THE IMPLICATIONS OF SPATIAL VARIABILITY IN SURFACE SEAL HYDRAULIC RESISTANCE FOR INFILTRATION IN A MOUND AND DEPRESSION MICROTOPOGRAPHY

Soil surface crusting has a major impact on water infiltration and ero sion in many soils. Considerable progress has been made in describing crusting processes and in modelling the impact of crusting on infiltra tion. Most studies, however, have neglected the high spatial variabili ty in crust characteristics observed in the field. The objective of th is experiment was to determine the influence of runoff depth on infilt ration rate in the presence of a surface seal varying in hydraulic cha racteristics with microtopography. The Blosseville silt loam has a low aggregate stability and forms crusts readily. The Villamblain silty c lay loam has a greater aggregate stability due to its greater clay and organic matter contents, and it is more resistant to aggregate breakd own processes under rainfall. Samples of the soils were sieved to reta in aggregates less than 2.0 cm and packed in 50 X 50 X 15 cm soil tray s. The trays were surrounded by a 10 cm soil border to compensate for splash loss. After molding the surface into a mound and depression mic rotopography, the samples were subjected to simulated rainfall at an i ntensity of 22.8 mm h(-1). Hourly measurements of surface roughness sh owed that the original roughness was smoothed out due to the infilling of depressions by sediments detached from the mounds. For the final h our, runon was added to the top of the soil tray to increase the runof f rate and depth. For both soils, infiltration rate increased more tha n could be attributed to the increased pending pressure head. The chan ge in infiltration rate was particularly great for Villamblain. The me asurements of hydraulic resistance showed that structural crusts had a lower hydraulic resistance than sedimentary crusts. They also showed that the crusts formed on Villamblain were of a lower hydraulic resist ance than those of Blosseville. It appears that small changes in runof f depth can significantly increase infiltration rate when structural c rusts of lower hydraulic resistance are inundated. The effect was less important in Blosseville which formed seals of relatively high hydrau lic resistance everywhere. The results provide a suitable explanation for field observations of increasing infiltration rate with either inc reasing rainfall intensity or runoff rate. The results also have impli cations for the relationships between surface roughness, surface water storage, and infiltration, (C) 1998 Elsevier Science B.V. All rights reserved.