Spatial variation of soil hydrologic parameters.

Abstract

The objectives of this study were to determine the variance of selected soil properties within a soil mapping unit, to examine variance components, to correlate a pore size distribution index with other measured parameters, and to test the sensitivity of calculated unsaturated hydraulic conductivity to variation in the pore size distribution. The study area was 40 hectares, all mapped as the Pima clay loam soil type, on the University of Arizona Experimental Farm at Marana, Arizona. The soil was sampled randomly using a hierarchal scheme at a depth of 50 cm. Electrical conductivity of the soil extract, sodium adsorption ratio, bulk density, texture, and pore size distribution were determined for each sample. The results indicated that there was significant variation for all parameters among fields, except electrical conductivity and sodium adsorption ratio. A large number of samples, 40 to 60, was needed to estimate the mean within 10%. The pore size distribution index correlated Significantly with bulk density but had poor correlation with other parameters. The Pima clay loam is alluvial in origin and stratified in nature which may explain the significant variation and poor correlation of the measured parameters. The calculated unsaturated hydraulic conductivity for the Pima clay loam varied from 0.0008 cm/hr to 0.0011 cm/hr at a moisture content of 30% and from 0.320 cm/hr to 0.500 cm/hr at 50% moisture based on a variability of one standard deviation on either side of the average moisture characteristic curve. Application of elaborate simulation models of water movement in soils may be limited by variability of soil properties and the variance determined in this study will help predict the sources of variability for models in the future.