Environmental Engineering Reference
In-Depth Information
density for those points with only depth measurements. The amount of snow water
equivalent, commonly abbreviated as SWE, is calculated from the product of depth
and density. Details on snow course measurements are described in WMO
(
1994
:117-122).
In many studies, the amount of precipitation is recorded without differentiating
whether it is rain or snow. Local air-temperature data can be used to separate
rainfall from snowfall. The transition from snow to rain normally occurs at
temperatures between 1 and 3
C (e.g., Dingman
2002
:108).
3.4.5 Calibration and Maintenance
Precipitation gages need to be recalibrated at specific intervals to operate at their
stated accuracy. Detailed calibration procedures are specific to the instrument, and
are usually found in the manufacturer's operation manual. Calibration is simple for
manual gages; it involves accurate measurement of the gage orifice to confirm that
orifice dimensions are as stated in the manufacturer's specifications. If not, a
custom multiplier can be applied to the data. For weighing gages, known weights
of water are added to the gage to validate the gage reading. For tipping-bucket
gages, a known volume of water is slowly poured through the gage (e.g., 5-10 tips
per minute) and the total number of tips is used to determine the amount of
precipitation (mm) per tip, which can be different from the manufacturer's specifi-
cation by as much as 10 % (Marsalek
1981
).
Precipitation gages need to be periodically cleaned (at least once every field
season) to prevent clogging of the funnel opening, accumulation of dust in the
tipping buckets, and to minimize friction of all moving parts. Gages should be
checked to ensure they remain level. Any potential obstruction, such as fallen
woody debris or vegetation growing close to the orifice, should be removed.
Recording instrument calibrations and servicing in a maintenance record is useful.
Such information should ideally be kept in a metadata file accessible in the same
area as the data. This advice applies to collection of all automated data related to
wetland hydrology and eliminates confusion during subsequent data processing and
analysis, or when someone other than the operator uses the data.
3.4.6 Spatial Variability and Interpolation
Precipitation can have large spatial variability due to variation in elevation, topog-
raphy, prevailing wind direction, and distance from moisture sources or from urban
areas. For example, the amount of precipitation may systematically decrease on the
lee side of mountain ranges. Urban “heat islands” may have complex effects on the
local distribution of precipitation. Convective storms have a relatively small area of
influence, often resulting in a large difference in rainfall between sites located
