In designing subsurface drainage system the depth as well as proper spacing of drains to provide suitable water table profile for optimum soil moisture and salinity levels in root zone are important considerations. But usually, after installation of drains in the field, the drain spacing estimated through Hooghoudt equation does not provide suitable water table profile and this may cause either stress or water-logging conditions for plants. To overcome this, a correction factor can and must be used in Hooghoudt equation. To estimate the correction factor for Hooghoudt equation for subsurface drainage sysytem in Roudasht, Isfahan an experimental subsurface drainage farm with an area of one hectare, located about 65 km east of Isfahan was employed. In this experimental farm, water table profiles between two drains under different drainage coefficients were plotted. The statistical analysis of the collected data showed that water table positions and drainage coefficients can be related through a sixth order polynomial with a correllation coefficient of more than 80 percent. Using this polynomial equation and estimating the water table height above the drains, the Hooghoudt equation can then be used to estimate the drain spacing in actual field conditions.
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