| Semi-arid areas of Northern China are the transitional and sensitive zone between the semi-humid and arid areas, which are suffering the severe aridity trend, ecological environments deterioration, and the increasing dust storm and haze, as a result of the climate change and human activities. It seriously affected people’s life and productions in this area, and has also become one of the key issues of sustainable development strategy that need to be addressed urgently in China. Therefore, investigating the physical and biochemical processes between the land surface and the atmosphere interactions, and exploring the mechanism and strategy of the land-atmosphere interactions over the arid and semi-arid areas, will significantly improve the prediction of the regional climate models, protect its fragile ecological environment, and reduce the negative impact on human activities.The long-term characteristic of energy budget, hydrological cycle, the CO2flux exchange and the main land surface parameters, as well as the effect of precipitation on the land-atmosphere interactions in different climate background and years, were investigated by using the continuous observation data from the Semi-arid Climate and Environment Observatory of Lanzhou University (SACOL), the degraded grassland and the cropland surface in Tongyu, and the intensify experiment in Jingtai hill and farmland and in Xinglong mountain cropland (XM) in this study. The main results are as follows:(1) SACOL can well representative for the climate characteristic of the study area, by comparison with53-year climate data in Yuzhong meteorological stations. The53-yr-average annual precipitation at the Yuzhong meteorological station was37.2mm, while the6-yr-average annual precipitation was428.2mm. It was significantly larger than the climatic average, but had large inter-annual variability, and more than80%of the precipitation occurred in the Julian day121～306. The annual total potential evaporation and actual evapotranspiration were780-1020mm,280～490mm, respectively. Total precipitation, precipitation intensity, and its spatial and temporal distribution will change the characteristics of soil moisture. Meanwhile, the energy partitioning and ecological environment of this region will be affected. However the increasing total annual precipitation cannot always decrease the impact of drought on the semi-arid areas. (2) As result of the differences in terrain conditions and the local water vapor, although the isolated oasis Xinglong Mountain and SACOL were affected by the same weather system, the characters of precipitation distribution were significantly different. Not only the probability of the precipitation occurs and heavy rainfall occurs at XM were larger than at SACOL, but also the average precipitation intensity was higher too, and total precipitation at XM is25.9%more than SACOL. The water vapor pressure deficit of Xinglongshan was obviously lower than SACOL, and the air at XM was more humid, as the sufficient water vapor conditions at XM. In addition, the net radiation, which is used to heat the atmosphere, evaporate water vapor and transfer the heat from surface layer to the deep, was much less than it at SACOL during the growing season, it was mainly affected by cloud.(3) The variation of soil moisture was mainly depend on the distribution, intensity and duration of precipitation, and the soil moisture had bimodal characteristics at SACOL, as result of the snow melt in spring and the precipitation during the growing season. In the case of abundant rainfall, surface soil moisture was rapidly increased, while the deep layer followed behind. Surface soil moisture will also decrease rapidly, when there was only little precipitation or long-term without precipitation. But the soil moisture below0.80m deep almost not affected, and only increased after a period of long-term sustainability heavy precipitation. The most difference between SACOL and XM is the vertical structure of soil moisture, that soil moisture decreased with soil depth, conversely XM increased (0～0.50m). In the arid and semi-arid regions, soil moisture can not only changed the surface albedo and soil thermal properties, but also change the aerodynamic roughness indirectly, and then affected bulk momentum transfer coefficient, bulk sensible heat transfer coefficient and energy partitioning by influencing vegetation growth conditions.(4) The surface energy balance unclosure is common in the observation analysis over the semi-arid area. The energy imbalance ratio was usually about30～50%. It can be reach55%during our analysis at XM. While given the surface heat storage calculated by TDEC, the energy closure rate can improve approximately8%. Whereas the energy closure in autumn was3%less than it in spring and summer, it may be due to the influence of the vegetation canopy. As a result of the stability of atmospheric and the uncertainty of surface soil heat flux, the energy balance rate was lowest in the whole year. Under natural land surface in the semi-arid area, the mean energy closure ratio was about0.68. Although, the energy closure was very poor at night, compared with the whole day analysis, it only affected the closure about2%. High closure errors are unacceptable in the atmospheric models, so it was need to be further study, especially the closure difference due to the regional diversity. (5) Whether semi-arid region over the Loess Plateau, or semi-arid region in northeast China, sensible heat flux was dominant in the energy partitioning, accounted for35～40%of the net radiation, latent heat flux accounted for21～23%, while the surface heat flux was lowest only6-11%, but had obviously seasonal variation. Associating with the precipitation in summer and autumn, soil moisture increased, and the proportion of sensible heat flux was significantly decreased, while the proportion of latent heat flux was increased among the available energy, and gradually exceeded the sensible heat flux, then the Bowen ratio was close to1.0or less1.0. The total amount, distribution and intensity of precipitation, followed by better soil water retention, as well as relatively moist local circulation, which can provide sufficient water for the latent heat of evaporation and the underlying surface plants growing, it was the main mechanism that the isolated oasis can be maintained in the semi-arid area over Loess Plateau.(6) With the temperatures raised and soil moisture increased during Julian day180-306, plant growth reached its peak, the rate of carbon absorption was also higher than that in the first half year, but when it started is mainly dependent on the spatial and temporal distribution, intensity of precipitation, and soil moisture. In the case of year, which was much less precipitation and higher potential evaporation, as well as soil moisture cannot provide sufficient water for evaporation, or precipitation was much in the first half year, but low in the second half year, were both not conducive to the growing of underlying surface plant. The maximum monthly averaged CO2flux appeared at SACOL, only when the soil moisture at SACOL reached it maximum during the growing season, with high relationships with inter-annual variability and monthly total amount precipitation. But the maximum monthly averaged CO2flux strongly depended on the crop phenology of the underlying surface at XM. Therefore, the maximum was not appeared at the month when the soil moisture was highest at XM, it seems that the affect of soil moisture was not much sensitive to the growth of crops at XM man that at SACOL.(7) Through the analysis of relationship between the soil moisture and other variables or parameters in different semi-arid region, differ underlying surface vegetation type, and the isolated oasis in the Loess Plateau, it can be sufficient described that the soil moisture is not only the most important and sensitive variable on energy budget, hydrological cycle and CO2flux exchange, but also the most important characterization factor on land-atmosphere interactions in the climate change over the arid and semi-arid regions. Not only reflected on that the soil moisture can directly represent the dry and wet, drought conditions, but also on that the soil thermodynamic parameters was directly changed by the variation of soil moisture. Surface soil moisture can directly affected the surface albedo and energy partitioning in short temporal scale, while the soil moisture in deep layer can be affected the energy and hydrological cycle in large temporal and regional scale. However, the soil moisture is mainly dependent on the spatial and temporal distribution and intensity of the regional precipitation over the arid and semi-arid areas. |
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