Mechanisms and Control of Percolation Losses in Irrigated Puddled Rice Fields

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Abstract
The low water use efficiency of irrigated lowland rice is partly due to water loss by percolation. Mechanisms of percolation losses were studied in a puddled rice field with a permeable subsoil using simulation models and field experiments. Inclusion of small nonpuddled areas (1.5 m2 per 100 m2 of puddled soil) within the field with 5‐cm ponding water depth (PWD) increased field water loss from 2.7 mm d−1 to 15 mm d−1. Under‐bund percolation rate (lateral movement of ponded water into the bunds, then vertically down to the water table) was about 10 mm d−1 in a 25 by 100 m field. A one‐dimensional mechanistic soil‐water balance model, SAWAH, accurately simulated the measured water losses in a homogeneously puddled rice field only when lateral flow toward nonpuddled spots and bunds was prevented. Maintaining shallow PWD did not significantly affect percolation loss through uniformly puddled soil but greatly reduced losses in nonpuddled spots and under‐bund percolation. Sealing the bund walls with puddled soil material will decrease the horizontal conductivity of the bunds and may further reduce under‐bund percolation.

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