Drainage Rates and Water-Table Depths

Precipitation and evapotranspiration data for 27 years from three stations in the Ottawa and St. Lawrence Lowlands have been used together with a hydrologic model of the drainage process to obtain the duration of particular water table heights to be expected. An IBM 370/158 computer was used to make day-by-day calculations of the expected water table heights for a range of conditions including drainage coefficients from 3 mm/day to 15 mm/day, drain depths of 1 m, 1.4 m, and 1.8 m, drainable porosities from 0.03 to 0.11, available soil moisture in the root zone from 60 mm to 151 mm. A statistical frequency analysis of the output was made. The results from these computations were prepared in tabular and graphical form to show the effect of drainage rate on water table height. By using a criterion for the number of days the water table may be at or less than a particular depth, one can select a design drainage rate.