Program Description

The Hydrologic Cycle

Water is essential to life on Earth, plays a key role in the development and functioning of society, and is a high priority resource for sustainable development. Many aspects of the Earth’s water cycle are not well understood or simulated by climate models.  One thrust of the Water System effort is to reduce this uncertainty through improved understanding and parameterization of the water cycle in climate models. Another component of the Water System effort is to examine the impact of a changing water system on water management and policy.  A complete understanding of the water system requires an approach that extends beyond atmospheric analysis to include ground and surface water cycling and the water resources implications associated with change in an evolving Earth system.

Key Water System Questions:

Diurnal Cycle of Summertime Rainfall Occurrence Over the Continental U.S. (after Carbone and Tuttle, 2007)

• Do current climate models properly represent the coupling between the land surface, planetary boundary layer, and clouds in their parameterizations?  If not, what needs to be improved?
• Which dynamical and physical factors exert the greatest influence on the occurrence of warm season continental precipitation, and how will these change on a warmer planet?
• Which dynamical and physical factors exert the greatest influence on the occurrence of cool season precipitation over orography, and how will these change on a warmer planet?
• What are the key processes controlling the diurnal cycle of precipitation globally?  How do human perturbations impact this diurnal cycle?
• Will climate warming lead to increased floods and droughts?
• What are the inter-relationships between changes in land use, precipitation, surface water, and ground water storage in aquifers?
• How does management of water resources (dams, irrigation, etc.) impact the Earth System, including its interaction with biogeochemical cycles?

Hydrometeorological Component

An overarching goal is to improve the representation of the water cycle in climate models. While climate models typically predict temperature with reasonable confidence, predictions of precipitation are notably weak.  Our focus has been on the diurnal cycle of warm season precipitation including diagnoses of rainfall climatology downwind of major mountain ranges over continental regions.  These global and regional studies have shown that such regions are frequented by organized convection, which propagates and may produce up to 70% of the observed warm season precipitation.  These systems, however, are inadequately represented in current global and regional climate models.  IPCC4 projections of precipitation trends highlight these regions as having the largest variability or uncertainty.   This component of the program will focus on two areas during the next 5 years: 1) the development of an improved parameterization of convective precipitation over continental regions globally, and 2) the role of soil moisture in modulating the quantity of convective precipitation over continents.

Projected precipitation changes in global climate models over regions where people live mostly show low confidence. This is especially true in the northern hemisphere summer and the southern hemisphere winter (red outlines). These are also regions with coherent patterns of propagating convection, a focus of Water Cycle research. (Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change)

Hydrologic Component

While precipitation is a key component of the water cycle, water processes at the land surface and below ground play a similarly important role.  The terrestrial hydrologic cycle is driven by water and energy fluxes at scales smaller than the grid spacing of climate system models.  Efforts during the next five years will focus on improved representation of hydrologic processes, such as routing and infiltration of water, as part of land models within regional and global climate system models, and the emergent Earth system modeling efforts.  This will include water processes at the land surface and below, as well as the important interactions with carbon and nitrogen cycles.

Water Resources Component

TIIMES and ISSE will jointly conduct coordinated Water System research that will span predictions in the physical water cycle, water resources and societal impacts.  An umbrella program, “Societal, Water, the Atmosphere and Natural Systems” (SWANS), is currently emerging.  The first project, “Colorado Headwaters”, focuses on critical questions concerning the effect of climate change on snow processes in the western cordillera of North America.  It employs both high resolution models and observations, and addresses the resulting implications for water management and policy.  This five-year project is a major focus of the Water System program.  More generally, studies of hydrological extremes are a shared priority for TIIMES/ISSE research, including the study of floods and droughts in future climate scenarios and the fate of aquifers under stress from climate change and human exploitation.

Multidisciplinary Collaboration

NCAR’s Water System Program is a multidisciplinary collaboration housed in TIIMES that includes participants from four NCAR Divisions: ATD - CGD - MMM - RAL.

Program Lead - Contact

Dr. Roy Rasmussen
Email: rasmus@ucar.edu
Phone: 303.497.8430    Fax: 303.497-8401
Mailing Address: NCAR, P.O. Box 3000, Boulder, CO USA 80307-3000