Geoscience Reference
In-Depth Information
highest annual losses, estimated to be about
2000mm, are in the western Pacific and central
Indian Ocean near 15°S (cf.
Figure 3.30
); 2460MJ
m
-2
yr
-1
(78W m
-2
over the year) are equivalent
to an evaporation of 1000mm of water/cm
2
).
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 vapor
pressure in the air to its saturation value. The land
maximum occurs more or less at the equator
owing 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 380mm
in Scotland to 500mm 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 120-150mm in these
parts of the country, necessitating considerable
use of irrigation water by farmers. The annual
moisture budget can also be determined approxi-
mately by a bookkeeping method devised by
C. W. Thornthwaite, where potential evapotran-
spiration is estimated from mean temperature.
Figure 4.7
illustrates this for stations in western,
central and eastern Britain (cf.
Figure 10.25
). In the
winter months, there is an excess of precipitation
over evaporation; this goes to recharging the soil
moisture, and further surplus runoff. In summer,
when evaporation exceeds precipitation, soil
moisture is initially used to maintain evaporation
at the potential value, but when this store is
depleted there is a water deficiency as shown in
Figure 4.7
for Southend.
In the United States, monthly moisture
conditions are commonly evaluated on the basis
of the Palmer Drought Severity Index (PDSI).
This is determined from accumulated weighted
differences between actual precipitation and the
calculated amount required for evapotranspira-
tion, soil recharge and runoff. Accordingly, it takes
account of the persistence effects of droughts.
The PDSI ranges from ≥4 (extremely moist) to
≤
-4 (extreme drought).
Figure 4.8
indicates an
oscillation between drought and unusually moist
conditions in the continental USA during the
period October 1992 to August 1993.
ins
mm
ins
mm
ins
mm
6
150
6
150
6
150
CARDIFF
BERKHAMSTED
SOUTHEND
254mm
4
100
4
100
4
100
74mm
71mm
64mm
102mm
165mm
121mm
71mm
2
2
2
50
50
50
102mm
102mm
198mm
102mm
38mm
0
0
0
0
0
0
J
FM
A
M
JJ
A S O N D
J
J
FM
A
M
J J
A S O N D
J
J
FM
A
M
J J
A S O N D
J
Water surplus
Water deficiency
Soil moisture utilization
Soil moisture recharge
Potential evapotranspiration
Precipitation
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: After Howe (1956). Courtesy of the Royal Meteorological Society.
