The source region of the Yellow River located in the middle east of Tibetan Plateau. The total area is about 52 000 km2, mainly covered by grassland (79%), unused land (16%) and water (4%). The increasing utilization of the land in this area have been keeping increasing the risk of environmental degradation. The land use/cover data (1985 and 2000) provided by the Data Center for Resources and Environmental Sciences of Chinese Academy of Sciences, have been used to analyze the land cover change of the Yellow River. DEM (1:250 000) data，roads, residents data (1:100 000) and the collecting field data were used to analyze grassland degradation distribution character. The Arcgis 9 software was use to convert data types and do the overlay, reclass, zonal statistic analysis. Results show that grassland degradation is the most important land cover change in the research region, which occupied 8.24% of the whole region. And human activities are the main reasons of the grassland degradation in the source region of Yellow River: a) the degradation rate is higher in the sunny slope and lower in the northern slope; b) The grassland degradation rate goes down while the elevation rise up with the correlation coefficient of -0.93; c) The nearer to the residents the higher degradation rate, especially in the range of 11km, of which the correlation coefficient of the distance and degradation rate is -0.99; d) In the range of 4km, the degradation rate goes down with the distance to the roads with the correlation coefficient of -0.98. Though there are some physical reasons, human activities are the most important driven forces of the grassland degradation in the source region of the Yellow River since 1985. The ultimate way to resolve the degradation problems is to decrease the population who depends on the livestock production and to lessen the society demand of the GDP incoming from the grassland. To keep the sustainable development, the management and dynamic process of the grassland ecosystem are in dire need of enhanced.
Arid and semi-arid ecosystems exhibit a spatially complex biogeophysical structure. According to arid western special climate-vegetation characters, the fractional cover of photosynthetic vegetation (PV), non-photosynthetic vegetation (NPV), bare soil and water are unmixed, using the remote sensing spectral mixture anaysis. We try the method to unmix the canopy funcation sturcture of arid land cover in order to avoid the differentiation of regional vegetation system and the disturbance of environmental background. We developed a modified production efficiency model NPP-PEM appropriate for the arid area at regional scale based on the concept of radiation use efficiency. The model referring to the GLO-PEM and CASA model was driven from remotely sensed observations. The model calculates not just the conversion efficiency of absorbed photosysnthetically active radiation but also the carbon fluxes that determine net primary productivity (NPP). We apply and validate the model in Kaxger and Yerkant river basins in arid western China. The NPP of the study area in 1992 and 1998 was estimated based on the NPP-PEM model. The results show that the improved PEM model, considering the photosysnthetical activation of heterogeneous functional vegetation, is in good agreement with field measurements and the existing literature. An accurate agreement (R2 = 0.85, P < 0.001) between estimates and ground-based measurement was obtained.
Guangdong in South China has the largest area of mangrove wetlands in China. Mangrove forests have important ecological functions in protecting the environment. However, the mangrove wetlands have been largely lost because of intensified human activities in the study area. Remote sensing can be conveniently used to inventory mangrove forests because field investigation is very difficult. In this study, a knowledge-based system is developed to retrieve spatio-temporal dynamics of mangrove wetlands using multi-temporal remote sensing data. Radar remote sensing data are also used to provide complementary information for the quantitative analysis of mangrove wetlands. Radar remote sensing is able to penetrate mangrove forests and obtain the trunk information about mangrove structures. The integration of radar remote sensing with optical remote sensing can significantly improve the accuracies of classifying mangrove wetland types and estimating wetland biomass.
An RDI WH600 ADCP has been deployed on a moving vessel for the water and suspended sediment fluxes measurements in the Changjiang Estuary. The acoustic signals recorded have a significant relationship with the suspended sediment concentrations (SSCs) obtained by in-situ water sampling during the measurements. Then the acoustic SSC models established were used to convert ADCP acoustic signals into high-resolution SSCs along the cross-sections at Xuliujing and the bar shoal at South Trough in the Changjiang Estuary. The analysis of current velocity profiles shows that the study area is characterized by the rectilinear current at the Xuliujing area. The maximum water and suspended sediment fluxes present during the flood period. However, since the ebb period is much longer than the flood period and the river discharge is huge, the water and suspended sediment fluxes are larger during the ebb than those during the flood. Thus, the net water and suspended sediment transport towards the sea. For example, the net water and suspended sediment are 0.71×109 m3 and 2.5×108 kg during a tidal cycle on November 12, 2003, respectively. Nevertheless, the current is with a rotary pattern at the bar shoal at South Trough of the Changjiang Estuary. The surveyed data also shows that the water and suspended sediment fluxes are larger during the flood than those during the ebb. We also find that a significant relationship exists between the water discharge measured at Datong Station and that at Xuliujing cross-sections from 1998 to 2004. Hence, the derived equations could be used to estimate the daily water discharge through Xuliujing, which can represent the discharge into the sea from the Changjiang River. And the historic water discharge into the sea can be retrieved by using this method. In addition, the annual water discharge into the sea is slightly larger than the annual discharge at Datong Station. However, during the dry period (water discharge < 104 m3/s at Datong Station), the discharge into the sea is smaller than that measured at Datong Station, which may be resulted from the human activities.
Based on the long-term monthly discharge series of Yichang, Hankou and Datong stations, this paper has explored the discharge series structure of periodicity, trend and abrupt changes as well as its long-term momery with Singular Spectum Analysis, and Detrended Fluctuation Analysis in employment. The results revealed that: 1) there is a significant 15a periodicity in the discharge series of the Yangtze River, and the 15a periodicity has been disturbed since the 1970s; 2) two abrupt and negative changes of Yichang series respectively in 1926 and 1970 have been detected in Hankou series in 1954; two abrupt changes occurred in Datong series respectively in 1955 and 1988, the former negative and the latter positive; and 3) the performance of DFA revealed a long memory in the discharge series of the Yangtze River, and the larger the upslope contributing area is, the stronger the long memory, which exhibits a cumulative effect.
The relationship among precipitation, vegetation and erosion is an important, unsolved issue in the field of earth surface processes. Based on data from the Loess Plateau of China, some nonlinear relationships between forest cover (Cf), rainfall erosivity (Ir) and annual precipitation (Pm) has been found. A threshold has been identified at Pm = 450 mm, that is, when Pm is smaller than 450 mm, Cf is low and basically does not vary with Pm; when Pm exceeds 450 mm, Cf increases rapidly. Furthermore, two thresholds are identified in the relationship between rainfall erosivity and annual precipitation. When Pm is smaller than 300 mm, Ir is low and basically does not vary with Pm. When Pm exceeds 300 mm, Ir increases rapidly; when Pm becomes larger than 530 mm, the rate at which Ir increases with Pm becomes higher. Based on these nonlinear relationships, the nonlinear picture of variation between erosion intensity and annual precipitation, i.e., the erosion intensity increases with annual precipitation to a peak and then declines, is explained. The implication of these thresholds for erosion control on the Loess Plateau is discussed.
Based on data of cross-section measurements obtained during the period 1973-1997, the development of the secondary suspended channel in the lower Yellow River has been studied. It has been found that the period 1973-1985 was the main period in which the secondary suspended channel was formed in the reach above Gaocun station; but in the reach below, the process can be divided into two periods of time. In the period 1973-1985 with high water discharges, large quantities of sediment deposited on the floodplain near the natural levee, resulting in a larger lateral slope of the flood-plain. In the period 1985-1997 with low water discharges, large quantities of sediment deposited within the main channel, and as a result, the channel shrank and the situation of the secondary suspended channel became even more serious.
Complex erosion by wind and water, which is also called aeolian-fluvial interactions, is an important erosion process and landscape in arid and semiarid regions. The effectiveness of links between the wind and water process, spatial environmental transitions and temporal environmental change are the three main driving forces determining the geomorphologic significance of aeolian-fluvial interactions. As a complex interrelating and intercoupling system, complex erosion by wind and water has spatial-temporal variation features. The process of complex erosion by wind and water can be divided into palaeoenvironmental process and contemporary process. Early work in drylands has often been attributed to one of two schools advocating either an "aeolianist" or a "fluvialist" perspective, so it was not until the 1930s that the research on complex erosion by wind and water has been conducted. There are several obstacles restricting the research of complex erosion by wind and water. Firstly, how to transform in different temporal and spatial scales is still unsettled; and secondly, the research methodology is still immature. In the future, the mechanism and control of erosion, the complex soil erodibility in wind and water erosion will be the emphasis of research on complex erosion by wind and water.
Degree-day factor is an important parameter of degree-day model which is a widely used method for ice and snow melt computation. Spatial variations of a degree-day factor have great impacts on the accuracy of snow or ice melt modeling. This paper analyzes the spatial variability characteristics of the degree-day factors obtained from the observed glaciers of different regions in western China. Results clearly show that the regional patterns of the degree-day factors are detectable on the observed glaciers of western China due to the unique climatic environment and heat budget of the Tibetan Plateau and the surrounding areas; the factors of maritime glaciers are higher than those of the sub-continental and extremely continental glaciers because of the ablation area covered with a thin debris layer in the maritime glacier; for a single glacier, the degree-day factors are subject to significant small-scale variations. In general, the regional patterns of the degree-day factors are detectable on observed glaciers due to the unique climatic environment and heat budget of the Tibetan Plateau and the surrounding areas. Low degree-day factors can be expected for cold-dry areas, whereas, high degree-day factors can be expected for warm-wet areas in western China.
The microclimate of a desert oasis in the hyperarid zone of China was monitored using micrometeorological methods and compared with those of areas adjacent to forested land. Differences in ground-level photosynthetically active radiation (PAR) on clear, cloudy and dust storm days and their subtending causes are analysed and discussed. Desert oases serve the ecological functions of altering solar radiation, adjusting near-ground and land surface temperatures, reducing soil temperature differences, lowering wind velocity, and increasing soil and atmospheric humidity. The total solar radiation in the interior of the oasis was roughly half of that outside a forest canopy. During the growing season, air temperatures in Populus euphratica Oliv. (poplar) and Tamarix ramosissima Ledeb. (tamarisk) forests were 1.62oC and 0.83 oC lower, respectively, than those in the areas around the forests. Furthermore, the taller the forest cover, the greater the temperature drops; air temperatures in the upper storey were greater than those in the lower storey, i.e., air temperature rose with increasing height. Over the growing season, the relative humidities of the air in the poplar and tamarisk forests were 8.5% and 4.2% higher, respectively, than those in areas around the forests. Mean wind velocity in poplar-forested lands was 0.33 m s-1, 2.31 m s-1 lower than that in the surrounding area. During dust storm days the PAR was significantly lower than on cloudy or clear days, when it was high and varied in an irregular manner.
