A new two‐step stochastic modeling approach: Application to water transport in a spatially variable unsaturated soil

A new two‐step stochastic modeling approach based on stochastic parameter inputs to a deterministic model system is presented. Step I combines a Stratified sampling scheme with a deterministic model to establish a deterministic response surface (DRS). Step II combines a Monte Carlo sampling scheme with the DRS to establish the stochastic model response. The new two‐step approach is demonstrated on a one‐dimensional unsaturated water flow problem at field scale with a dynamic surface flux and two spatially variable and interdependent parameters: The Campbell [1974] soil water retention parameter (b) and the saturated hydraulic conductivity (Ks). The new two‐step stochastic modeling approach provides a highly time efficient way to analyze consequences of uncertainties in stochastic parameter input at field scale. The new two‐step approach is competitive in analyzing problems with time consuming deterministic model runs where the stochastic problem can be adequately described by up to two spatially variable parameters.