A FLUCTUATING WATER-TABLE MODEL FOR THE MANAGEMENT OF A CONTROLLED-DRAINAGE SUBIRRIGATION SYSTEM

A model (FWTMOD) was developed as a simplified method to predict daily changes in watertable depth for a controlled-drainage and subirrigati on system. The model was based upon a fluctuating watertable equation derived by de Zeeuw and Hellinga of The Netherlands. This equation onl y requires two parameters to describe daily watertable fluctuation: (1 ) a reaction factor for a receding or rising watertable, and (2) a rec harge coefficient to account for the portion of rainfall which causes the watertable to rise. For a given water management system, these par ameters can be estimated from observed (measured) watertable responses for a representative number of rainfall events. The model was calibra ted and tested to evaluate its application in simulation of a subsurfa ce drainage system. Climatological and watertable data from a field dr ainage experiment at the Louisiana Agricultural Experiment Station's B en Hur Research Farm, Baton Rouge, Louisiana, was used to calibrate th e model. Monthly averages of the daily reaction factor of watertable f luctuations and the daily recharge coefficient of soil moisture were d etermined from three years of data from the field experiment. Recharge coefficients were multiplied by a correction factor to account for an tecedent soil moisture conditions prior to the onset of rainfall event s. The reciprocal of moisture deficit in the soil profile was used to estimate the correction factor. The standard deviation of differences between the observed and FWTMOD simulated daily watertables was 23.1 c m for a seven-year evaluation period, 1981 to 1987. For comparison pur poses, the more complex and comprehensive drainage simulation model DR AINMOD was also used, with a resulting 20.8 cm standard deviation of d ifferences between the observed and predicted daily watertables. In ge neral, the simpler model FWTMOD predicted daily watertable fluctuation s nearly as well as DRAINMOD for conventional subsurface drainage in a Commerce silt loam soil.