Capillary Length and Field Capacity in Draining Soil Profiles. Issue 5 (27th May 2019)

• Record Type: Journal Article
• Authors: Aldrees, A.; Nachabe, M.
• Is Part Of: Water resources research. Volume 55:Issue 5(2019)
• Page Start: 4499
• Page End: 4507
• Publication Date: 2019-05-27
• Subjects: Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2018WR024288
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: The capillary length ( λ s ) and time ( t s ) are dynamic scalars that emerge routinely in the infiltration problem when gravitational and pressure gradients forces are involved. During drainage, however, capillary gradients oppose gravity and retain soil moisture close to surface. In this case, the pull of capillary gradients increases with drainage time and offsets gravity resulting in a quasi‐hydrostatic pressure distribution and negligibly small drainage flux in the profile. In this paper, it is proposed to anchor the dynamic concept of field capacity—the attainment of a small negligible drainage flux—in the physics of soil moisture redistribution as influenced by gravity and capillary forces. Similar to infiltration, this dynamic approach grounds the concept of field capacity in soil hydrology and allows its estimation from readily measured intrinsic physical characteristics such as λ s, t s, and K s . Finally, we exploit an analytical solution by Broadbridge and White (1988, https://doi.org/10.1029/WR024i001p00145 ) to track the drainage front as soil water redistributes in an initially saturated soil profile. While initially large, the downward migrating drainage front decelerates with time reaching near steady state condition at t ≈ 1, 000 t s . Quasi‐hydrostatic pressure matric head and water content profiles develop above the drainage front. Key Points: Similar to infiltration, drainage is controlled by soil capillary length and time scales A drainage front determines the attainment of field capacity in a profile