Comparing Four Colocated Rain Gauges under Varying Weather Conditions

Six months of rainfall data collected by two (automated) tipping bucket rain gauges and two CoCoRaHS style collection gauges are compared. The tipping bucket gauges are of two different styles: a Texas Electronics TE-525 and Hydrological Services TB3 with syphon mechanism. In Location 1, three gauges are lined up: the TE is positioned 6 m to the right of the TB3 and the CoCoRahs gauge is installed 2 m to the right of the TE gauge. All 3 gauge rims are 1.5 m above ground level. A second CoCoRaHS gauge is installed in Location 2, approximately 36 m away from Location 1. This gauge’s rim is approximately 2 m above ground level. Both sites are extremely well sheltered as they are surrounded by trees and buildings, with the closest obstruction 20 m away from Location 1 and 10 m from Location 2. Both sites are co-located with a South Alabama Mesonet weather station in southwest Alabama that measures 11 meteorological parameters at 1-minute intervals.

Wind is a factor in causing gauge under-collection in rain gauges of all types and has been extensively studied. Since the gauges are co-located with a Mesonet station that measures wind speed at 2 and 10 m elevation, the extend of this effect at this location can be quantified and adjusted using results from prior studies. It is expected the effect will be minimal given the sheltered location of the site.

Narrow gauges suffer from under-collection in large drop rain storms which are common in the north-central Gulf Coast area. While the CoCoRaHS gauges are narrower than the tipping bucket gauges it is hoped this effect is minimal. By comparing differences between rain gauges during the cold and warm seasons, the effects from large drop storms on under-collection can at least be qualitatively determined.

As is well documented in the literature, tipping bucket rain gauges are notorious for under-collection due to two main reasons: 1) splashing as the bucket tips and 2) evaporation of rain remaining in a bucket that is not full enough to tip after a rain event ends. The loss due to splashing while tipping has been quantified in previous studies and correction factors will be used to adjust for this effect. Using the above corrections for wind and splashing-while-tipping, the effects from evaporation can be quantified by comparing tipping bucket versus CoCoRaHS daily (and longer) rainfall totals. An observer visits the site on a daily basis to monitor whether water is present in the buckets. An additional evaporation factor may be the syphon in the TB3 gauge. It has been observed that water remains in the syphon after a rain event. This water will add to the evaporation effect in this type of gauge. These effects will be observed in the field and will also be quantified in lab experiments. In the field, evaporation rates will be correlated with temperature, humidity, and solar radiation. Since the gauges are visited on a daily basis, animal activity, debris, and vandalism are minimal factors in gauge under-collection.