Observational Experimental Study Of Land Surface Process In Semi-arid And Rainfed Agriculture Region

The characteristic of surface radiation, energy balance,albedo,bulk momentum transfer coefficient CD and bulk sensible heat transfer coefficient CH between surface and atmosphere over rainfed field are analyzed based on the data of land-surface process field experiment in the Dingxi arid meteorology and ecological environment experimental station.The maximum of instantaneous global radiation is beyond 1000W/m2 and the ten-days average intensity is 338.90W/m2 because of high atmospheric transparency over semi-arid rainfed agriculture region of loess plateau between June and August.Global radiation, reflected global radiation and surface long wave radiation emerge the characteristic of daily variations on a typical clear day and cloudy day, whereas atmospheric long wave radiation has not this characteristic.Meanwhile,net radiation, latent heat flux sensible heat flux and soil heat flux appear the same characteristic as the mean daily variations above spring wheat field and bare soil on a typical clear day and cloudy day. Moreover, evapotranspiration consumes mainly net radiation on a clear day and sensible heat flux is greater than latent heat flux on a cloudy day above spring wheat field. Soil heat flux from atmosphere to soil is about 5% of net radiation on a clear and cloudy day and soil discharge energy to atmosphere on a rainy day above the same field. In brief, average latent heat, sensible heat and soil heat flux is 40%. 50% and 10% of net radiation respectively. On the contrary, the latent heat and sensible heat on a clear day is 60% and 19% of net radiation above bare soil surface. Similarly, the net radiation on a cloudy day is mainly used up evaporation of soil. The integrated value of soil heat flux is negative on a cloudy day andthat shows the soil emits the heat, which is different on a clear day. In short, average latent, sensible and soil heat flux is 67%, 21 % and 6% of net radiation respectively.The character of surface radiation and energy expended over soil temperature variation is that the temperature variation at surface, 5cm depth and 10cm depth are approximately sine curve on clear and cloudy day while this way the temperature variation at 20cm depth shows small extent anti-phase variation and the temperature under 50cm is no diurnal. In contrast, the temperature at 5m, 10cm and 20cm shows linear decrease and the soil-surface temperature is concaved on rainy day.The daily albedo variation, which is big in the morning and afternoon and smallest at midday, emerge parabola above spring wheat and bare soil surface. The biggest diurnal albedo is 0.24 and the smallest 0.11 .Moreover, the peak occurs on the day that radiation is very strong and the soil surface is dry and the valley appears on the day that rainfall is much more before this day and the soil water content of upper layer is high. In brief, the albedo of spring wheat and bare soil is 0.17 and 0.20.The mean bulk momentum transfer coefficient CD and bulk sensible heat transfer coefficient CH is 3.37*10-3 and 4.47*10-3,which is slightly larger than that above desert Gobi and grassland of plain and little than that above plateau. On one hand, the more unstable atmosphere is, the larger bulk transfer coefficients are under unstable conditions, on the other hand, and bulk transfer coefficients decrease as wind speed increases. Furthermore, bulk transfer coefficients also emerge obviously daily variations and are greater on the daytime and less in night.