Geoscience Reference
In-Depth Information
Table 3.3 Effective film thickness f t for different plants (kg m 2 or mm)
Species
Thickness
Big bluegrass
0.203
Slough grass
0.102
Monterey pine
0.0762
Baccharis pilularis (Coyote Brush,
evergreen ground-cover shrub)
0.1778
Chaparral (dense thicket, California)
0.152
Annual ryegrass
0.127
between 0.6 and 1.0; a rough estimate of S ic can be obtained from
S ic = cf t La
(3.22)
where La is the leaf area index, which is the area (one side) of foliage per unit area of ground
surface, and f t is the maximum storage of water per unit area of foliage. Table 3.3 shows
a few values of f t for different plant species, collected by Merriam (1961); these values
suggest that 0.2 kg m 2 can be taken as an upper value. Table 2.9 shows values of the leaf
area index La for a number of plant communities.
Thus, with these assumptions, Equation (3.21) can also be written in terms of the leaf
area index as
L i = 0.2 nc (La + D )
(3.23)
3.5
RELIABILITY OF OPERATIONAL PRECIPITATION
MEASUREMENTS
Precipitation was probably the first hydrologic variable to be measured regularly on a
routine basis, and in many places in the world such measurements started more than a
century ago. Thus for a variety of purposes in hydrology, the availability of this historic
data base presents a useful opportunity. In principle the measurement of precipitation
should be a simple matter. It is important to be aware, however, that most of the available
records of precipitation from the past suffer from substantial systematic error and that
caution is required in their use. This has, of course, been known for a long time (see, for
example, Larson and Peck, 1974; McGuiness and Vaughan, 1969; Neff, 1977; Golubev
et al ., 1992; Duchon and Essenberg, 2001), but it is only in relatively recent years that
steps have been taken to remedy the situation. Although much remains to be done to
solve the archival precipitation data correction problem (Groisman and Legates, 1994),
a better understanding is gradually emerging.
Standard precipitation gages are usually placed with their orifice at some height above
the ground (from 0.5 m on up, depending on the type), primarily for convenience and to
avoid raindrop splash and snow drift. Thus one of the main factors is the distortion of the
wind field by the presence of the precipitation gage as an obstacle, which results in a wind
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