Direct confirmation of the external sources was sought, and an indirect attempt to quantify it was made, via the water balance calculations analyses and discussion. Water balance components were calculated for the two sub-catchments at Great Hatfield and South Field as well as for the entire Catchwater Drain catchment. These water balance analyses showed that the predominant low flow contributions to total stream flow during dry weather periods, came from the sandy deposits in the Great Hatfield area, and that these contributions were able to sustain stream flow during dry weather periods, especially in July, August and September. However, the observed low flow contributions greatly exceeded the water balance estimates of low flow over the two dry periods of 1987 and 1988. Also, the observed runoff at the outlet of the Catchwater Drain catchment in dry periods, which analyses of channel flow conditions and measurements of discharge showed came from the sandy areas at Great Hatfield, was much higher than the runoff estimated from the water balance for all the sandy areas combined. In this way both the existence and the scale of the external source of low flow from the sandy areas were confirmed. Furthermore, it seemed reasonable to infer, from the water balance analyses, that the external source occurred at some depth below the Catchwater drain catchment and was probably the underlying Chalk aquifer.