An Accurate Small Orifice Rain Gage

Abstract

An accurate, inexpensive, small-orifice rain gage (modified Victor-rain gage) with an orifice diameter of 20.2 mm was developed for use in rainfall networks and in agrometeorological and hydrological studies. It has a large capacity and can be installed and read easily. Comparison with standard rain gages showed good agreement under various conditions of exposure and with different observation techniques. Small amounts of rainfall are likely to be overestimated, owing to a film of water adhering to the glass water container of the gage, whereas hail tends to be underestimated. If the gage is intended for storage, the funnel tube has to reach nearly to the bottom of the inner vessel.

Precipitation

“…The overall comparison showed a 7% increase in catch for the plastic gage. Kalma et al (1969) also noted that the small orifice gages caught more rain than the larger, standard gages. Roper (1975) noted that the plastic gages collected dew and fog, which was not recorded by the standard gage.…”

“…The economics of constructed materials ordinarily mandates that smaller is better, but a lower limit of orifice diameter exists for rain gages if they are to function reliably and accurately. The lower limits are determined by susceptibility to evaporation and splash losses, ability to catch hail (Kalma et al, 1969), storage capacity, and measurement sensitivity.…”

“…The two smaller gages caught more rainfall in storms up to 5 mm than the larger two gages. Kalma et al (1969) noted that gages with a 2.92 cm sharp-edge brass orifice caught more rainfall, especially in light rainfall events. Their gages were equipped with a funnel and a glass tube as an inner measuring container.…”

“…Wetting loss also occurs due to adhesion of the water to the sides and bottom of the measuring container after it is emptied. If rain falls before this water is evaporated, the next rainfall event may be slightly overestimated (Kalma et al, 1969). Wetting loss increases as the area of gage material increases; for example, gages with a high magnification of catch have a larger wetting loss than gages that have a direct reading scale.…”

Locally modified surface winds on linear dunes as derived from directional raingauges

“…(c) Raingauges along the transect mentioned above, which eventually consisted of up to 38 measuring points, of which up to 25 points (on the dune slopes) were equipped with both a regular and an inclined raingauge. All raingauges along the transect, including those on the VPMs, were of the small-orifice type (6.75 cm 2 , after Kalma et al (1969)) and were installed about 35-45 cm above the ground in order to minimize windaffected measurement errors (Sharon, 1980). The efficiency of the latter was tested at the time by comparing resulting measurements with those in identically located pit gauges designed to eliminate that error (Sharon, 1980: 171, 174-176).…”