Assessing and Measuring Wetland Hydrology - Wetland Techniques - page 105

Fig. 3.5 Examples of precipitation gages equipped with a windshield. (a) a weighing gage
equipped with an Alter shield. (b) a non-recording gage equipped with a Nypher shield
3.4.3 Effects of Wind
Precipitation gages that project above the ground surface generate wind eddies that
tend to reduce the catch of the smaller raindrops and snowflakes (Dingman
2002 :109). This can be a major source of error in precipitation measurements.
The amount of measured precipitation relative to “true” precipitation is referred to
as gage-catch deficiency. The degree of deficiency generally increases with wind
speed and can be on the order of 20 % (Yang et al. 1998 ). Several types of wind
shield are commonly installed around a precipitation gage to reduce wind eddies
above the gage orifice (Fig. 3.5 ). Even with a wind shield, catch efficiency is
significantly less than one, especially for snowfall. Empirical correction formulas
can compensate for the negative bias caused by wind. Coefficients are usually
determined by fitting a regression curve to data that relate precipitation to wind
speed (Fig. 3.6 ). Unfortunately, these curves are fitted to what often are noisy data
sets (e.g., Goodison et al. 1998 :36-37). Therefore, protection from wind should be a
high priority in selecting a site for measuring precipitation, particularly for
measurements of snowfall.
3.4.4 Snowfall Measurement
Measurements of snowfall using a precipitation gage have additional challenges
even if wind effects are minimized. If a weighing gage is used, the reservoir needs