Abstract
Compared to rain, dew is an important supplementary source of water for the survival of certain plants and animals in drylands. However, the hydrology of dew has not yet been fully investigated due to difficulties in measuring the amount and duration of it. In this study, a 3-year in-situ monitoring experiment was conducted from 2014 to 2016 in the semi-arid Sanyuan County, Shaanxi Province of China, using a leaf wetness sensor (LWS) and four associated meteorological instruments. Results showed that the average annual total dewfall was 32.8 mm with a daily maximum of 0.88 mm. The majority of daily dew occurred in the night from 18:00 to 8:00 with the maximum condensation rate occurring at around 4:00. The maximum dew residence time was about 18 h/day on the dew days in all seasons. However, the actual dew production period was about 14 h in spring (March–May), autumn (September–November), and winter (December–February), and only 11 h in summer (June–August). The maximum intensity and amount of dew always occurred in autumn (with an average amount of 12.2 mm or 37% of the annual total), followed closely by spring (11.4 mm, 35%), with much less in summer (6.6 mm, 20%) and winter (2.6 mm, 8%). The annual dew distribution by months showed a double crest variation, with two peaks in April–May and October–November, and two valleys in January–February and July. Comparatively, annual dewfall is only about 1/18th of the rainfall in this region, but the number of dew days (224 days, or 61% of year) is 2.6 times that of rain days (87 days, 24%), making dew a critical supplementary source of water for mitigating dry periods and supporting native plants and animals. Rain and dew are highly complementary as dew occurs in cloudless nights while the rain occurs in different and on much fewer occasions in the region. The dew amount was highly and positively correlated to the relative humidity of the air above the threshold of 81% (r = 0.78, p < 0.01), negatively correlated to the difference between air temperature Ta and dewpoint Td, when (Ta − Td) is less than 4 °C (r = −0.66, p < 0.01), and weakly correlated to wind speed (0.2 to 2 m·s−1), wind direction, surface soil moisture, and temperature. In the Sanyuan region, two general wind directions, 30°–90°and 210°–270°, were more favorable for the formation of dew.
Highlights
The formation of dew is a common and natural process in which water vapor in the air condenses on a cooler substrate and transforms into liquid water [1]
Numerous studies in recent decades have aimed at understanding the hydrology of dew, the hydrological significance, as well as utilization methods of dew
Results show that dew plays a more important role for water budgets in arid areas than in humid areas
Summary
The formation of dew is a common and natural process in which water vapor in the air condenses on a cooler substrate and transforms into liquid water [1]. The yield is relatively small, dew itself as a viable water resource occurs naturally and frequently in many locations globally, in arid and semi-arid areas and during dry periods in humid areas [4,5]. The frequency and duration of dew in Wageningen, Netherlands were 70% and 16.4 h/day, respectively, even though the average annual amount of dew only accounted for approximately 4.5% of the total precipitation (more than 800 mm) [6]. Results show that dew plays a more important role for water budgets in arid areas than in humid areas. In a semi-humid area such as the Sanjiang Plain in NE China, the average amount of dew accounted for 4.9% of the annual precipitation (410 mm) [12].
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