Spatial Variability and Measurement Scale of Infiltration Rate on an Agricultural Landscape

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 (I s ) 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 m 2 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 cm 2 Geostatistical analyses show a decrease in the sample variance of the I s values and an increase in spatial correlation of I s with depth. They also suggest that an area >10, 7.2‐cm diam. rings (i.e., approximately >400 cm 2 ) 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 I s 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 I s 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.