Contact

Justin Sheffield
Dept. Civil & Environ. Eng.
Princeton University
Princeton
NJ 08544, USA
tel: (609) 258-1551
fax: (609) 258-2799

Welcome to my web pages. Here you will find summaries of my research and current projects. Click on the links to the right to access these. Below are some recent publications, results, photos and other new stuff.

My main research interests center around the global land surface hydrological cycle and particularly the question of whether the hydrologic cycle is intensifying in response to climate change and global warming. The potential effects of intensification on the environment and society in terms of changes to the availability or absence of water, such as through flooding or drought, is relatively unknown but potentially large and damaging. In an attempt to address these issues my research has looked at a number of inter-related themes, including historic and future drought occurrence, soil moisture memory and precipitation recycling, teleconnectivity at local and remote scales and regional climate impacts.

Global Estimates of Land Evapotranspiration

Dec 2011. Estimates of global evapotranspiration from multi-model and multi-sensor data

We have developed estimates of global land evapotranspiration based on satellite remote sensing data for 1984-2006. The retrieved data are from multiple remote sensing datasets and using three process-based retrieval models, which represents a first consistent estimate of the uncertainties in global evapotranspiration from remote sensing.

Abstract: Estimating evapotranspiration (ET) at continental to global scales is central to understanding the partitioning of energy and water at the earth's surface and the feedbacks with the atmosphere and biosphere, especially in the context of climate change. Recent evaluations of global estimates from remote sensing, upscaled observations, land surface models and atmospheric reanalyses indicate large uncertainty across the datasets of the order of 50% of the global annual mean value. In this paper, we explore the uncertainties in global land ET estimates using three process-based ET models and a set of remote sensing and observational based radiation and meteorological forcing datasets. Input forcings were obtained from International Satellite Cloud Climatology Project (ISCCP) and Surface Radiation Budget (SRB). The three process-based ET models are: a surface energy balance method (SEBS), a revised Penman–Monteith (PM) model, and a modified Priestley–Taylor model. Evaluations of the radiation products from ISCCP and SRB show large differences in the components of surface radiation, and temporal inconsistencies that relate to changes in satellite sensors and retrieval algorithms. In particular, step changes in the ISCCP surface temperature and humidity data lead to spurious increases in downward and upward longwave radiation that contributes to a step change in net radiation, and the ISCCP data are not used further. An ensemble of global estimates of land surface ET are generated at daily time scale and 0.5 degree spatial resolution for 1984–2007 using two SRB radiation products (SRB and SRBqc) and the three models. Uncertainty in ET from the models is much larger than the uncertainty from the radiation data. The largest uncertainties relative to the mean annual ET are in transition zones between dry and humid regions and monsoon regions. Comparisons with previous studies and an inferred estimate of ET from long-term inferred ET indicate that the ensemble mean value is reasonable, but generally biased high globally. Long-term changes over 1984–2007 indicate a slight increase over 1984–1998 and decline thereafter, although uncertainties in the forcing radiation data and lack of direct linkage with soil moisture limitations in the models prevents attribution of these changes

Vinokullo, R. V., R. Meynadier, J. Sheffield, and E. F. Wood, Multi-model, multi-sensor estimates of global evapotranspiration: climatoloyg, uncertainties and trends, Hydrol. Proc., doi : 10.1002/hyp.8393, (http://onlinelibrary.wiley.com/doi/10.1002/hyp.8393/abstract). Link to Hydrological Processes Article

Research on Trends in Drought over China

Aug 2011. Research on Drought in China highlighted in Nature Climate Change

Our recent article in Journal of Climate looks at multi-model simulations of agricultural drought over China since 1950. The analysis shows that over the past 60 years severe droughts in China were frequent and grew increasingly common, even though many were not widely reported. The study, led by Aihui Wang, of the Institute of Atmospheric Physics in Beijing, China, looked at changing soil moisture levels across the country between 1950 and 2006 using four models. After checking that the models showed only limited variation in their outputs, the results were combined to generate a geographic map of moisture variations for the period. Overall, 37% of China's area became drier, while 22% got wetter. Northern and central regions experienced the most severe drying trends, suggesting an increasing risk of agricultural failure in the future if the trend continues. Indeed, the winter drought that hit the northeast in 2008–2009 led to economic losses of $2.3 million and left more than 10 million people struggling with water shortages. Climate models predict that China's summer monsoon will become more intense everywhere, which may offset the macroscale drying trend.

Wang, A., D. P. Lettenmaier, and J. Sheffield, 2011: Soil moisture drought in China, 1950-2006, J Climate, 24 (13), 3257-3271, Jan. 27, 2011, doi: 10.1176/2011JCLI3733.1 Link to J. Climate Article

Link to Nature Climate Change Article

New book on global drought published

April 2011. "Drought: Past Problems and Future Scenarios", by Justin Sheffield and Eric F. Wood, Earthscan, UK, p192

Drought is one of the likely consequences of climate change in many regions of the world. Together with an increased demand for water resources to supply the world's growing population, it represents a potentially disastrous threat to water supplies, agriculture and food production, leading to famine and environmental degradation. Yet predicting drought is fraught with difficulty.

The aim of this book is to provide a review of the historical occurrence of global drought, particularly during the 20th century and assess the likely potential changes over the 21st century under climate change. This includes documentation of the occurrence and impacts of major 20th century drought events and analysis of the contributing climatic and environmental factors that act to force, prolong and dissipate drought. Contemporary drought is placed in the context of climate variability since the last ice age, including the many severe and lengthy drought events that contributed to the demise of great civilizations, the disappearance of lakes and rivers, and the conversion of forests to deserts.

The authors discuss the developing field of drought monitoring and seasonal forecasting and describe how this is vital for identifying emerging droughts and for providing timely warning to help reduce the impacts. The book provides a broad overview of large scale drought, from historic events such as the US Dust Bowl and African Sahel, and places this in the context of climate variability and change. The work is soundly based on detailed research that has looked at drought occurrence over the 20th century, global drought monitoring, modelling and seasonal prediction, and future projections from climate models.

Available from Earthscan publishers and other online booksellers.

High Resolution Evapotranspiration for Mexico

April 17 2010: We have developed a high resolution (~ 10km) dataset of evapotranspiration for Mexico for 1984-2006 based on remote sensing data and this has been published in the Journal of Hydrometeorology. Evapotranspiration is calculated using a modified version of the Penman-Monteith algorithm, with input radiation and meteorological data from the International Satellite Cloud Climatology Project (ISCCP) and vegetation distribution derived from Advanced Very High Resolution Radiometer (AVHRR) products. The ISCCP data are downscaled to 1/8th degree resolution using statistical relationships with data from the North American Regional Reanalysis (NARR). The final product is available at 1/8th degree, daily, for 1984-2006 for all Mexico. Comparisons are made with the NARR, off-line land surface modeling and measurements from approximately 1800 pan stations. The remote sensing estimate follows well the seasonal cycle and spatial pattern of the comparison datasets, with a peak in late summer at the height of the North American Monsoon and highest values in low-lying and coastal regions. The spatial average over Mexico is biased low by about 0.3 mm day-1, with a monthly rmse of about 0.5 mm day-1. The underestimation may be related to the lack of a model for canopy evaporation which is estimated to be up to 30% of total evapotranspiration. Uncertainties in both the remote-sensing based estimates (due to input data uncertainties) and the true value of evapotranspiration (represented by the spread in the comparison datasets) are up to 0.5 and 1.2 mm day-1, respectively. This study is a first step in quantifying the long-term variation in global land evapotranspiration from remote sensing data.

Sheffield, J., E. F. Wood, and F. Munoz-Arriola, 2010: Long-term regional estimates of evapotranspiration for Mexico based on downscaled ISCCP data. J. Hydrometeor., 11(2), 253-275.

Updated Global Meteorological Forcings Dataset Released for 1948-2006

Jul 15 2009: Our global meteorological forcing dataset has been extended to 1948-2006 and is now available for download. This replaces the previous version due to significant upgrades to the underlying data and methods: i) extension to 2006; ii) improved sampling procedure for correction of rain day statistics; iii) use of latest versions of CRU (TS3.0), SRB (V3.0) and TRMM products; iv) improved consistency between specific and relative humidity and air temperature; v) improved inter-annual variability in downward shortwave radiation.

The data can be downloaded from http://hydrology.princeton.edu/data.php or from the NCAR CISL Research Data Archive (dataset ds314.0) at http://dss.ucar.edu/datasets/ds314.0/

Large Scale Drought Events 1950-2000

May 13 2009: A paper just published in Journal of Climate uses observation-driven simulations of global terrestrial hydrology and a cluster algorithm that searches for spatially connected regions of soil moisture, to identiy 296 large-scale drought events (greater than 500 000 km2 and longer than 3 months) globally for 1950–2000. Using a severity–area–duration (SAD) analysis, we identified and characterized the most severe events for each continent and globally at various durations and spatial extents. An analysis of the variation of large-scale drought with sea surface temperatures (SSTs) revealed connections at interannual and possibly decadal time scales. Three metrics of large-scale drought (global average soil moisture, contiguous area in drought, and number of drought events shorter than 2 years) are shown to covary with ENSO SST anomalies. At longer time scales, the number of 12-month and longer duration droughts follows the smoothed variation in northern Pacific and Atlantic SSTs. Globally, the mid-1950s showed the highest drought activity and the mid-1970s to mid-1980s the lowest activity. This physically based and probabilistic approach confirms well-known droughts, such as the 1980s in the Sahel region of Africa, but also reveals many severe droughts (e.g., at high latitudes and early in the time period) that have received relatively little attention in the scientific and popular literature.

Sheffield, J., K. M. Andreadis, E. F. Wood, and D. P. Lettenmaier, 2009: Global and continental drought in the second half of the 20th century: severity-area-duration analysis and temporal variability of large-scale events, J. Climate, 22(8), 1962-1981.

Closing the Terrestrial Water Cycle from Satellite Remote Sensing

Apr 3 2009: Our paper published in Geophysical Research Letters looks at the potential to monitor the terrestrial water cycle using only satellite remote sensing. It is now possible to regularly monitor the major components of the water balance (precipitation, evaporation, water storage and streamflow) from space but the question remains as to how accurate these are and whether they are consistent, i.e. is there closure of the water balance. Using precipitation data from TRMM and CMORPH, evaporation based on EOS (AIRS, MODIS, CERES) data and water storage change from GRACE, we derive runoff as a residual of the water balance and compare this with observed runoff and streamflow for the Mississippi basin. The results show large residual errors (non-closure) mainly due to high biases in the precipitation data. Removal of systematic biases reduces the error significantly, but the remaining errors and uncertainties in the individual components are often larger than the streamflow itself. Further research is needed to reduce errors in remotely sensed precipitation and uncertainties in other components.

Sheffield, J., C. R. Ferguson, T. J. Troy, E. F. Wood, and M. F. McCabe, 2009: Closing the terrestrial water budget from satellite remote sensing, Geophys. Res. Lett., 36, L07403, doi:10.1029/2009GL037338.

Experimental Global Terrestrial Water Cycle Monitor

Jan 15 2009: Our experimental Global Terrestrial Water Cycle Monitor has come online (see link). The monitor forms the basis of the African Drought Monitor (see below), providing near realtime monitoring of land surface hydrological conditions for global land areas, excluding Greenland and Antarctica.

Experimental African Drought Monitor

Jun 18 2008: Our experimental African Drought Monitor (ADM) has come online (see http://hydrology.princeton.edu/monitor). The ADM is operated by the Land Surface Hydrology Group at Princeton University with support from the International Hydrology Program of UNESCO. The system provides near realtime monitoring of land surface hydrological conditions. The hydrologic cycle is modeled using the VIC model which is forced by a combined model/observation dataset of meteorological forcings (precipitation, temperature, etc). Precipitation is currently taken from the PERSIANN dataset. Temperature and windspeed are taken from GTS gauge reports. The monitor is updated every day at 2 days behind realtime. Available outputs include water budget components (precipitation, evapotranspiration, runoff, snow and soil moisture) and derived products such as current drought conditions.

Future Changes in Global Drought

18 May 2008: Our paper published in Climate Dynamics shows that drought will increase globally over the 21st century. We analyzed soil moisture from 8 climate models that participated in the latest IPCC assessment (AR4) and calculated changes in drought frequency, severity and spatial extent globally and regionally. The regions projected to be hardest hit are the Mediterranean, southwest US, central America, southern Africa and Australia: regions that currently suffer from drought. The main culprit is descreasing precipitation, coupled with warmer temperatures that lead to increased evaporation. Although the climate models generally predict wetter conditions in high northern latitudes, these will be offset somewhat by earlier and faster spring melt and increased summertime evaporation. The time frame for these changes to be noticeable (statistically different form current climate variability) is of the order of a few decades in some regions.

The figure on the right shows the changes in drought frequency, duration and severity for the world and 20 regions for the SRES B1, A1B and A2 future climate scenarios. The bars indicate the range across the climate models.

Sheffield J., and E. F. Wood, Projected changes in drought occurrence under future global warming from multi-model, multi-scenario, IPCC AR4 simulations, Climate Dynamics, 13 (1), 79-105, doi:10.1007/s00382-007-0340-z.

Global Meteorological Forcing Dataset

15 July 2006: We have recently completed a 50-yr (1950-2000) global forcing dataset for driving hydrologic and other terrestrial models. The data are derived by combining reanalysis fields with a suite of observational datasets. The dataset is available for download from here.

This image shows June precipitation overlayed on Google Earth. Precipitation is downscaled and bias corrected in terms of monthly totals and number of rain days.

Sheffield, J., G. Goteti, and E. F. Wood, 2006: Development of a 50-yr high-resolution global dataset of meteorological forcings for land surface modeling, J. Climate, 19 (13), 3088-3111

Northeast Climate Impacts Assessment

A study into current and future climate changes and impacts in the Northeast US been just been released. We carried out the modeling and analysis of hydrologic impacts of increasing temperatures and changing precipitation under different emission scenarios.

Hayhoe, K., C. Wake, T. Huntington, L. Luo, M. D. Schwartz, J. Sheffield, E. F. Wood, B. Anderson, J. Bradbury, A. DeGaetano, T. J. Troy, and D. Wolfe, 2007: Past and future changes in climate and hydrological indicators in the U.S. Northeast. Climate Dynamics, 28 (4), 381-407, 10.1007/s00382-006-0187-8.

Global Hydrology Simulations

This image shows global snow depth as simulated by our land surface model and overlayed on Google Earth. The simulation was forced by our global 50-yr forcing dataset that has just been released.