Challenges and Opportunities in the Hydrologic Sciences (2012) / Chapter Skim
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2 The Water Cycle: An Agent of Change
2 The Water Cycle: An Agent of Change
Pages 45-82
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From page 45... ...
Humans have become major agents alongside nature in the functioning of the water cycle, through water management and changes in the land, atmosphere, and ocean that alter natural water processes. Humans also are altering Earth's climate, which produces further changes in the water cycle.
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From page 46... ...
Yet an opportunity exists for models providing plausible scenarios of the impact of climate change and land use change on the regional water cycle. Stepping away from the contemporary and future perspective, examination of water flow and storage during periods ranging over the past decades, centuries, millennia, and into deep geologic time offers opportunity to understand how Earth's water cycle evolved to its present state.
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From page 47... ...
The discovery of terrestrial extrasolar planets potentially broadens that perspective even more. RESEARCH OPPORTUNITIES In this section, the committee discusses several research opportunities for the hydrologic sciences and presents underlying research questions.
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From page 48... ...
. In particular, population growth and development in arid and semi-arid regions where surface water is scarce have led to large reservoirs, diversion projects, and intensive groundwater pumping.
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From page 49... ...
The hydrology of the land surface is affected directly by warming temperatures due to changes in snow and ice cover and shifts in vegetation patterns. The causes of hydrologic replumbing (land use change, hydrologic storage, climate change, etc.)
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From page 50... ...
In some cases, agricultural return flows have become an important source of water, as exemplified well in the Cienega de Santa Clara, an open-water wetland in northern Mexico, which is supported by the return flows from the lower Colorado River basin. The Cienega is threatened by the Yuma Desalting Plant through which return flows would be diverted to the plant for desalination.
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From page 51... ...
. Oil shale and gas production, along with mining, have used a smaller portion of the freshwater in the United States, at 5 percent of the total withdrawn from surface and groundwater supplies.
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In recent years, with dwindling surface water supplies, groundwater has become a primary source of water supply for the NCP. According to Zheng et al.
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SOURCE: China Geological Survey (2009)
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From page 54... ...
Although ethanol production could help achieve the national long-term goal of weaning the nation off greenhouse gas producing fuels, it comes at a cost to water resources, with regard to both water availability and water quality. Water is consumed not only in the production of crops to generate biofuels but also in the refineries that produce ethanol.
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From page 55... ...
Planning for future water resources in these regions should evolve in the face of an anticipated reduction in precipitation. Research is needed, for example, to determine optimal measures for water management as precipitation declines.
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As the glaciers recede, basic research probing the major impacts on water supplies and flow regimes should be pursued. What modeling efforts are needed to better understand the connection between changes in the upstream glacier-water catchments and available water resources downstream?
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From page 57... ...
Runoff projections require coupling of climate models at both the global and regional scales, which feed into hydrologic models and often then into planning and operations models. Furthermore, it is now well known that important feedbacks to climate can occur from terrestrial hydrologic processes such as soil moisture and groundwater fluctuations, necessitating coupling of hydrologic and climate models for improved climate projections.
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From page 58... ...
How can management strategies be refined in the face of known uncertainties in regional climate variability and change? What educational programs are needed to train the next generation of hydrologic and climate scientists who are capable of both integrating the appropriate sets of hydrologic and climate measurements and fully coupling hydrologic and climate models to estimate future water resources availability?
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From page 59... ...
Spaceborne sensors can be deployed that map the state of surface soil moisture, surface temperature, water storage in Earth's crust (Box 2-3) , and other conditions related to the hydrology of the land surface that impose important constraints on the estimation of evaporation and recharge.
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A striking example of an application is the calculation of the amount of water flowing through the Amazon River basin (Figure 2-5)
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Discharge fluxes, from groundwater to Earth's surface, occur to streams and rivers, lakes, and the ocean, to the land surface itself in springs and seeps, or directly to plant roots that emit water into the atmosphere via transpiration. Groundwater discharge also can originate from recharge at distant locations in other watersheds connected by long, groundwater flow paths.
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From page 62... ...
Groundwater fluxes at interfaces can often be inferred using remote sensing, typically during times when the temperature of discharging groundwater is much different than the temperature of the surface water body. These data provide information about broad patterns of discharge.
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From page 63... ...
The committee presents two important problems that further challenge predictive understanding of hydrologic variability across a range of scales, from small subbasins of a few square kilometers to large basins of the order of tens of thousands of kilometers. The first relates to the dynamic connectivity of landscapes, from hillslopes to the subsurface and to fluvial river networks, and how it may manifest in scaling theories of hydrologic response.
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in the Iowa River Basin. The 32,400 km2 Iowa River basin has 29 daily recording U.S.
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interact with those at the larger scale (networks) to describe the entire hydrologic response of river basins and the possible emergent scaling laws for specific rainfall-runoff events remains elusive.
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. A challenge of theoretical and practical interest is to develop a predictive understanding of the two-way interactions between human impacts on the landscape and the climate system and how these impacts may manifest in the hydrologic response.
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From page 67... ...
Understanding causes and patterns of changing hydrology is necessary for understanding changes in streamwater quality and aquatic life -- posing new challenge frontiers at the interface of hydrologic sciences with sister natural sciences as well as with management and policy. Can humaninduced hydrologic changes be regulated to minimize changes in stream ecosystem functioning?
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From page 68... ...
that include both natural variability and human impacts on climate. Slowly varying natural climate behavior is driven by similar conditions in the world's oceans and their interactions with the atmosphere.
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From page 69... ...
. These rapid changes in climate that affect the cryosphere and hydrologic variability present a number of challenges and research opportunities for the hydrologic sciences.
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70 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 2-9 Flowing meltwater on the surface of the Greenland ice sheet. Some of this water goes into the ice sheet through features called moulins, eventually reaching the bottom, where it may promote more rapid flow of the ice.
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From page 71... ...
GCMs currently do not reproduce well the observed low-frequency variability, necessitating the use of both long records of natural variability and information from GCMs for plausible projections of future climate and hydrology. ENSO is one of the best-understood, large-scale drivers of regional climates and hydroclimatic variability, and it influences climate in many parts of the world.
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From page 72... ...
The observed and reconstructed SWE2-10 records are plotted as anomalies from the long-term average. These records indicate that wet conditions in the northern bitmapped, uneditable Rockies and Yellowstone region tended to coincide with dry conditions in the Colorado River basin over the past centuries.
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From page 73... ...
Finally, how can paleoclimate data be best used to disentangle the low-frequency natural variability from trends due to climate change, and can this information be used to inform the range of hydroclimatic conditions that can be expected in the future? Hydrologic systems, particularly the large reservoirs that contain water storage that equals several years of accumulated flow, may have different time scales of response compared to the combined effects of seasonal, annual, and multiyear climate variability.
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From page 74... ...
Refinement of records extending back hundreds to many thousands of years is needed to provide clarity on how natural processes, acting alone or in conjunction with human-caused factors, may yield rapid climate change in the future. Paleoclimate analyses that document abrupt climate change under natural climate variability (Overpeck and Cole, 2006)
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From page 75... ...
. The committee suggests two important research questions that focus on surface water processes.
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From page 76... ...
For example, how do hydrologists distinguish gullies formed by dry avalanches versus wet debris flows versus bedload transport in surface water flows? Early in the manned exploration of Earth's moon, satellite imagery revealed sinuous channels (also called rills)
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From page 77... ...
CONCLUDING REMARKS The aspects of the water cycle highlighted in this chapter present numerous scientific challenges, from understanding the near-surface flux terms foundational to Earth's metabolism and the global-scale natural drivers of hydroclimatic variability, to extending lessons learned on Earth throughout the universe. These are among the most important of the wide range of water cycle topics.
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From page 78... ...
2006. A multimodel ensemble approach to assessment of climate change impacts on the hydrology and water resources of the Colorado River basin.
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From page 79... ...
Water Resources Research 45:W04428.
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2010. Global depletion of groundwater resources.
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From page 81... ...
Hydrologic Sciences Journal 57(1)
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