Drainage of a Tropaqualf Before and After Water Extraction by a Crop

To improve production of crops planted after lowland rice (Oryza sativa L.) in monosoonal Asia, there is a need to understand more clearly the drainage pattern during the transition from the wet to the dry season. For crops of 75 to 80 d maturity, the top 750 mm of soil contains the bulk of the root system. This study determined the flux of water across the 750-mm plane and the water table recession patterns during the transition from wet to dry season and again after a crop had removed water from the soil. The study was conducted in three phases on a 9-m by 9-m plot of an Aeric Tropaqualfs. In the first phase, the soil was field saturated, covered with plastic sheeting, and drained for 30 d. In the second phase, a cowpea (Vigna unguiculata L. Walp) — sorghum (Sorghum bicolor L. Moench) intercrop was grown for 42 d. In the third phase, 22 mm of water was applied and soil was again covered and drained for 15 d. Volumetric water changes were determined by neutron scattering at 150-, 300-, 600-, 900- and 1200-mm depths. Tensiometers were placed at the same depths. The plot was surrounded by flow barriers to a 750-mm depth. Drainage through the 750-mm plane during the first phase amounted to only 11 mm. A perched water table formed in and above the B horizon at 600 mm. The ground water table receded at 30 mm d⁻¹ after it appeared below the perched water table on day 10. During the second phase roots penetrated at about 25 mm d⁻¹ suggesting that the initial high water content was not a major impediment to root development. After a 22-mm irrigation, the drainage rate during the third phase was 1.2 mm d⁻¹, with drainage apparently coming both from applied water and water remaining in the profile prior to irrigation. The results suggest both opportunities and problems for using soil water remaining after rice harvest during the transition from wet to dry season.