Abstract
Occult precipitation describes forms of precipitation that are typically not captured by traditional rain gauges, like fog, and represent a small, but potentially important component of total precipitation through the interception of water droplets onto plant surfaces. Occult precipitation is notoriously difficult to quantify, and there has been debate over the potential contributory role that fog has in the high water yields associated with Otago tussock grasslands. To ascertain how much occult precipitation was occurring as fog and horizontally-driven rain, a passive fog collector was situated over a tipping bucket rain gauge and monitored along with meteorological conditions at a field site in the Saint Marys Range in Central Otago, New Zealand. Over a two-year period fog contributed 32.2 mm, or 1.5% of total precipitation, whereas horizontal rain that formed as a precursor to rain (identified here as frontal fog) contributed 181.7 mm and was equivalent to 8.3% of the total precipitation recorded. Collectively, this occult precipitation contributed 9.7% of the total precipitation, and was more frequent during spring and autumn.