Dynamics and biophysical controls of nocturnal water loss in a winter wheat-summer maize rotation cropland: a multi-temporal scale analysis

Abstract

Nocturnal water loss (NWL), the evapotranspiration minus dew at night, has been identified to play a significant role in the surface water and energy balance. However, few studies have examined the variability in NWL in croplands with high water consumption across different timescales. Here, based on the eddy-covariance measurements of latent heat flux during nighttime, the dynamics and biophysical controls of NWL were examined in a winter wheat-summer maize rotation cropland in the North China Plain over a 5-year period (2008-2012) at nocturnal, seasonal and inter-annual scales. Besides, its relationship to drought was also revealed. Nocturnally, NWL showed a decreasing tendency from 20:00 to 05:00, with large diurnal variations in May. Seasonally, NWL was higher for wheat than that for maize. Interannually, the annual average daily NWL showed the same variation as the NWL during the wheat growth across the five years. The NWL was dominantly controlled by the difference between surface temperature and air temperature (Ts-Ta) nocturnally; Seasonally, NWL was influenced by vapor pressure deficit (VPD), wind speed (Ws) and site bulk surface conductance (gs) for wheat, and by VPD, gs and net radiation (Rn) for maize; NWL was mainly regulated by Ts-Ta and soil water content at 40 cm below the ground (VWC40) interannually. VPD and gs controlled NWL more during dry years. NDVI exhibited stronger control on NWL for wheat in wet years. The most favorable conditions for NWL during dry months were at Ta<16 ℃, VPD <0.8 kPa, and VWC40 <0.46 m3 m−3. NWL accounted for 4.0% of precipitation during the non-dry period but 19.6% during drought. Our results highlight the importance of biophysical factors in regulating NWL of the cropland ecosystem, and more attention should be paid to the influence of NWL on the water and carbon cycles in the context of climate change.