Mark D. Ankeny

Spatial analysis of hydraulic conductivity measured using disc infiltrometers

Spatial variability of surface hydraulic properties and the extrinsic (e.g., traffic, cropping, etc.) and intrinsic (e.g., soil type, pore size distribution, etc.) factors associated with these properties are important for infiltration and runoff processes in agricultural fields. Disc infiltrometers measured infiltration at 296 sites arranged on two parallel transects. To examine and differentiate the factors contributing to spatial structure under different field conditions these measurements were made in the corn rows, no-track interrows, and wheel track interrows of the field using four different soil water tensions * (0, 30, 60, and 150 mm). Unsaturated hydraulic conductivity (K) and saturated hydraulic conductivity (K,) were maximum in the corn rows and minimum in wheel track inter-rows, with no-track interrows intermediate. Exponents (CZ parameters) of K, and K relationships (K = K, exp -aV) for corn rows and no-track interrows were not significantly different from each other but were significantly different from Q for the wheel track interrows a t P = 0.01 level. Spatial variability o f K and K, values showed some pseudoproportional effect in nugget variance for all three field conditions. No-track interrows clearly showed an inverse trend for semivariogram o f K with changing tension (q) values, whereas differences were found for corn rows and wheel traffic interrows. The spatial structure of (Y for all three field conditions were mostly white noise. Under corn rows, in addition to random variation, a small five-row periodic variation at th e P = 0.20 level, matching the five-row traffic configuration, was discovered. The spatial structure of a was influenced by soil type for the no-track interrows. Spatial structure was absent in wheel track interrows, indicating the destruction of pore structure due to compaction.

Infiltration and macroporosity under a row crop agricultural field in a glacial till soil

Previous field-scale infiltration studies showed difference in the magnitude and the trend of spatial variation of infiltration rates under different soil water tensions. In different studies the differences in infiltration rates are caused by management practices, relative field positions, and soil and topographic setups, hence warranting further site-specific infiltration studies. In this study, variability in infiltration rate (I ψ ) at four soil water pressure heads, ψ, were investigated in a no-tillage agricultural field under corn rows, nontrafficked interrows, and trafficked interrows in a central Iowan glacial till soil. Automated disc infiltrometers were used to measure infiltration at 0-, 30-, 60-, and 150-mm tensions at 296 sites arranged on two parallel transects perpendicular to corn rows. Mean infiltration rates at different soil water tensions were found maximum under corn row, minimum for trafficked interrow, and intermediate for nontrafficked interrow positions. Maximum variability was found for larger pores (those conducing water at 0-mm tension) under all three surface positions (corn row, CV = 85% ; trafficked interrow, CV = 95% ; nontrafficked interrow, CV = 124%). Infiltration at saturation (0-mm tension) showed a different scale of heterogeneity than infiltration at other (30-, 60-, and 150-mm) tensions, and approximately 90% of the saturated flux moves through macropores (>1-mm diameter) that constitute less than 3% of the total surface area at three field positions. Spatial analysis of I ψ indicated a larger proportion of random variations under all three field positions in the glacial till soil. In addition to the large random noise, a small spatial structure of 7.6 to 11.4-m range was found for I ψ (at all four tensions) under corn row position, and only for I 150 under (nontrafficked and trafficked) interrow positions.