PHYSICS OF SURGE IRRIGATION .2. RELATIONSHIP BETWEEN SOIL PHYSICAL AND HYDRAULIC PARAMETERS

A physical model was developed in the laboratory to explore the mechan ism of consolidation due to soil-water matric suction in two intermitt ently wetted soils: a micaceous sand (Poudre) and a sandy clay loam (G reeley). Consolidation was more pronounced in the Greeley than in Poud re soil. As matric suction increased from 0 to 1 m water, the average void ratio decreased by 4% in Poudre and 13% in the Greeley soil. In P oudre sand, this led to 20% reduction in measured average saturated pe rmeability. In the clayey Greeley soil, cracks persisted during rewett ing and caused saturated permeability, which was decreased by 50% at l ower matric suction, to increase significantly following periods of dr ainage at higher matric suction. When no special precautions were take n to eliminate air entrapment, permeability was significantly reduced by entrapped air bubbles and consolidation was reversed as entrapped b ubbles expanded upon rewetting. Air entrapment is regarded as a potent ial factor to reduce intake rate in surge flow, in particular in coars e-textured soils with no appreciable consolidation. Intermittent wetti ng causes soil pore size and availability to be altered by factors suc h as consolidation, air entrapment and expansion, and cracks. The phys ical nature of these factors implies that soil permeability could be a ltered in opposing ways.