It was assumed that the remaining components of nitrogen
mass balance were nitrates. Water-soluble nitrates may be either
absorbed by the crop through evapotranspiration flux or emitted to
the water compartment as pollutants via deep percolation and
drainage. Following Pathak et al. (2004), it was also assumed that
the proportion of nitrates bound to drain or leach to the surface and
groundwater compartments during the crop cycle equalled the
proportion of water that was unused by crops in the paddy system.
Therefore, a water mass balance was needed to ascertain water use
efficiency and to determine percolation and drainage components.
Runoff was considered nil because in common conditions, paddy
fields are flat and managed in a way that prevents water from
spilling over bunds; farmers maintain water depth between
defined minimal and maximal ponding conditions (generally 0e
150 mm). However, at times, and especially at the end of the
cropping season, farmers drain the fields off.