Aggregate stability and rain-impacted sheet erosion of air-dried and prewetted clayey surface soils under intense rain

Stability of aggregates at the immediate soil surface affects the detachment, transport, and deposition of soil particles. The aggregate stability (expressed as mean weight diameter, MWD) and erosion rate were tested for clayey surface soils at two different water contents. The soils consisted of six Oxisols, one Ultisol, three Vertisols, one Alfisol, and one Aridisol. The MWD was determined by wet sieving of 4.76 to 8.00 aggregates, which were either air-dried (MWDd) or capillary prewetted at −0.5 kPa for 24 h (MNDw). Air-dried and prewetted (saturated from the bottom up for 2 h) soils were subjected to a 110-mm h rain until a steady-state infiltration was reached. Runoff, sediment loss, and infiltration (for prewetted soil only) were monitored throughout the rain. The MWDd ranged from 0.18 to 4.80 mm, and the MWDw ranged from 0.44 to 5.66 mm. Prewetting increased the MWD for the oxidic or kaolinitic Oxisols and Ultisols and for one smectitc/kaolinitic Vertisol. The MWDw was positively related to CEC for highly weathered soils and negatively related for swelling soils, whereas base saturation was negatively related for both groups of soils. The ΔMWD, which expresses the increase in stability caused primarily by a decrease in slaking, was greatest for soils rich in kaolinite and Fe and Al oxyhydroxides and low in organic carbon. The steady-state infiltration rate (Is) ranged from 1.8 (illitic soil) to 54.8 mm. h (sandy loam kaolinitic soil). The Is was consistently less for swelling than for highly weathered soils, and it was related to the sand/clay ratio, sand, and water retained between −33 and −1500 kPa. The increased aggregate stability with prewetting was generally not reflected in smaller amounts of erosion and runoff, possibly because of the faster rate of wetting than for the stability test, a higher water table for the prewetted soil than for the air-dried soil, and the inclusion of smaller aggregates in the erosion tests. Prewetting decreased the time to ponding for three highly weathered Oxisols and the illitic Alfisol, increased steady-state runoff rate (Qw) for five highly weathered soils, increased sediment loss rate (Qs) for the illitic and for one highly weathered soil, and increased Qs on a stable Oxisol. Soil textural properties (usually clay and sand/clay) and “binding” agents (organic carbon and Fe and Al oxyhydroxides) were the significant independent variables included in the stepwise multiple linear regressions for Is, Qs, and Qw