edu/meec/module06/Packing.htm • http://research.eeescience.utoledo.edu/lees/papers_PDF/Saxton_1986_SSSAJ_files/Fig_6.gif • http://www.eol.ucar.edu/projects/cases/maps.html • http://gis.esri.com/library/userconf/proc99/proceed/papers/pap365/p3654.gif • http://weather.msfc.nasa.gov/surface_hydrology/surface_hydrology_inverse_model.html

38. Questions????