SPATIAL VARIABILITY OF HYDRAULIC CONDUCTIVITY IN A CULTIVATED FIELD AT DIFFERENT TIMES

Farming practices and climatic patterns may alter soil surface hydraul ic conductivity (K) both spatially and at different times during a sea son. This study quantified spatial variability and spatial dependence of ponded and tension K measurements in a cultivated held at different times. Paired small-base infiltrometers (76-mm diameter) were used at four pressure heads (5, -30, -60, and -150 mm) to measure K across a transect four times during one and a half growing seasons (1-2 July 19 91, 12-13 Aug. 1991, 29-30 Apr. 1992, and 28-29 May 1992). A natural l og transformation best normalized ponded K data, but In(K + 1) was bet ter for the tension data. Periodicity was apparent for K at a head (h) of -150 mm, with a significant period of 46 m for July 1991, 147 m fo r April 1992, and 96 m for May 1992. The periods for the three dates m ay be three different harmonics. The July periodicity was consistent w ith expected wheel traffic patterns from the previous harvest; K measu rements would have coincided with wheel tracks every 46 m for the July 1991 measurement date. Periodicity for other dates had no apparent ca use. For ponded K, only within-pair data were spatially correlated (0. 6-0.8 m). At a head of -150 mm, K values were spatially correlated ove r distances of 6.6, 16.8, and 0.6 m for the measurements of July 1991, April 1992, and May 1992, respectively. Correlation distances were in termediate for K--30 and K--60. Since spatial correlation and periodic ity varied with measurement date, one set of transect measurements was not adequate to describe spatial variability of K.