The errors in Z-R Relationships include: Anomalous propagation (Causes overestimates) and partial beam filling which further results in underestimation, attenuation, and incorrect hardware calibration.
More errors in Z-R relationships include: Variations in drop size distribution, mixed precipitation types, bright banding due to precipitation near and below the melting level, and hail. Sleet, wet snow, and bright banding can cause overestimation of precipitation amounts where strong winds below the beam cause an underestimation. Evaporation is also an error at long ranges, which is seen as a donut around the radar, if the donut grows, it won't precipitate, and vice versa. Coalescence (merging of droplets) and snow also throw off values. Relate snow reflectivity to those measurements of ground observers.