A Monte Carlo Analysis of the hydrologic effects of spatial variability of infiltration

Abstract

Monte Carlo simulation is employed to analyze the effects of random distribution of soil infiltration properties on the hydrologic performance of catchment areas. Soil hydraulic parameters are distributed with log normal distribution as reported from previous field measurements. Although independent sampling units are assumed, a discussion is presented on the nature and effect of spatial dependence. The composite infiltration rate of an area is simply the average, at an instant in time, of the infiltration rate of all points within that area. Rigorous treatment of the topic is a forbidding problem in stochastic differential equations. Monte Carlo simulation is used here to study the effects of random distribution of soil properties on distribution of ponding time, composite areal rate, and bias due to variability in space. More accurate treatment for watershed studies must also consider interaction of various points on the surface by way of overland flow paths. A distributed watershed simulation model is used for this purpose to demonstrate hydrograph bias due to deterministic spatial variability, and from random variation, plus the relation of bias to rainfall rate. An analysis is also made of the partial area contribution in early parts of storm runoff caused solely by random variation in surface soil properties.