Rapid Measurement of FieldSaturated Hydraulic Conductivity for Areal Characterization<link href="#vzj2vzj20070159-note-0001"/>

142 F hydraulic conductivity (Kfs) and its variation over land surfaces are important to pedologic, hydrologic, and ecologic problems that require knowledge of soil moisture or of the partitioning of precipitation into infi ltration, overland fl ow, and evapotranspiration. Th e nature and variability of this partitioning aff ect diverse problems such as slope stability, soil erosion rates, and the ecology of water-limited landscapes. Field-saturated hydraulic conductivity is the hydraulic conductivity of the soil when it has been brought to a near-saturated state by water applied abundantly at the land surface, typically by processes such as ponded infi ltration or copious rainfall or irrigation. Th is type of wetting normally traps air in a signifi cant fraction of the pores, both large and small. Because the conductance of channels with trapped air is much less than it would be if the channels were completely water-saturated, Kfs is less than the conductivity of a totally saturated state attainable by artifi cial means. Contributions to the value of Kfs are often dominated by the largest water-containing pores, commonly including macropores such as interaggregate cracks, wormholes, rootholes, and burrows. Th ough repeatability is an issue because diff erent episodes of wetting may produce diff erent microscale arrangements of pore water and trapped air, in this paper we assume that for practical purposes there is a repeatable Kfs that ideally is characteristic of the soil and independent of the measurement method. Various types of infi ltrometers are used to measure Kfs at the land surface (Angulo-Jaramillo et al., 2000; Reynolds et al., 2002b); they diff er in attributes such as the time, eff ort, and specialized equipment required, the volume and degree of wetting of the soil whose average Kfs the measurement represents, and the measurement uncertainty in the results. One general class applies water directly to the soil surface at known positive pressure, typically by means of a singleor double-ring infi ltrometer (e.g., Bouwer, 1966; Youngs, 1987). Another, implemented as various versions of tension infi ltrometer, applies water at negative pressure through a membrane in contact with the soil (e.g., Ankeny et al., 1988; Perroux and White, 1988). Another type that has recently come into some use is the hood infi ltrometer, which can supply water at positive or negative pressure directly to the soil surface (Schwärzel and Punzel, 2007). Other methods using instrumentation in boreholes (e.g., Bouwer, 1961; Reynolds et al., 1983), or laboratory measurement on retrieved core samples (e.g., Reeve, 1957; Klute and Dirksen, 1986), can give an indication of Kfs at depth. In this paper, however, our concern is surface-applied methods, which may relate most directly to infi ltration capacities and runoff . Arguments have been put forward for diff erent modes of application producing a Kfs value most representative of the soil-water conditions of interest, a matter which depends on the Rapid Measurement of Field-Saturated Hydraulic Conduc vity for Areal Characteriza on