Environmental Engineering Reference
In-Depth Information
Bubble gages can drift or fail if sediment covers the bubble orifice. The system
will fail if the pressurized-gas source is exhausted or if the battery supply is
insufficient to power the pump. Output from submerged pressure transducers can
drift due to cable or wire stretch, slippage of the point from which the cable or wire
is suspended, or simply due to sensor electronic drift (formerly, a common prob-
lem). Non-contact sensors that make use of radar or sonar are sensitive to air
temperature and sensor height above the water surface. Floating debris, wind-
generated waves, rain or snow, and other falling debris (e.g., pollen) can further
degrade data quality.
Errors can result from sensor location as well. If the sensor is not located in the
deepest part of the wetland basin, the sensor might indicate that the wetland has
gone dry while there is still standing water in a deeper portion of the wetland. For
larger wetlands, strong wind can generate a seiche, or oscillation of water stage
associated with piling of water on the downwind side of the wetland. The effects of
seiche can be minimized by averaging multiple measurements over a period longer
than the characteristic period of oscillation, which is roughly proportional to the
length of the wetland, and inversely proportional to the square root of the depth of
water (Wilson
1972
).
3.4 Precipitation
Precipitation is the main driver of most wetland water budgets through direct
application to a wetland surface and indirect inputs via surface runoff and ground-
water discharge (Winter and Woo
1990
). Accurate measurements or estimates of
precipitation are essential. Compared to other water-budget components, precipita-
tion measurement at a given location (i.e., a point measurement) using a properly
designed precipitation gage is relatively straightforward and accurate. However,
there are a host of issues that can introduce error in these simple point measure-
ments, such as poor installation or maintenance of instruments. Scaling up from
point data, sparse data, or off-site data to precipitation distributed over a watershed
also increases the uncertainty of a representative value. In the following sections,
we will present methods for making point measurements, indicate potential sources
of errors, and then present methods for scaling from point measurements to
determining precipitation on a watershed scale.
3.4.1 General Consideration for Point Measurements
To obtain representative values of precipitation over an area of interest, the choice
of measurement site, the type and exposure of the instrument, the prevention of
evaporation loss, and the reduction of wind effects and splashing all are important
considerations (WMO
1994
:91). Ideally, a precipitation gage should be located
