FOG DEPOSITION IN TALL TUSSOCK GRASSLAND, SOUTH ISLAND, NEW-ZEALAND

Hill cloud microphysics was examined with a forward scattering spectro meter probe over tall tussock grassland at Swampy Summit near Dunedin, New Zealand. In a 4-month period, cloud water drop diameter spectra w ere obtained for 17 cloud-ground intersection events that varied from 1 to 61 h in duration, Rain fell in most events, Easterly conditions m ade up 60% of the events and had a median cloud drop diameter of 13.9 mu m and 0.14 g m(-3) liquid water content, For southwest winds, media n drop diameter and liquid water content were significantly smaller at 9.8 mu m and 0.05 g m(-3). Aerodynamic conductance for momentum trans fer, measured with a three-dimensional sonic anemometer, varied linear ly with wind speed (R-2 = 0.7) between 50 and 70 mm s(-1) for wind spe eds from 4 to 9 m s(-1). Combining this relation with the cloud liquid water content measurements, cloud water deposition rates were estimat ed onto the 0.8-m-tall tussock canopy of one-sided leaf area index = 3 . These estimates were in the range 0.02-0.26 mm h(-1) with a median o f 0.05 mm h(-1) which is the same as the median rate derived from the water balance of a large weighing lysimeter containing eight tussock p lants. However, the water balance included a number of small quantitie s with determination of evaporation and rainfall being particularly di fficult. Consequently, the lowest cloud water deposition rates for the water balance were sometimes negative, Nevertheless, even the maximum estimated rates incorporated into an annual calculation suggest cloud water deposition is unlikely to be a significant component of the wat er balance of a tall tussock grassland catchment. In the uplands of so uthern New Zealand, changing land use from tall tussock to pastoral ag riculture with short vegetation is unlikely to alter streamflows via t he cloud water deposition process.