Root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate

The root‐zone moisture replenishment mechanisms are key unknowns required to understand soil hydrological processes and water sources used by plants. Temporal patterns of root‐zone moisture replenishment reflect wetting events that contribute to plant growth and survival and to catchment water yield. In this study, stable oxygen and hydrogen isotopes of twigs and throughfall were continuously monitored to characterize the seasonal variations of the root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate in South Australia. The two studied hillslopes (the north‐facing slope [NFS] and the south‐facing slope [SFS]) had different environmental conditions with opposite aspects. The twig and throughfall samples were collected every ~20 days over 1 year on both hillslopes. The root‐zone moisture replenishment, defined as percentage of newly replenished root‐zone moisture as a complement to antecedent moisture for plant use, calculated by an isotope balance model, was about zero (±25% for the NFS and ± 15% for the SFS) at the end of the wet season (October), increased to almost 100% (±26% for the NFS and ± 29% for the SFS) after the dry season (April and May), then decreased close to zero (±24% for the NFS and ± 28% for the SFS) in the middle of the following wet season (August). This seasonal pattern of root‐zone moisture replenishment suggests that the very first rainfall events of the wet season were significant for soil moisture replenishment and supported the plants over wet and subsequent dry seasons, and that NFS completed replenishment over a longer time than SFS in the wet season and depleted the root zone moisture quicker in the dry season. The stable oxygen isotope composition of the intraevent samples and twigs further confirms that rain water in the late wet season contributed little to root‐zone moisture. This study highlights the significant role of the very first rain events in the early wet season for ecosystem and provides insights to understanding ecohydrological separation, catchment water yield, and vegetation response to climate changes.     

Seasonal variation of carbon exchange of typical forest ecosystems along the eastern forest transect in China

The long-term and continuous carbon fluxes of Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical evergreen coniferous forest (QYZ), Dinghushan subtropical evergreen mixed forest (DHS) and Xishuangbana tropical rainforest (XSBN) have been measured with eddy covariance techniques. In 2003, different responses of carbon exchange to the environment appeared across the four ecosystems. At CBS, the carbon exchange was mainly determined by radiation and temperature. 0°C and 10°C were two important temperature thresholds; the former determined the length of the growing season and the latter affected the magnitude of carbon exchange. The maximum net ecosystem exchange (N _EE) of CBS occurred in early summer because maximum ecosystem photosynthesis (G _PP) occurred earlier than maximum ecosystem respiration (R _e). During summer, QYZ experienced severe drought and N _EE decreased significantly mainly as a result of the depression of G _PP. At DHS and XSBN, N _EE was higher in the drought season than the wet season, especially the conversion between carbon sink and source occurring during the transition season at XSBN. During the wet season, increased fog and humid weather resulted from the plentiful rainfall, the ecosystem G _PP was dispressed. The Q ₁₀ and annual respiration of XSBN were the highest among the four ecosystems, while the average daily respiration of CBS during the growing season was the highest. Annual N _EE of CBS, QYZ, DHS and XSBN were 181.5, 360.9, 536.2 and ?320.0 g·C·m^?2·a^?1, respectively. From CBS to DHS, the temperature and precipitation increased with the decrease in latitude. The ratio of N _EE/R _e increased with latitude, while R _e/G _PP, ecosystem light use efficiency (L _UE), precipitation use efficiency and average daily G _PP decreased gradually. However, XSBN usually escaped such latitude trend probably because of the influence of the south-west monsoon climate which does not affect the other ecosystems. Long-term measurement and more research were necessary to understand the adaptation of forest ecosystems to climate change and to evaluate the ecosystem carbon balance due to the complexity of structure and function of forest ecosystems.     

Photosynthetic characteristics of dominant tree species and canopy in the broadleaved Korean pine forest of Changbai Mountains

Based on the light-photosynthesis response measurement at leaf level, combined with over-and under-canopy eddy covariance measurements, research on photosynthetic characteristics of single trees and forest canopy was conducted. The relationship between light intensity and photosynthetic rates for leaves and canopy can be well fitted by a non-rectangular hyperbola model. Mongolian oak presented a high light compensation point, L _cp (28 μmol·m^?2·s^?1), a light saturation point L _sp (>1800 μmol·m^?2·s^?1), and a maximal net photosynthetic rate P _max (9.96 μmol·m^?2·s^?1), which suggest that it is a typical heliophilous plant. Mono maple presented the highest apparent quantum efficiency α (0.066) but the lowest, L _cp (16 μmol·m^?2·s^?1), L _sp (≈800 μmol·m^?2·s^?1), and P _max (4.51 μmol·m^?2·s^?1), which suggest that it is heliophilous plant. Korean pine showed the lowest α value but a higher P _max, which suggest that it is a semi-heliophilous plant. At the canopy level, the values of both α and P _max approached the upper limit of reported values in temperate forests, while L _cp was within the lower limit. Canopy photosynthetic characteristics were well consistent with those of leaves. Both showed a high ability to photosynthesize. However, environmental stresses, especially high vapor pressure deficits, could significantly reduce the photosynthetic ability of leaves and canopy.     

Seasonal and annual variation of CO2 flux above a broad-leaved Korean pine mixed forest

Long-term measurement of carbon metabolism of old-growth forests is critical to predict their behaviors and to reduce the uncertainties of carbon accounting under changing climate. Eddy covariance technology was applied to investigate the long-term carbon exchange over a 200 year-old Chinese broad-leaved Korean pine mixed forest in the Changbai Mountains (128°28′E and 42°24′N, Jilin Province, P. R. China) since August 2002. On the data obtained with open-path eddy covariance system and CO₂ profile measurement system from Jan. 2003 to Dec. 2004, this paper reports (i) annual and seasonal variation of F _NEE, F _GPP and R _E; (ii) regulation of environmental factors on phase and amplitude of ecosystem CO₂ uptake and release Corrections due to storage and friction velocity were applied to the eddy carbon flux.

L_AI and soil temperature determined the seasonal and annual dynamics of F_GPP and R_E separately. V_PD and air temperature regulated ecosystem photosynthesis at finer scales in growing seasons. Water condition at the root zone exerted a significant influence on ecosystem maintenance carbon metabolism of this forest in winter.

The forest was a net sink of atmospheric CO₂ and sequestered −449 g C·m⁻² during the study period; −278 and −171 gC·m⁻² for 2003 and 2004 respectively. F _GPP and F _RE over 2003 and 2004 were −1332, −1294 g C·m⁻². and 1054, 1124 g C·m⁻² respectively. This study shows that old-growth forest can be a strong net carbon sink of atmospheric CO₂.

There was significant seasonal and annual variation in carbon metabolism. In winter, there was weak photosynthesis while the ecosystem emitted CO₂. Carbon exchanges were active in spring and fall but contributed little to carbon sequestration on an annual scale. The summer is the most significant season as far as ecosystem carbon balance is concerned. The 90 days of summer contributed 66.9, 68.9% of F _GPP, and 60.4, 62.1% of R _E of the entire year.

     

Investigation of three-dimensional circulation and hydrography over the Pearl River Estuary of China using a nested-grid coastal circulation model

The Pearl River Estuary (PRE) in South China's Guangdong Province is a subtropical estuary with highly irregular topography and dynamically complicated circulations. A nested-grid coastal circulation modelling system is used in this study to examine dynamic responses of the PRE to tides, meteorological forcing and buoyancy forcing. The nested-grid modelling system is based on the Princeton Ocean Model and consists of three downscaling subcomponents: including an outer-most model with a coarse horizontal resolution of ~7 km for simulating tidally forced and wind-driven surface elevations and depth-mean currents over the China Seas from Bohai Sea to the northern South China Sea and an innermost model with a fine resolution of ~1.2 km for simulating the 3D coastal circulation and hydrography over the PRE and adjacent coastal waters. Model results during the winter northeast monsoon surge in January and super typhoon Koryn in June of 1993 are used to demonstrate that the 3D coastal circulation and hydrographic distributions in the PRE are affected by tides, winds and buoyancy forcing associated with river discharge from the Pearl River with significant seasonal and synoptic variabilities.     

A cluster-optimizing regression-based approach for precipitation spatial downscaling in mountainous terrain

Precipitation temporal and spatial variability often controls terrestrial hydrological processes and states. Common remote-sensing and modeling precipitation products have a spatial resolution that is often too coarse to reveal hydrologically important spatial variability. A statistical algorithm was developed for downscaling low-resolution spatial precipitation fields. This algorithm auto-searches precipitation spatial structures (rain-pixel clusters), and orographic effects on precipitation distribution without prior knowledge of atmospheric setting. It is composed of three components: rain-pixel clustering, multivariate regression, and random cascade. The only required input data for the downscaling algorithm are coarse-pixel precipitation map and a topographic map. The algorithm was demonstrated with 4 km × 4 km Next Generation Radar (NEXRAD) precipitation fields, and tested by downscaling NEXRAD-aggregated 16 km × 16 km precipitation fields to 4 km × 4 km pixel precipitation, which was then compared to the original NEXRAD data. The demonstration and testing were performed at both daily and hourly temporal resolutions for the northern New Mexico mountainous terrain and the central Texas Hill Country. The algorithm downscaled daily precipitation fields are in good agreement with the original 4 km × 4 km NEXRAD precipitation, as measured by precipitation spatial structures and the statistics between the downscaling and the original NEXRAD precipitation maps. For three daily precipitation events, downscaled precipitation map reproduces precipitation variance of the disaggregation field, and with Pearson correlation coefficients between the downscaled map and the NEXRAD map of 0.65, 0.71, and 0.80. The algorithm does not perform as well on downscaling hourly precipitation fields at the examined scale range (from 16 km to 4 km), which underestimates precipitation variance of the disaggregation field. For a scale range from 4 km to 1 km, the algorithm has potential to perform well at both daily and hourly precipitation fields, indicated from good regression performance.     

Time-lag effects of vegetation responses to soil moisture evolution: a case study in the Xijiang basin in South China

The time-lag effects of droughts on vegetation responses vary significantly across a large-scale river basin. The spatio-temporal response characteristics obtained are important for decision making processes on the allocation and transportation of regional water resources in mitigating drought impacts. Here we consider the Xijiang (West River) basin in South China as a case study, which has experienced severe drought events since the beginning of the 21st century. A threshold level approach is employed to identify the major drought events over the basin in the first decade of this century. The vegetation responses to land soil water evolution are examined, particularly for the severe drought events occurred. The time-lag effects of the vegetation responses within the basin range within 0–96 days. The lower reaches of the headwater sub-basins in the west part of the Xijiang basin are identified as the regions with short time-lag effects. The enhanced vegetation index (EVI) shows consistent responses to the soil water evolution in conjunction with the climate aridity in this area, which is the drought-vulnerable area in the Xijiang basin.     

碳酸盐岩储层缝洞发育带预测:以塔河油田为例

塔河油田碳酸盐岩为古岩溶喀斯特化与受后期构造叠置改造的孔、洞、缝等组成的非均质储层,储层具有规模不同、空间形态不规则、充填性质各异的复杂组合特征以此为对象通过分解地震波场不同特征信号,开发了一系列针对不同储层目标的叠前叠后地震预测技术,不同特征地震预测结果经综合叠置验证分析,实现了残丘体上叠置的孔、洞、缝及古地貌的精细刻画及表征,展示了缝洞空间纵横交错的叠置关系,提高了复杂储层空间展布规律的认识及精确定位。     

川北硅质岩型铼多金属矿铼的赋存状态

川北硅质岩型铼多金属矿是国内首次发现的铼矿床新类型,矿石中具有Re、Se、V、Mo等多种有价元素富集,综合利用价值较好.本次通过X射线荧光光谱分析(XRF)、X射线衍射(XRD)、能谱分析(EDS)、电子探针(EPMA)、扫描电子显微镜(SEM)等多种手段开展矿石工艺矿物学及铼的赋存状态研究.结果 表明,矿石中主要矿物包括石英、伊利石、黄铁矿等,并含有大量碳质,Re、Se富集与S紧密相关,V富集与Si、A1紧密相关.黄铁矿电子探针分析结果显示,测点含硒平均值0.021％、铼含量平均值为0.024％,表明铼、硒主要以类质同象形态赋存于黄铁矿中.钒主要赋存于含钒伊利石等黏土矿物中.