Hydraulic gradient and wetting rate effects on the hydraulic conductivity of two calcium vertisols

Understanding the combined role of intrinsic (e.g., clay content) and extri nsic soil conditions in determining the saturated hydraulic conductivity (K ) is a key factor in improving soil and irrigation management. Hydraulic gr adient effects on K have been investigated on two Ca vertisols (Chromic Hap loxerert) from Yizreel and Kedma, Israel. Samples were packed in columns an d 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 electrolyt e concentrations (TEC) of 0.5, 0.01 M Cl-, and deionized water (DW). The av erage hydraulic conductivity at the end of the leaching with the 0.5 M solu tion ((K) over bar(0.5)) Of the two vertisols increased with a decrease in prewetting rate. The effect of prewetting rate was more pronounced in Yizre el, where the high clay content (70.3% clay) resulted in a more stable stru cture. Upon leaching with the 0.01 M solution, (K) over bar(r0.01) first de creased and then increased. This increase in (K) over bar(r0.01) was explai ned by cohesive bond formation, which increased with an increase in soil cl ay content and,vith increased proximity between the clay particles. High hy draulic gradient enhanced clay to clay contacts, and a steep increase in (K ) over bar(r0.01). Following fast prewetting, the effect of the hydraulic g radient on cohesive bond formation was more pronounced in Yizreel than in K edma (46.5% clay). Leaching with DW decreased the (K) over bar(rDW), regard less of the prewetting treatment or soil clay content, suggesting that swel ling was the governing mechanism in the reduction of (K) over bar(rDW).