The Global Water Cycle
USGCRP FY 2002 Accomplishments

USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

The Global
Water Cycle

Observing and Monitoring the Climate System

Communications

International Research and Cooperation

The following are some of the USGCRP's major accomplishments related to the global water cycle during Fiscal Year 2002.

Decision-Support Product Testing:

Collaborative exercises were developed whereby water managers and water users test experimental products developed by the Water Cycle program in parallel with normal operations to evaluate product utility. In particular, experimental Land Data Assimilation System (LDAS) products have been developed that will be compared with traditional algorithms used to determine releases from multipurpose reservoirs in the Upper Columbia Basin. Other test sites include the Madison and Jefferson headwaters basins of the Upper Missouri. In addition, a framework for scientist-stakeholder interaction to improve water management was designed for the headwaters of the Red River in southwest Oklahoma in conjunction with the Hydrology for Environment, Life and Policy program.

Precipitation Measurement:

Precipitation is the most important variable in developing products for water resource managers on the seasonal and sub-seasonal time scales, yet it is not accurately predicted. Although it is a major component in the climate system, tropical precipitation has not been quantified with a high degree of accuracy. The accurate and detailed three-dimensional observation data provided by the Tropical Rainfall Measuring Mission (TRMM) are improving precipitation predictions and providing information on extreme precipitation events in the remote tropical areas and over the oceans. Combining TRMM data with data from earlier satellites has substantially reduced uncertainty in tropical precipitation estimates from about 50 percent to about 20 percent. In addition, analysis of TRMM observations has shown that dust storms originating in northern Africa, urban air pollution, and biomass burning all result in smaller cloud droplets and change precipitation processes in and near the affected areas.

Water Cycle Observing Systems:

Variations in the water cycle lead to variations in the productivity of many sectors including hydropower production, coastal fisheries, agriculture, and forestry, to name just a few. Better monitoring and prediction of water cycle variations will allow for better management of these resources. The recently launched EOS-Aqua satellite carries a suite of observing instruments that enable it to deliver estimates of important water cycle variables, including evaporation from the oceans, water vapor, clouds, precipitation, soil moisture, snow cover, and ice on the land and sea. Aqua also will measure related variables, such as changes in radiant energy, aerosols, vegetation cover on the land, phytoplankton and dissolved organic matter in the oceans, and air, land, and water temperatures. The cloud properties measured by Aqua will complement and enhance those measured by the EOS-Terra satellite, because the afternoon viewing time of Aqua provides more clouds than the morning viewing time of Terra.

In addition, Aqua is carrying sophisticated temperature and moisture sounding instruments and a modern microwave imager. A multitude of water cycle and climate studies are expected to utilize data from the Terra and Aqua satellites. Moreover, advances in data processing enable investigators to use data streams from sensors on separate but complementary satellites to separate dynamic processes at various scales. Research that utilized data from space-flight trials over forests, agricultural areas, and grasslands demonstrated the value of remotely sensed measurement of soil moisture. Accurate characterization of soil moisture is critical for modeling hydrologic and atmospheric processes, but until satellite-based instruments were developed, soil moisture data were so sparse soil moisture estimates contributed substantially to output uncertainty. In another study, an algorithm successfully produced measurements of soil moisture on a seasonal timescale from satellite data, and demonstrated the potential of this technology for global mapping of soil moisture.

Earth's Radiant Energy:

Many of the uncertainties in the projections of the warming effects of increasing atmospheric carbon dioxide arise from the inability to adequately represent the cloud and water vapor radiative feedback processes in models. Advances were made in measuring the Earth's surface energy and describing the interactions of water and energy (heat) in the water cycle. On a global scale, an analysis of satellite data showed that in the tropics, over the period 1985-2000, the thermal radiation emitted by the Earth to space increased, while reflected sunlight decreased. The size of these changes is significant and indicates a decrease in cloud cover accompanying a warming trend in the region. Complementary analyses of upper tropospheric humidity, cloud amount, surface air temperature, and vertical velocity indicate the changes could be associated with an observed long-term (decadal) strengthening of atmospheric circulation in the tropics. Such strengthening leads to more humid, cloudy conditions and more intense convection in certain predictable regions of the tropics and drier (and less cloudy) conditions in other tropical regions and subtropical regions. Improved understanding of this relationship leads to reduced uncertainty in climate model projections of regional impacts. The recently completed data set of decadal Surface Radiation Budget Climatology was used to identify these patterns in tropical weather systems.

Figure 5.2.  The "Blue Marble" -- Global Water and Energy Cycles.  (a) Western Hemisphere; (b) Eastern Hemisphere

Data Assimilation:

The predictions made with many applied models, such as for predicting crop yields, insect populations, health, reservoir management, fire potential and management, all require good input data. Data assimilation is an advanced method for using measurements and models in combination to provide improved input data for these models. Researchers have developed a more effective method of assimilating remotely sensed data to estimate ground heat, sensible heat at the surface, and changes in heat associated with evaporation at regional scales. Progress in data assimilation is needed because the observational networks of the future are very likely to consist primarily of remotely sensed, rather than in situ, data and because current measurements of these variables are either poor or nonexistent. The assimilated data provide a consistent version of observations and model initial conditions in numerical simulations and forecasts. In the Mississippi River Basin, for example, combining high-resolution models and data through data assimilation systems has improved, by as much as 15 percent, the ability to estimate regional water and energy variables. These improvements will enable regional models to do a better job of simulating runoff and streamflow.

Land-Surface Processes:

Predictions of water availability for hydropower, reservoir management and flood planning depend on how precipitation is partitioned when it reaches the ground and, for snow, how water moves after the snow melts. Improved representations of cold season and land surface processes, such as snow cover and ground frost, produced capabilities for more accurate predictions of regional winter and spring temperatures and the amount and timing of runoff in the Missouri and Mississippi River basins in the winter and spring months. These improvements were incorporated into climate models and operational numerical weather forecast procedures. Regional studies also have demonstrated the effects of Pacific Ocean surface temperatures on the weather that influences snow pack and stream flow in the Pacific Northwest. Modifying models to include these effects is expected to improve water resource predictions. Other integrated studies assessed the effect of different climate histories on ground water levels in the Santa Clara-Calleguas region of southern California.

Water Vapor :

Because water vapor is by far the most abundant of the greenhouse gases, accurate water vapor measurements are essential for understanding many atmospheric processes and representing them in climate models. Reliable measurement tools are needed to measure water vapor accurately. Field campaigns at the Atmospheric Radiation Measurement Southern Great Plains field site evaluated the accuracy of water vapor measuring instruments in the upper troposphere, where cold, dry conditions make accurate measurements particularly challenging. They demonstrated the accuracy of three different instruments, which produce measurements that agree within 2 percent, and showed that the other instruments tested will require more work to reach the same level of agreement. Improving the characterization of water vapor flows was identified as essential to improving the predictive capability of water cycle models and their usefulness to water resource management. Studies of variability in the flow of global water vapor revealed a clear pattern in the three-dimensional distribution of water vapor, recurring daily or more often like ocean tides.

Water Quality:

Not only does poor water quality limit the water available for use by society, it also affects the productivity of ecosystems. Recent field studies investigated the accumulation of pollutants in ponds and streams, particularly in bed sediments, and subsequent movement of nutrients from land through coastal wetlands. Based on the field results, models are being developed that simulate how aeration, binding, pollution, and storm energy interact in pollution loading to coastal waters. Integrated field and modeling studies in hill-slope environments elucidated flow paths and chemical reactions in the dynamic mixing zones near the soil surface. Water quality in streams and rivers is largely determined by chemical weathering and reactions along runoff flow paths down and through hill slopes and floodplains. Research producing better definition of these pathways and temporary storage locations -- their physical, chemical and microbiological characteristics -- is building the science needed to predict the impacts of land and its uses and of climate change on water quality and aquatic ecosystems.