Geoscience Reference
In-Depth Information
R
n
E
= ————
L
(1
b)
and 10 to 20°S, in the belts of the constant trade winds
(see Figures 4.5B and 4.6). The highest annual losses,
estimated to be about 2000 mm, are in the western
Pacific and central Indian Ocean near 15°S (cf. Figure
3.30); 2460 MJ m
-2
yr
-1
(78 W m
-2
over the year) are
equivalent to an evaporation of 900 mm of water. There
is a subsidiary equatorial minimum over the oceans,
as a result of the lower wind speeds in the doldrum
belt and the proximity of the vapour pressure in the air
to its saturation value. The land maximum occurs more
or less at the equator due to the relatively high solar
radiation receipts and the large transpiration losses from
the luxuriant vegetation of this region. The secondary
maximum over land in mid-latitudes is related to the
strong prevailing westerly winds.
The annual evaporation over Britain, calculated
by Penman's formula, ranges from about 380 mm in
Scotland to 500 mm in parts of south and southeast
England. Since this loss is concentrated in the period
May to September, there may be seasonal water deficits
of 120 to 150 mm in these parts of the country neces-
sitating considerable use of irrigation water by farmers.
The annual moisture budget can also be determined
approximately by a bookkeeping method devised by
C.E. Thornthwaite, where potential evapotranspiration
The most satisfactory climatological method devised
so far combines the energy budget and aerodynamic
approaches. In this way, H.L. Penman succeeded in
expressing evaporation losses in terms of four meteo-
rological elements that are measured regularly, at least
in Europe and North America. These are net radiation
(or an estimate based on duration of sunshine), mean air
temperature, mean air humidity and mean wind speed
(which limit the losses of heat and vapour from the
surface).
The relative roles of these factors are illustrated
by the global pattern of evaporation (see Figure 4.6).
Losses decrease sharply in high latitudes, where there is
little available energy. In middle and lower latitudes
there are appreciable differences between land and
sea. Rates are naturally high over the oceans in view of
the unlimited availability of water, and on a seasonal
basis the maximum rates occur in January over the
western Pacific and Atlantic, where cold continental
air blows across warm ocean currents. On an annual
basis, maximum oceanic losses occur about 15 to 20°N
ins
mm
150
ins
mm
150
ins
mm
150
6
6
6
CARDIFF
BERKHAMSTED
SOUTHEND
(254 mm)
4
100
4
100
4
100
(74 mm)
(64 mm)
(71 mm)
(102 mm)
(165 mm)
(121mm)
(71 mm)
2
50
2
50
2
50
(102 mm)
(102 mm)
(198 mm)
(38 mm)
(102 mm)
0
0
0
0
0
0
JFM MJJ
A
A S O N D
J
J F M
A
M J J
A S O N D
J
J F M
A
M J J
A S O N D
J
WATER SURPLUS
POTENTIAL EVAPOTRANSPIRATION
PRECIPITATION
WATER DEFICIENCY
SOIL MOISTURE UTILIZATION
SOIL MOISTURE RECHARGE
Figure 4.7
The average annual moisture budget for stations in western, central and eastern Britain determined by Thornthwaite's
method. When potential evaporation exceeds precipitation soil moisture is used; at Berkhamsted in central England and Southend on
the east coast, this is depleted by July to August. Autumn precipitation excess over potential evaporation goes into replenishing the soil
moisture until field capacity is reached.
Source
: From Howe (1956). Reprinted from
Weather
, by permission of the Royal Meteorological Society. Crown copyright ©.
