The Impact Of Human Activities On Land And Atmosphere Interaction Over Cropland In The Arid Area

Land and atmosphere interaction is an important part of the earth science system, exchange law of energy and material between land and atmosphere that are accurately depicted is extremely significant to improve the accuracy of weather and climate prediction and even global change research. In recent years, land use and land cover changed by human activities on impact of environment and climate is one of the hot and difficult problems in the interaction between land and atmosphere research.In order to improve efficiency of agricultural production over arid area in northwest china, several kinds of measures on preventing sand wind and saving water are invented and spreaded. So far two measures, shelterbelts and plastic mulch, are widely used. In order to suppress soil evaporation and to improve water use efficiency in agricultural production, plastic mulch technology is generally used. The mulch is a white plastic film that is impermeable and airtight. The plastic mulch is not only significantly different from the soil and vegetation in physical properties, but also blocks water cycle process in the interaction between land and atmosphere, these two aspects will not only have an impact on surface albedo, but which will further affect distribution of energy and material in the land and atmosphere interaction. Meanwhile, in order to prevent the wind and dust to destruction of the oasis, rows of aspen are planted around the cropland. And to prevent sand wind efficiently, a row of aspen alternate with another row of aspen. As a result, cropland underlying surface with sparse shelterbelts have been formed, which is different from the dense forest underlying surface. The sparse shelterbelts in the oasis can decelerate the wind speed, which will change the micrometeorological characteristics, such as, surface roughness length and wind profile, influencing the atmospheric turbulence characteristics, thereby affecting the exchange law of energy and material in the land and atmosphere interaction.Land cover change by human activities over cropland underlying surface will affect characteristics of land and atmosphere interaction. Study on characteristics of land and atmosphere interaction over such typical cropland underlying surface is significant to go deep into understanding impact of human activities on land and atmosphere interaction. It can also help to improve and develop land process models that are applicable to the arid area, which will improve accuracy of the environment and climate prediction. It is also has a vital practical significance to the improvement of ecological environment. Using observational data obtained from the Gulang Heterogeneous Underlying Surface Layer Experiment(GHUSLE), the impact of human activities on characteristics of land and atmosphere interaction is studied over cropland underlying surface in the arid area in northwest china.The main conclusions in this study are summarized as following:(1) Using observational data from cropland station in GHUSLE, the variational characteristics of surface albedo of cropland covered with plastic mulch was analyzed at first, and then compared to the existing results; finally the parameterization scheme of surface albedo of cropland covered with plastic mulch was developed. The results show that:①Albedo change in early period in crop growth is same as that of bare soil covered with plastic mulch, its average value is0.26; albedo change in lush period in crop growth is same as that of dense vegetation, its average value is0.19.②During the dynamic crop growth period, albedo is between0.26and0.19, albedo increases with the rise of soil moisture, this is so significantly different from the existing relationship of surface albedo which decreases with the rise of soil moisture.③The existing parameterization scheme of albedo is not applicable on cropland covered with plastic mulch, while the developed parameterization scheme of albedo of cropland covered with plastic mulch is able to accurately depict dynamic change of albedo during the dynamic crop growth period.(2) Using observational data from cropland station in GHUSLE, the constant flux layer, dimensionless variance of velocity and spectra of velocity were studied and wind speed, temperature, specific humidity profiles and Businger-Dyer similarity relationship applicability were analyzed in the surface layer over cropland underlying surface with sparse shelterbelts. The results show that:①Sparse shelterbelts significantly change the airflow characteristics in the surface layer, the airflow are divided into the upper level in which the aerodynamic roughness is0.9m and the lower level in which the aerodynamic roughness is0.24m.②The surface layer is not a constant flux layer over cropland underlying surface with sparse shelterbelts in the oasis; the surface layer is closer to a constant flux layer as the windspeed increases.③The relationship between the dimensionless variance of velocity and the stability basically meets the1/3power relation for both above and within canopy layer of the shelterbelts. In approximate neutral stratification, the dimensionless variance of velocity at u, v, w direction above canopy layer of the shelterbelts are2.49,2.28,1.24; within canopy layer of the shelterbelts are3.30,2.80,1.41.④Above the shelterbelts, the contribution of energy is primarily low frequency eddy, the energy spectra of velocity at u, v, w direction meet the-1/3power relationship with frequency, the eddy corresponding to the spectra of velocity is anisotropy; within the shelterbelts, the contribution of energy is primarily high frequency eddy, the energy spectra of velocity at u, v direction meet the-1/3power relationship with frequency, while at w direction meet the-1/5power relationship with frequency, the eddy corresponding to the spectra of velocity is anisotropy.⑤82.10%of wind profile over cropland underlying surface with sparse shelterbelts in the oasis showed a’turning point’, which means the wind profile is discontinuous. Businger-Dyer similarity function can’t describe the vertical distribution of wind in the surface layer well; while the profile of temperature and specific humidity is continuous, Businger-Dyer similarity function can describe the vertical distribution of potential temperature and specific humidity well.The conclusions above show that human activities have significant impact on characteristics of land and atmosphere interaction over cropland underlying surface in the arid area of northwest china. Simultaneously these create the particularity of characteristics of land and atmosphere interaction over cropland underlying surface in the arid area of northwest china, such as albedo, vertical distribution of turbulent fluxes and wind profiler et al. Compared to the existing research results, these characteristics are significant different.