18
R.G. Lawford
values of net radiation occur in Baja California , with values reaching 120
to 130W/m
2
in latitudes from 25°N to 30
0 N .
The energy budgets at most latitudes are quite well balanced. However ,
at latitudes of 19°S to 22°Sand 24°S to 29°S and several other 1° latitude
bands, the sums of the sensible and latent heat fluxes exceed the net
radiation, indicating that either ground fluxes are significant or that
uncertainties exist in the technique for generating the values, possibly
because the sensible heat fluxes are overestimated.
The Water Balance on the West Coasts of South and North America
In Equation 1.1, the precipitation and evapotranspiration terms are
generally large and control the water balance in a particular area. Precipitation and stream flow are measured regularly. As noted earlier, the
storage-term parameters, when averaged over a year or longer , can be
neglected at most latitudes , because changes in net water storage tend
toward zero, except for the small component that infiltrates the groundwater system. Over longer intervals (>10 years) , one would anticipate
that the flux of water into the ground-water system would balance the flux
of water out of the ground-water system to the surface.
If the storage term is neglected , runoff represents the difference between the precipitation and evaporation. When precipitation exceeds
evaporation by a large amount, runoff will be large. For this investigation ,
runoff estimates from three independent sources were compared , namely,
Henning 's (1989) surface energy budget estimates (SEB) , Korzun et al.'s
(1978) World Water Balance (WWB) estimates and L'vovich's (1974)
World Water Resource s and their Future (WWRA) estimates. Figure
1.10 shows the runoff computed using each of these techniques . Although
SEB and WWRA runoff estimates are of the same order of magnitude ,
some differences were found. As shown in Figure 1.10, SEB values
reached a maximum of 1600mm at 56°N, with secondary maxima
occurring at 48°N, 40
0S
, 52°S and between 5
0S
and 15°N. Similar peaks
were found in the WWRA data set, with maxima centered at 52°N,
lOON, 39°S, and 48°S to 50
0S
. However, the WWB runoff values were
larger, with maximum values two to three times greater than the values
estimated by the other two studies in the areas of relative precipitation
maxima. (Runoff values as high as 3000mm in North America and 4900
mm in South America were estimated , values that are much higher than
the precipitation averages for the same latitude.) The differences are
particularly noteworthy for latitudes between 35°Sand 50°S. Runoff
values in excess of annual precipitation suggest that either the runoff
values are too high or that the precipitation is not accurately represented
at the latitudes being considered , or , possibly, both. Underestimates of
precipitation are most likely to arise in mountainous terrain where precip-
R.G. Lawford
values of net radiation occur in Baja California , with values reaching 120
to 130W/m
2
in latitudes from 25°N to 30
0 N .
The energy budgets at most latitudes are quite well balanced. However ,
at latitudes of 19°S to 22°Sand 24°S to 29°S and several other 1° latitude
bands, the sums of the sensible and latent heat fluxes exceed the net
radiation, indicating that either ground fluxes are significant or that
uncertainties exist in the technique for generating the values, possibly
because the sensible heat fluxes are overestimated.
The Water Balance on the West Coasts of South and North America
In Equation 1.1, the precipitation and evapotranspiration terms are
generally large and control the water balance in a particular area. Precipitation and stream flow are measured regularly. As noted earlier, the
storage-term parameters, when averaged over a year or longer , can be
neglected at most latitudes , because changes in net water storage tend
toward zero, except for the small component that infiltrates the groundwater system. Over longer intervals (>10 years) , one would anticipate
that the flux of water into the ground-water system would balance the flux
of water out of the ground-water system to the surface.
If the storage term is neglected , runoff represents the difference between the precipitation and evaporation. When precipitation exceeds
evaporation by a large amount, runoff will be large. For this investigation ,
runoff estimates from three independent sources were compared , namely,
Henning 's (1989) surface energy budget estimates (SEB) , Korzun et al.'s
(1978) World Water Balance (WWB) estimates and L'vovich's (1974)
World Water Resource s and their Future (WWRA) estimates. Figure
1.10 shows the runoff computed using each of these techniques . Although
SEB and WWRA runoff estimates are of the same order of magnitude ,
some differences were found. As shown in Figure 1.10, SEB values
reached a maximum of 1600mm at 56°N, with secondary maxima
occurring at 48°N, 40
0S
, 52°S and between 5
0S
and 15°N. Similar peaks
were found in the WWRA data set, with maxima centered at 52°N,
lOON, 39°S, and 48°S to 50
0S
. However, the WWB runoff values were
larger, with maximum values two to three times greater than the values
estimated by the other two studies in the areas of relative precipitation
maxima. (Runoff values as high as 3000mm in North America and 4900
mm in South America were estimated , values that are much higher than
the precipitation averages for the same latitude.) The differences are
particularly noteworthy for latitudes between 35°Sand 50°S. Runoff
values in excess of annual precipitation suggest that either the runoff
values are too high or that the precipitation is not accurately represented
at the latitudes being considered , or , possibly, both. Underestimates of
precipitation are most likely to arise in mountainous terrain where precip-