Detecting fog deposition to tussock by lysimetry at Swampy Summit near Dunedin, New Zealand

Fog deposition on narrow leaved snow tussock canopy has been suggested as an important component of the water balance and therefore as a reason for retaining this species as cover in water supply catchments in southern New Zealand. The objective of this study was to identify fog deposition periods and amounts using a large weighing lysimeter at a fog-prone tussock grassland site at 736 m elevation near the city of Dunedin on the east coast of the South Island. The lysimeter water balance was solved for the sum of the unknown fog deposition (F) and evaporation (E), using measured rainfall (P), storage change (ΔS), and drainage outflow (Q): (F + E) = ΔS - P - Q Periods when the sum of these two unknowns is positive were considered as possible periods of fog deposition. There were 40 such days in 3 years of record and 70mm of apparent fog deposition. This is less than 2% of measured rainfall, and we cannot discount the possibility that the amount calculated could be the result of undercatching by the rain gauges. The weather on the 40 days is, however, indicative of cloud-ground intersection at the site. The median fog deposition rate was 0. 05+/0.03 mm hr⁻¹ which is similar to measurements elsewhere over forest, pasture, and moorland. We conclude that fog deposition is not important in terms of water supply from tussock catchments in southern New Zealand.