INFILTRABILITY AND MEASUREMENT OF PHYSICA L CHARACTERISTICS OF CRUSTSDEVELOPED ON WET OR DRY SOIL AGGREGATES UNDER SIMULATED RAINS

The sealing of a soil surface under rain reduces the infiltrability of cultivated soils. This phenomenon is largely conditioned by the soil constitution and the initital water content These 2 factors were studi ed in the laboratory using a bed of soil aggregates. Silty (L) and a c lay-silty (AL) soils were chosen. Soils were initially dry or wet and then received a series of rains (26 mm/h, 23 J/m(2)/mm). Between 2 sub sequent rains, the infiltrability was measured under a large amount of water (120 mm/h, 3 J/m(2)/mm), in order to demonstrate that the reduc tion in the infiltrability occurs before the phenomenon can be actuall y measured under rain. Aggregates of the dry - L, dry AL and wet-L soi ls break into fragments. The wet - AL soil shows abrasion of aggregate s. The reduction in soil infiltrability, which is related to crust for mation, appears after a greater quantity of rains if the soil was init ially wet than when it was initially dry. The decrease in infiltrabili ty then proceeds slowly in all cases, and the final infiltrability ran king is: wet AL > dry - L = wet - AL > dry - L. The porosity of the cr ust was analysed using mercury porosimetry, bulk volume measurements a nd the volumetric water contents at the pressure potentials ranging fr om -0.1 to -10 m. The crust porosity analysis shows that a structural porosity exists within the crusts. Three hypothesis are made to explai n the final infiltrability order: interconnected structural pores perf orm the water transfer under rain (AL - wet); air can be caught in str uctural pores due to desaturation between subsequents rains (dry - AL) ; or structural pores remain saturated but are not interconnected (dry - L and wet - L).