Geoscience Reference
In-Depth Information
1
Precipitation, P
Evaporation, E = b E p
b
Field
Capacity
d
0
d max
0
Runoff, R
Water
Depth
d max
d
= P
E
t
d
If d
>
d max d
d max and R = ( d
d max )
Figure 24.1 Schematic diagram of the SVATS used in early studies of the effect of land surfaces on weather and climate
based on the 'Budyko Bucket.
Originally, land surface models (e.g., Manabe et al. , 1965; Shukla and Mintz,
1982) were as simple as they could be. They ignored energy storage in the soil-
vegetation-atmosphere interface (Table 24.1, value 6), and did not calculate a
detailed surface water partition (Table 24.1, value 7) or carbon dioxide exchange
(Table 24.1, value 8). They simply assumed typical fixed values of aerodynamic
roughness length, albedo, and surface emissivity and applied these to all continental
surfaces to calculate values 1, 2, 3, 4 and 5 in Table 24.1.
In early models, energy sharing between latent and sensible heat and (implicitly)
the surface temperature (requirements 4 and 5) was calculated using a simple
'Budyko Bucket' model (Budyko, 1948; 1956), see Fig. 24.1. A form of potential
evaporation rate was assumed and calculated using the Penman-Monteith
equation assuming zero surface resistance and an aerodynamic resistance equal to
that for momentum transfer in neutral conditions with the assigned aerodynamic
roughness length. The actual evaporation rate was then calculated at any point in
time by making a running water balance of the water level in a hypothetical
'bucket' located at the land surface, which is filled by precipitation and emptied by
evaporation and, when the water depth, d , stored in the bucket exceeds a critical
value, d max , also by runoff. The actual evaporation rate was assumed to vary linearly
with the fractional fill of this bucket between zero and the calculated potential
evaporation rate.
Early evidence for land-surface influences on climate came through model
experiments made with General Circulation Models (GCMs) which used the
simple model just described. These experiments involved making imposed
changes in the values of one of the (few) parameters used in these simple SVATS
and comparing the model-simulated climates before and after such a change.
The first such study was made by Charney et al. (1975) and was motivated by
the hypothesis (Fig. 24.2a) that expansion of desert regions, especially those in
the Sahel region of Africa, might result from a human-induced, land-surface
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