Field assessments on the accuracy of spherical gauges in rainfall measurements

In an effort to reduce wind effect on rainfall catch to a minimum level, Chang and Flannery (2001. Hydrological Processes 15: 643-654) designed two spherical orifices to modify the standard gauge and other gauges in use today. Because of the spherical shape, the two orifices will catch rain with an effective diameter always equal to the actual diameter. regardless of wind speed and direction. This report covers the testing of spherical gauges at two different locations, one at the City Landfill, Nacogdoches, TX, and the other at the NWS Forecast Office, Shreveport, LA. Based on 131 storms at Nacogdoches and 94 storms at Shreveport, observed between May 1998 and February 2001, the results showed: (1) spherical gauges recorded an average 6-9% greater than standard gauge and 3-4% less than pit gauge, only 1-2% less than reported in the original study; (2) the catch of spherical gauges was not significantly affected by three gauge heights at 0(.)91, 1(.)83, and 2(.)74 m above the ground, but catch by the standard gauge decreased with increasing gauge height; (3) improvements of the spherical gauges were most significant for larger storms and for winds at higher speeds; (4) the spherical gauge with cylinders recorded 1-2% more rainfall than the spherical gauge with vanes; and (5) correlation coefficients between catch deficiencies and wind speed were low and weak because of the distance and height of the existing wind sensor. Owing to greater surface wetting and evaporation loss, the spherical gauges may underestimate rainfall catch by standard gauge for small storms (generally less than 5.0 mm), especially on hot summer afternoons and for smaller storms. However, the underestimates do not overshadow the merits of spherical gauges, because the differences are too small to be of hydrologic significance. Using polyethylene or other synthesized materials to construct spherical orifices may improve the catch for small storms. The results of the study agreed with the previous claims that spherical,gauges are effective in reducing wind effects on rainfall measurements. The spherical gauges could greatly improve the accuracy of hydrologic simulations and the efficiency on the designs and management of water resources. They are suitable for large-scale applications. Copyright (C) 2004 John Wiley Sons, Ltd.