Site-Specific Management of Flow and Transport in Homogeneous and Structured Soils

Abstract

Among research publications in soil science, few have had a greater impact than those by Nielsen et al. (1973) or Biggar and Nielsen (1976). According to Science Citation Index, the former paper, entitled “Spatial variability of field-measured soilwater properties,” has been cited by scientific peers over 390 times. The 1976 paper, entitled “Spatial variability of the leaching characteristics of a field soil,” has been cited over 232 times. Experimental work presented in both papers represents the first-ever attempt at a large field-scale study of steady-state water and solute transport (Wagenet, 1986). Among the seminal findings of these two papers were as follows: (1) extensive spatial variability existed in soil hydraulic and solute transport properties within a relatively homogeneous field (important in the work of Pilgrim et al., 1982; Addiscott and Wagenet, 1985; Feddes et al., 1988; van dcr Molen and van Ommen, 1988); (2) soil water content, bulk density, and soil particle size exhibited normal frequency distributions, while distributions for hydraulic conductivity, hydraulic diffusivity, pore water velocity, and hydrodynamic dispersion were lognormal (work extended by van der Pol et al.. 1977; Rao et al., 1979); (3) frequency distributions were far superior to field-average parameter values (especially for lognormally distributed properties) in describing field transport behavior (demonstrated by Rao et al., 1979; Trangmar et al., 1985); (4) a simple unit hydraulic gradient method was shown to estimate saturated hydraulic conductivity accurately (results extended by Libardi et al., 1980; van Genuchten and Leij, 1992); (5) good correspondence was found between solute velocity and pore water velocity (key assumption in Jury and Fluhler, 1992); (6) and theoretical predictions of a linear relation between hydrodynamic dispersion and pore water velocity were shown to be obeyed at the field scale (result used widely by solute transport modelers, as discussed in Nielsen et al., 1986). The seminal works by Nielsen et al. (1973) and Biggar and Nielsen (1976) produced several new directions in soil science and vadose zone hydrology research. The most interesting was a series of papers that rejected the theoretical basis and practicality of using deterministic equations, and instead introduced stochastic approaches to describe field-scale water and solute fluxes.