The difference between ground soil and air temperature (Ts-Ta) was analyzed and studied by using the data of ground and air temperature of 99 stations over the Qinghai-Xizang (Tibet) Plateau from 1960 to 2000, and its spatial distribution and time changing tendency have been diagnosed by principal component analysis and power spectral analysis methods. The results show that the values of (Ts-Ta) are the biggest in June and the smallest in December. The first three loading eigenvectors reflecting the main spatially anomalous structure of (Ts-Ta) over the Qinghai-Xizang Plateau contains the contrary changing pattern between the northwestern and the southeastern regions, the pattern response of the sea level elevation and the geography, and the pattern response of the distribution of the permafrost. There are four patterns of time evolution including the patterns of monotonous increasing or decreasing trends, the basic stability pattern and the parabola pattern with the minimum value. (Ts-Ta) has a periodic variation of about 2 years. According to the spatial distribution of the third loading eigenvector of (Ts-Ta) over the Qinghai-Xizang Plateau in cool season, the permafrost response region and the seasonal frozen ground response region are identified.
Freeze-thaw erosion is the third largest soil erosion type after water erosion and wind erosion. Up to now, being restricted by many factors, few researches on freeze-thaw erosion have been done at home and abroad, especially on the assessment method of freeze-thaw erosion. Based on the comprehensive analysis of impact factors of free-thaw erosion, this paper chooses six indexes, including the annual temperature range, annual precipitation, slope, aspect, vegetation and soil, to build the model for relative classification of freeze-thaw erosion using weighted and additive methods and realizes the relative classification of the freeze-thaw erosion in Tibet with the support of GIS software. Then a synthetic assessment of freeze-thaw erosion in Tibet has been carried out according to the relative classification result. The result shows that the distribution of freeze-thaw erosion area is very extensive, accounting for 55.3% of the total land area of Tibet. The spatial differentiation of freeze-thaw erosion with different intensities is obvious and the difference in distribution among different freeze-thaw areas is also obvious.
Standardization of tree-ring data is one of the most important procedures in dendroclimatology. We used abundant Qilian juniper (Sabina przewalskii Kom) tree-ring samples along the eastern margin of the Qaidam Basin to approximate the growth trend to generate a total growth trend curve. We used samples that contained complete pith reaching to a growth culmination in 40-60 years to estimate the total growth trend curve, and then employed the generalized negative exponential function to fit the curve. Usually, most sample cores cannot reach the tree pith for various reasons and it is difficult to determine the trees' cambial ages. An empirical model of initial radial growth (IRG) was developed to estimate the number of rings missing from the pith based on the same tree-ring data and the IRG model explained 90.9% of the variance. When constructing the chronology, the cambial age of the first ring from the pith is regarded as 1 year. Then the ages of the rings in samples without piths were determined by the estimated numbers of missing rings in the cores. Standardization was accomplished by dividing each tree-ring series by the corresponding values of the total growth trend curve. The chronologies developed by this method can preserve more low-frequency variability. This method helps to construct more reliable tree-ring width chronologies.
Tree-ring standardized chronologies are developed with 78 cores obtained from eastern and western Mt. Helan. Both STD and RES chronologies correlate negatively with the different periods of early half year temperature, especially January to July (August) (JJ/JA) temperature. It has evident physiological meaning that wide rings were formed when JJ(JA) temperature was low and narrow rings were formed when JJ(JA) temperature was high. Based on this analysis, we reconstructed mean temperature from January to August in the period 1776-1999, and the explained variance is 43.3% (F = 21.422，p < 0.001). The comparatively high temperature periods in reconstruction are 1805-1818, 1828-1857, 1899-1907, 1919-1931, and 1968-1995; and the comparatively low temperature periods are 1858-1872, 1883-1895, and 1935-1953. Ten year moving average curve presents three slow uplifting periods of 1766-1853, 1862-1931, and 1944-1995. Each temperature increase was followed by a sudden temperature decrease of about ten years, that is to say, the JA temperature has the character of slow increase and sudden decrease. The 70- and 10.77-year periodicities detected in the tree-ring growth series correspond respectively to the Gleissberg (80a) and Schwabe (11a) periodicity of solar activity, and the 2.11-2.62 year cycles are considered to be influenced by QBO (Quasi-Biennial-Oscillation) and the local environmental change.
In 2005, when studying the process of historic sandy desertification on Erdos Plateau, we found that a lot of ancient city ramparts were fortified by alternate layers of sand and soil, and these cities included Shi'er Lian Ancient City (located in Zungar Banner, Inner Mongolia, China), Dabaodang Ancient City (located in Shenmu county, Shaanxi province, China), Bayanhurihu Ancient City (located in Otog Front Banner, Inner Mongolia, China) and so on. They are located in deep desert now. Several researchers have thought that the fate of these ancient cities is associated with natural environmental conditions. However, our experimental result does not support this view. The discovery sheds new light into when and how sandy desertification occurred and provides new historic insight for eco-environment conservation in this district. Through the experiment, some conclusions are drawn as follows: (1) The sandy layer in the ramparts consisting of aeolian-sand which should deposit before the cities were built. Considering the time when these cities were built, we could conclude that the desert scene had existed before the Han Dynasty. Some researchers pointed out that the existence of these cities were associated with good natural conditions when they were built, while our experimental result did not support this view. Accordingly, the existence of these cities could not be an evidence to prove good environmental conditions on Erdos Plateau in historical times. (2) It is an effective way to analyze the environmental change and process of man-land relationship through the fate of the ancient cities, relics, traces of human activities and changes in wasteland reclamation. However, it is not accurate to quantify the time when sandy desertification occurred through the time when these relics were abandoned. Undoubtedly, farming activities in historical times to a certain extent destroyed vegetation on Erdos Plateau, thereby accelerating the process of sandy desertification, but it is not the main factor leading to sandy desertification before the Ming Dynasty. (3) It is credible that sandy layer in the ramparts as a direct sign to interpret the environmental background, so it is very useful for us to explain the process of sandy desertification in historical times. Through the discovery, mobile sand dunes, fixed sand dunes, meadows in low humid lands, lakes, and so on, can coexist on Erdos Plateau in prehistory and human history. Up to now, these environments still coexist in the district.
Based on the data of annual mean groundwater level, we divided the groundwater levels into six environmental gradients and data were sampled repeatedly six times from vegetation plots for each gradient of groundwater level. Through analysis of the changes in vegetation coverage, species diversity and structure of plant communities along the gradients of groundwater levels, the results were obtained to explain the degraded process of vegetation. Meanwhile, we analyzed the mechanism of vegetation degradation. The results showed that: (1) Degradation of richness of herbaceous plants happened at a depth greater than 4 m of groundwater level, while that of woody plants started at a depth of 8 m groundwater level. (2) The decrease of vegetation coverage was due to decline of herbaceous coverage. The threshold depth of groundwater level for coverage degradation was 4 m. This depth was the same as the depth of groundwater for degradation of species diversity. However, when the depth of groundwater was under 6 m, the decrease of vegetation coverage was due to decrease of coverage of woody plants. (3) Changes of two typical plant communities in vertical structure and species were characterized by the degraded succession of plant communities. In the process of degradation of Phragmites communis the dominant species was replaced, but the dominant species of the Populus euphratica community, namely Populus euphratica, retained its dominant position in the process of degradation. (4) The degradation of desert riparian ecosystem was resulted from interior vulnerability of the ecosystem and extrinsic human disturbance. The increase of population and requirements caused the increase in human disturbance, while the immediate reason of vegetation degradation was human overconsumption of the environmental resource - water, which was a dominant factor for vegetation survival. In the process of degradation of ecosystem, plant functional types and intensity of disturbance impacted the extent of vegetation degradation.
By the wind tunnel experiment, we studied the deflation rates of 8 different sediments in arid regions of China, discussed the sources of aeolian sand and its influence on the development of sand dunes and formation of desert from the view of dynamics of wind erosion. Results of the study indicated that the average deflation rates of 8 typical sediments in arid regions of China can be arranged down: lacustrine sand > alluvial sand > weathered particles developed from sandshale > pluvial sediments > fluvioglacial sand > weathered particles developed from granite > slope deposit > glacial sediments. The deflation rates exhibited strong positive correlations with the erodible fraction—sand (0.063-2mm) content and sorting features. In contrast, the deflation rates have obvious negative correlations with the silt clay (< 0.063 mm) and gravel (> 2 mm) content. According to the deflation rates, the 8 typical sediments can be divided into four categories: (1) lacustrine sand and alluvial sand, which are prone to wind erosion, and are the main sources of aeolian sand; (2) weathered particles developed from sandshale, pluvial sediments and fluvioglacial sand, with the larger deflation rates, being the minor source of aeolian sand; (3) weathered particles developed from granite and slope deposit having the smaller deflation rates, only providing a small amount of aeolian sand; and (4) glacial sediments with a strong anti-erodibility, hardly offering any aeolian sand. Other than the strong wind conditions, the exposure of extensive lacustrine sand induced by the desiccation of inland lake basin, as well as the pre-sorting of sediments by flowing water are key factors in the development of deserts in China. It is the possible reason that deserts in China are mostly distributed along the inland lake basin and riverbank in a material light.
The Salawusu Formation of Milanggouwan section in Salawusu River Valley includes 7 layers of paleo-mobile dune sands, and 4 layers of paleo-fixed and semi-fixed dune sands. We have observed the structure of these dune sands and analyzed the grain-size, surface textural features and several main chemical elements of them, the results showed that: (1) these dune sands have some eolian structure characteristics which are similar to the recent dune sands; (2) they are also similar to the recent dune sands in grain-size components, and parameters of Mz, σ, Sk and Kg, as well as in several main chemical components; (3) the scatter plots of Mz-σ, SiO2-Al2O3+TOFE and the probability curves of grain-size showed that these paleo dune sands are different from paleosols and fluvio-lacustrine facies but are consistency with recent dune sands; and (4) the quartz sands have well roundness and some surface textural features such as dish-shape pits, crescent-shape pits, pockmarked pits, upturned cleavage plates, siliceous precipitates and siliceous crevasses, indicating that they had been carried for a long time by the wind. As the 11 layers of paleo dune sands possess the eolian characteristics in structure, grain-size, surface textural features and chemical elements, the origin of their formation should be eolian.
Visual interpretation remarks were established by taking TM images in 2000 and 2005 as information source. Then desertification dynamic in Horqin Sandy Land in recent five years is monitored with the help of images and data processing function of GIS. The results showed that desertified land changed from 22423.1 km2 in 2000 to 22422.4 km2 in 2005, only at a reducing rate of 0.14 km2·a-1. This indicated that the constant aggravation of the desertification trend in the study area has been basically controlled and tends to be in a relatively stable state. The degree of transformation dynamics of desertification is different for different types of land, desertification degree has been mitigated evidently for the primary desertified land, and the difference is 958.9 km2 between reversal area and deteriorative area. At the same time, there are 113.3 km2 of non-desertified land desertification at a rate of 22.7 km2·a-1. Therefore, great attention should be paid to this phenomenon.
The sand dredging and its impacts on riverbed evolution and changes of the tidal dynamic in the Dongjiang River network and lower reaches of the Dongjiang River are analyzed in this paper. The huge amount of sand, totally 0.332 billion m3 from 1980 to 2002, was mined from the riverbed of the Dongjiang River network and lower reaches of the Dongjiang River. The main impacts of sand dredging on the riverbed evolution include increasing substantially of the channel capacity, lowering of the average riverbed elevation, and deepening of the water depth and decreasing of the longitudinal riverbed gradient. The main tidal dynamic changes are as follows: (1) In the upper part of the Dongjiang River network and lower reaches of the Dongjiang River, the tidal water level drops apparently, the tidal range widens, the flood tidal duration goes longer and the amplitudes of the main tidal components become bigger. (2) The tidal wave spreads faster. (3) The limits of the tidal level, tidal current and salt water move upwards. (4) The tidal dynamic in the upper part of the Dongjiang river network and lower reaches of the Dongjiang River intensifies greatly. The above changes are mainly caused by the dramatic change of riverbed topographic alteration shaped as a result of the huge amount of sand dredging.
