Field infiltration measurements in grassed roadside drainage ditches: Spatial and temporal variability

Roadside drainage ditches (grassed swales) are an attractive stormwater control measure (SCM) since they can reduce runoff volume by infiltrating water into the soil, filter sediments and associated pollutants out of the water, and settle solids onto the soil surface. In this study a total of 722 infiltration measurements were collected in five swales located in Twin-Cities, MN and one swale located in Madison, WI to characterize the field-saturated hydraulic conductivity (K_fs) derived from the infiltration measurements of these swales. Measurements were taken with a falling head device, the Modified Philip Dunne (MPD) infiltrometer, which allows the collection of simultaneous infiltration measurements at multiple locations with several infiltrometers. Field-saturated hydraulic conductivity was higher than expected for different soil texture classes. We hypothesize that this is due to plant roots creating macropores that break up the soil for infiltration. Statistical analysis was performed on the K_fs values to analyze the effect of initial soil moisture content, season, soil texture class and distance in downstream direction on the geometric mean K_fs value of a swale. Because of the high spatial variation of K_fs in the same swale no effect of initial soil moisture content, season and soil texture class was observed on the geometric mean K_fs value. But the distance in downstream direction may have positive or negative effect on the K_fs value. An uncertainty analysis on the K_fs value indicated that approximately twenty infiltration measurements is the minimum number to obtain a representative geometric mean K_fs value of a swale that is less than 350 m long within an acceptable level of uncertainty.