The paper discusses water balance data for a small (16 km2) boulder clay catchment. In order to check the accuracy of the calculations, water balances are cast for different, overlapping periods of time within the same run of data (e.g. three-year, annual, seasonal, monthly, ten-day and storm-period). For each period the residual values are satisfactorily low (cf. 12 mm for the three-year balance and zero for the storm-period balance). A further series of checks is provided by rewriting the water equation in a number of different ways to enable estimated and measured values of certain water balance components to be compared. It is concluded that the measured values seem to be accurate and that the catchment appears to be watertight.