USGCRP
Home  Research Elements & Cross-cutting Activities
The
Global Water Cycle
Recent Accomplishments |
| Search |
Updated 1 December 2007
|
The
Global Water Cycle |
|
The Global Water CycleRecent Accomplishments Archived News Postings (June 2000 - July 2005)
Additional Past Accomplishments:
|
 |
The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2008 edition of the annual report, Our Changing Planet). These research results address the strategic research questions on the global water cycle identified in the CCSP Strategic Plan. Reducing Uncertainty in the Predictions/Projections of Climate Change, and in the Global and Regional Water/Energy Cycle.1 An evaluation of of two modeling approaches, the multi-scale modeling framework (MMF) and the traditional Community Atmospheric Model (CAM), compared the models' simulations with observations. The evaluation showed that distributions of cloud fraction, precipitation intensity, and downwelling solar radiation flux at the surface from the MMF run were more consistent with observations than those from the CAM run (see Figure 4). This is attributed to the improved representation of convective (e.g., thunderstorm) clouds in the MMF compared to the conventional climate model.
Water Cycle over High Latitudes and Polar Regions. 2,3 Consistent with evidence of warmer temperatures, earlier spring green-up of vegetation and longer growing seasons at high latitudes, the atmospheric water cycle over polar regions shows a trend toward an earlier transition from winter to summer moisture recycling patterns during spring over North America and Europe. This conclusion is supported by findings from the Gravity Recovery and Climate Experiment (GRACE) gravity anomaly satellite, and other observations showing a thinning of the Greenland ice sheet and accelerated ice discharge. Model projections of future climate with
double and triple
atmospheric carbon dioxide concentrations suggest a general increase in precipitation in high-latitude river basins driven by increased transport of moisture into the basins, and higher rates of
evaporation driven by rising temperature.
Trajectory Shifts in the Arctic and Subarctic Freshwater Cycle.4 Manifold changes in the
freshwater cycle of the high-latitude lands and oceans have been reported the past few years. A synthesis by researchers of these changes in freshwater sources and in the ocean freshwater storage illustrates the complementary and
concurrent pattern and magnitude of these changes over the past 50 years. Increasing river discharge anomalies and excess net precipitation on the ocean contributed about 20,000 km3 of freshwater to the Arctic and high-latitude North Atlantic oceans from minimal annual rates in the 1960s to maximal rates in the 1990s. Sea ice attrition provided roughly another 15,000 km3, and glacial melt added about 2,000 km3. The sum of inputs from these freshwater sources above the long-term average matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995. The changes in freshwater inputs and oceanic storage occurred in
conjunction with the amplifying North Atlantic Oscillation, a large-scale pattern of high and low pressure, and rising air temperatures. Freshwater may now be accumulating in the Arctic Ocean and will likely be transported southward if and when the North Atlantic Oscillation enters a new phase.
Colorado River Basin Water Management.5 Recent studies of past climate and streamflow conditions have broadened understanding of long-term water availability in the Colorado River, revealing many periods when streamflow was lower than at any time in the past 100 years of recorded flows. Past water management decisions have been based largely on the gage record, and there has been an implicit assumption that there is a single value of the river's average annual flow–about 18.5 km3 per year–around which interannual flow variations occur. Even though the basin experienced wet and dry periods, river flows and weather conditions were expected to return to a "normal" state, largely defined by the climate of the early and middle 20th century. However, recent reconstructions based on tree rings demonstrate that Colorado River flows occasionally shift into decade-long periods in which average flows are lower, or higher, than the supposed mean value. These reconstructions reinforce the point that the gage record covers only a small subset of the range of natural hydroclimatic variability in the river basin over several centuries. The basin's future hydrology thus may not be reasonably characterized by the gage record alone. That information, along with two important trends–a rapid increase in urban populations in the West and significant climate warming in the region–will require that water managers prepare for possible reductions in water supplies that cannot be fully averted through traditional means. Successful adjustments to these new conditions will require strong and sustained cooperation among the many entities involved in Colorado River water management and science programs.
Climate-Driven Variability and Trends in Mountain Snowpack in Western North America.6,7 A recent study applied a regression analysis approach to snowpack and climate station data spanning the period 1960 to 2002 for the western North American region (Oregon, California, Nevada, and Colorado, among others). The study used 1 April snowwater equivalent (SWE) to represent a cumulative, simplified summary of the
previous several months' weather: snow deposition, snow melting or ablation, and rain events that may either partially melt snow or be absorbed in the snowpack, increasing SWE. For most snow course locations in the West, the study found that long-term variations in spring SWE are reasonably well explained by summaries of seasonal climate at nearby stations. Day-to-day details of snow accumulation, ablation, and melt are generally of secondary importance, except where correlations between observed and climate-derived SWE are low.  During the second half of the 20th century, and likely since 1916, winter and spring warming in the West have reduced spring snowpack at most locations. Increases in precipitation appear to have offset this loss in some places since mid-century, notably in the southern Sierra Nevada mountains, where large increases have occurred. Some of the interannual variability and long-term trends can be explained as a response to variability and change in North Pacific climate, especially as represented by the North Pacific Index (NPI), which responds to the oceanic variations of the El Niño Southern Oscillation and Pacific Decadal Oscillation, which are large-scale
patterns of climate variations. However, NPI can only account for about half of the trends in the Pacific Northwest since mid-century (and less elsewhere or from earlier starting points), in rough agreement with modeling results. The remaining portion clearly includes the influence of the warming observed throughout the West, which is largely unrelated to the Pacific climate variability and may well represent human
influences on climate. That is, even after accounting for the role of known patterns of climate variability, there is a substantial
downward trend in overall snowpack in the West that is consistent with observed warming. Even a conservative estimate (0.3°C per decade) of the likely future
warming rate for the western mountains in winter would, by 2100, move the 0°C isotherm to where the 3°C isotherm now lies: Most of the western mountains would be in the transient snow zone, in which snow accumulates and melts repeatedly
during the snow season. In the future, intraseasonal behavior of snowpack will likely change from a steady accumulation to alternating accumulation and loss due to warmer conditions. Simple regression-based methods currently used by water resource planners for forecasting seasonal volumetric streamflow will have to be revised or replaced by more sophisticated methods that can account for the changing role of temperature both in determining the quantity of spring snowpack (the subject of this study) and the rate at which it melts.
Vegetation, Soil Moisture, and Water Table Interactions.8,9The fraction of rainfall that recharges groundwater and ends up as streamflow tends to increase as the fraction of land devoted to agriculture decreases. Conversely, when the extent of cultivation increases a greater fraction of rainfall goes into evapotranspiration, potentially driving a long-term drop in groundwater levels. This is true in areas without crop irrigation. Where irrigation taps surface or groundwater, depletions may be even faster. Modeling of the coupled groundwater-surface water-soil vegetation system shows that shallow water tables can be either a sink or source of water relative to surface soil moisture, depending on the balance of infiltration versus evaporation, while deep water tables have little impact on surface fluxes. Thus, intense agriculture can amplify surface water stresses by increasing the propensity of soil moisture to decouple from a depleting water table, particularly during drought conditions.The Effect of Irrigation on Land Surface Temperatures.10As seen from space, the region identified as the Umatilla Ordnance Depot in northeastern Oregon provides a striking example of temperature contrast caused by irrigation and vegetation type. Suited to hot, dry summers and an average annual precipitation of 200 mm, the native plants that grow on the depot use little water. Poplars–fast-growing commercial trees used as inexpensive lumber and pulp for paper–use large amounts of water, making them one of the most heavily irrigated crops in the Columbia River Basin. In nature, poplars grow in wetlands or along riverbanks, where they have access to water, and the commercial poplar plantation is located close to the river for the same reason. Figure 5 shows the difference in water use that distinguishes the two ecosystems.
Fifty-Year High-Resolution Global Data Set of Meteorological Forcings for Land Surface and Hydrological Modeling.11 Understanding variability of the terrestrial hydrological cycle is central to determining the potential for extreme events and its susceptibility to future change. In the absence of long-term, large-scale observations of the components of the hydrological cycle, modeling can provide consistent fields of land surface fluxes and states. To enable such an integrated analysis, researchers have created a global,
50-year, 3-hourly, 1° data set of meteorological forcings that can be used to drive models of land surface hydrology. The data set was constructed by combining a suite of global observation-based data sets with the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis. Known systematic errors (biases) in the reanalysis precipitation and near-surface meteorology have been corrected using observation-based data sets of precipitation, air temperature, and
radiation, among others. Wind induced undercatch of frozen precipitation is removed using the results of the World Meteorological Organization's Solid Precipitation Measurement Intercomparison. Precipitation is reduced in scale to 1° using statistical relationships developed with the Global Precipitation Climatology Project (GPCP) daily product. Disaggregation in time from daily to 3-hourly is accomplished similarly, using the Tropical Rainfall Measuring Mission (TRMM) 3-hourly real-time data set. Other meteorological variables (downward shortwave and longwave radiation, specific humidity, surface pressure, and wind speed) are downscaled in space while accounting for changes in elevation. The data set was evaluated against the bias-corrected forcing data set of the second Global Soil Wetness Project (GSWP2). The final product can be used to drive models of terrestrial hydrologic and ecological processes for the study of seasonal and interannual variability and for the evaluation of coupled models and other land surface prediction schemes.
Additional Past Accomplishments:
References1) Ovtchinnikov, M., T. Ackerman, R. Marchand, and M. Khairoutdinov, 2006: Evaluation of the Multiscale Modeling Framework using data from the Atmospheric Radiation Measurement Program. Journal of Climate, 19(9), 1716-1729, doi:10.1175/JCLI3699.1.2) Dirmeyer, P.A. and K.L. Brubaker, 2006: Trends in the Northern Hemisphere water cycle. Geophysical Research Letters, 33, L14712, doi:10.1029/2006GL026359. 3) Rignot, E. and P. Kanagaratnam, 2006: Changes in the velocity structure of the Greenland Ice Sheet. Science, 311, 986-990, doi:10.1126/science.1121381. 4) Peterson, B.J., J. McClelland, R. Curry, R.M. Holmes, J.E. Welsh, and K. Aagaard, 2006: Trajectory shifts in the Arctic and sub-Arctic freshwater cycle. Science, 313, 1061-1066. 5) NRC, 2007: Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability. National Academy Press, Washington, DC, USA, 159 pp. 6) Mote, P.W., 2006: Climate-driven variability and trends in mountain snowpack in Western North America. Journal of Climate, 19, 6209-6220. 7) Hamlet, A., P.W. Mote, M.P. Clark, and D.P. Lettenmaier, 2005: Effects of temperature and precipitation variability on snowpack trends in the western United States. Journal of Climate, 18, 4545-4561. 8) Jayawickreme, D.H. and D.W. Hyndman, 2007: Evaluating the influence of land cover on seasonal water budgets using NEXRAD rainfall and streamflow data. Water Resources Research, 43, W02408, doi:10.1029/2005WR004460. 9) Miguez-Macho, G., Y. Fan, C. Weaver, R. Walko, and A. Robock, 2007: Incorporating water table dynamics in climate modeling, Part II: Formulation, validation, and soil moisture simulation. Journal of Geophysical Research, 112, D13108, doi:10.1029/2006JD008112. 10) Mildrexler, D.J., M. Zhao, and S.W. Running, 2006: Where are the hottest spots on Earth? EOS, Transactions, American Geophysical Union, 87(43), 461-467. 11) Sheffield, J., G. Goteti, and E.F. Wood, 2006: Development of a 50-year high-resolution data set of meteorological forcings for land surface modeling. Journal of Climate, 19, 3088-3111. 12) See science.arm.gov/clasic/ 13) See asp.labworks.org/ 14) Acker, J.G. and G. Leptoukh, 2007: Online analysis enhances use of NASA earth science data. EOS, Transactions, American Geophysical Union, 88, 14-17. 15) Gettelman, A., W.D. Collins, E.J. Fetzer, A. Eldering, F.W. Irion, P.B. Duffy, and G. Bala, 2006: Climatology of upper tropospheric relative humidity from the Atmospheric Infrared Sounder and implications for climate. Journal of Climate, 19, 6104-6121. 16) Gettleman, A., E.J. Fetzer, A. Eldering, and F.W. Irion, 2006: The global distribution of supersaturation in the upper troposphere from the Atmospheric Infrared Sounder. Journal of Climate, 19, 6089-6103.
|
|
