Challenges and Opportunities in the Hydrologic Sciences (2012) / Chapter Skim
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3 Water and Life
3 Water and Life
Pages 83-122
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From page 83... ...
Although the amount and timing of water supply ultimately constrains life on Earth, living organisms collectively influence the water cycle and the global climate. Vegetation blankets the majority of Earth's land surface, altering its albedo (reflection of solar energy)
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From page 84... ...
3.1. Deep Time Landscapes Landscapes, hydrologic processes, ecosystems, and climate have co-evolved throughout Earth's history and across all spatial scales.
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From page 85... ...
Terrestrial organisms permanently changed hydrologic processes in at least two fundamental ways. With the development of stomata, about 400 million years ago, plants could lift water from deep soil reservoirs without desiccation and transfer it back to the atmosphere, greatly increasing this mass flux (Figure 3-1)
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From page 86... ...
Figure 3-1 bitmapped, uneditable about a dominance of subsurface flow to river channels for much of the global runoff that reaches channels. Erosion processes and rates also must have changed, altering landscape evolution.
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From page 87... ...
These interactions are less obvious, but no less important, in more humid, nutrient-limited ecosystems, where the amount, timing, and routing of water supply can substantially affect nutrient availability and soil carbon storage. Across the full gradient of annual precipitation, the key link between the biosphere and the hydrosphere is soil moisture.
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From page 88... ...
Alternatively, plants themselves may create spatially variable conditions favorable to their survival by influencing soil characteristics that alter surface water infiltration, moisture retention, and erosion. Much remains to be discovered about controls on pattern and process.
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From page 89... ...
What are the effects of topography, geology, and land history on soil moisture patterns and dynamics? Field studies of rooting depth of vegetation, direct observation of water transport in roots, and results from climate modeling all point to the importance of deep water sources (several meters below the ground surface)
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From page 90... ...
Although soil maps can provide guidance to estimating soil properties, there are no comparable data to estimate the depth of weathered rock that may serve as a moisture reservoir. Recently the zone from the canopy top through the soil and down into the underlying weathered bedrock and groundwater has been referred to as the "Critical Zone" (Box 3-1)
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From page 91... ...
How can results of local mechanistic studies of soil and weathered bedrock be upscaled to watershed hydrologic and regional climate models? How have vegetation assemblages, landscapes, climate, and the hydrologic systems that drive them co-evolved?
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From page 92... ...
92 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 3-2 Digital image of the Gabilan mesa area, south of San Francisco, California, showing the strong aspect control on forest distribution. Image is derived from airborne laser swath mapping data collected by the National Center for Airborne Laser Mapping (NCALM)
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From page 93... ...
Ecosystems, hydrologic processes, and regional climate can co-evolve to create a self-sustaining system, but one that if disturbed may not recover. Perhaps the most important such system on Earth is the Amazon rainforest.
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From page 94... ...
. In 2009, an unprecedented peat fire in the Arctic released enough carbon to offset the annual carbon sequestration potential of the entire Arctic tundra biome (Figure 3-3C)
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From page 95... ...
Conversion of native vegetation to croplands typically increases evapotranspiration rates and enhances cooling. Biological soil crusts, composed of soil particles and various microorganisms including cyanobacteria, green algae, and bacteria, are estimated to be the dominant ground cover in some arid lands.
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From page 96... ...
3.3. Subsurface Ecosystems and Hydrologic Processes Subsurface ecosystems create and direct hydrologic pathways, release gases to the atmosphere, and control access to moisture and nutrients by aboveground ecosystems.
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From page 97... ...
Models that explicitly account for biotic activity will allow for better predictions of soil evolution and expansion of the understanding of related hydrologic and biologic interactions in the subsurface. How are subsurface biota controlled by and influencing hydrologic processes?
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From page 98... ...
, and these plant community changes can in turn influence soil biota." SOURCE: Reprinted, with permission, from Wardle et al.
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From page 99... ...
Hydrologic processes connect relatively deep groundwater, as well as the rhizosphere, to the atmosphere. Soil developed on sediment deposited by rivers (most obviously on Earth's the great lowland floodplains)
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From page 100... ...
Subsurface ecosystems and the interdependence between vegetation and belowground biota are a frontier area of research in which water abundance, seasonality, and spatial distribution play a first-order role. Should models directed at the coupled evolution of vegetation assemblages, topography, and local climate account for the equally evolving soil biota and their influence on hydrologic and biogeochemical properties?
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From page 101... ...
A central challenge for the hydrologic and ecological community is to find common ground in understanding the complex ways that flow regimes impact critical geomorphic and ecological processes and in turn the maintenance and dispersal of organisms in aquatic ecosystems. What are the critical components of river hydrologic regimes that dictate composition and dynamics of aquatic ecosystems?
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From page 102... ...
. In addition to its role in shaping a river system's physical habitat, the flow regime is itself an important determinant of the distribution, abundance, and life history traits of river and floodplain (riverine)
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From page 103... ...
and climate change. As flow regimes change, the winning evolutionary strategies of the past may prove less competitive so that native species may be lost and new species may become dominant or invade aquatic ecosystems from which they were previously excluded.
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From page 104... ...
to document, explain, and predict changes that inevitably will come. In part because of recent major reports, for example, the Arctic Climate Impact Assessment (ACIA, 2005)
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How will physical and biological responses to desertification in turn alter local and regional climate? What controls the low flow extent of stream networks?
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From page 106... ...
In order to understand and predict ecological responses to altered flow regimes it is critical to be able to both measure and model low and zero flow hydrology. Low flow hydrology has received relatively little attention from researchers (Smakhtin, 2001)
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From page 107... ...
What will be the effect of predicted climate change on the low flow stream network? How will aquatic and riparian ecosystems respond to low flow channel network contraction and to reduced perennial flow?
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From page 108... ...
. At one point, Lake Lanier, Atlanta's main water supply, contained only 90 days of water supply.
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From page 109... ...
WATER AND LIFE 109 FIGURE 3-6 "CUMMING, Georgia -- With water supplies rapidly shrinking during a drought of historic proportions, Governor Sonny Perdue declared a state of emergency Saturday for the northern third of the state of Georgia and asked President [George W.] Bush to declare it a major disaster area.
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From page 110... ...
What will make wetlands restoration work? As an environment that transitions between land and water, wetlands are the ultimate theater for the interaction of biota and hydrologic processes.
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From page 111... ...
How can we manage the hydrologic regimes of natural, restored, and constructed wetlands to most effectively retain and remove contaminants or to protect species dependent upon wetland ecosystems for all or part of their lives? How do vegetation development and subtle flow dynamics lead to distinct self-organized patterns of vegetation assemblages?
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From page 112... ...
Engineered avulsions, in which river flood flow is directed into coastal basins, had been proposed, but doubts were raised about sufficient sediment supply, the high rates of subsidence, and the anticipated effects of climate change induced sea-level rise. Figure 3-8 shows an example, however, where numerical model ing performed as part of the delta dynamics integrated research project of the National Center for Earth Surface Dynamics demonstrates the possibility of land building.
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From page 113... ...
of wetlands is typically not included in projects, so the opportunity to learn and make adaptive management decisions is limited. Redesign may be the more common option for large wetlands.
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From page 114... ...
Wetlands produce distinct landforms, vegetation patterns, and ecological interactions that invite model development -- and much work has been done to explore essential controls on wetlands attributes. Nonetheless, wetlands, especially large lowland systems, remain challenging to study because of their size, access difficulties, and the dispersed nature of ecologic and hydrologic processes (Harvey et al., 2009)
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From page 115... ...
. River restoration projects typically attempt to reestablish the water and habitat quality of degraded streams using one of two overarching approaches, either focusing on reestablishing hydrographs and hydrologic connectivity (hydrologic restoration)
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From page 116... ...
The large number of river restoration projects offers many opportunities for testing basic hydrological, biogeochemical, and ecological understanding. If river sediments and their contaminants are responsible for downstream ecosystem decline, can restoration measures increase upstream retention?
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From page 117... ...
Collaborative field studies are essential, because many questions of process remain and can only be understood at the scale and richness of dynamics found in the natural world. Ecological processes introduce different time scales than do hydrologic processes.
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From page 118... ...
The magnitude and timing of water delivery shape biological systems but the quality of water is essential to all organisms. The next chapter argues that the ability to have clean water for ecosystems and humans requires understanding, predicting, and managing water quality.
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From page 119... ...
2009. Is it feasible to build new land in the Mississippi River Delta?
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From page 120... ...
2001. Low flow hydrology: A review.
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From page 121... ...
2005. Wetland resources: Status, trends, ecosystem services, and restorability.
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