The Hooghoudt equation is widely used as a simple means of specifying drain spacing when designing networks of parallel drains in drainage systems, based on estimating the maximum water table height between two drains. It is shown via comparison with a numerical model that the Hooghoudt equation can overestimate water table height and therefore yield drain spacings that may be too wide. This is because the Hooghoudt drain-spacing equation in fact has a concealed dependency on Van Genuchten soil-water retention curve parameters, which can bias the water table estimates unless adjustments are made explicitly. A modification of the Hooghoudt equation is presented that incorporates two new dimensionless coefficients to make allowance for this dependency. The modified expression yields improved accuracy as measured against the numerical reference model.