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Modeling Your Soil-Water Budget

Modeling Your Soil-Water Budget. Jerry P. Henzel & Cyndy L. Henzel. Soil Moisture Data. 31,499 Measurements in 173 Schools. Principals of Hydrology Suggest:. Soil moisture & runoff depend on the amount and timing of precipitation Precipitation is localized & specialized

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Modeling Your Soil-Water Budget

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  1. Modeling Your Soil-Water Budget Jerry P. Henzel & Cyndy L. Henzel

  2. Soil Moisture Data • 31,499 Measurements in 173 Schools

  3. Principals of Hydrology Suggest: • Soil moisture & runoff depend on the amount and timing of precipitation • Precipitation is localized & specialized • Varies from month to month

  4. Principals of Hydrology Suggest: • Varies from month to month • Varies from place to place • Leads to a Variable Amount of Water Available • Properties of soil affect amount stored Texture, porosity, conductivity, sequence & thickness of horizons

  5. Sum of the Parts Sum of All Local Parts Global Hydrologic Cycle

  6. The Water Budget? • The Water Budget is an accounting of all water on a local scale i.e. local hydrologic cycle

  7. Hypothesis • Can the Soil Water Budget model can be used as an indicator to show soil water content in the soil where such data is not available?

  8. The Water Budget Equation Accounting of Input, Output & Storage P = AE + S + ST P = Incoming Precipitation AE = Actual Evaporation S = Surplus or Leftover water ST Soil Moisture Recharge

  9. What We Need To Know? • To find soil water content we need to know how much was evaporated and how much ran-off. • The leftover is what is left in the soil

  10. How Much is Lost to Evaporation? • Calculate it • Evaporation depends on the amount of heat (i.e. temperature) • Use temperature and latitude (duration of daylight) to find the maximum possible evaporation (Potential Evapotranspiration)

  11. How Much Ran-off? • We use field capacity? • How much can a soil hold? • Once it reaches it maximum the rest is run-off

  12. Field Capacity Examples • Sand 12* • Loamy Sand 14 • Sandy Loam 23 • Loam 26 • Silt Loam 30 • Silt 32 • Silty Clay Loam 34 • Silty Clay 36 • Clay 36 *Cm/Meter of Soil Source: ASCE, 1990, Table 2.6 p.21

  13. What We Know • The Amount of precipitation • Calculate the potential evapotranspiration • Calculate the run-off • Horizon depth/field capacity • With the above and a formula we can calculate the soil water content

  14. The Soil Water Budget

  15. Research Project

  16. Defining Time & Space • For 1996-1998 • 3 Years of soil moisture data • 3 Years of temperature data • 3 Years of precipitation data • Spatially • Different water budget regimes • Dry < 50 cm/year • Wet > 90 cm/year

  17. Methodology • Use Modeling Your Water Balance • Gather Soil Moisture Content Data • Compare the Above • Visually • Statistical Test of Correlation • Statistical Test of Significance

  18. The Coefficient of Correlation • Index of Relationship • Low values indicate a low degree of relationship • High values indicate a high degree of relationship • Scale runs from –1 - 0 - +1 • Does not indicate causality

  19. Assumptions for Pearson’s r • Distributions are unimodal • Distributions are asymmetrical • Violations do not affect r very much Havlicek & Peterson, Psychological Bulletin 84 • Very Robust

  20. Frequency Distributions

  21. Assumptions for Spearman’s r • Distribution Free • Well – tried alternative • Approximates the Pearson Product Moment • Quick & Easy to calculate

  22. Is This Significant? • What is the probability the result is a result of a chance association? • Significant Values for 24 Degrees of Freedom • .05 - .343 • .001 - .485 • At 99.9% confidence level we can state that result is not a result of a chance association

  23. Conclusion • The soil water budget model can be used as an indicator to show soil water content in the soil where such data is not available

  24. Further Research • Expand your spatial sampling • Experiment with Field Capacity values • Investigate the correlations when runoff is high • Further test correlations with ‘extreme’ samples

  25. Further Research • Permanent Wilting Point (PWP) • Plants cannot get to the water • Evaporation will not take place • Large in clayey soils • Depth of soil for field capacity • (Field capacity - PWP * Thickness of Horizon)

  26. What Your Student Learn • Earth Systems • Math • Statistics • Spreadsheet Skills • Modeling • Graphing • Critical Thinking Skills

  27. Applications of the Water Budget • Estimate Soil Moisture Content • Calculate irrigation needs • Which Sands/Clays can support traffic • Calculate Evapotranspiration • Essential in estimating potential vegetation types • Essential for estimating crop yields • Evaluate Local Environments • Water resource potential • Climatic classification

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