Page 166 - Hydrogeology Principles and Practice

HYDC05 12/5/05 5:35 PM Page 149

Groundwater investigation techniques 149

Fig. 5.9 Mean annual rainfall over the
United Kingdom, 1961–1990, compiled by
the UK Meteorological Office. The mean
annual rainfall varies enormously over the
British Isles from in excess of 3000 mm in
parts of the western highlands of Scotland
to about 500 mm in parts of East Anglia
and the Thames Estuary. However, there
is much less difference in the number of
rain days (defined as days when 0.2 mm or
more of rain falls). On average, the drier
areas have between 150 and 200 rain days
annually, while the wetter areas have just
over 200 rain days. In most areas, the
month of December has the highest
number of rain days.

5.3.2 Evapotranspiration measurement and Wind speed is also a factor in determining the rate at
estimation which vapour is carried away from the water surface.
Evaporation can be measured directly using an evap-
In the hydrological cycle, evaporation of water oration tank or pan. Tanks are buried in the ground,
occurs from free water surfaces such as lakes, reser- whereas pans are set above ground on a small plinth.
voirs and rivers with the rate dependent on factors The British standard tank has dimensions of 1.83 m
such as water temperature and the temperature and × 1.83 m and is 0.61 m deep. The United States
humidity of the layer of air above the water surface. National Weather Service (NWS) Class A pan, with a