| The loess hilly region of China is belong to semi-arid climate,and the shortage of water resources is the key factor restricting the sustainable development of agriculture in this region.Over the years,unreasonable afforestation has caused a large area of soil desication.Jujube(Ziziphus jujuba Mill.)is tolerant of drought and is of significant economic value.It has been widely planted in the Loess Plateau region,but the phenomenon of soil desication also appeared in rainfed jujube planations.In arid and semi-arid ecosystems where soil moisture is scarce,norainfall water inputs(fog,dew,and water vapor adsorption)is a common and significant water resources.In the loess hilly region,the variability of land surface water processes affect rapid responses to land surface heating and eco-physiological processes.However,little is known about the quantitative characteristics of no-rainfall water inputs as water sources in a rainfed jujube plantation.It is particularly important to study eco-hydrological effect of non-rainfall water in rainfed jujube planations,this study is of great significance for a deep understanding of the water and heat balance processes of the soil-plant-atmosphere continuous,also is helpful to develop and utilize non-rainfall water resources and can provide a basis for maintaining the stability of ecological function in rainfed jujube planations.The rainfed jujube in the loess hilly region was taken as the research samples.Dew intensity and duration were monitored using dielectric leaf wetness sensors(LWS),Also Leaf area index,microclimate within canopy,soil water content and temperature,sap flow,and meteorological variables were respectively measured using winscanopy canopy analysis equipment,VP-4 sensors,GS-3 sensors,thermal dissipation probes(TDP)and weather station during jujube growing seasons from 2016 to 2020.Firstly,The meteorological data(1980～2017)in the loess hilly region were used to reveal the drought trend during the jujube growing seasons.Secondly,based on the formation mechanism of non-rainfall water,the data were used to analyze the characteristics of component of non-rainfall water and then to discuss the spatial and temporal distribution characteristics,relationship and influencing factors of non-rainfall water components.Thirdly,The variation rules of transpiration and evaporation,the relationship between non-rainfall water and evapotranspiration and contribution of non-rainfall water to evapotranspiration were analyzed to clarify the role of non-rainfall water in regional hydrological cycle.At the same time,the change rules of non-rainfall water and soil hydraulic redistribution and their relationships were studied under different soil water conditions basied on soil desication in the rainfed jujube planation.Finally,The Relative Humidity(RH),Dew Point Temperature(DPD),Classification and Regression Tree(CART),and Neural Network(NN)models were calibrated and validated by machine learning(decision tree and artificial neural network)for different scenarios(dew,rainfall,dew+rainfall)on half hourly and daily scales,and then the appropriate models for non-rainfall water at the hourly and daily scales was selected.The main conclusions are as follows:(1)At the monthly scale,precipitation increased in July,September and October,and reference crop evapotranspiration increased from May to August.Drought increased from May to June,but decreased from July to October.At the annual scale,the drought year was as high as 56.76%,and the annual precipitation can not meet the water consumption of jujube during growing seasons.(2)Non-rainfall water inputs in rainfed jujube planations were found to be composed of canopy dew and water vapor adsorption,their occurred more frequently and were quantitatively more stable than rainfall events.The occurrence days of canopy dew were lower than those of water vapor adsorption in the same period,but the leaf wetness duration(LWD)caused by canopy dew was significantly higher than that of water vapor adsorption(P<0.05);Canopy dew intensity reached the maximum value at 5:00～6:30,and water vapor adsorption mainly occurred from 6:30 to 9:00.Canopy dew intensity was higher than water vapor adsorption rate,canopy dew accounted for about 81%of non-rainfall water.(3)Dew was different in different spatial positions of jujube canopy.The frequency occurrence of dew at the top was higher than that at the middle and bottom of canopy position,up to 87.01%;dew duration(LWD)at the top was higher than that at the middle and bottom of the canopy position,and LWD at the middle and bottom of the canopy pisition was close,the LWD at the top of the canopy position had a good linear relationship with LWD at the middle and bottom of the canopy position.The accumulated amount of canopy dew(Iimax)at the top of the canopy position was higher than that at the middle and bottom of the canopy position,the Iimax at the middle and bottom of the canopy position changed from year to year.In years with heavy rainfall,The Iimax between different canopy positions showed a power function relationship.The Iimax at the top of canopy position was significantly correlated with LWD at the top,middle and bottom of canopy positions(P<0.05),The correlation between Iimax at the middle and bottom of canopy positions and LWD at the top,middle and bottom of canopy positions varied from year to year.LWD was affected by meteorological factors and soil water content,and the correlation between Iimax and meteorological factors varies with different canopy positions and years.(4)The occurrence frequency,duration and amount of canopy dew decreased with the increase of soil water deficit.Canopy dew amount and duration under ND condition were significantly higher than those under LD,MD and SD conditions(P<0.05).Hydraulic redistribution was affected by non-rainfall water,The frequency of hydraulic redistribution increased with the increase of soil water deficit in 20～60 cm soil layer,and it reached the maximum value under SD condition;Hydraulic redistribution was influenced by canopy dew,canopy dew enhanced hydraulic redistribution under the ND condition,but inhibited hydraulic redistribution under LD,MD and SD conditions.the mutual effects of canopy dew and hydraulic redistribution lessened soil water deficit,making it an important ecohydrological process affecting the response of rainfed jujube plantations to soil desiccation.(5)Canopy dew decreased night sap flow(18:00～6:00),but water vapor adsorption had no effect on evaporation.The maximum daily contribution of canopy dew to transpiration and evapotranspiration was more than 80%.The contribution of canopy dew and water vapor adsorption to evapotranspiration of jujube was 18.4%during the whole jujube growing season.Non-rainfall water inputs have potential positive ecological effects on rainfed jujube plantations with soil desiccation and are important components of the hydrological cycle in this region.(6)The RH,DPD,CART,and NNM empirical models calibrated by machine learning method can be used to well estimated dew duration on half hour and day scales in the scenario of dew-only than other scenarios(rain and all scenario).Considering the accuracy,practicability and number of variables of the model,calibrated RH model for dew-only scenario should be used to estimate dew duration for rainfed jujube planation. |
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