Modeling Gross Primary Production by Integrating Satellite Data and Coordinated Flux Measurements in Arid and Semi-Arid China

Assessing large-scale patterns of gross primary production (GPP) in arid and semi-arid (ASA) areas is important for both scientific and practical purposes.Remote sensing-based models,which integrate satellite data with input from ground-based meteorological measurements and vegetation characteristics,improve spatially extended estimates of vegetation productivity with high accuracy.In this study,the authors simulated GPP in ASA areas by integrating moderate resolution imaging spectral radiometer (MODIS) data with eddy covariance and meteorological measurements at the flux tower sites using the Vegetation Photosynthesis Model (VPM),which is a remote sensing-based model for analyzing the spatial pattern of GPP in different land cover types.The field data were collected by coordinating observations at nine stations in 2008.The results indicate that in the region during the growing season GPP was highest in cropland sites,second highest in woodland sites,and lowest in grassland sites.VPM captured the temporal and spatial characteristics of GPP for different land covers in ASA areas.Further,Enhanced Vegetation Index (EVI) had a strong liner relationship with GPP in densely vegetated areas,while the Normalized Difference Vegetation Index (NDVI) had a strong liner relationship with GPP over less dense vegetation.This study demonstrates the potential of satellite-driven models for scaling-up GPP,which is a key component for studying the carbon cycle at regional and global scales.

Simulation of the regional climatic effect of irrigation over the Yellow River Basin

本文基于新设计的灌溉参数化方案,并将其耦合到区域气候模式WRF之中,利用此模式对黄河流域进行为期10年的区域气候模拟,结果显示引入灌溉之后将减小模拟的土壤湿度,感热和潜热的偏差。灌溉导致地表增加额外的水分,从而导致土壤湿度和蒸发增加,地面空气冷却,感热减小和潜热增加,同时导致行星边界层也发生变化。揭示了农业灌溉在区域气候研究中的重要性和增强对人类活动在调节陆-气-云相互作用的理解。

Evaluation of AMSR-E——Derived Soil Moisture over Northern China

In this study,the authors evaluated two re-motely sensed surface soil moisture datasets derived from the Advanced Microwave Scanning Radiometer of the Earth Observing System (AMSR-E) over northern China.The soil moisture datasets were derived from algorithms developed by the National Snow and Ice Data Center (NSIDC) and jointly developed by the Vrije Universiteit Amsterdam and NASA Goddard Space Flight Center (VUA-NASA).The NSIDC and VUA-NASA products were compared to in situ soil moisture data from nine enhanced coordinated observation stations.The VUA-NASA dataset presented a strong correlation with top layer in situ soil moisture observations,and the correla-tion coefficients ranged from 0.34 to 0.73 (p<0.01).The correlation coefficients decreased as the observed soil layer depth increased.The correlation coefficients be-tween the NSIDC retrievals and the top layer in situ ob-servations were between 0.10 and 0.62 (p<0.01).Fur-thermore,VUA-NASA soil moisture variations agreed well with in situ soil moisture dynamics and responded sensitively to precipitation events.In contrast,the NSIDC dataset failed to capture signals of soil moisture dynamics.The analyses demonstrated that the VUA-NASA product was capable of representing soil moisture conditions over northern China.

Evaluation of the WRF Model with Different Land Surface Schemes: A Drought Event Simulation in Southwest China during 2009–10

The authors examined the performance of version 3.4.1 of the Weather Research and Forecasting Model(WRF) with various land surface schemes in simulating a severe drought event in Southwest China. Five numerical experiments were completed using the Noah land surface scheme, the Pleim-Xiu land surface scheme, the Noah-MP land surface schemes, the Noah- MP scheme with dynamic vegetation, and the Noah-MP scheme with dynamic vegetation and groundwater processes. In general, all the simulations reasonably reproduced the spatial and temporal variations in precipitation, but significant bias was also found, especially for the spatial pattern of simulated precipitation. The WRF simulations with the Noah-MP series land surface schemes performed slightly better than the WRF simulation with the Noah and Pleim-Xiu land surface schemes in reproducing the severe drought events in Southwest China. The leaf area index(LAI) simulated by the different land surface schemes showed significant deviations in Southwest China. The Pleim-Xiu scheme overestimated the value of LAI by a factor of two. The Noah-MP scheme with dynamical vegetation overestimated the magnitude of the annual cycle of the LAI, although the annual mean LAI was close to observations. The simulated LAI showed a long-term lower value from autumn 2009 to spring 2010 relative to normal years. This indicates that the LAI is a potential indictor to monitor drought events.

Comparison of trends and frequencies of drought in central North China and sub- Saharan Africa from 1901 to 2010

This study compares the trends and frequencies of drought between central North China(CNC) and(SSA) for the periods 1901–2010 and 1951–2010. The Standardized Precipitation Evapotranspiration Index(SPEI) and Self-Calibrating Palmer Drought Severity Index(sc-PDSI) are used to assess the drought trends and frequencies. In general, the results exhibit downward trends of drought index values and upward trends of drought frequencies over CNC and SSA. A high rate of the trends' slopes for the drought index and a low rate of the frequencies' slopes is found over CNC with respect to SPEI and sc-PDSI. Furthermore, some abrupt changes are revealed after applying the sequential Mann–Kendall test to detect change points. These findings offer insight into the trends and frequencies of drought over the regions studied. Further analysis needs to be undertaken to understand the mechanisms underlying the occurrence of drought in these areas.

Spatial and Temporal Variability of Water Availability in the Yellow River Basin

The changes in hydrological processes in the Yellow River basin were simulated by using the Community Land Model(CLM,version 3.5),driven by historical climate data observed from 1951 to 2008.A comparison of modeled soil moisture and runoff with limited observations in the basin suggests a general drying trend in simulated soil moisture,runoff,and precipitation-evaporation balance(P-E) in most areas of the Yellow River basin during the observation period.Furthermore,annual soil moisture,runoff,and P-E averaged over the entire basin have declined by 3.3%,82.2%,and 32.1%,respectively.Significant drying trends in soil moisture appear in the upper and middle reaches of the basin,whereas a significant trend in declining surface runoff and P-E occurred in the middle reaches and the southeastern part of the upper reaches.The overall decreasing water availability is characterized by large spatial and temporal variability.

Intra-Annual Variability of Diurnal Cycle Precipitation over China from 1960–2000

Characteristics of diurnal cycle precipitation over China are investigated using twice-daily observations by the China Meteorological Administration during 1960–2000. Characteristics investigated include nighttime/daytime precipitation amount(PA), intensity, and frequency. Geographically, the region is separated into western and eastern China by the 110°E longitude. Our analysis shows that there generally is more night-time than daytime precipitation in western China, particularly in the Sichuan Basin. Over eastern China, the opposite holds true, particularly along the southeast coast. Regional average monthly daytime and night-time precipitation peaks in the same month for both western and eastern China. Over western China, monthly night-time precipitation is always greater than that during daytime, but the night-time precipitation frequency(PF) is only greater in non-summer(June–August) months. Over eastern China, daytime precipitation is greater than that in the night-time during the warm season(May–August) in both amount and frequency. The night-day difference(night-time minus daytime) in PA over western China is mainly influenced by precipitation intensity, while over eastern China the night-day difference in rainfall amount is mostly driven by PF.

A Preliminary Analysis of the Relationship between Precipitation Variation Trends and Altitude in China

Characteristics of the relationship between precipitation variation trends (PVT) and altitude were analyzed using monthly mean precipitation data from 526 observation stations in China from 1961 to 2008.With respect to elevation,China was divided into three subregions,below 200 m,200-1500 m,and above 1500 m.The results showed that the correlations between annual PVT and altitude are different among the three regions.In the region below 200 m in elevation,the best relationship has a correlation coefficient of-0.19 (0.49),passing the 90% (99.9%) significance level south (north) of 35 N.However,the correlation coefficient is close to zero,and the latitude strongly governs the spatial distribution of the amplitude of annual PVT in the 200-1500-m elevation region.In most of the Tibetan Plateau,where the elevation is greater than 1500 m,there is a weak negative correlation.The Mann-Kendall method was used to test the trend of regional mean annual precipitation,which indicated that the annual mean precipitation had no obvious trend of change in China due to the reverse significant variation trends in different areas of the country.