PROCESS-BASED ANALYSIS OF AGGREGATE STABILITY EFFECTS ON SEALING, INFILTRATION, AND INTERRILL EROSION

Aggregate stability indices are correlated with soil erosion rates. Al though aggregate stability influences soil erodibility and aggregate b reakdown product size distributions, these indices are usually unsatis factory because of interacting physicochemical considerations affectin g infiltration and erosion. We examined the spatial variability in str uctural and depositional seal composition, and the influence of breakd own fragment size distribution on seal hydraulic resistance and interr ill erosion. Two Alfisols, Villamblain clay loam and Blosseville silt loam, were sieved to retain <2.0-cm aggregates and packed in 0.25-m(2) soil trays. Soil trays were rained upon for 4 h at 23 mm h(-1). Infil tration, runoff, and subseal pressure head were measured throughout th e experiment, and surface roughness and depositional seal area were me asured at selected times during the event. Aggregate stability, hydrau lic resistance, and fragment size distribution of the seal matrix were also determined. Mean weight diameter (MWD) of aggregates after mecha nical breakdown by stirring was greater for Villamblain (1.70 mm) than for Blosseville (0.92 mm) (r(2) = 0.94, P < 0.001). Blosseville seals had finer fragments than those of Villamblain (r(2) = 0.85 P < 0.001) : fewer than 28% of fragments >1 mm and more than 40% <0.1 mm, compare d with 60 and 15% for Villamblain. Depositional seals had finer fragme nts than structural seals for both soils. For the same runoff rate (15 mm h(-1)), soil loss was greater for Blosseville (2 g m(2) min(-1)) t han for Villamblain (0.4 g m(2) min(-1)) since breakdown products were finer and more transportable. Seal hydraulic resistance was correlate d (r(2) = 0.53, P < 0.01) with MWD of seal fragments.