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Quantifying Rates of Soil Water Evaporation: Implementing a Soil Water Oxygen Isotope Model in Matlab Erik Oerter EPS 209 27 April 2011

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Project Motivation: Soil Water Oxygen Isotopes Soil Water originates as meteoric water, preserving the isotopic signature of precipitation Hydrogen and Oxygen isotopes undergo Rayleigh fractionation as water evaporates Enrichment in 18 O relative to precipitation The 18 O/ 16 O can be sampled: Directly as soil water (contemporary water) Indirectly as CaCO 3 (ancient water) Use 18 O/ 16 O to estimate evaporation rate Climate Proxy

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Evaporation Rate via Soil Water Oxygen Isotopes 18 O/ 16 O of water at z in soil profile Depth in soil profile Evap. Rate Depth of evap. front Diff coef. of water in soil 18 O/ 16 O of water at depth 18 O/ 16 O of water at evap. front (Barnes and Allison, 1988, J. of Hydrology) L = Total depth of soil profile

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Project Idea: Fit modeled Oxygen isotope profile to observed data and find the evaporation rate Model 18 O profile Observed 18 O profile Data from Wang et al., 1996

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Project Approach: m-file Sequence of Events 1)Load in observed data as text file 2)Initialize variables - Make an initial guess for evaporation rate - Populate a vector with evap guesses (~100) Specify the step between evap guesses - Initialize a matrix for calculated 18 O profiles (# depth increments x # evap guesses) 3)Loop over all evap guesses, calculating 18 O profiles - Now have 100 18 O profiles…

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Project Approach: m-file Sequence of Events

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Some Results Diffusion Coefficient of Water in Soil = 1.5*10 -10 m 2 /sec Starting Evap Rate Guess = 5x10 -11 m/sec (0.15 cm/year) E guess step = 1*10 -12 m/sec Best fit Evap rate = 1.02*10 -10 m/sec (0.32 cm/year)

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Future Work This interpretation of the Barnes and Allison Model: - Single wet soil zone with the evaporating front at the surface Barnes and Allison envisioned a dry zone of soil overlying the wet zone and evaporating front at some depth Implement the two zone scheme - Allow for a migrating (+/- depth) evaporation front Allow for resetting of the 18 O input value (Allow it to rain on the soil)

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