| Occurrence of agricultural chemicals leaching to ground water at rates faster than expected has raised concerns. In spite of prior research, there are important factors involved in water and chemical transport through the vadose zone which we do not understand. One factor that may lead to rapid transport of chemical to ground water is preferential flow of water and solute in soil. Traditional theory of uniform water movement in homogeneous and isotropic soil is facing serious challenges in field applications. Most field soils are structured to some extent and contain various types of macropores. To better understand the nature of water flow in field soils, relationships between soil hydraulic properties and morphological features were sought in 18 soils in the Claypan Area, Blackland Prairies, and Coast Prairie Major Land Resource Areas of Texas. Dye-tracing study in two Vertisols first demonstrated the importance of morphological features in characterizing preferential flow in structured soils. It was found that, when water was supplied at tensions {dollar}<{dollar}24 cm, infiltration first proceeded into the soil through slickenside fissures, root channels, living roots, cracks/fissures, and interpedal pores, then diffused into the soil matrix. Stained water flow paths showed that water often moved several times deeper than distance explained by classical theory of water movement in soil. Effective porosity in structured soils under flow at tensions {dollar}<{dollar}24 cm was usually {dollar}<{dollar}5-10% of a soil's total porosity, but the fraction of total water flux at 0 cm tension contributed by macropores (active at {dollar}le{dollar}3 cm tension) was 76% {dollar}pm{dollar} 18% (mean {dollar}pm{dollar} standard deviation) in the 97 soil horizons investigated. In comparison, micropores (active at {dollar}>{dollar}24 cm tension), which often dominated a soil's total porosity, usually contributed {dollar}<{dollar}10% to the total water flux. A two-line regression model, with a break-point at 3 cm tension, generally fit paired (lnQ, h) data, where Q is apparent steady flux at supplied tension h. The slope of each (lnQ, h) curve at tension intervals of 0 to 3 cm and 3 to 24 cm indicated the degree or tendency of preferential flow along macropores and mesopores, respectively. These slopes are named macropore flow index and mesopore flow index, respectively. Soil morphological features, including texture, pedality, macroporosity, initial moisture, and living roots in soil were shown to be indicative of soil hydraulic properties and their spatial and temporal dynamics. Empirical equations were established in this study that linked quantified soil morphological properties to hydraulic parameters (hydraulic pedotransfer functions). Such quantitative technical translation from soil morphology to hydraulic properties would provide access to large existing databases for the derivation of water and chemical transport parameters needed in models. Finally, this study also documented spatial and temporal variability and depth function of hydraulic properties in some major Texas soils. The information gathered through this study would assist critical linkage for evaluation of soil resources relative to nonpoint and point source ground water and surface water pollution. |
