| The arid zone in central Ningxia is a typical arid inland area with sparse vegetation,scarce surface water sources,deep groundwater,long sunshine and strong evaporation,which is one of the areas with the most serious desertification in China.Surface sand coating can effectively restrain water evaporation and reduce soil water loss,which is one of the important ways to solve the problem of strong evaporation.Therefore,it is particularly important to clarify the law of soil evaporation in different stages and the factors affecting evaporation in different stages.It is particularly important to explore the relationship between soil water evaporation and influencing factors in different stages.Based on this,this paper takes the soil water evaporation in the arid zone of central Ningxia as the research object.in view of the unclear calculation method of soil water evaporation at different evaporation stages in this area,the soil water evaporation process is simulated by the method of field experiment.progress has been made in the following aspects:(1)Through the field experiment to observe the sand surface temperature,water surface temperature and air temperature in a long sequence of hours,it is found that there are some rules among them.The change rules of water surface temperature and surface temperature are basically the same under the same air temperature,both increase with the increase of air temperature,and show a trend of first increasing and then decreasing in time series.Using the air temperature to fit the water surface temperature and the compacted sand surface temperature,it is found that both of them show an exponential relationship,the correlation of the water surface temperature calculation model is more than 0.95,and the compacted sand surface temperature calculation model is 0.76.This experiment can provide some theoretical support for regional agricultural guidance which can not detect water surface temperature and surface temperature.(2)By setting up an evaporation dish to observe the water surface evaporation,it is found that there is a significant correlation between the water surface evaporation and the wind speed.The wind speed is segmented and the relationship between the corresponding wind speed and the water surface evaporation is studied.A method suitable for calculating water surface evaporation in the arid zone of central Ningxia is established.The method R~2 for calculating water surface evaporation is more than 0.90.When the wind speed is less than 4m/s,the water surface evaporation increases with the increase of wind speed,and it shows an exponential growth trend when the wind speed is less than lm/s,corresponding to R~2 more than 0.96.when the wind speed is in 1m/s-4m/s,the water surface evaporation increases linearly,R~2 is more than 0.90.when the wind speed is greater than 4m/s,the water surface evaporation increases abruptly,but still increases with the increase of wind speed,R~2 is more than 0.80.When the wind speed is greater than 4m/s,the water surface evaporation increases suddenly,but still increases with the increase of wind speed.In this experiment,the calculation model of water surface evaporation based on wind speed in the arid zone of central Ningxia is established,which can be used to judge the potential evaporation.(3)The first stage process of soil evaporation was observed by Markov flask combined with field soil column experiment.it was found that both water surface evaporation and soil evaporation of the first stage showed an increasing trend before 16 hours,and the evaporation of water surface was larger than that of soil in the first stage of compacted sand in each period,and the difference was 0.02～0.19mm.The model of soil evaporation in this stage is established from the point of view of mass and energy balance,the mechanism of the first stage of soil evaporation in pressed sand is revealed,and the effects of air temperature,wind speed and solar radiation on evaporation are explored.The accuracy of the model is high,R~2 is 0.85.(4)The second stage of soil evaporation was observed by measuring the quality of the micro-lysimeter and the water content of each soil layer.at this stage,the soil evaporation showed a fluctuating and decreasing trend,and when the evaporation reached 30 days,the change of evaporation was small,and the surface water content decreased slowly.compared with the surface layer,the soil moisture content increased.By establishing the relationship between evaporation and soil water diffusivity and water content,it was found that there was a strong correlation between evaporation and soil water diffusivity,R~2 reached more than 0.91.Through the simulation and verification tests of the established model in different hydrological years,it is found that the calculated evaporation is in good agreement with the measured evaporation.Among them,the water content in the rainy year is more abundant,and there is a certain error between the calculated evaporation and the measured evaporation,and there is a good agreement between the calculated evaporation and the measured evaporation when the weather fluctuation is small,R~2 reaches 0.87;the water distribution in the normal year is more uniform,and the fitting degree of the calculated evaporation is higher when it is less than 2mm/d,and R~2 reaches 0.94;in the dry year,the soil water content is lower,and the overall fitting degree is the highest,R~2 reaches 0.95.In dry year,the soil water content is relatively low,and the overall fitting degree is the highest,with R~2 reaching 0.87.(5)The applicability of the model was judged by comparing the difference between the measured value and the first and second stage calculation model of soil evaporation to simulate the whole soil evaporation process.The results showed that in the process of simulating intact soil evaporation,the calculated values fitted well with the measured values,and the average absolute error was 0.19mm.Thus,the feasibility and effectiveness of the soil evaporation calculation model in the arid zone of central Ningxia established in this paper are further verified. |
PDF Full Text Request