| Abstrakt: |
Steady-state infiltration rates were measured at three instrument scales within a 4.0 x 4.0-m field plot. The three scales were 4.0 x 4.0 m (scale SL, n= 1), 1.0 x 1.0 m (scale SL/4, n= 16), and 0.25 x 0.25 m (scale SL/16, n= 256). After a 30-day ponding period under a constant hydraulic head, infiltration measurements were made at each measurement scale. Even though the entire area was sampled at each instrument scale, the average infiltration rate decreased with decreasing size of infiltrometer. The infiltration rate measurements were adjusted assuming an “apparent” stagnation zone of 4 cm along the boundaries of each infiltrometer. This adjustment produced average infiltration rates of 1.96 cm/h for each of the smaller measurement scales. This value was in excellent agreement with the final value measured at scale SLupon completion of the experiment (1.97 cm/h). However, the value for the “apparent” stagnation zone was found to be valid only in an average sense, because the correlation between the adjusted average value of the SL/16measurements and the adjusted value of the SL/4measurements was not significant. The 256 measurements at scale SL/16were neither normal nor lognormally (natural log (ln)) distributed, although the latter provided a somewhat better representation. Too few measurements were available to determine the probability distribution for scale SL/4. The regularized semivariogram (range of 0.7 m) for scale SL/16was decon-voluted to provide estimates of the point semivariograms for both the actual and adjusted measurement scales, leading to spatial ranges of 0.21 and 0.30 m for actual and adjusted, respectively. With or without adjustment, dispersion variance analyses illustrated that the spatial structure estimate obtained from measurement scale SL/16was inconsistent with that obtained from measurement scale SL/4. Thus, we conclude that the infiltrometer instrument fundamentally alters the infiltration process in such a way that measurements are only meaningful in a relative sense. |
