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Quantifying Flow Processes during Land Soaking of Cracked Rice Soils

IRRI, P.O. Box 933, 1099 Manila, Philippines

M. C. S. Wopereis

West Africa Rice Development Association, B.P. 96, Saint-Louis, Senegal

*Corresponding author (T.Tuong{at}CGNET.COM).

ABSTRACT

Knowledge of water flow into cracks of dry soil is needed to understand high water loss in rice (Oryza sativa L.) fields. A methodology is presented to quantify flow processes during land soaking of dry, cracked rice fields. Crack geometry is determined from measurements of crack depth and width. The advance of the wetting front, the water table depth, and the soil moisture content are monitored at regular distances from the irrigation source. Bypass flow losses (water that flows through the cracks to the subsoil) are estimated from the difference between water input (irrigation and rainfall) and water stored in cracks, on the soil surface, and in the topsoil matrix (0–0.2 m) plus losses due to evaporation. Lateral drainage to the surroundings is determined from the amount of bypass flow and the increase in water content of the subsoil (deeper than 0.2 m). The method was applied at two Epiaqualf sites with a relatively permeable subsoil (saturated hydraulic conductivity 0.1–0.5 m d^–1) in the Philippines. During land soaking, water moving in the crack networks was about 7 m ahead of the surface water front, recharging the water table through bypass flow. At the monitoring sites, the recharge rapidly raised the water table nearly 2 h before the arrival of the advancing surface water front. The amount of water that bypassed the topsoil accounted for 41–57% of total water applied; 66–74% of the bypass flow was lost to the surroundings through lateral drainage. Reducing water losses during land soaking may greatly increase the water use efficiency of irrigated rice production systems.

Received for publication December 8, 1994.

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