Recent assessment of the resource potential of the Yazor Gravel Aquifer near Hereford (Liddament & Clark 1980) has provided evidence which supports the concept of physical controls proposed by Wellings & Bell (1980, 1982), and suggests that the same controls govern the movement of water through unsaturated granular material as well as Chalk.
The Yazor Brook drains an area of about 50 km2 west of Hereford and north of the River Wye. The lower and central parts of the Yazor Valley are infilled by superficial glacial and post-glacial sediments up to about 15 m in thickness. A significant proportion of these sediments are ill-sorted glacial-outwash sands and gravels which make up the local, but important, Yazor Gravel Aquifer System. A mathematical model of the Yazor Gravel Aquifer was constructed but there were great problems in modelling the water balance of the system. The major problem was in quantifying the recharge to the aquifer, because the traditional Meteorological Office approach, using the soil moisture deficit (SMD) concept, did not appear to work.
The behaviour of the water table in two observation wells equipped with continuous water level recorders help to show quite clearly that, in the Yazor Valley, the traditional SMD concept cannot be applied entirely. Recharge can take place where an SMD exists and considerable evapotranspirative losses can occur by upward flux from quite deep water tables.
Through each summer all, or almost all, rain should be soaked up, removing the SMD. In