The water balance of lowland rice

Because of the flooded nature of lowland rice, its water balance and water productivity are different from those of other cereals such as wheat and maize. Water inputs to lowland rice fields are needed to match the outflows by seepage, percolation, evaporation, and transpiration (Figure 1). Seepage is the lateral subsurface flow of water and percolation is the down flow of water below the root zone. Typical combined values for seepage and percolation vary from 1-5 mm d-1 in heavy clay soils to 25-30 mm d-1 in sandy and sandy loam soils. Evaporation occurs from the ponded water layer and transpiration is water loss from the leaves of the plants. Typical combined evapotranspiration rates of rice fields are 4-5 mm d-1 in the wet season and 6-7 mm d-1 in the dry season, but can be as high as 10-11 mm d-1 in subtropical regions before the onset of the monsoon. Total seasonal water input to rice fields (rainfall plus irrigation) varies from as little as 400 mm in heavy clay soils with shallow groundwater tables to more than 2000 mm in coarse-textured (sandy or loamy) soils with deep groundwater tables. Around 1300-1500 mm is a typical value for irrigated rice in Asia. Outflows of water by seepage and percolation account for about 25-50% of all water inputs in heavy soils with shallow water tables of 20-50 cm depth, and for 50-85% in coarse-textured soils with deep water tables of 150 cm depth or more.