DEW AND GUTTATION - FORMATION AND ENVIRONMENTAL SIGNIFICANCE

Observations were made of dew and guttation droplets on the grass Holc us lanatus L. (Yorkshire fog) at a site in rural Norfolk, UK, in 1985. The mean overnight dewfall was 0.14 +/- 0.02 mm (N - 40 nights). Gutt ation droplets found on grass blade tips had an average diameter of 1. 49 +/- 0.16 mm (N = 1200 measurements), compared with 0.20 +/- 0.02 mm for true dew droplets (N = 550 measurements). The average total volum e of guttation water exuded per grass blade per night was 1.0 +/- 0.3 x 10(-7) dm3, which represents about 0.1 mm of precipitation; guttatio n supplied about the same amount of water as dewfall to a short grass surface. About 8% of the mean daily June August net radiation in south ern England would be needed to evaporate the average dew and guttation -derived leaf wetness, which totalled 0.25 +/- 0.04 mm. Guttation amou nt was significantly correlated with soil temperature and moisture (P < 0.001, r2 = 0.874). The diameter of dew droplets was proportional to gravimetrically measured dewfall amount. The average total surface ar ea of dewdrops on short grass was about 11 m2 m-2 ground area. The cor responding average for guttation was about 5 M2 M-2 ground area. The m odal pH of a mixture of dewfall and guttation was 5.0-5.2. Chemical an alyses of mixed samples of dew and guttation showed a ten-fold super-s aturation with respect to CO2. The equilibrium partial pressure Of SO2 in these bulked samples was very low (10(-4) 10(-3) Pa), an order of magnitude lower than atmospheric concentrations. The half-life Of SO2 with respect to oxidation in the dew and guttation mixtures was a few hours.