Moutier, M., Université Catholique de Louvain, Unité de Génie Rural, Place Croix du Sud 2, B-1348 Louvain-la-Neuve, Belgium
Shainberg, I., Inst. of Soils, Water, and Environmental Sciences, Agricultural Research Organization (ARO), Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel
Levy, G.J., Inst. of Soils, Water, and Environmental Sciences, Agricultural Research Organization (ARO), Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel

Understanding the combined role of intrinsic (e.g., clay content) and extrinsic soil conditions in determining the saturated hydraulic conductivity (K) is a key factor in improving soil and irrigation management. Hydraulic gradient effects on K have been investigated on two Ca vertisols (Chromic Haploxerert) from Yizreel and Kedma, Israel. Samples were packed in columns and subjected to two prewetring rates (4.5 and 70 mm h-1) and two hydraulic gradients (2.8 and 11.6 for Yizreel; 3.1 and 13.6 for Kedma). Saturated K was determined during leaching with CaCl2 solutions having total electrolyte concentrations (TEC) of 0.5, 0.01 M Cl-, and deionized water (DW). The average hydraulic conductivity at the end of the leaching with the 0.5 M solution (K̄0.5) of the two vertisols increased with a decrease in prewetting rate. The effect of prewetting rate was more pronounced in Yizreel, where the high clay content (70.3% clay) resulted in a more stable structure. Upon leaching with the 0.01 M solution, K̄(r0.01) first decreased and then increased. This increase in K̄(r0.01) was explained by cohesive bond formation, which increased with an increase in soil clay content and with increased proximity between the clay particles. High hydraulic gradient enhanced clay to clay contacts, and a steep increase in K̄(r0.01). Following fast prewetting, the effect of the hydraulic gradient on cohesive bond formation was more pronounced in Yizreel than in Kedma (46.5% clay). Leaching with DW decreased the K̄(rDW), regardless of the prewetting treatment or soil clay content, suggesting that swelling was the governing mechanism in the reduction of K̄(rDW).Understanding the combined role of intrinsic (e.g., clay content) and extrinsic soil conditions in determining the saturated hydraulic conductivity (K) is a key factor in improving soil and irrigation management. Hydraulic gradient effects on K have been investigated on two Ca vertisols (Chromic Haploxerert) from Yizreel and Kedma, Israel. Samples were packed in columns and subjected to two prewetting rates (4.5 and 70 mm h-1) and two hydraulic gradients (2.8 and 11.6 for Yizreel; 3.1 and 13.6 for Kedma). Saturated K was determined during leaching with CaCl2 solutions having total electrolyte concentrations (TEC) of 0.5, 0.01 M Cl-, and deionized water (DW). The average hydraulic conductivity at the end of the leaching with the 0.5 M solution (K̄0.5) of the two vertisols increased with a decrease in prewetting rate. The effect of prewetting rate was more pronounced in Yizreel, where the high clay content (70.3% clay) resulted in a more stable structure. Upon leaching with the 0.01 M solution, K̄r0.01 first decreased and then increased. This increase in K̄r0.01 was explained by cohesive bond formation, which increased with an increase in soil clay content and with increased proximity between the clay particles. High hydraulic gradient enhanced clay to clay contacts, and a steep increase in K̄r0.01. Following fast prewetting, the effect of the hydraulic gradient on cohesive bond formation was more pronounced in Yizreel than in Kedma (46.5% day). Leaching with DW decreased the K̄rDW, regardless of the prewetting treatment or soil clay content, suggesting that swelling was the governing mechanism in the reduction of K̄rDW.