Xiaomang Liu

Recent changes in pan-evaporation dynamics in China

[1] Pan‐evaporation (Epan) as the indicator of atmospheric evaporative demand has decreased worldwide with climate change in the last decades, which is called “Pan Evaporation Paradox”. This study investigates the recent changes in Epan dynamics in China using the observed Epan records for the period 1960–2007. The records show that Epan decreased in China from 1960 to 1991 by �5.4 mm yr �2 . The attribution results show that the significant decreases (P 0.1). The results show that increasing air temperature dominated the change in Epan, which offset the effect of wind speed and led to the increase in Epan. Citation: Liu, X., Y. Luo, D. Zhang, M. Zhang, and C. Liu (2011), Recent changes in pan‐evaporation dynamics in China, Geophys. Res. Lett., 38, L13404, doi:10.1029/2011GL047929.

Assessment of climate change impacts on hydrology and water quality with a watershed modeling approach

The assessment of hydrologic responses to climate change is required in watershed management and planning to protect water resources and environmental quality. This study is designed to evaluate and enhance watershed modeling approach in characterizing climate change impacts on water supply and ecosystem stressors. Soil and Water Assessment Tool (SWAT) was selected as a base model, and improved for the CO2 dependence of potential evapotranspiration and stream temperature prediction. The updated model was applied to quantify the impacts of projected 21st century climate change in the northern Coastal Ranges and western Sierra Nevada, which are important water source areas and aquatic habitats of California. Evapotranspiration response to CO2 concentration varied with vegetation type. For the forest-dominated watersheds in this study, only moderate (1-3%) reductions on evapotranspiration were predicted by solely elevating CO2 concentration under emission scenarios A2 and B1. Modeling results suggested increases in annual average stream temperature proportional to the projected increases in air temperature. Although no temporal trend was confirmed for annual precipitation in California, increases of precipitation and streamflow during winter months and decreases in summers were predicted. Decreased streamflow during summertime, together with the higher projected air temperature in summer than in winter, would increase stream temperature during those months and result in unfavorable conditions for cold-water species. Compared to the present-day conditions, 30-60 more days per year were predicted with average stream temperature >20°C during 2090s. Overall, the hydrologic cycle and water quality of headwater drainage basins of California, especially their seasonality, are very sensitive to projected climate change.

Dramatic decrease in streamflow from the headwater source in the central route of China's water diversion project: Climatic variation or human influence?

The Danjiangkou Reservoir is the headwater source of the central route of Chinas South to North Water Diversion Project (SNWDP). Average annual streamflow into the Reservoir was 40.97 km(3) from 1951 to 1989, while it was 31.64 km(3) from 1990 to 2006. Between the two periods, the average annual streamflow was reduced by 9.33 km(3), accounting for 71.8% of the proposed amount of water diversion of the central route (13 km3 per year). The sharply decreasing streamflow would inevitably have negative impacts on the implementation of the SNWDP. The reasons for the decrease in streamflow should be investigated before developing any adaption strategies. In this study, the impacts of climatic variation and human activities on streamflow were evaluated by a climate elasticity method. The results show that the impact of climatic variation (indicated by precipitation and potential evapotranspiration) was responsible for 84.1-90.1% of the streamflow reduction, while human activities or other indentified uncertainties contributed 9.9-15.9% of the streamflow reduction. The observed 69.89 mm decrease in average annual precipitation contributed 81.6-87.3% of the decrease in streamflow. According to the observed data during the study period, the planned water diversion could lead to an ecological disaster of the downstream area of the Danjiangkou Reservoir in certain years. We suggest that the water diversion from the Danjiangkou Reservoir should be conducted in an adaptive manner to avoid such an adverse consequence, instead of the current plan of a fixed annual amount of water.

Spatial and temporal change in the potential evapotranspiration sensitivity to meteorological factors in China (1960–2007)

Potential evapotranspiration (E0), as an estimate of the evaporative demand of the atmosphere, has been widely studied in the fields of irrigation management, crop water demand and predictions in ungauged basins (PUBs). Analysis of the sensitivity of E0 to meteorological factors is a basic research on the impact of climate change on water resources, and also is important to the optimal allocation of agricultural water resources. This paper dealt with sensitivity of E0 over China, which was divided into ten drainage systems, including Songhua River basin, Liaohe River basin, Haihe River basin, Yellow River basin, Yangtze River basin, Pearl River basin, Huaihe River drainage system, Southeast river drainage system, Northwest river drainage system and Southwest river drainage system. In addition, the calculation method of global radiation in Penman-Monteith formula was improved by optimization, and the sensitivities of Penman-Monteith potential evapotranspiration to the daily maximum temperature (STmax), daily minimum temperature (STmin), wind speed (SU2), global radiation (SRs) and vapor pressure (SVP) were calculated and analyzed based on the long-term meteorological data from 653 meteorological stations in China during the period 1960–2007. Results show that: (1) the correlation coefficient between E0 and pan evaporation increased from 0.61 to 0.75. E0 had the decline trends in eight of ten drainage systems in China, which indicates that “pan evaporation paradox” commonly exists in China from 1960 to 2007. (2) Spatially, Tmax was the most sensitive factor in Haihe River basin, Yellow River basin, Huaihe River drainage system, Yangtze River basin, Pearl River basin and Southeast river drainage system, and VP was the most sensitive factor in Songhua River Basin, Liaohe River basin, Northwest river drainage system while Rs was the most sensitive factor in Southwest river drainage system. For the nation-wide average, the most sensitive factor was VP, followed by Tmax, Rs, U2 and Tmin. In addition, the changes in sensitivity coefficients had a certain correlation with elevation. (3) Temporally, the maximum values of STmax and SRs occurred in July, while the maximum values of STmin, SVP and SU2 occurred in January. Moreover, trend analysis indicates that STmax had decline trends, while STmin, SU2, SRs and SVP had increasing trends.

Investigation of the probability of concurrent drought events between the water source and destination regions of China's water diversion project

In this study, we investigate the concurrent drought probability between the water source and destination regions of the central route of Chinas South to North Water Diversion Project. We find that both regions have been drying from 1960 to 2013. The estimated return period of concurrent drought events in both regions is 11years. However, since 1997, these regions have experienced 5years of simultaneous drought. The projection results of global climate models show that the probability of concurrent drought events is highly likely to increase during 2020 to 2050. The increasing concurrent drought events will challenge the success of the water diversion project, which is a strategic attempt to resolve the water crisis of North China Plain. The data suggest great urgency in preparing adaptive measures to ensure the long-term sustainable operation of the water diversion project.

GRACE-Based Hydrological Drought Evaluation of the Yangtze River Basin, China

AbstractIn this study, hydrological drought in the Yangtze River basin (YRB) is characterized based on Gravity Recovery and Climate Experiment (GRACE) total water storage (TWS). An artificial neural network approach is applied to extend the GRACE TWS observations (2003–12) to a longer TWS time series (1979–2012), which is well matched (Nash–Sutcliff efficiency of 0.83) to the GRACE data. Hydrological drought is identified by water storage deficit (WSD; the shortfall in TWS from the average value) in three consecutive months. The method builds on previous research by considering potentially ineffective interdrought events and by characterizing drought recovery time from a multidecadal TWS time series. The results show that the YRB was in hydrological drought 29 times during 1979–2012, and the three subbasins of the YRB (upper, middle, and lower YRB) experienced between 21 and 28 hydrological drought events during the same period. The drought recovery time, defined as the time required for WSD to recover to...

Assessment of the Influences of Different Potential Evapotranspiration Inputs on the Performance of Monthly Hydrological Models under Different Climatic Conditions

AbstractPotential evapotranspiration (PET), which determines the upper limit of actual evapotranspiration (AET), is a necessary input in monthly hydrological models. In this study, the sensitivities of monthly hydrological models to different PET inputs are investigated in 37 catchments under different climatic conditions. Four types of PET estimation methods (i.e., Penman–Monteith, Hargreaves–Samani, Jensen–Haise, and Hamon) give significantly different PET values in the 37 catchments. However, similar runoff simulations are produced based on different PET inputs in both nonhumid and humid regions. It is found that parameter calibration of the hydrological model can eliminate the influences of different PET inputs on runoff simulations in both nonhumid and humid regions. However, the influences of parameter calibration on the simulated water balance components, including AET and water storage change (WSC), are different in nonhumid and humid regions. In nonhumid regions, simulated runoff, AET, and WSC ar...

Evaluation of streamflow simulation results of land surface models in GLDAS on the Tibetan plateau

The Global Land Data Assimilation System (GLDAS) project estimates long-term runoff based on land surface models (LSMs) and provides a potential way to solve the issue of non-existent streamflow data in gauge-sparse regions such as the Tibetan Plateau (TP). However, the reliability of GLDAS runoff data must be validated before being practically applied. In this study, the streamflows simulated by four LSMs (CLM, Noah, VIC, and Mosaic) in GLDAS coupled with a river routing model are evaluated against observed streamflows in five river basins on the TP. The evaluation criteria include four aspects: monthly streamflow value, seasonal cycle of streamflow, annual streamflow trend, and streamflow component partitioning. The four LSMs display varying degrees of biases in monthly streamflow simulations: systematic overestimations are found in the Noah (1.74 ≤ bias ≤ 2.75) and CLM (1.22 ≤ bias ≤ 2.53) models, whereas systematic underestimations are observed in the VIC (0.36 ≤ bias ≤ 0.85) and Mosaic (0.34 ≤ bias ≤ 0.66) models. The Noah model shows the best performance in capturing the temporal variation in monthly streamflow and the seasonal cycle of streamflow, while the VIC model performs the best in terms of bias statistics. The Mosaic model provides the best performance in modeling annual runoff trends and runoff component partitioning. The possible reasons for the different performances of the LSMs are discussed in detail. In order to achieve more accurate streamflow simulations from the LSMs in GLDAS, suggestions are made to further improve the accuracy of the forcing data and parameterization schemes in all models.

Change in global radiation and its impacts on the water cycle in the Yangtze River basin from 1961 to 2010

This study analyzes changes in global radiation in the Yangtze River basin during 1961 to 2010. Global radiation decreased significantly from 1961 to 1989 and increased from 1990 to 2010, which has been described as “from dimming to brightening”. The analysis indicates that changes in fuel consumption were responsible for the variation in global radiation, while changes in meteorological variables had a limited contribution. The impact of changing global radiation on the water cycle is discussed. The change in global radiation was possibly the main cause of changes in potential evapotranspiration, which also decreased from 1961 to 1989 and increased from 1990 to 2010.