Physics of Surge Irrigation I. Quantifying Soil Physical Parameters

Soil consolidation caused by soil-water matric forces was produced in a CO2-filled laboratory apparatus in Poudre sand and Greeley sandy clay loam soils. As matric suction increased intermittently (wetting and draining) from 0 to 1 m of water, both soils consolidated from an initial height of 59.5 mm, Poudre sand by 1.4 mm and the Greeley soil by 5.1 mm. Soil desaturation led to significant trapping of soil gas-phase (CO2) upon rewetting. However, the trapped bubbles quickly dissolved in water and had no lasting effect on consolidation. Similar experiments conducted in air verified air entrapment upon rewetting of drained soils. The trapped air bubbles persisted during subsequent draining and wetting cycles implying a long-term effect on pore space with a tendency to expand as rewetting of drained samples approached surface ponding. The expansion of air bubbles in Poudre sand totally reversed consolidation and led to a net increase in void ratio, possibly causing what others have observed as “rebound” of drained soils upon rewetting in surge flow. More than 90% of the final consolidation occurred during the first 10 min of drainage, reconfirming surge flow field observations, suggesting less additional consolidation benefit for longer dewatering periods.