Hongjie Xie

Water level variation of Lake Qinghai from satellite and in situ measurements under climate change

Lake level elevation and variation are important indicators of regional and global climate and environmental change. Lake Qinghai, the largest saline lake in China, located in the joint area of the East Asian monsoon, Indian summer monsoon, and Westerly jet stream, is particularly sensitive to climate change. This study examines the lakes water level and temporal change using the ice, cloud, and land elevation satellite (ICESat) altimetry data and gauge measurements. Results show that the mean water level from ICESat rose 0.67 m from 2003 to 2009 with an increase rate of 0.11 m/yr and that the ICESat data correlates well (r2 = 0.90, root mean square difference 0.08 m) with gauge measurements. Envisat altimetry data show a similar change rate of 0.10 m/yr, but with ∼0.52 m higher, primarily due to different referencing systems. Detailed examination of three sets of crossover ICESat tracks reveals that the lake level increase from 2004 to 2006 was 3 times that from 2006 to 2008, with the largest water level increase of 0.58 m from Feb. 2005 to Feb. 2006. Combined analyses with in situ precipitation, evaporation, and runoff measurements from 1956 to 2009 show that an overall decreasing trend of lake level (−0.07 m/yr) correlated with an overall increasing trend (+0.03°C/yr) of temperature, with three major interannual peaks of lake level increases. The longest period of lake level increase from 2004 to 2009 could partly be due to accelerated glacier/perennial snow cover melt in the region during recent decades. Future missions of ICESat type, with possible increased repeatability, would be an invaluable asset for continuously monitoring lake level and change worldwide, besides its primary applications to polar regions.

An IDL/ENVI implementation of the FFT-based algorithm for automatic image registration ☆

Abstract Georeferencing images is a laborious process so schemes for automating this process have been under investigation for some time. Among the most promising automatic registration algorithms are those based on the fast Fourier transform (FFT). The displacement between two given images can be determined by computing the ratio F 1 conj(F 2 )/|F 1 F 2 | , and then applying the inverse Fourier transform. The result is an impulse-like function, which is approximately zero everywhere except at the displacement that is necessary to optimally register the images. Converting from rectangular coordinates to log-polar coordinates, shifts representing rotation and scaling can also determined to complete the georectification process. A FFT-based algorithm has been successfully implemented in Interactive Data Language (IDL) and added as two user functions to an image processing software package—ENvironment for Visualizing Images (ENVI) interface. ENVI handles all pre- and post-processing works such as input, output, display, filter, analysis, and file management. To test this implementation, several dozen tests were conducted on both simulated and “real world” images. The results of these tests show advantages and limitations of this algorithm. In particular, our tests show that the accuracy of the resulting registration is quite good compared to current manual methods.

GIS-based NEXRAD Stage III precipitation database: automated approaches for data processing and visualization

This study develops a geographical information system (GIS) approach for automated processing of the Next Generation Weather Radar (NEXRAD) Stage III precipitation data. The automated processing system, implemented by using commercial GIS and a number of Perl scripts and C/C++ programs, allows for rapid data display, requires less storage capacity, and provides the analytical and data visualization tools inherent in GIS as compared to traditional methods. In this paper, we illustrate the development of automatic techniques to preprocess raw NEXRAD Stage III data, transform the data to a GIS format, select regions of interest, and retrieve statistical rainfall analysis over user-defined spatial and temporal scales. Computational expense is reduced significantly using the GIS-based automated techniques. For example, 1-year Stage III data processing (~9000 files) for the West Gulf River Forecast Center takes about 3 days of computation time instead of months of manual work. To illustrate the radar precipitation database and its visualization capabilities, we present three application examples: (1) GIS-based data visualization and integration, and ArcIMS-based web visualization and publication system, (2) a spatial-temporal analysis of monsoon rainfall patterns over the Rio Grande River Basin, and (3) the potential of GIS-based radar data for distributed watershed models. We conclude by discussing the potential applications of automated techniques for radar rainfall processing and its integration with GIS-based hydrologic information systems.

Development and assessment of combined Terra and Aqua snow cover products in Colorado Plateau, USA and northern Xinjiang, China

This study presents our methods to produce multi-day Terra, Aqua, and Terra-Aqua moderate resolution imaging spectroradiometer (MODIS) snow cover composite images respectively from daily Terra MODIS, Aqua MODIS, and both Terra and Aqua MODIS snow cover products, with flexible starting and ending dates and a user-defined cloud cover threshold. The methods are applied to Colorado Plateau, USA and northern Xinjiang, China. The statistical comparison gives the following results. For the 2003-2004 hydrologic year, the daily Terra-Aqua composite images exhibited ~10-15% less on annual mean cloud cover and ~1-4% more on annual mean snow cover, compared with their daily Terra or Aqua counterparts. Using 10% cloud cover as a user-defined threshold, we produced 152 (northern Xinjiang) and 162 (Colorado Plateau) multi-day Terra-Aqua composite images for the 2003-2004 hydrological year, respectively. On average, it is 2.4 and 2.2 days per composite image for northern Xinjiang and Colorado Plateau, respectively. The multi-day Terra, Aqua, and Terra-Aqua composite products result in similar annual mean snow covers (~15% for the Colorado Plateau and ~30% for the northern Xinjiang), as those from the corresponding standard NASA 8-day products, ~3 times as those from the standard NASA daily products. The lower snow cover percentage retrieved from the daily standard products is mainly due to larger cloud cover in the daily products. The Aqua products always have lower annual mean snow cover and higher annual mean cloud cover than those of the Terra products. The daily Terra-Aqua composite products have higher agreement with ground measurements than either of the standard NASA daily Terra or Aqua products. The multi-day Terra-Aqua composite products have much higher agreement with ground measurements than that of the standard daily products and have similar agreement as that of the standard 8-day products. Therefore, those new composite products generated from our methods are a significant contribution to the current MODIS snow cover product series.

Estimating surface temperature changes of lakes in the Tibetan Plateau using MODIS LST data

Temperature changes over the Tibetan Plateau (TP) exhibit a dependence on altitude, as observed from meteorological station data and Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data. However, little is known about the changes in water surface temperature (WST) of lakes in the TP under the warming climate conditions over the past few decades. In this study, lake WST was examined using a MODIS/Terra 8 day LST (nighttime) product (MOD11A2) over the period 2001–2012. It was found that 52 lakes included in the analysis of WST exhibited an average rate of change of 0.012 ± 0.033°C/yr. Of these 52 lakes, 31 lakes (60%) displayed a temperature increase with a mean warming rate of 0.055 ± 0.033°C/yr and 21 lakes (40%) displayed a temperature decrease with a mean cooling rate of −0.053 ± 0.038°C/yr. The rates of change in WST for 13 lakes were statistically significant, and these included nine warming and four cooling lakes. Of the 17 lakes with nearby weather stations, nine lakes (53%) showed faster warming than nearby air/land. The warming lakes could be attributed to locally rising air and land surface temperatures as well as other factors such as the decreased lake ice cover. The cooling lakes were mostly located at high elevations (>4200 m), and the trend could have been due to increased cold water discharge to the lakes from accelerated glacier/snow melts. Therefore, both warming and cooling lake temperatures in the TP were possibly the result of increased air temperatures (0.036 ± 0.027°C/yr) under global warming conditions.

Application of a distributed hydrologic model to the november 17, 2004, flood of bull creek watershed, Austin, Texas

This study presents a hydrologic analysis of a flood event that took place over a small urbanizing watershed in Austin, Texas. The physically based, distributed-parameter gridded surface subsurface hydrologic analysis (GSSHA) hydrologic model was used to simulate the watershed response to a very high-intensity rain event. The hydrologic model was forced by both gauge-observed and multisensor precipitation estimator (MPE) rainfall input. Observed discharge was compared to GSSHA-generated hydrograph under various degrees of representation of the watershed physiography. In addition, simulation hydrographs by GSSHA using five different model grid sizes were compared in order to evaluate the effect of grid size on model predictions. The simulation hydrograph for the model using a 30-m grid cell generally compared well to the observed flow data once the effects of storm water detention were simulated. The comparison of simulation results from models using 30, 60, 90, 120, and 150 m grid size highlighted the los...

Extensive and drastically different alpine lake changes on Asia's high plateaus during the past four decades

Asias high plateaus are sensitive to climate change and have been experiencing rapid warming over the past few decades. We found 99 new lakes and extensive lake expansion on the Tibetan Plateau during the last four decades, 1970-2013, due to increased precipitation and cryospheric contributions to its water balance. This contrasts with disappearing lakes and drastic shrinkage of lake areas on the adjacent Mongolian Plateau: 208 lakes disappeared, and 75% of the remaining lakes have shrunk. We detected a statistically significant coincidental timing of lake area changes in both plateaus, associated with the climate regime shift that occurred during 1997/1998. This distinct change in 1997/1998 is thought to be driven by large-scale atmospheric circulation changes in response to climate warming. Our findings reveal that these two adjacent plateaus have been changing in opposite directions in response to climate change. These findings shed light on the complex role of the regional climate and water cycles and provide useful information for ecological and water resource planning in these fragile landscapes.

Suitable remote sensing method and data for mapping and measuring active crop fields

The objective of the study was to examine suitable remote sensing methods and data for mapping and measuring the acreages of active crop lands in order to improve irrigation management. We compared classification results from a supervised classification method and a method using normalized difference vegetation index (NDVI) with additional pre‐classification processing. IKONOS and Landsat Enhanced Thematic Mapper Plus (ETM+) images were tested to see if high spatial resolution remote sensing data would have significant advantages in distinguishing between active and fallow lands. The classification achieved an overall accuracy of 93.63%. The results showed that the supervised classification did not have a clear advantage over the simple method using NDVI at the level of distinguishing between active crops and fallow lands. The result suggests that using ETM+ instead of IKONOS high spatial resolution imageries is appropriate because of the high cost of IKONOS imageries and image heterogeneity of agricultural fields. It was shown that pre‐processing with a mask to exclude the non‐agricultural objects blended with agricultural fields is critical.

Lake volume and groundwater storage variations in Tibetan Plateau's endorheic basin

The Tibetan Plateau (TP), the highest and largest plateau in the world, with complex and competing cryospheric‐hydrologic‐geodynamic processes, is particularly sensitive to anthropogenic warming. The quantitative water mass budget in the TP is poorly known. Here we examine annual changes in lake area, level, and volume during 1970s–2015. We find that a complex pattern of lake volume changes during 1970s–2015: a slight decrease of −2.78 Gt yr−1 during 1970s–1995, followed by a rapid increase of 12.53 Gt yr−1 during 1996–2010, and then a recent deceleration (1.46 Gt yr−1) during 2011–2015. We then estimated the recent water mass budget for the Inner TP, 2003–2009, including changes in terrestrial water storage, lake volume, glacier mass, snow water equivalent (SWE), soil moisture, and permafrost. The dominant components of water mass budget, namely, changes in lake volume (7.72 ± 0.63 Gt yr−1) and groundwater storage (5.01 ± 1.59 Gt yr−1), increased at similar rates. We find that increased net precipitation contributes the majority of water supply (74%) for the lake volume increase, followed by glacier mass loss (13%), and ground ice melt due to permafrost degradation (12%). Other term such as SWE (1%) makes a relatively small contribution. These results suggest that the hydrologic cycle in the TP has intensified remarkably during recent decades.

Spatio-Temporal Change of Snow Cover and Its Response to Climate over the Tibetan Plateau Based on an Improved Daily Cloud-Free Snow Cover Product

Using new, daily cloud-free snow-cover products, this study examines snow cover dynamics and their response to climate change. The results demonstrate that the daily cloud-free snow-cover products not only posses the advantages of the AMSR-E (unaffected by weather conditions) and MODIS (relatively higher resolution) products, but are also characterized by high snow and overall classification accuracies (~85% and ~98%, respectively), substantially greater than those of the existing daily snow-cover products for all sky conditions and very similar to, or even slightly greater than, those of the daily MODIS products for clear-sky conditions. Using the snow-cover products, we analyzed the snow cover dynamics over the Tibetan Plateau and determined that the maximum number of snow-covered days (SCD) in a year followed a decreasing tendency from 2003 to 2010, with a decrease in snow-covered area (SCA) equivalent to 55.3% of the total Tibetan Plateau area. There is also a slightly increasing tendency in the maximum snow cover area (SCA), and a slightly decreasing tendency in the persistent snow cover area (i.e., pixels of SCD > 350 days) was observed for the 8-year period, which was characterized by increases in temperature (0.09 °C/year) and in precipitation (0.26 mm/year). This suggests that, on the Tibetan Plateau, changes in temperature and precipitation exert a considerable influence on the regional SCD and SCA, as well as the distribution of persistent snow cover.