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Determining representative infiltration rate parameters for use in modeling field‐scale flow and transport processes is difficult because of the spatial variability of soil properties. To determine how steady‐state infiltration rate variability is affected by support scale, steady‐state infiltration rates (Is) were measured at three spatial scales (local, hillslope, and landscape) along a 710‐m transect on a swell–swale landscape in Indiana. Spatial variability at the local scale was studied using measurements in a 1 × 1 m2 array of 100 ring infiltrometers (7.2‐cm diam.) for three soils at three horizons each. Studies were conducted at the hillslope and landscape scales using three nested infiltrometers of sizes 20 × 20, 60 × 60, and 100 × 100 cm2 Geostatistical analyses show a decrease in the sample variance of the Is values and an increase in spatial correlation of Is with depth. They also suggest that an area >10, 7.2‐cm diam. rings (i.e., approximately >400 cm2) is needed to provide a representative measurement area (RMA; i.e., area needed to filter out smaller‐scale heterogeneities) at the local scale. Hillslope‐ and landscape‐scale tests indicate that Is measurements with infiltrometers require an infiltrometer with a support area greater than the local‐scale RMA to show the spatial correlation of the larger scales. In addition, these infiltrometer measurements may not provide appropriate effective Is estimates at these greater scales unless they are averaged over a domain that extends across the landscape's range of variability, estimated from the computed semivariograms to be 120 to 200 m for this study.

or matric suction head), both r and h are used interchangeably in this discussion as an indication of relative Scaling of soil hydraulic properties is a convenient way to characterpore size. A positive sign for h indicates that it repreize soil variability in a unified manner. Scaling techniques implicitly sents suction. Using Eq. [1], the scaling factor for the assume geometric similitude for the soils being scaled; however, the meaning of similar media is somewhat ambiguous. This study focuses ith soil (␣ i ) is defined as on the question of how to define geometric similarity. This questionis addressed using the physically based scaling (PBS) technique proposed by Kosugi and Hopmans to coalesce 247 soil water retentionThe scaling factor approach provides a convenient curves (WRCs) measured from soil cores collected across eight differway to coalesce multiple WRCs into a single reference ent counties in Indiana, USA. These soil cores represented 29 different WRC. Alternatively, when scaling factors are known, soil series and included seven broad textural classes. Although all the Eq. [1] and [2] provide a convenient way to estimate 247 WRCs could be scaled together, the RMSEs of the scaling results unscaled capillary pressure heads from the reference were improved when WRCs within each textural group were scaled capillary pressure head values. Since the introduction separately. Thus, soil texture may be used as a preliminary guide to of similar media concepts by Miller and Miller (1956), group similar soils. This study shows that the sample standard deviavarious scaling techniques have been proposed to scale tions of the pore-size distribution should be used to quantify geometric similarity among soils. Specifically, the coefficient of variation among soil hydraulic properties (Warrick et al., 1977; Simmons standard deviations (CV ) for selected WRCs may be used to demaret al., 1979; Rao et al., 1983; Ahuja and Williams, 1991; cate the limit of similarity among soils. Our results show that the Claustnizer et al., 1992; Pachepsky et al., 1995; Kosugi CV Յ 10% may be used as a working definition for similarity in soils.and Hopmans, 1998). Most approaches compute scaling factors by minimizing the mean sum-of-squared-deviations between the observed and reference soil water Abbreviations: K-S, Kolmogorov-Smirnov; LNR, lognormal reten-