INFILTRATION AND MACROPOROSITY UNDER A ROW CROP AGRICULTURAL FIELD IN A GLACIAL TILL SOIL 1

Abstract

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 (IΨ) 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 IΨ 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 IΨ (at all four tensions) under corn row position, and only for I150 under (nontrafficked and trafficked) interrow positions.