Studies on dew formation under semi-arid conditions

Abstract

In this study results of dew recordings by means of the Kessler dew recorder above various soil surfaces under semi-arid climatic conditions in the interior of South Africa are reported. Dew formation on lawn was heaviest in autumn but lowest in late winter and spring. Because dew deposition totalled approximately 12 mm over the year—or only 2% of the average annual rainfall—on 167 days, it was concluded that dew under the local conditions could not have a significant bearing on the water economy of the soil. However, in late summer and autumn dew occurred on 20 days per month and dew formation continued for 12–15 h at night. It was assumed that these conditions may reduce transpiration stress and favour the germination and spread of pathogenic spores on the plant surfaces. Compared with the amount of dew on lawn (100%), dewfall in growing wheat approached 200% when the sampling plate was placed slightly below the surface of the crop. The ratio between dew above uncropped soil and on lawn was approximately 55% throughout the year. It is emphasized that continuous dew recordings in crops are useful for field studies concerned with the effect of weather conditions on plant growth and plant diseases. In addition, standard recordings of dew occurrence on lawn are also necessary for reference purposes and for evaluations of the relationship between weather conditions and dew formation.