Modeling The Water Infiltration And Evaporation In Unsaturated Zone Of The Extremely Arid Area

Soil water is an important constraint factor of the stability of unsaturated zone of extremely arid area and dune.Precipitation is the main recharge source of soil water in desert area,and which type of precipitation event will supply soil water and groundwater is a hotspot of the research in unsaturated zone of arid desert field.However,because of the limit of observation and the difficulty of conducting experiments,people still have little knowledge about the water infiltration rule in unsaturated zone of extremely arid aeea.Therefore,new researches and findings on the water infiltration rule in unsaturated zone of extremely arid aeea brings essential scientific influence on the field of the regional water balance.Based on the observation and simulation experiments in the Lanzhou University eco-hydrological laboratory in Dunhuang,we conduct the related experiments and analyze the precipitation feature in the “rainfall season”,soil moisture,temperature and the dynamics of soil water potential between 2014 and 2016 on the basis of the precipitation and climate factor in this region.After that,we analyze the water balance rule between the evaporation and infiltration,and simulate the soil water movement.Finally,we could reveal the rule of different types of precipitation between evaporation,infiltration and recharge in extremely arid area.This could provide the statistic support and scientific research to the rule of water cycle,sustainable utilization of water resources and the stability of desert in the unsaturated zone of extremely arid aeea.This paper shows:The observation of annual precipitation in research area is between 37.52 to 50 mm,and the variation range is between 3 to 103.8mm.There is a positive correlation relationship between the precipitation and the altitude of observation spot,and the precipitation variability decreases with the increase of altitude.The type of precipitation in this region is summer-rainfall,and the amount of precipitation in summer accounts for 56.69% of the annual rainfall;the maximum of precipitation mainly happens in June and July;the rainfall in spring and summer is over 80.38% of the total;the day-scale precipitation is little and its time is short;0-5mm precipitation,as the main precipitation events,occupies 91.28% of the total precipitation events.The diurnal precipitation events are clarified to CP,OAP,RP and EP by the boundary of day precipitation of 5mm,10 mm and 25 mm.We could see that CP is the main type of precipitation,and the probability of RP and EP are low by the observation from 2014 to 2016.By the artificial modelling precipitation experiments and the observation of natural rainfall,we find that the soil from 0 to 5 mm is impacted greatly by the precipitation events in the research area,and the precipitation over 3 mm could influence the moisture content in the 0-5mm sand zone,and this zone is the area where the volumetric water content changes most.In this zone,the water content gets to peek immediately after rainfall,and then decrease rapidly.During 3 to 5 days,the water content in this area becomes the initial number,and in the condition of none natural water recharge,the surface water content ranges in 0.056±0.003m3m-3,which is the steady condition of dry sand water content in laboratory.40 cm is the lower limiting depth of infiltration influence of CP,OAP and RP events in this region,and the sensor in the soil from the depth of 50 to 120 cm only respond to the heavy rainfall events with hysteresis quality,for example,the response time of the water content in 120 cm soil zone after the 35 mm rainfall event is 520 hours.Similarly,the depth of infiltration increases as the precipitation rises,and there is good exponential curve fitting relationship between the infiltration depth and time changes.The dynamic change of soil water potential proves that the moisture movement,OAP and over OAP could make the response of soil water potential(under 20 cm)change,and the form and moving down of zero flux potential after rainfall events are the fundamental power to drive the movement of soil moisture.The temperature of ground surface and the air temperature have close relationship,and the impact to ground temperature from air temperature decreases with the depth increasing.The precipitation events bring the sharp change of ground temperature,and the depth influenced is until 120 cm.The actual evaporations of the observation period from 2014 to 2016 respectively consume 89.9%,98.6% and 155.4% of natural rainfall in the same period.Soil water content produced in the rainy season of 2014 and 2015 accounted for 10.2% and 1.4%.The soil water is in the red after observation in 2016.The accumulate evaporation capacity of CP precipitation event is more than 127.5% of the same period's precipitation and the precipitation will evaporate rapidly within 1 to 3 days until it is entirely consumed.OAP precipitation events accumulate evaporation consumption is more than 99.8% of the same period's precipitation.In the extreme drought area,CP and OAP event both belong to invalid precipitation scope.The precipitation of PR event during the observation period evaporation accounted for more than 68.7% of the same period's precipitation,which was an important constituent for producing water storage in rainy season.These two simulative EP events accumulated evaporation capacity,which accounted for 65.37% and 84.91% of the same period's precipitation in interval after 20 days and 22 days.In addition,water storage of rainy season accounted for more than 15.89% of the same period's precipitation.Under the different depth of soil cumulative infiltration volume,it showed that in the absence of heavy precipitation event period,CP,OAP could not produce effective water storage.Soil water only in the surface layer of the RP events within 40 cm,consumption evaporates.Extremely rainfall events only produces infiltration in 50 cm or deeper,such as 30 mm and 35 mm could reach to 3.3mm and 4mm respectively 666 h and 497 h after the event,infiltration capacity was up to 50 mm.The study found that,day scale actual evaporation after the rain on evaporation of the first day or precipitation change with time t after the rain to the derivative of the correlation function,the fitting area actual evaporation experience formula: Ea=(0.9447E1 + 0.4097)/t,which can better simulate the Dunhuang area day scale actual evaporation rate after the rain.Through observing soil water balance on May 1,2015 and April 30,2016,we can see that the surface of soil water change had obvious phase differences: from May to September(water storage),it is water supplement;from September to December,it is heavy losing water stage;from December to next May,it is the weak water losing stage.When the total annual precipitation of observation period is 102.82 mm,the actual evaporation cumulative consumption was 100.94 mm.Cumulative evaporation consumption accounted for 97.5% of the total precipitation during this period,the soil water storage was only 1.88 mm,which further evidence that it derived from the heavy rain events.The coupling of non-isothermal module and gas migration in the model HYDRUS-1D,response results of the model to the depth of the different types of precipitation,and the response amplitude has good simulated result.The test station model developed soil water migration,based on this,soil water content and temperature can obtain a better simulation results.At the same time,the study find that under the natural condition of rainfall is 55.8 mm,actual consumption is 53.9mm,infiltration supplementary amount is 1.9mm,which accounted for 3.4% during the precipitation of corresponding period.The natural experiment simulation results further show that heavy rain events is an important source of deep soil water supply and the recharge rate of natural state are consistent with the results of experiment stations observation simulation condition.Through 1954-2013 scale precipitation,infiltration simulation results also find that the water balance has obvious loss and surplus in the unsaturated zone of extremely arid aeea area.In addition,at the end of the year with no infiltration is about 30 years.The regional simulation average annual evaporation capacity and the actual rainfall are respectively 38.03 mm and 38.87 mm.The annual average infiltration volume is only 0.84 mm.Combined with the research results;infiltration simulation value was slightly higher than the actual observed value.Annual average infiltration accounted for fewer than 2.16% of the precipitation in the extreme arid desert environment,which could not form area effective supply.