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RICE IN SRI LANKA - Water Management

Methods of water loss

Water could be lost through

    • Evaporation (E) from surfaces
    • Transpiration (T) from leaves
    • Seepage (S) and percolation (P) of the soil
    • Bund leakages 
    • Runoff from the field

Rates of S and P (in comparison with ET) is relatively stable in a given period within a given AER with a uniform climate but varies from place to place. S and P rates are mainly governed soil profile characters (much greater in sandy than clay soils), topography and depth to the water table. The rates of S and P is about 6 mm/day in well drained and 3 mm/day in poorly drained soils. Reddish brown earth (RBE) soils have greater S and P compared to Low Humic Gley (LHG). Although wet land preparation consumes much water, puddling reduces the amount loss via S and P by clogging the pores and forming a hardpan below the plough layer. Poorly constructed bunds and crab holes increase seepage. S and P vary with the edaphic environment which could be partially controlled through proper management.

However, ET is determined mainly by the vapor pressure deficit and the canopy size which is beyond the control of a farmer. Bund leakages and runoff from the field is totally under the farmers’ control. Therefore, the main determinants of water requirement (WR) are evapotranspiration, seepage and percolation (S & P) rates, which could be summarized as follows.

WR = E + T + (S + P)