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Dew is an important water source for plants in arid and semi-arid regions. However, information on dew is scarce in such regions. In this study, we explored dew formation, amount, and duration of rain-fed jujube (Zizyphus jujube Mill) trees in a semi-arid loess hilly region of China (i.e., Mizhi County). The data included dew intensity and duration, relative humidity, temperature, and wind speed measured from 26 July to 23 October, 2012 and from 24 June to 17 October, 2013 using a micro-climate system (including dielectric leaf wetness sensors, VP-3 Relative Humidity/Temperature Sensor, High Resolution Rain Gauge, and Davis Cup Anemometer). The results show that atmospheric conditions of relative humidity of >78% and dew point temperature of 1°C-3°C are significantly favorable to dew formation. Compared with the rainfall, dew was characterized by high frequency, strong stability, and long duration. Furthermore, heavy dew accounted for a large proportion of the total amount. The empirical models (i.e., relative humidity model (RH model) and dew point depression model (DPD model)) for daily dew duration estimation performed well at 15-min intervals, with low errors ranging between 1.29 and 1.60 h, respectively. But it should be noted that the models should be calibrated firstly by determining the optimal thresholds of relatively humidity for RH model and dew point depression for DPD model. For rain-fed jujube trees in the semi-arid loess hilly regions of China, the optimal threshold of relative humidity was 78%, and the optimal upper and lower thresholds of dew point depression were 1°C and 5°C, respectively. The study further demonstrates that dew is an important water resource that cannot be ignored for rain-fed jujube trees and may affect water balance at regional scales.

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Leaf wetness duration (LWD) affects the ability of tree canopy to regulate ecohydrological processes. Jujube (Ziziphus jujuba Mill.) is one of the main plants with significant impact on water cycle process and flux characteristics in the Loess Plateau region of China. However, little is known about LWD in rainfed jujube plantations in the loess hilly region of China. For the causes of leaf wetness in rainfed jujube plantations, dew-only, rain and the combined scenario were investigated in terms of LWD and meteorological variables for the 2012, 2013, 2017 and 2018 jujube growing seasons. The results were as follows: The leaf wetness events occurred frequently, and the days of leaf wetness driven by dew-only and rain accounted for 88.3% of the total number of days in a regular jujube growing season. LWD driven by dew-only was significantly lower than that driven by rain (p < 0.05). The correlation between LWD and meteorological factors varied with different scenarios. In dewonly scenario, the four calibrated models (RH, DPD, CART and NN) accurately predicted LWD. In rain scenario, only the calibrated NN model performed well. In the combined scenario, the calibrated RH, CART and NN empirical models performed nearly equally; all well estimated LWD. On the basis of accuracy, practicability and variable count, the calibrated RH, NN and RH models for dew-only, rain and the combined scenario can reliably estimate LWD in rainfed jujube plantations in the loess hilly region of China.

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