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(a)
(b)
M&S
M&S
1.5
1.5
Woodland
Shrubland
1.0
1.0
0.5
0.5
(a)
(b)
0.0
0.0
10
100
1000
10000
10
100
1000
10000
Surface resistance (s m 1 )
Surface resistance (s m 1 )
(c)
M&S
1.5
Grassland
1.0
0.5
(c)
0.0
10
100
1000
10000
Surface resistance (s m 1 )
Figure 22.12 Comparison between measured values of a and daily average surface resistance over (a) woodland,
(b) shrubland, and (c) grassland covers in southern Arizona, (Redrawn from Shuttleworth et al., 2009, published
with permission.)
although originally developed using initiation fields taken in the humid
climate of the Netherlands may have more general applicability.
Arguably the most significant aspect of Figs 22.11 and 22.12 is that they sug-
gest that for values of surface resistance less than about 100 s m −1 (a value typical
of many unstressed natural surfaces), the ABL feedback processes described
earlier in this section seem reasonably effective at constraining the evaporation
rate to be approximately 25% greater than the equilibrium evaporation rate.
This result is broadly consistent with the proposal of Priestley and Taylor (1972)
that Equation (22.30) is an 'appropriate framework' for apportioning surface
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