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Bias in Ponded Infiltration Estimates Due to Sample Volume and Shape

Soil Scientist, USDA-ARS, Columbia Plateau Conservation Research Center, P.O. Box 370, Pendleton OR 97801

View larger version (21K): [in a new window]   Fig. 1. Intake curves for one winter wheat–fallow plot (upper graph) and one no-till pea–winter wheat plot (lower graph). In both plots two cylinders of each diameter were measured for 4 h, at which time the wetting front was well below the bottom of the cylinders. The lines were plotted as straight segments between data points without smoothing or averaging. Infiltration rates in this experiment were measured between 0.5 and 1 h.

View larger version (19K): [in a new window]   Fig. 2. Ponded quasi-steady state infiltration rates measured using cylinders of 20-, 30-, and 45-cm diameter (306-, 705-, and 1596-cm2 cross-sectional area) driven 25 cm deep. The three measurements were made side by side in 56 research plots (n = 56 for each diameter) with treatments ranging from seedling winter wheat planted into plowed summer fallow to wheat stubble in a no-till, pea–winter wheat rotation. Box plots are shown to the left of each data set.

View larger version (15K): [in a new window]   Fig. 3. Box plots of water infiltration data divided into treatment and crop status conditions producing high and low infiltration rates. Cylinders with diameters of 20, 30, and 45 cm were used in side-by-side measurements under diverse soil management practices. Error bars to right of box plots show upper and lower confidence limits at 10% calculated using the uniformly minimum variance unbiased estimator method of Land (Parkin and Robinson, 1992).

View larger version (111K): [in a new window]   Fig. 4. Photos of cross sections from plots with high and low infiltration rates. The blue dye (not visible in black-and-white copies of this figure) was added to the ponded water for 10 min before the water was removed. The 31 mm h–1 example was a 20-cm diameter cylinder in a plowed treatment in wheat stubble. The 112 mm h–1 example was a 30-cm cylinder in a no-till treatment in seedling winter wheat.

View larger version (30K): [in a new window]   Fig. 5. Infiltration rate changes as cylinders are driven deeper into the soil in the concentric cylinder experiment. Numbers indicate the depth (centimeters) of the bottom of the cylinder at the time an infiltration rate was determined. The 30-cm cylinders were single. The 20- and 45-cm cylinders were concentric, as in a standard double-ring infiltration test.

View larger version (20K): [in a new window]   Fig. 6. Infiltration rate of 45-cm diameter cylinders driven 25 cm deep, with and without a sheet-metal divider gently inserted after quasi-steady state ponded infiltration was attained. Arrows show timing when the divider depth (cm) was increased. The vertical divider almost completely divided the soil in the cylinders across a diameter.