Study Of The Characteristics Of Dew Formation And Its Influencing Factors In The Mu Us Sandy Land

The Mu Us Sandy Land is a semi-arid area,where evaporation is high and precipitation is scarce and unevenly distributed.The environmental damage and waste of water resources caused by the intensive development of the mineral extraction and agriculture and livestock industry in the Mu Us Sandy Land have been increasing seriously.At the same time,the ecological environment is more fragile and sensitive because sand prevention and desertification control measures are highly dependent on water resources.Although the amount of dew is small,it continues to serve as a crucial water source in semi-arid regions,playing a vital role in maintaining local ecological balance and supporting the survival of drought-resistant vegetation.However,due to the complicated process of dew formation and the difficulty in observing dew amount,the mechanisms of dew formation are still unclear.In addition,the role of dew in the ecosystems and water balance also is not clear due to a lack of long-term observation data.Therefore,studies are needed to further solve the above research gaps.To this end,we carried out field experiments in the Mu Us Sandy Land.A large-scale weighing lysimeter was set up to continuously observe the amount of dew(including frost,dew,and fog,note that we collectively refer to them as dew)under Artemisia ordosica and bare soil conditions during the period from 1 October,2020 to 30 September,2021.We explored the dew characteristics,and analyzed the process of dew formation and the influence of dew amount on water balance,as well as the significance of dew for the survival of Artemisia ordosica.Meanwhile,we designed micro-lysimeters to observe the water vapor sources of dew in September 2022.Additionally,the Boosted Regression Tree(BRT)model was used to analyse the influence of meteorological factors on dew formation.The contribution of meteorological factors on the dew amount was also quantified based on BRT models.The main conclusions can be drawn as follows:(1)We investigated the characteristics of dew amount under Artemisia ordosica and bare soil conditions at different time scales.The formation processes and the water vapor sources of dew are analyzed.The results show that the amount of dew under Artemisia ordosica and bare soil conditions was 47.10 mm and 35.83 mm,respectively.The ratio of dew amount from Artemisia ordosica and bare soil to precipitation were 19.6%and 14.9%,respectively.The dew amounts in colder months were larger than in warmer months.The number of days that dew occurred was 214 under Artemisia ordosica condition and 162 under bare ground condition,which accounted for 58.6%and 44.4%of the total experimental period.In addition,the daily dew amount ranging from 0 to 0.3 mm accounted for 78.0%(Artemisia ordosica)and 72.2%(bare ground)of the total formation days.The contribution of atmospheric water to dew was less than that of soil vapor.The ratio of contributions from atmospheric water and soil vapor to the dew was 2:3.(2)We explored the conditions of dew formation and analyzed the correlation relationship between night(20:00～08:00 of the next year)meteorological factors and dew amount at different time scales(daily and monthly).The BRT model was used to quantify the influence of meteorological factors on dew formation and provide the conditions of meteorological factors contributing to dew formation.The results show that the relative humidity was positively correlated with dew amount at both daily and monthly scales,while wind speed and air temperature at night were negatively correlated with dew amount at both daily and monthly scales.The correlation between nighttime meteorological factors and dew amount on a monthly scale is greater than that on a daily scale.The BRT results also clearly show that relative humidity,wind speed,net radiation,and air temperature were key factors to dew formation.Under Artemisia ordosica conditions,the contributions of meteorological factors to dew formation were:relative humidity(43.4%)>net radiation(21.7%)>wind speed(15.2%)>air temperature(12.2%)>vapor pressure deficit(7.5%).Under bare soil conditions,the contribution of meteorological factors to dew formation was:relative humidity(54.1%)>net radiation(14.9%)>wind speed(13.7%)>air temperature(10.2%)>vapor pressure deficit(7.1%).Based on the results of marginal effects,it was found that,dew formation can be promoted when the relative humidity was greater than 53%,the wind speed varied from 0.5 m/s to 1.80 m/s,and the net radiation was less than-37.0 W/m~2 under Artemisia ordosica condition.When the nighttime relative humidity was greater than 59%,the wind speed varied from 0.60m/s to 1.45 m/s,and the nighttime net radiation was less than-40.0 W/m~2,dew formation can be promoted under bare ground condition.(3)The significance of the amount of dew on the survival of Artemisia ordosica during the soil drought period was analysed.The results show that Artemisia ordosica was in soil drought period from 4 December 2020 to 3 February 2021 and from 21 May 2021 to 18 August 2021(The relative extractable water content less than 0.4).The summer drought period is longer than the winter drought period.During the winter drought period,the soil water content remained at about 0.08 cm~3/cm~3 in each soil layer.During the extreme drought period in summer(21 July2021 to 18 August 2021),the soil water content at depths of 20～60 cm remained at about 0.06cm~3/cm~3.This indicates that plant roots had difficulty obtaining water from the soil during the drought periods.During the extreme drought periods,the ratio of dew to precipitation was55.9%.(4)The role of dew in the water balance in the Mu Us Sandy Land was analyzed.The results show that the water balance error was 25.3%if the dew amount was ignored in under Artemisia ordosica conditions.After adding the dew amount to the water balance equation,the water balance error was reduced to 5.7%.The water balance error can be reduced from 29.8%to 14.9%if the dew amount was considered.Therefore,the dew amount could effectively improve the accuracy of the water balance and reduce the error caused by the part of water entering the atmosphere through evaporation and returning to the surface.The result has significance for improving the accuracy of water resource assessments.