Comparison of Soil Wetness Indices for Inducing Functional Similarity of Hydrologic Response across Sites in Illinois

Abstract

The comparative ability of four soil wetness indices to normalize soil moisture dependence of rootzone fluxes across a range of sites in Illinois is investigated. The soil wetness indices examined are various transformations of the water stored in the top 30 cm of soil (W). The wetness index parameters used in the transformations include porosity, wilting point, and the maximum observed, mean, and standard deviation of W. The long-time average dependence of rootzone outflow (the sum of evapotranspiration, drainage, and runoff) on each index is measured using a new estimation technique based on conditional averaging of precipitation according to soil moisture level. For each soil wetness index, the dependence of mean outflow on that index is estimated at 17 sites and then averaged. The variance around the average curve is used as a measure of the ability of each index to induce functional similarity. Soil wetness indices based on statistical properties of soil moisture data (e.g., maximum observed, means, and variances) are found to be more successful at inducing functional similarity than are indices based on independent soil properties (e.g., porosity) that did not reduce the variance of moisture-outflow data below the baseline variance found using W as the metric of soil wetness. Indices based on the statistical properties of the data are found to be preferable both to untransformed soil moisture observations and to indices based on soil properties for normalizing spatially varying soil moisture?flux relations.