Fluctuating watertable 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 subirrigation system. The model was based upon a fluctuating watertable equation derived by de Zeeuw and Hellinga of The Netherlands. This equation only requires two parameters to describe daily watertable fluctuation: (1) a reaction factor for a receding or rising watertable, and (2) a recharge coefficient to account for the portion of rainfall which causes the watertable to rise. For a given water management system, these parameters can be estimated from observed (measured) watertable responses for a representative number of rainfall events. The model was calibrated and tested to evaluate its application in simulation of a subsurface drainage system. Climatological and watertable data from a field drainage experiment at the Louisiana Agricultural Experiment Station's Ben Hur Research Farm, Baton Rouge, Louisiana, was used to calibrate the model. Monthly averages of the daily reaction factor of watertable fluctuations and the daily recharge coefficient of soil moisture were determined from three years of data from the field experiment. Recharge coefficients were multiplied by a correction factor to account for antecedent soil moisture conditions prior to the onset of rainfall events. 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 cm for a seven-year evaluation period, 1981 to 1987. For comparison purposes, the more complex and comprehensive drainage simulation model DRAINMOD was also used, with a resulting 20.8 cm standard deviation of differences between the observed and predicted daily watertables. In general, the simpler model FWTMOD predicted daily watertable fluctuations nearly as well as DRAINMOD for conventional subsurface drainage in a Commerce silt loam soil.