Revisiting "Ground Truth"—Assessment of Wind and Out-of-Level Collector Funnel Effects on Rain Gague Catch

Gauge measurements are widely used as the true rainfall reaching the land surface for a variety of applications, ranging from calibration of remotely sensed rainfall estimates to water budget studies. The quality of rain gauge data may be affected by human or animal interference, mechanical or electrical failure, or debris settling inside the collector funnel hindering rainwater from reaching the measuring device. Less obvious sources of potential rainfall measurement errors relate to the wind effect on the rain gauge catch and whether the instrument's funnel rim is level. These two particular effects are revisited here. It is shown that out-of-level gauges may either miss some or catch too much rainwater in windy conditions, with the associated errors potentially amounting to tens of percent of the total catch.