Biophysical controls of dew formation in a typical cropland and its relationship to drought in the North China Plain

Abstract

Dew, as an important contribution of non-rainfall water (NRW), is found to be an important hydrological component for ecosystem water stress alleviation and further ecosystem functions maintenance during rainless seasons. The North China Plain (NCP) is facing severe water shortage mainly due to intensive agricultural irrigation. However, few studies have examined the variations and controlling mechanism of dew in the typical croplands predisposed to drought. Here, seasonal variation in dew was examined during growing period in a wheat and maize rotation cropland in the NCP over 3 years (2008–2010), based on the eddy-covariance measurements of latent heat flux (LE) during nighttime, and the ecosystem drought was defined to occur when the relative extractable water content (REW) in the soil dropped below 0.4. Results showed that annual (from April to September) mean dew amount for this cropland was 7.61 ± 0.02 mm, accounting for 2.5 % of precipitation (PPT), and the annual mean dew amount during drought period was 3.11 mm, accounting for 3.4 % of PPT. Seasonal variation in dew was mainly regulated by relative humidity (RH) and surface stomatal conductance (gs), furthermore, dew was influenced by gs and vapor pressure deficit (VPD) during the wheat growing period, and by gs during maize growing period. 51.1 %–72.0 % of drought events occurred during wheat growing season. Compared to maize growing season with higher nocturnal evapotranspiration (ETN), the higher frequency of dew occurrence during wheat growing season had reduced the ETN amount. Drought timing, duration, and degree together influenced dew amount. Long-term drought (e.g. >1-month drought) would induce an increase of dew amount after drought period. Our findings clarified the temporal dynamics in dew across different crop growing periods (such as wheat and maize); and contribute to the sustainable utilization of non-rainfall water (especially dew) in cropland in the context of extreme climate (such as droughts) in the future.