STEADY-STATE INFILTRATION AS A FUNCTION OF MEASUREMENT SCALE

Steady-state infiltration rates were measured at three instrument scal es within a 4.0 x 4.0-m field plot. The three scales were 4.0 x 4.0 m (scale S(L), n = 1), 1.0 x 1.0 m (scale S(L/4), n = 16), and 0.25 x 0. 25 m (wale S(L/16), n = 256). After a 30-day ponding period under a co nstant hydraulic head, infiltration measurements were made at each mea surement scale. Even though the entire area was sampled at each instru ment scale, the average infiltration rate decreased with decreasing si ze 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 r ates of 1.96 cm/h for each of the smaller measurement scales. This val ue was in excellent agreement with the final value measured at scale S (L) upon completion of the experiment (1.97 cm/h). However, the value for the 'apparent' stagnation zone was found to be valid only in an av erage sense, because the correlation between the adjusted average valu e of the S(L/16) measurements and the adjusted value of the S(L/4) mea surements was not significant. The 256 measurements at scale S(L/16) w ere neither normal nor lognormally (natural log (ln)) distributed, alt hough the latter. provided a somewhat better representation. Too few m easurements were available to determine the probability distribution f or scale S(L/4). The regularized semivariogram (range of 0.7 m) for sc ale S(L/16) was deconvoluted to provide estimates of the point semivar iograms for both the actual and adjusted measurement scales, leading t o spatial ranges of 0.21 and 0.30 m for actual and adjusted, respectiv ely. With or without adjustment, dispersion variance analyses illustra ted that the spatial structure estimate obtained from measurement scal e S(L/16) was inconsistent with that obtained from measurement scale S (L/4). Thus, we conclude that the infiltrometer instrument fundamental ly alters the infiltration process in such a way that measurements are only meaningful in a relative sense.