We consider steady state unsaturated flow in bounded, randomly heterogeneous soils under the influence of random boundary and source terms. Our aim is to predict pressure heads and fluxes without resorting to Monte Carlo simulation, upscaling, or linearization of the constitutive relationship between unsaturated hydraulic conductivity and pressure head. We represent this relationship through Gardner’s exponential model while treating its exponent, alpha, as a random constant and saturated hydraulic conductivity, K_s, as a spatially correlated random field. We linearize the steady state unsaturated flow equations by means of the Kirchhoff transformation and integrate them in probability space to obtain exact integro-differential equations for the conditional mean and variance-covariance of transformed pressure head and flux. After approximating these equations recursively to second order in the standard deviation, S_Y, of Y = ln K_s, we solve them by finite elements for superimposed mean uniform and divergent flows in the vertical plane, with and without conditioning on measured Y values. Comparison with Monte Carlo solutions demonstrates that whereas our nonlocal solution is nominally restricted to mildly nonuniform media with S_Y << 1, it yields remarkably accurate results for strongly nonuniform media. This accords well with a previous theoretical analysis, which shows that the solution may remain asymptotic for values of log-conductivity variance as large as 2.
Conditional moment analysis of steady state unsaturated flow in bounded, randomly heterogeneous soils
GUADAGNINI, ALBERTO;
2002-01-01
Abstract
We consider steady state unsaturated flow in bounded, randomly heterogeneous soils under the influence of random boundary and source terms. Our aim is to predict pressure heads and fluxes without resorting to Monte Carlo simulation, upscaling, or linearization of the constitutive relationship between unsaturated hydraulic conductivity and pressure head. We represent this relationship through Gardner’s exponential model while treating its exponent, alpha, as a random constant and saturated hydraulic conductivity, K_s, as a spatially correlated random field. We linearize the steady state unsaturated flow equations by means of the Kirchhoff transformation and integrate them in probability space to obtain exact integro-differential equations for the conditional mean and variance-covariance of transformed pressure head and flux. After approximating these equations recursively to second order in the standard deviation, S_Y, of Y = ln K_s, we solve them by finite elements for superimposed mean uniform and divergent flows in the vertical plane, with and without conditioning on measured Y values. Comparison with Monte Carlo solutions demonstrates that whereas our nonlocal solution is nominally restricted to mildly nonuniform media with S_Y << 1, it yields remarkably accurate results for strongly nonuniform media. This accords well with a previous theoretical analysis, which shows that the solution may remain asymptotic for values of log-conductivity variance as large as 2.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
