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3 Water and Life Water and life are inseparable and interacting. Ecosystems depend on and drive hydrologic processes, giving rise to distinct patterns of biotic com- munities, and a climate system strongly coupled through vegetation to moisture in the ground. INTRODUCTION The evolution of life on Earth likely began with the formation of liquid water and has been shaped by the flow of water ever since. Water is essential for all living organisms, and, on land, the magnitude of water supply and the timing of water delivery structures biological systems at all spatial and temporal scales. Over geologic time scales, hydrologic change has been a major force of natural selection. Across modern Earth, annual precipitation and temperature explain much of the variation in the stature and composition of vegetation, and the pattern of hydrologic connections constrains the distribution of many organisms as they migrate to complete their life cycles. 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 sur- face, altering its albedo (reflection of solar energy), recycling its water, and mediating its gaseous and aqueous chemistry. Biotic communities directly alter landscape properties (such as topography, permeability, weathering geochemistry, and erodability), fundamentally changing soil formation, ero- sion processes, runoff paths, river morphology, and in turn the water and nutrient availability that sustains biotic communities. The currency of these interactions, which take place over molecular to global scales, is water. Recently ecologists, geomorphologists, biogeochemists, soil scientists, and hydrologic scientists have found that a common frontier of their fields lies at the nexus of life and water. New cross-disciplinary research is emerg- 83
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84 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES ing with a surge of literature already illuminating progress and ways for- ward and applying disciplinary names such as ecohydrology, ecological climatology, and hydroecology. One measure of this surge is the increased frequency of published articles using the terms ecohydrology and hydro- ecology, terms that weren't widely used prior to 1990. Today, a Web of Science search for these terms shows the growth of these interdisciplinary publications over the past 20 years, with the majority of that growth taking place from 2000 to the present. The breadth, strength, depth, and impor- tance of this topic--the co-action of life and water on Earth--ensures that the great opportunities for discovery will be pursued. RESEARCH OPPORTUNITIES This chapter discusses six research topics and associated exemplary questions. The topics are not meant to be exclusive but are among the most important in the subject area. The central theme is the idea of bidirection- ality (i.e., water affects life, which affects water). This interaction can be over a short time period of, e.g., a growing season, or over geologic time in which evolution occurs. Studies suggest that action at the finest scale, such as the controls on moisture availability to root hairs, can have consequences for the large scale, such as regional climate. Hence, local mechanistic un- derstanding is needed, and therefore the significant challenge of upscaling should be tackled. Interactions can lead to patterns, from repeating patches of vegetation to the self-organized development of ridge-and-valley topog- raphy. Such patterns invite theory, and these two patterns in particular have driven much research. The coupling, bidirectionality, and internal dynamics of these patterns can lead to a high sensitivity to change and to the potential for irreversible change (e.g., see D'Odorico and Porporato, 2006). 3.1. Deep Time Landscapes Landscapes, hydrologic processes, ecosystems, and climate have co-evolved throughout Earth's history and across all spatial scales. Early Earth history is nearly as mysterious as the geologic evolution of other planets. To inform understanding of early Earth, scientists have limited preserved bedrock, some chemical records, and scant fossil evidence. Nonetheless, early life clearly interacted with the physical system. The fossil record suggests that as the evolution of life exploded and proceeded, the fossil record points to evolutionary inventions that altered how the planet works. Much is yet to be learned.
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WATER AND LIFE 85 How have hydrologic processes affected the co-evolution of life and planet Earth? Although a finite set of hydrologic processes are involved in Earth's water cycle, the relative rates and importance of the processes have changed dramatically throughout Earth's history. These changes corresponded and contributed to major evolutionary steps in emergence of landscapes and their ecosystems and the climate system (including the oceans). Three ex- amples illustrate this point. Early Earth is a great puzzle. The early Sun was 70 percent as bright as today (Kasting, 2010), and consequently the Earth should have been a frozen sphere for nearly one-half of its history. Instead geologic evidence makes it clear that oceans were present very early, and it is possible that Earth was even warmer than present. But then, approximately coincident with the rise of atmospheric oxygen from photosynthesis, the first known global glaciation occurred approximately 2.4 billion years ago. Since then Earth has periodically experienced "ice house" conditions of large-scale glaciations. Numerous hypotheses explain the "faint young sun" puzzle, but three-dimensional climate simulations that explicitly account for hy- drologic processes of runoff, storage, and evaporation must be developed to fully explore them. Another puzzle for hydrologic science arises from the "snowball Earth" event about 650 million years ago, during which it has been argued that much of Earth's ocean and terrestrial surface was covered with ice. This event is of particular interest because its termination was marked by an explosion of multicellular life, known as the Cambrian explosion. Why did Earth's water freeze and then thaw? There was also a significant rise in oxy- gen after the snowball Earth period ended. Recently it has been proposed that this rise in oxygen resulted from accelerated erosion of high mountains that flushed nutrients to the sea, greatly increasing photosynthesis. These events highlight the need for further understanding of the relationships among climate, topography, hydrology, erosion, and ecosystems. Perhaps the most radical change in hydrologic processes after the emer- gence of continental landmasses was the evolution of land plants and the subsequent diversification of terrestrial life. Terrestrial organisms perma- nently 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). This acceleration of the hydrologic cycle profoundly affected climate processes. The spread of vegetation and fauna across landscapes led to more intense weathering and the development of deeper conductive soils, which must have systematically increased near-surface storage of water and brought
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86 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 3-1 The evolution of roots, vascular tissue, and stomata through the De- vonian Period permitted plants to gain access to water stored deeper in the soil and to transfer it into the atmosphere. This increase in transpiration generated increases in precipitation over continents, significantly influencing the water cycle and climate processes. SOURCE: Reprinted, with permission, from Berry et al. (2010). © 2010 R02116 by Elsevier. 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. All current landscapes and ecosystems emerged under this new, biologically mediated hydrologic sys- tem. More focused research is needed to understand how the hydrologic feedbacks between organisms and the physical environment shape the co- evolution of landscapes, and the extent to which vegetation controls hydrol- ogy at local and regional scales.
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WATER AND LIFE 87 3.2. The Hydrology of Terrestrial Ecosystems Hydrology plays a critical role in driving the environmental pat- terns that exist and evolve on Earth. Dominant vegetation type or biomes, standing biomass, and annual primary production (energy fixation from sunlight by plants) vary with mean annual precipitation. Within biomes, inter- and intra-annual varia- tion in the timing and amount of precipitation can exert strong control on primary productivity and vegetation structure. In systems where vegetation growth is strongly water limited, small changes in the magnitude and timing of precipitation can lead to dramatic changes in vegetation composition and productivity. 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. Soil moisture fuels bare-earth evaporation and plant transpiration. Yet, evapotranspiration and soil moisture dynamics are the two primary unknowns in water budgets of landscapes. Soil moisture dynamics drive soil respiration (the flux of carbon dioxide, CO2, from the soil to the atmosphere) and are a significant con- tributor to the global CO2 budget, but soil moisture has proven extremely difficult to predict. Climate models that find agreement in predicting future temperatures and rainfall may nonetheless generate widely different predic- tions of soil moisture. Reducing this critical uncertainty in understanding the mechanisms of feedbacks between vegetation and climate is central to the ability to effectively predict future climate and vegetation dynamics at local and global scales. Among the barriers to building meaningful predictions of soil mois- ture are the challenges to linking small-scale transport processes in plants and soils with local atmospheric processes. In a review article, Katul et al. (2007) suggest that two primary barriers limit the progress of soil-plant- atmosphere interactions mediated primarily by hydrologic fluxes. First, scientists have limited ability to describe water movement at the very smallest scales where fine plant roots interact with soil water, where small tubes (xylem) carry water in plants, and where water diffuses through plant tissue. Second, once scientists achieve appropriate microscopic descriptions, the appropriate methods to extrapolate them to larger spatial scales and longer time scales are lacking (what is known in the field as an "upscaling" problem). Understanding how water molecules move through soils and plant tissues and developing the scaling laws necessary for extrapolating this understanding to ecosystem scales is a challenge.
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88 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES Hydrologists have made significant progress in understanding how vegetation responds to and controls local and regional hydrology in arid and semi-arid climates, where water limitation is a major determinant of vegetation patterns. Here competition for limited water leads to spatially structured vegetation, which may simply trace the topographic controls on water paths. 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. Hydrologists, ecologists, and geomorphologists have found common cause in studying the water mediated interactions among climate, vegeta- tion, and landscapes within terrestrial ecosystems. These ecosystems include the entire food web of animal life they support, but the dependency of ani- mals on precipitation is often indirect and tied to precipitation variability and seasonality. In water-limited landscapes, correlations between ungulate populations (for example) and rainfall have been found, but even here density-dependent interactions, such as competition for food, may obscure simple climate dependencies (Owen-Smith, 2006). Understanding how the loss of native species or the addition of invasive species to ecosystems may alter vegetation and climate is equally important as understanding how changing climates may affect terrestrial organisms. How do soil and rock moisture vary across landscapes and in turn drive biotic, geochemical, erosional, and climatic processes? Water in unsaturated soils, typically referred to as soil moisture, is returned to the atmosphere through bare soil evaporation and plant tran- spiration. This evapotranspiration is approximately 57 percent of the total land precipitation (Van der Ent et al., 2010), and it uses up about 50 per- cent of the total solar energy absorbed by the land surface (e.g., Seneviratne et al., 2010). Soil moisture influences climate as a source of moisture, and in various ways it influences latent and sensible heat fluxes. Soil moisture influences water potential gradients (and thus infiltration rates and unsatu- rated subsurface flow rates), thereby influencing runoff paths and the re- sulting erosion during storms. Geochemical reactions are partially paced by water content and associated microbial activity. Soil moisture controls and is regulated by vegetation; hence, understanding moisture dynamics is cen- tral to related studies. Soil respiration varies with seasonal moisture in the soil. Despite the central role that soil moisture plays in Earth surface pro- cesses and ecosystems, the spatial and temporal dynamics of soil moisture are poorly documented, and theory predictions have had limited success. Hydrologists have played a leading role in mapping soil moisture and developing theory about its distribution across landscapes. Remote sens-
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WATER AND LIFE 89 ing technology for mapping moisture dynamics, while providing valuable observations, only penetrates a few centimeters into the ground. Therefore, field observations and theory remain essential to obtaining estimates of soil moisture patterns and dynamics. Ground-based technology for mapping soil moisture to significant depths is advancing and will be important in cre- ating field data sets to test remote sensing and model-based predictions of soil moisture dynamics. For example, naturally produced neutrons and their thermalization are utilized in the Cosmic Soil Moisture Observing System1 (COSMOS) to provide larger footprint measures of soil moisture (Zreda et al., 2008). Distributed temperature sensing (DTS), in which optical fibers are "planted" across agricultural fields at varying depths, is now provid- ing soil moisture estimates using either the natural diurnal heat flux within the shallow soils, or by actively heating the cable over its entire length to form an enormous heat dissipation sensor. New approaches for assessing both the spatial and depth distribution of soil moisture are beginning to fill the gap between point sensors and remote sensing, yet this major data gap remains. Climate models are beginning to reach a resolution where the topographic effects on moisture redistribution can be treated. Large differences remain in how such models treat the water holding capacity of soils and how soil moisture regulates evapotranspiration. 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 im- portance of deep water sources (several meters below the ground surface). Sufficiently deep roots can lift deeper water to near surface soils, increas- ing moisture availability to shallow roots. In some places, soils are thick and can provide this deep water, but in others, especially places underlain by bedrock, the moisture available to plants may reside in the underlying fractured rock. In seasonally dry, hilly landscapes with thin soils, vegetation may be sustained by moisture extracted from weathered bedrock beneath the soil. The importance of this so-called "rock moisture," and the ground- water that lies beneath it, in providing water to vegetation is relatively unexplored. To improve hydrologic, climate, geochemical, and ecological models, field observations and theory are needed to explain and predict the spatial variation in the thickness and properties of the soil mantle and the underly- ing conductivity of weathered rock across landscapes. Climate models rely on compilations of soil thickness and texture properties extracted from soil surveys, but as the finer-scale topography of hills and valleys enter into climate models, soil data spatially mapped onto this topography will 1 See http://cosmos.hwr.arizona.edu/.
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90 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES be needed. Very little is known quantitatively about this mostly invisible mantle, especially with regard to the spatial variation in depth of weather- ing of bedrock across landscapes. 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). Six Critical Zone Observatory field projects are currently BOX 3-1 Critical Zone Observatories In 2001 the National Research Council report Basic Research Opportunities in Earth Science (NRC, 2001) proposed the term Critical Zone to describe "the heterogeneous, near-surface environment in which complex interactions involv- ing rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources." The report recommended "integrative studies" of the Critical Zone as one of six "important problem areas spanning a wide range of future activity in Earth science." This challenge was embraced by the research community, and by 2007 the National Science Founda- tion (NSF) funded three Critical Zone Observatories (CZOs) for an initial 5-year investigation and subsequently added three more in 2009. This effort has inspired a corresponding program in Europe referred to as SoilTrEC (Soil Transformation in European Catchments)a and rapidly expanding international collaborations across the globe. The CZOs are envisioned as field environmental laboratories to explore the chemical, physical, and biological processes that shape Earth. The three main goals are (1) to develop a unifying theoretical framework of Critical Zone evolution that integrates new understanding of coupled hydrological, geochemi- cal, geomorphological, and biological processes; (2) to develop coupled systems models to predict how the Critical Zone is driven by anthropogenic effects, climate, and tectonics; and (3) to develop an integrated data-measurement framework to document processes and test hypotheses.b The six established CZOs are located in (1) the Southern Sierra (California), (2) the Jemez River and SantaCatalina Mountains (New Mexico and Arizona), (3) Boulder Creek (Colorado), (4) the Susquehanna Shale Hills (Pennsylvania), (5) the Christina River basin (Delaware and Pennsylvania), and (6) the Luquillo Mountains (Puerto Rico). Each of these CZOs is managed by different multidisci- plinary teams, and the mix of sites offers opportunities to explore different aspects of the Critical Zone. These CZOs form a national network and meet, coordinate, and collaborate as a shared program. a See http://www.soiltrec.eu/. b See http://criticalzone.org/.
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WATER AND LIFE 91 funded by the National Science Foundation (NSF) across the United States, wherein hydrologic processes and development of the soil and weathered bedrock zone are studied intensively in conjunction with biogeochemical and geomorphic processes. 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? We live on a patterned Earth. Across the planet, vegetation is banded into distinct bioclimatic zones of differing dominant vegetation. Within a zone, topography and geology can drive moisture, slope stability, and soil and mineralogical differences that structures vegetation assemblages. Sys- tematic differences emerge with respect to orientation of hills (i.e., aspect), with, for example, more forested, moisture-demanding vegetation facing north in northern latitudes (Figure 3-2). The visible co-organization of vegetation and topography, in which topographically structured moisture availability drives dominant vegetation assemblages (most prominent in water-limited environments), has attracted many researchers. These pat- terns are being examined at least three ways: (1) how vegetation patterns are driven by water stress, (2) how vegetation patterns may be used to docu- ment water availability and transpiration, and (3) how vegetation patterns may, in turn, affect hydrologic and erosional processes, thereby altering soil and topographic evolution. Highly structured vegetation patterns also occur that do not correspond to strong topographic and soil control but, instead, are argued to be emer- gent features that arise from competing effects of facilitative and competi- tive processes within the vegetation community. In some cases remarkable vegetation patterns of repeating bands, spots, and mosaics develop. In water-limited environments where overland flow occurs, vegetation cluster- ing can have such facilitative effects as inducing water infiltration, trapping nutrients, providing shade, and protecting against herbivory. A considerable body of theory has been advanced to predict these self-organized emer- gent vegetation patterns. The challenge is to provide definitive tests of the theory. The vegetation patterns themselves--without extensive field work to confirm driving mechanisms--may reveal very little about their origin and therefore provide an insufficient test of theories. A frontier area of research is to expand these inquiries into humid regions where spatially structured water availability is less apparent, but vegetation patterns still form (Rodriguez-Iturbe et al., 2007). Fire pattern and history may strongly dictate vegetation patterns in both arid and humid landscapes, either emphasizing or obscuring water availability differences
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92 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 3-2 Digital image of the Gabilan mesa area, south of San Francisco, Cali- fornia, showing the strong aspect control on forest distribution. Image is derived from airborne laser swath mapping data collected by the National Center for Air- borne Laser Mapping (NCALM) and then colorized and shaded to reveal patterns of vegetation and topography. The distance between each valley is approximately R02116 160 m. SOURCE: Reprinted, with permission, from Ionut Iordache, NCALM, and Whipple (2009). © 2009 by Nature Figure 3-2 Publishing Group. bitmapped, uneditable
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WATER AND LIFE 93 and other disturbances (e.g., extreme storms, grazing, insect outbreaks, and invading species) can take significant roles. The inclusion of multispecies, food-web, and disturbance-driven processes in coupled models of climate, hydrologic, and vegetation pattern development is an area of expanding and exciting research. What are the hydroecological interactions that rein- force the spatial patterns across diverse landscapes and varying hydrocli- mate regimes? All landscapes and their ecosystems have experienced climate change. Some systems may currently be legacies of a previous climate state under which they became established and now exist with limited resilience to further change. 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. A significant fraction of all terrestrial evaporation (and transpiration) is returned as precipitation over land (e.g., Van der Ent et al., 2010). Some argue that wholesale cutting of the forest or the effects of global warming could disrupt the self-sustaining hydrologic cycle of the Amazon, leading to widespread soil drying and a shift from mesic (having a moderate supply of water) forests to drier grasslands. Scientists need to understand how modern hydrosphere-biosphere feedbacks, like those in the Amazon, have evolved to maintain current vegetation patterns in order to anticipate future states. How will vegetation communities and their regional climate co-evolve with climate variability and change? Landscapes evolve as channels erode down, steepening adjacent hill- slopes, which, through this connection, may eventually develop a form that erodes at a rate similar to that of the channel. The competition of advective processes driven by runoff (which tend to predominate in channels) and diffusive processes (which tend to predominate on hillslopes) can lead to a regular ridge-and-valley topography with distinct wavelengths (Figure 3-2). Nearly all landscapes evolve under a biota mantle, yet explicit account- ing for the effects of vegetation (or the assemblages of biota in the soil) in geomorphic models is just beginning. Do topography, vegetation (and their animal ecosystems), and the hydrologic processes that connect them co-organize over geomorphic time scales? How can scientists predict abrupt change in terrestrial ecosystems? Of great concern is the possibility that future state changes may be irre- versible, and that the approach to state changes will be nonlinear or abrupt and thus very difficult to predict. The tight feedbacks between vegetation and climate set the stage for the potential for rapid transitions in vegeta- tion dynamics. As global and regional climate changes, vegetation will both respond to and affect the climatic regime. Global climate models predict
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112 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES BOX 3-3 Mississippi Delta Restoration The Mississippi River Delta has lost about one-third of its original wetland since European settlement of North America. This wetland provides many ecosys- tem services, most notably freshwater habitat and storm surge protection. At the current rate of land loss and shoreline migration, it is estimated that New Orleans will be exposed to the open sea by 2090 (Fischetti, 2001). The reduction of sediment supply to the delta due to dam construction in the Mississippi watershed has contributed to this land loss. But presently the main cause of land loss is the confinement of the river to levees, which have created an efficient "pipeline" of sediment (and nutrients) directly out to the Gulf of Mexico. The levees prevent the river spilling into adjacent coastal basins, where sediment deposition would provide the mineral base for wetlands vegetation growth and thus sustain the delta level. Without this resupply the coastal wetlands drown because of natural and anthropogenic (due to hydrocarbon extraction) subsid- ence and sea-level rise. The final result is open water with no remaining wetlands ecosystem services. 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. This model suggests that effective land building can be done without threatening navigation or demanding a large fraction of the Mississippi flood flow. Basic research is needed to guide such a major restoration project. Recently NSF funded a "Delta Dynamics Collaboratory" that will support an intensive field observatory (the Wax Lake Delta, a recent, actively growing delta about 100 km west of the main Mississippi delta) and a modeling activities center associ- ated with the NSF supported Community Surface Dynamics Modeling System (CSDMS) at the University of Colorado. The interaction of sediment supply and wetland ecosystems dynamics will be a primary focus. hydrologic change (climate induced or managed) prevent or exacerbate the spread of invasive species in wetlands? How will the critically important role of wetlands in global carbon cycling change as a result of climate change or direct hydrologic alteration? Although many restoration efforts are under way, there is considerable controversy about their effectiveness. It is argued that constructed wetlands are not a substitute for naturally formed ones, because they fail to perform
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WATER AND LIFE 113 FIGURE 3-8 View of the delta of the lower Mississippi River below New Orleans, showing predictions of the new land (delta surface) that could be built over 100 years starting from 2010. Two diversions are considered: Barataria Bay and Breton Sound. The calculation is based on a "base case" scenario: a subsidence rate of 5 mm per year and sea-level rise rate of 2 mm per year. The inset shows results for a "best case," subsidence rate of 1 mm per year and sea level rise rate of zero mm per year, and a "worst case," with corresponding values of 10 and 4 mm per year. For the sake of clarity, land losses in the part of the deltaic wetlands not subject to diversion are not estimated or shown. SOURCE: Reprinted, with permission, from Kim et al. (2009). © 2009 by the American Geophysical Union. the same hydrological, ecological, and biogeochemical functions. Monitor- ing the effectiveness of restoration (and redesign) 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 op- tion for large wetlands. In this case, the original wetlands system cannot be reconstituted because of permanent changes in land use and hydrologic routing (e.g., through dams, diversions, and drains). California's San Fran-
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114 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES cisco Bay Delta Estuary, the Florida Everglades, and the lower Mississippi River wetland systems are prime examples of systems with strong con- straints, multiple goals, and a complex community of users. Although exceptionally to produce challenging, coupled models of hydrological, eco- logical, geochemical, and geomorphological processes are needed to enable communities to make reasonable comparisons about the costs and benefits of alternative management decisions. Wetlands research presents a great opportunity to explore and discover the subtle linkages among water level (flow, duration, and frequency), vegetation establishment and growth, soil chemical evolution, aquatic ver- tebrate and invertebrate dynamics, and morphologic evolution of subdued and emergence landforms. 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 at- tributes. Nonetheless, wetlands, especially large lowland systems, remain challenging to study because of their size, access difficulties, and the dis- persed nature of ecologic and hydrologic processes (Harvey et al., 2009). Remote sensing data (including current and future satellites for mapping water level and storage) and high-resolution topographic data (of ground and water surfaces), coupled with process oriented field studies, can reveal underlying mechanisms and processes. What are the key controls on large wetland system function and services? What are the minimum features re- quired to build mechanistic models linking ecosystem, biogeochemical, and hydrologic processes that will increase the efficacy of wetlands restoration? Can wetlands be constructed, restored, or redesigned to provide resilience to the consequences of global change? What will make river restoration work? In the United States and throughout the world, restoration of rivers and streams is an increasingly common approach to managing freshwaters. This trend reflects a growing awareness of river degradation and societal desires for waterways that provide beneficial human uses while sustaining biodi- versity and ecosystem goods and services. Rivers drain landscapes and thus their form and dynamics are linked to the cumulative land use activities across their watersheds. Land use alters the water runoff rate and stream temperature, sediment supply (size and amount), and water chemistry that a river receives and passes downstream. Rivers are straightened, dredged, leveed (preventing access to adjacent floodplains), confined in hardened banks, stripped of in-channel woody debris and riparian vegetation, di- verted, and covered. The combined effects of altered flow, sediment, and nutrients regimes together with altered physical states have led to significant degradation of river biodiversity and to significant increases in the delivery
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WATER AND LIFE 115 of pollutants to estuarine and off shore environments. Non-point-source contamination, channel degradation, and aquatic species loss or decline are among the most common motivations for undertaking stream restora- tion (Bernhardt et al., 2007). 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) or focusing on restor- ing habitat form (hydromorphological restoration). Hydrologic restoration includes "reestablishing part of the historic flow regime, removing levees to recover floodplain functionality, scheduling wa- ter releases from reservoirs to restore native vegetation and riparian func- tions, and in some cases even removing flow blockages or reconnecting river reaches that have been fragmented" (Bernhardt and Palmer, 2011). Dam removal, in particular, has become a widespread practice, often resulting from relicensing assessments. A central goal of hydrologic restoration is to reestablish natural processes such that the river system will create flow and morphologic dynamics critical to maintaining ecosystems. Hydromorpho- logical restoration places greater emphasis on increasing channel stability and in-stream habitat by altering channel form and structure along a river reach in order to restore biodiversity and ecological function. Commonly this approach employs introduction of in-channel structures (using boulders or large woody debris). Whole reaches of channels may be redesigned to a fixed channel pattern to meet some desired form and assumed function. But restoration projects have also been designed with the goal of chan- nels recovering their morphologic dynamics, such as shifting laterally and thereby creating increased habitat complexity. Reports monitoring the ef- fectiveness (pre- and post-restoration quantitative sampling) of restoration outcomes are rare (Bernhardt et al., 2005), but some hydrologic restoration efforts have been success stories (e.g., Hall et al., 2010). In contrast, there is limited evidence of demonstrable ecological improvements resulting from hydromorphological restoration projects (Bernhardt and Palmer, 2011), de- spite the fact that these types of projects are extremely common worldwide. Restoration projects are mostly implemented without detailed under- standing of the watershed context and potential channel morphodynamics. Typically, limited, if any, ecological field studies precede project imple- mentation. This is especially true in hydromorphological studies, which commonly rely on the assumption that adding structures to channels or redesigning a channel to a particular form will create an ecological benefit. Restoration projects commonly lack two critical components: (1) an eco- logical assessment to determine limiting factors to species success in order to define what restoration would be most effective and (2) analysis of sedi- ment supply and consequences for restoration strategy (e.g., Rosenfeld et al., 2010).
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116 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES Forty-five percent of the nation's assessed rivers are classified as en- dangered or impaired (EPA, 2000); thus, considerable work is still ahead. The restoration measures taken will vary greatly and depend on the eco- system, the river and its watershed, and practical constraints. There is a need for development of quantitative models that explicitly link hydrologic, geomorphic, geochemical, and ecologic processes and that enable users to ask "what if" questions regarding restoration measures in a changing environment. 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? In a watershed where a large portion of its network of rivers is degraded and water quality is poor, what are the most effective proce- dures to restore habitats and aquatic biodiversity? How can key barriers (physical, chemical, or temperature) to stream biota migration be efficiently identified and eliminated? What is necessary to restore or redesign a chan- nel such that it becomes a self-maintaining, morphologically dynamic river that passes water and sediment and supports a diverse ecosystem? Can scientists predict fish population dynamics for various restoration measures and anticipated effects of changing climate and land use? Such questions will naturally be best answered by interdisciplinary teams of scientists and engineers; and high-quality hydrologic research should be at the center of these inquiries. CONCLUDING REMARKS In the past 20 years a major shift has occurred in the hydrologic com- munity to embrace and explore all the ways water and life interact on Earth. Hydrologists are investigating how interactions that link subsurface, terrestrial, and freshwater environments are controlled by and contribute to ecosystem dynamics and the planet's evolving climate. Theory is rapidly developing, with mathematical models and process-rich numerical schemes being proposed. Opportunities for basic discovery occur at all scales, from the micrometer-scale processes surrounding root hairs to the regional and global scale of water exchange with the atmosphere. These opportunities require deep understanding of not only physics, chemistry, and biology of processes, but also natural history and modeling. If there is one common goal, then it is to gain enough understanding of these interactions, so that models, whether conceptual, analytical, or numerical, can provide reliable predictions of the future states of the Earth system. The more scientists are able to anticipate future states, the better society will be able to prepare for them or alter what might otherwise occur. Models are needed to predict consequences of local changes (e.g.,
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WATER AND LIFE 117 vegetation conversion for energy crops) as well as global changes (e.g., increasing CO2 in the atmosphere). It is vital that biotic processes be made explicit in such models. This advance, however, will reveal the uncertain future. Ecosystems are dynamic, time dependent, nonlinear, and complex. Biotic interactions, disease, behavior, and other attributes far different from purely physical or chemical processes challenge the goal of predictability. The pursuit of answers to the kinds of questions raised in this report, which explicitly seek to include life-water interactions, should contribute toward reaching this goal. 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 dif- ferent time scales than do hydrologic processes. Successive rainstorms can provide repeated data sets about how a particular landscape delivers water as runoff, for example. But that same landscape may undergo progres- sive changes in vegetation composition, when, for example exotic species invade, recovery occurs after a fire, or subtle climate shifts change the competitive edge from some species to others. Such shifts may take years to decades or longer to occur and feed back on the hydrologic and climate system. These slow dynamics call for commitments to long term collabora- tive studies. The development of inexpensive, easily deployed monitoring devices for ecosystem and hydrologic studies holds the promise of revealing processes over space and time. One of the greatest challenges faced by Earth scientists is the poor knowledge and inaccessibility of the hydrologically active ground beneath society's feet. In many environments most runoff occurs as subsurface flow. All moisture that gets pulled back to the atmosphere as transpiration as it sustains life is extracted from this near-surface region. Documenting sub- surface processes involves the technical challenges of finding ways of "see- ing" into the ground and of quantifying the properties of the subsurface, explored further in the following chapter. This documentation should also include analysis of subsurface ecologic processes. Observational tools and models are needed. Restoration, conservation, and redesign projects to create or regain desired ecosystem services are occurring across a wide range of scale from small seasonal ponds to the entire Mississippi River Delta. Done for practi- cal reasons, these projects are nonetheless invaluable experiments that test the ability to make predictions. Post project monitoring is a critical tool to for learning from these projects and making progressive improvements (NRC, 2006, 2011). As scientists look ahead, interdisciplinary approaches and perspectives will be needed to gain enough understanding of the interactions between water and life to predict the future states of the Earth system. Many of the
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