Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2003
Publication Date: 12/1/2003
Citation: Dicarlo, D.A. 2003. Drainage in finite-sized unsaturated zones. Advances in Water Resources. 26(12):1257-1266.
Interpretive Summary: The draining of water from the soil surface to the water table is a common, but commonly misunderstood, phenomena. One common misperception is that the water stops moving after a couple of days after rainfall. This paper explicitly calculates whether the water is moving or static. The calculation depends on the number of days since rainfall, the soil properties, and the length between the soil surface and the water table. Experimental results show close correlation to the equations developed. The results are important for travel times of surface water and possible contaminants to the ground water.
Technical Abstract: After the initiation of gravity drainage, water is often assumed to be either a) draining under unit gradient, or b) at capillary/gravity equilibrium. Both of these simplifications can be useful, but the regimes of validity of each assumption must be delineated. Water pressures are measured versus time and distance as water drains out of a 1.6 m long sand column to determine the relative effects of capillary and gravitational forces during drainage. For medium sized sands (0.15 ¿ 0.3 mm in diameter), the capillary pressure is constant in space in a large region of the column for over 12 days, and the water continues to flow under unit gradient for relatively long time scales. Similar results are seen for finer sands, but with a much faster approach to equilibrium. Numerical simulations and analytical estimates are presented and compare favorably to the measurements. Together, the experimental, theoretical and analytical results are used to calculate when capillary/gravity equilibrium is reached as a function of porous media properties and length of the unsaturated zone. The ratio of the length of the unsaturated zone to the bubbling pressure is a key parameter in determining the drainage regime, and that even for relatively short unsaturated zones the equilibrium time scale can be on the order of years.