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2 The Water Cycle: An Agent of Change Water has helped shape our planet to produce the world in which we now live. Knowing how water has acted throughout Earth's history and how water cycles function on other planets will broaden our understanding of how Earth's water cycle functions. This knowledge will allow us to better predict how human and natural factors will combine to produce the world we leave to our children and our children's children. INTRODUCTION Water plays multiple roles in the evolving physical system of the planet. It shapes the landscape as rivers and glaciers flow over the surface, waves break on shorelines, and freeze-thaw cycles crumble rocks. Water influences the movement of energy through the climate system as a greenhouse gas absorbing radiation and reemitting it to the surface; a reflector of sunlight when condensed into clouds, snow, and ice; and a transporter of heat when evaporating, circulating, and condensing; it influences the distribution of Earth's gravity field. The distribution of water affects the location and char- acter of life, the movement of Earth's crust, and even the rotation rate of the planet. The flow, phase changes, accumulation, and dispersal of water around the world--the water cycle--vary substantially with time and loca- tion. Thus, water does not respond passively to physical processes govern- ing Earth: it is a dynamic agent whose influence is central to processes that produced today's world and that will affect its evolution into the future. Human intervention in the water cycle alters water's dynamic role on the planet. 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. Hydrologic and related sciences require credible accounting of water to assess how the water cycle acts and will change. Such accounting is needed for timely and accurate prediction of natural hazards, including 45
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46 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES "abrupt" or irreversible changes, and relies on basic understanding of in- teracting processes, that is, understanding how the world works. Scientific and technological advances now offer exceptional opportu- nity to develop a comprehensive understanding of water's pervasive activity throughout the Earth system over periods ranging from early epochs to the present and future. Although many disciplines can contribute to this opportunity, hydrologic science plays a central role, as highlighted in this report. Over the past several decades, the knowledge of Earth as a system has grown considerably through new observational techniques, analysis methods, and computing tools, all of which have helped hydrology mature as a discipline (see Chapter 1). This maturation has given hydrologic sci- ence a leading role in advancing understanding of the water cycle and the processes that affect and are affected by it over a range of scales and envi- ronments. As part of this leadership, hydrologists and engineers have forged links with closely related disciplines, especially the atmospheric, soil, plant, and cryospheric sciences (which deal with snow, ice, and frozen ground) to develop a more comprehensive and coherent view of water as a central component in Earth's climate system. Admittedly, how water acts varies significantly across time scales rang- ing from seconds to decades and longer, and spatial scales from millimeters to planetary, thus presenting a very complex dynamic picture and a monu- mental task in monitoring all its storage and transport aspects. However, advances in observing systems and computing allow use of computational techniques such as data assimilation that could support development of this comprehensive view by merging observation sources into a unified, global portrayal of water with unprecedented temporal and spatial detail. New observing systems such as space-based platforms coupled with global networks of existing observing tools could produce global, real-time views of where water is and where it is going, in all its phases (Gao et al., 2010; Wong et al., 2011). The sensor revolution is in its nascent stages, but for the first time the promise of closing the global and regional water budgets with direct measurement of flux and storage components may be just within reach. Opportunities also exist to extend this portrayal of water into the future. Scientific advances have contributed to progress in understand- ing the interactions between water and other Earth system components, leading to modeling of the water cycle as part of a comprehensive Earth system simulation system. 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, exami- nation of water flow and storage during periods ranging over the past de- cades, centuries, millennia, and into deep geologic time offers opportunity to understand how Earth's water cycle evolved to its present state. Equally
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THE WATER CYCLE: AN AGENT OF CHANGE 47 important, understanding the range, frequency, and rate of change of past behavior provides a baseline of natural variability as well as a means to gauge the impact of humans on the water cycle, which can be helpful now and in forecasting the future. New methods of data acquisition and refine- ment of existing techniques yield an expanding set of paleoclimate data that shed new light on past hydrology. Further advances could provide longer views with yet finer temporal and spatial detail. Just as modeling has shed new light on contemporary and potential future water cycle behavior, modeling past climates guided by advances in paleoclimate reconstruction can further test the limits of knowledge, as expressed by models, while also revealing physical insight that complements proxy records. Stepping away from considering the Earth alone, the understanding of Earth's water history gains from comparison with alternative planetary evolution pathways. Advances in planetary science have broadened un- derstanding of where and how planets and moons form, both in the solar system and beyond. Awareness of the emergence and evolution within the solar system of "water cycles" based on water or other condensing constituents (e.g., methane) provides unparalleled opportunity to reveal cosmological principles that guided the formation of Earth and its water cycle. 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. The research opportunities are ordered as in the Introduction above, fo- cusing on challenges involving human influences and on contemporary and near-future hydrology (i.e., water-process measurements and model- ing), then considering challenges and opportunities involving hydroclimatic variability, from abrupt changes to long-term variability, including the paleoclimate perspective, and finally considering the opportunities posed by exohydrology. 2.1. Human Influences on Water Availability and Distribution The hydrologic cycle is being perturbed and "replumbed" through human activities. The hydrologic cycle has been described and depicted in a variety of different ways, most frequently as a natural system, even though it has long been altered by human activity. Alterations of the hydrologic cycle for ag-
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48 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES riculture, transportation, and domestic and industrial needs have amplified dramatically over the past century, along with building of infrastructure in the form of dams and canals (Figure 2-1). 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. These practices have had major impacts on surface and ground- water supplies, which have in turn impacted the downstream systems reli- ant on these supplies. FIGURE 2-1 Alteration of the water cycle in the form of dams and canals is em- bedded deeply within the modern global water cycle. In the United States, the geo- graphical extent of dams and reservoirs has increased dramatically over the past 200 years (top). This trend extends to developed parts of the world, with river regulation expanding rapidly in the 20th century (middle). As a result, human engineering and water use distort hydrographs (bottom). The left-hand graph is a purposeful interbasin transfer for hydroelectric production, the middle is the Aswan High Dam impact, and the right-hand is flow depletion R02116 due to cotton production in the Aral Sea contributing basin. SOURCE: Reprinted, with permission, from Vörösmarty et al. (2004). © 2004 by the American Figure 2-1 Union. Geophysical bitmapped, uneditable
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THE WATER CYCLE: AN AGENT OF CHANGE 49 The hydrologic cycle is altered by not only direct physical alteration, but also anthropogenic climate change; the most obvious symptom is the global redistribution of precipitation and the resulting change in surface water flow (Figure 2-2). In turn, changing the terrestrial branch of the wa- ter cycle impacts climate by altering the surface energy balance, changing evapotranspiration and surface reflectance characteristics. 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.) are not independent and can yield compounding and cascading effects. For example, dam construction in arid regions impacts downstream hydrology and ecosystems, and additional stresses due to climate change challenge dam operations that strive to meet competing needs and further impact conditions downstream. In addition to scientific issues, sociopolitical issues often take center stage. As an example, picture a semi-arid area of urban growth with limited water supply. Historically, water in many of these regions has largely sup- ported agriculture, but in recent decades, the water needs of urban centers have become more dominant. When this shift in water demand involves transfer of water rights, tension between urban and rural areas can impact FIGURE 2-2 Average aggregate (based on seven upper Rio Grande basin tributar- ies) streamflow by month for six climate change scenarios. Three climate change models represent the range of possible climate outcomes for New Mexico (wet, dry, and middle) in 2030 and 2080. These projections illustrate that peak flow and total streamflow decline for all climate scenarios in this basin. In 2080, there is a pronounced shift in the peak runoff month, by about 30 days. SOURCE: Re- printed, with permission, from Hurd and Coonrod (2008). © 2008 New Mexico State University.
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50 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES the amount of water available for agricultural communities. Infrastructure put into place to supply water for growing urban centers can impact sur- rounding ecosystems, creating an additional layer of tension. Finally, the impact of climate change introduces additional stresses on the hydrologic resources of the area. In recognition of the human influence on the Earth system and in the context of Earth history, many scientists now call present times the "Anthropocene." The traditional concept of the hydrologic cycle should perhaps by revised to consider humans as a significant and recognizable component. Research to further understand the human component of the water cycle, i.e., human-induced replumbing from both climate change and land use change, is recommended. This understanding is critical to provid- ing and maintaining water supplies for humans and ecosystems. How will water distribution and availability change because of hydrologic replumbing? Water-related infrastructure has allowed many parts of the world to flourish, but at a cost to the natural environment and with growing and unintended impacts on society and ecosystems (Box 2-1). The relationship between human consumptive use and available water is not fully under- stood, yet this information is essential to understand how distribution and availability will change in the near and distant future. The hydrologic community can shed light on this relationship by probing how replumbing perturbs hydrologic fluxes. What are the impacts of groundwater over- draft on the surrounding hydrologic regime? Finding answers to questions such as this one will further the larger, societal goals of encouraging the best conservation behaviors and pursuing water-efficient products, both of which should be accomplished with the best possible scientific information to assist fair, legal, and scientifically sound decision making. Conservation measures have been increasingly applied as water avail- ability has become more limited, but even measures designed to conserve water can have downstream impacts. 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. Thus, actions that change the natural hydrologic system often have complex and interacting impacts and can create competition for highly limited available water. These include actions with direct impacts on the water cycle, such as changes in water use, as well as those with indirect impacts, such as forest clearing. What are the downstream consequences of replumbing in terms of the
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THE WATER CYCLE: AN AGENT OF CHANGE 51 amount and rate of flow and seasonality? What are the repercussions on both society and natural environments? One recent focus has been on what is commonly called the "water- energy nexus" in which the human need for water is linked to energy just as tightly is the human need for energy linked to water. The procurement and delivery of water often requires energy for pumping, transport, desalina- tion, and treatment. Likewise, almost all sources of energy, except perhaps wind, require water for some aspect of production (e.g., extraction, cool- ing, or conveyance), and many energy sources use considerable amounts of water. Of the freshwater used by the United States, 39 percent is used for electricity from fossil fuels and nuclear energy, and of that, as of 1995, 71 percent of that amount was used solely for the generation of fossil-fuel electricity (Solley et al., 1998). 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. The increased use in the United States of biofuels, often touted as "green" energy sources (Box 2-2), provides another example. These uses highlight how increasing demands for energy correspond to increasing demands for water. The age of "separate but equal" resource planning for water and energy resources has passed--water is withdrawn and consumed during the life cycle of almost every energy source. What are the impacts of energy- related replumbing on water distribution and availability? How will climate change influence the delivery of moisture (i.e., the severity, duration, and extent of droughts and floods)? Climate change is expected to impact key hydrologic fluxes, most nota- bly precipitation, which translates to impacts on the severity, duration, and extent of droughts and floods. A clear picture of the manifestation of cli- mate change in floods and droughts has yet to emerge. Of course, increased vulnerability to hydroclimate extremes may be exacerbated by social and political factors, and having better scientific information may be of only limited value. For example, encroachment of construction in floodplains is a primary cause of increased damages (e.g., Pinter, 2005). Research op- portunities exist, challenging the hydrologic community to provide better scientific information upon which social-political action will be required. Flooding in the United States is linked to diverse regional climatologies of heavy rainfall. Extratropical cyclones, "atmospheric rivers" (Leung and Qian, 2009), rain on snow events, and convective storms are some of the most important flood agents in the western United States. Tropical cyclones, warm-season thunderstorm systems, and cold-season extratropical cyclones play important roles in the flood hydrology of the eastern United States (Smith et al., 2011), with their relative importance strongly dependent on
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52 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES BOX 2-1 Is the North China Plain Running Out of Groundwater? Groundwater overexploitation or persistent aquifer storage depletion is a worldwide phenomenon (Konikow and Kendy, 2005). Restricting their analysis to subhumid to arid areas, Wada et al. (2010) estimated that the total global groundwater depletion more than doubled from 1960 to 2000. The confluence of a multitude of factors, including rapid economic development, high population density, and climate change, makes the North China Plain (NCP) a compelling case study of a groundwater resource in peril (Zheng et al., 2010). The NCP refers to a relatively flat, low-lying alluvial plain in eastern China with a total area of 140,000 km2. It is home to the capital city Beijing and several other large cities including Tianjin and Shijiazhuang. Approximately 130 million people now live within the administrative borders of the NCP. With a population density of about 900 people per km2, the NCP is among the most densely populated regions in the world. The NCP is also critically important to Chinese economy, contributing about 12 percent of China's gross domestic product and producing more than 10 percent of China's total grain output. The amount of exploitable water resource per capita in the NCP is in the "absolute water scarcity" category according to the "water stress index" (Falken- mark et al., 1989). Meanwhile, annual precipitation has steadily decreased by approximately 100 mm since the 1950s. 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. (2010), more than 70 percent of the NCP's total water supply comes from groundwater to support the region's agricultural ir- rigation, industrial expansion, and population growth. The question is, how much longer can this be sustained? The NCP sits on an expansive Quaternary aquifer system. The thickness of the NCP aquifer is tens of meters on the western piedmont areas but increases to hundreds of meters toward the eastern coastal areas. The NCP aquifer is com- monly divided into two hydraulically connected units, referred to as the "shallow" aquifer and "deep" aquifer. During the "predevelopment" period until the 1950s, the water table of the shallow aquifer was usually no more than 3 m below the land surface in most of the NCP. Since the 1960s to 1970s, however, ever-increasing groundwater pumping has caused massive and continuing depletion in the NCP aquifer. According to the latest data from the China Geological Survey, the maxi- mum depths to water in the shallow and deep aquifers exceeded 65 m and 110 m, respectively, in the shallow and deep parts of the NCP aquifer. Since the 1970s, groundwater levels in many parts of the NCP aquifer have declined at a rate of more than 1 m annually (Figure 2-3). More than mere depletion of an invaluable natural resource, the overexploita- tion of the groundwater resource in the NCP has other severe environmental and ecological consequences, including dried-up rivers, land subsidence, seawater intrusion, and water quality deterioration (Figure 2-4). For the NCP's main river, the Haihe River, the annual runoff to the Bohai Sea has decreased by threefold since the 1950s. Moreover, much of the surface water has disappeared. More than 4,000 km of various river channels in the NCP have dried up and the total size of wetlands has decreased to 20 percent of their level in the 1970s. Land subsidence
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THE WATER CYCLE: AN AGENT OF CHANGE 53 has also exerted a heavy toll on the region's economic development, especially near the major industrial and coastal city of Tianjin where the maximum cumula- tive amount of land subsidence exceeded 3 m. For the NCP as a whole, a total area of 60,000 km2 has experienced a cumulative subsidence of 0.2 m or more. 42 40 38 36 Average monthly ground-water elevation 34 (meters above mean sea level) 32 30 28 26 24 22 20 18 16 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 FIGURE 2-3 The long-term decrease in groundwater elevation at an observation well near Shijiazhuang City, Hebei Province, northern China. SOURCE: China Geological Survey (2009). R02116 Figure 2-3 vector, editable FIGURE 2-4 Eco-environmental consequences of groundwater depletion in China: a bridge over a dried-up river (left); former riverbed used for farming (right). SOURCE: Photos courtesy of Chunmiao Zheng, University of Alabama.
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54 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES BOX 2-2 Water and Energy: Biofuels in the United States Motivated by an increasing national interest in energy independence as well as concerns about the impact of greenhouse gas producing fuels, the United States has taken legislative steps to encourage the development of technolo- gies that reduce dependence on these types of fuels. These steps include a new bioenergy program, which set the goal of developing technologies to generate biofuels that are price competitive with gasoline or diesel fuels. 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. Much of the water needed to cultivate biofuels relies on irrigation that taps limited surface and groundwater supplies, notably the Ogallala aquifer, and that compete with water supplies already used to support food production (NRC, 2007). An increase in the use of fertilizers to support additional crop yields for biofuel results in nutrient and pesticide pollution with corresponding impacts on water quality, including hypoxia, that endanger aquatic ecosystems (NRC, 2007). The average water consumed in ethanol production, based on data from 19 states that produced ethanol in 2007, is 142 million liters of water for each million liters of ethanol, but this number is highly variable from region to region, ranging from 5 to 2,139 million liters (Chiu et al., 2009). The toll on regions where irriga- tion is necessary is evident. In 2003, a U.S. General Accountability Office survey indicated that many of the states that currently produce ethanol will experience water shortages over the next decade. Some of the states most likely to undergo shortages include those that consume the largest amount of water in the cultiva- tion of corn and processing of ethanol, i.e., Colorado, Kansas, Oklahoma, and Wyoming (Chiu et al., 2009). As of 2008, corn was used to produce more than 95 percent of the U.S. sup- ply of bioethanol (EPA, 2008). Corn genetics research, water-conserving irrigation practices, and water pricing could help alleviate water stress in the production of corn-based ethanol (Chiu et al., 2009; NRC, 2007). Also, alternative sources of biofuels, such as sugar cane, oil seeds, the nonstarch parts of the corn plant, grasses, trees, and municipal wastes, may be less water intensive than corn and are being explored to determine their potential in terms of energy conversion efficiency and water quality impacts (NRC, 2007). Regardless of the source, the likely expansion of future production of biofuels has the potential to increase the demand for water in many parts of the United States (NRC, 2007). drainage basin scale (Miller, 1990). Mesoscale convective systems have been the agents of extreme flooding in the central United States, notably during the Great Flood of 1993 and the Iowa flood of June 2008 (Cole- man and Budikova, 2010). The Iowa floods were made worse by a heavy snow year resulting in greater than normal antecedent soil moisture for that
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THE WATER CYCLE: AN AGENT OF CHANGE 55 time of year which prevented the mesoscale precipitation from infiltrating. How heavy rainfall translates into flood frequency and magnitude requires substantial hydrologic insight. However, all of these processes leading to flooding may change as climate change alters the water cycle, so insight built from years of experience in watersheds is undermined by climate change and associated changes in heavy rainfall. Furthermore, changes to the landscape by humans will add to the challenge. Research is needed to assess how changing rainfall patterns coupled with changing land use can affect floods and their impact in the future. Another consequence of climate change is a potential expansion and further drying of a semicontinuous band of aridity in subtropical latitudes. Temperatures in all regions are projected to increase, but an intensification of the equatorial to subtropical circulation, called the Hadley cell circula- tion, is expected to result in a poleward expansion of the global latitudinal bands of aridity (Held and Soden, 2006; Lu et al., 2007) and with further reductions in precipitation. The impacts of this increased aridity will be felt in regions such as the U.S. Southwest and the Mediterranean region of Europe, with related impacts on water supplies for human and natural systems. On a more regional scale, research suggests that the winter-spring storm track over North America may be retracting poleward earlier in the season, leading to reduced spring precipitation in the western United States, a shift consistent with climate change projections under warmer condi- tions. Also, the moisture variability and occurrence of drought in regional climates in many parts of the world are strongly influenced by El Nińo/ Southern Oscillation (ENSO), a coupled ocean-atmosphere pattern of cir- culation in the tropical Pacific Ocean associated with climate in many parts of the world. In fact, the relationship between precipitation and ENSO is chaotic in some regions. In the California Sierra Nevada, for example, El Nińo years are either wet or dry but generally not near the median. But research results do not agree on the impact of climate change on ENSO, and therefore this issue remains to be resolved. Planning for future water resources in these regions should evolve in the face of an anticipated reduc- tion in precipitation. Research is needed, for example, to determine optimal measures for water management as precipitation declines. Additional factors can amplify the impacts of climate change. For example, depending on location and climate regime, glacier meltwater pro- vides essential water resources throughout the year, and in some locations it acts as a supplementary water source in the summer. Glacier loss due to climate warming and other factors could impact water resources in these areas, especially semi-arid and arid regions. Although many glaciers have exhibited recession over recent decades, with the highest retreat exhibited in glaciers at lower elevations, the rates of retreat can vary widely as regional factors (precipitation, aerosol concentration, etc.) affect glaciers in addition
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72 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES FIGURE 2-10Reconstructions of snowpack in the northern and central Rocky Mountains from tree-ring data. The graphs of the April 1 snow water equivalent (SWE) reconstructions show multiple watershed reconstructions (gray lines) within each larger region, with the regional average (orange line), smoothed (dark blue line). For the Northern Rockies and Greater Yellowstone region, the reconstructions are most reliable after 1376 (dotted vertical line). The 20th-century records of ob- served April 1 SWE are also plotted for each large region (black lines) and smoothed R02116 SWE periods highlighted in the full (cyan line). Shaded intervals show decadal-scale Figure paper. 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 Colo- rado River basin over the past centuries. However, this natural variability at 20- to 50-year time scales has been more synchronous because of late-20th-century warm- ing, resulting in a decline in snowpack across all three major watersheds. SOURCE: Reprinted, with permission, from Pederson et al. (2011). ©2011 by the American Association for the Advancement of Science.
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THE WATER CYCLE: AN AGENT OF CHANGE 73 circulation use land-based proxies that imply moisture transport from remote locations and have some utility. What types of paleoclimate data can be better exploited to indicate behavior for regions, such as the oceans, that are rela- tively data scarce? Notable features of past climate, such as the droughts of the medieval period, contain information that may be useful for anticipating the future, but understanding the global climate context for these droughts would be even more valuable. Can multiproxy records (records from mul- tiple sources that document climate, such as tree rings, lake sediments, and ice cores) of past climate, along with climate modeling, be used to simulate global climatic variability and its drivers during anomalous periods, such as the medieval period? Much focus has been placed on the tropical Pacific Ocean variability, and with good reason, but is it possible to develop robust ensembles of paleoclimate reconstructions of modes of climatic variability in other parts of the world as well? 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. Low-frequency variation in hy- droclimate superimposed on trends in temperature may affect the impact of drought in such managed hydrologic systems. Understanding the low- frequency variability and its impacts is critical for managing water supplies under a variety of scenarios. Can modeling be used to produce ensemble projections of hydroclimatic variables for use in water resource decision making? Some use has been made of paleoclimate data in assessing long- term reservoir operations under a broader range of conditions than that provided by the gauge records (e.g., Lower Colorado region, Bureau of Reclamation). How can these applications be expanded to use the low- frequency information in paleoclimate records, along with projections for future climate change, to present a plausible range of conditions? What causes "tipping point" transitions of the climate and what are the hydrologic implications? A tipping point with global implications is a transition from what is now occurring to an entirely new climate state or a point where abrupt climate change occurs. Earth's climate system has shown some evidence of regime behavior, most notably the potential to exist in at least two differ- ent states even with the same solar forcing, such as states with or without a thermohaline circulation. A shift in the climate regime from one state to another would have huge implications for many aspects of the hydrologic
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74 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES cycle, including the behavior of the cryosphere, surpluses and deficits in surface water reservoirs, and the frequency of extreme hydrologic events such as floods and droughts. As climate models have become more complex, the spread of their pro- jections of future climate has tended to widen, suggesting that increasing the number of processes included in the models reduces the predictability of the climate system. This also suggests that positive feedback processes not yet studied or modeled may amplify some fluctuations in the climate system. If the amplifications are large enough, they could push the climate system into new modes of behavior or induce an abrupt change. Further- more, paleoclimate records have revealed past episodes of rapid change to new climate regimes. There is value in understanding the past frequency of such "abrupt" episodes and the processes that caused them, but this requires relatively long records. 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 fac- tors, may yield rapid climate change in the future. Paleoclimate analyses that document abrupt climate change under natural climate variability (Overpeck and Cole, 2006) coupled with improved hydroclimate modeling will provide insights into causes and consequences of climate transitions and their hydrologic implications. Because models likely do not contain the feedbacks that trigger tipping points that are documented as abrupt changes in the paleoclimate record, scientists still lack the information needed to understand and anticipate tipping point transitions. What modeling im- provements and paleoclimate data are needed to understand and project potentially catastrophic abrupt changes in a warming climate? 2.5.Exohydrology The presence of water in and on planets changes everything-- from deep interior dynamics to the surface evolution of land- scapes to the potential for life. The recent exploration of Mars has popularized the idea of "fol- lowing the water" to look for life on other planets. Life occurs nearly everywhere on Earth's surface and, surprisingly, microbial life may occur even deep in Earth's wet underlying bedrock, perhaps as much as 5 km into the igneous rock underlying the oceans. There may even be more biomass in this deep rock reservoir than in the overlying oceans and on the entire land surface. Of course Earth is a water planet, with 71 percent of its surface area in oceans. Earth is blanketed in a watery atmosphere and washed by rain-
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THE WATER CYCLE: AN AGENT OF CHANGE 75 storms and snowmelt. What is less appreciated is that the dynamics and composition of the entire solid planet are affected by the distribution and abundance of water. There is a deep water cycle. The motion of mobile plates of crust that collide, creating mountains, and separate, making ocean basins, depends on water in the underlying mantle (with some of that wa- ter coming from subduction of wet ocean slabs). Water deep in the planet affects its chemical evolution and its internal dynamics. The abundance of granite on Earth records the extensive mixing of water with basalt and other rocks. The nature of volcanic eruptions and the movement of faults are strongly influenced by water. Ultimately, water weathers bedrock, and erodes, transports, and deposits mass, some of which is subducted with the ocean floors and enters the mantle. Hence, the water cycle, including the deep water cycle, strongly influences the composition and dynamics of the planet and likely the same is true on other water-rich planets (Marcy, 2009). When society explores other planets, then, a key goal is to quantify the abundance and dynamics of water, not only to determine the possibility of life elsewhere but also to understand how the entire planet operates. The recent discovery of planets beyond the solar system has led to new models of the positioning of planets and their size and composition relative to their sun. The presence of water is central to predicting the composition and dynamics of these planets, as well as to the potential for "habitability." The study of hydrologic processes on other planets could be termed "exohydrology" and it is only just beginning. As a sign of the current im- portance of this topic, the American Geophysical Union convened a session at the fall 2011 meeting titled "Follow the Water: Insights into the Hydrol- ogy of Solar System Bodies." There is also a literature developing on exo- hydrology (e.g., Andrews-Hanna and Lewis, 2011). Although exohydrology is necessary to understand the evolution and climate of other planets, it also offers a test of the understanding of how Earth works. Abundant new imagery has presented startling observations of river channels, alluvial fans, gullies, and deltas on a planet where surface liquid water is currently absent (Mars) and channels, lakes, and rain driven by condensing and evapo- rating methane (Titan). The committee suggests two important research questions that focus on surface water processes. These questions present challenges that are ripe for interdisciplinary research between hydrologic scientists, paleohydrologists, planetary scientists, and geomorphologists in exohydrology. What are the definitive signatures of rain and surface water transport on a planetary surface? Is there is a unique relationship between surface morphologic features and the mechanism that formed them, and if so can hydrologists specifi-
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76 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES cally assign a role to water? On Mars, channels that originate near drainage divides (the tops of ridges) suggest that, in the past, the planet had an atmo- sphere capable of producing precipitation (rain or snow). Can geomorphic features give us such unique interpretations? For example, how do hydrolo- gists distinguish gullies formed by dry avalanches versus wet debris flows versus bedload transport in surface water flows? Early in the manned ex- ploration of Earth's moon, satellite imagery revealed sinuous channels (also called rills). These channels are broadly distributed across the Moon and show morphologic features quite similar to river channels found on Earth. The first papers on these new observations proposed that they were possibly formed by meltwater from permafrost. Subsequent landings on the moon revealed that channels were formed by flowing lava. Sinuous rills, which possess morphology similar to river channels, have been mapped on Venus, likely formed there by flowing lava. On Titan sinuous channels and valley networks show great resemblance to features formed by flowing water, but in this case scientists can be certain that the fluid is not water but most likely methane. Despite the abundance of features on Mars that bear very strong resemblance to terrestrial water-driven landforms such as outburst channels, alluvial fans, and deltas, debate continues about the abundance, origin (rain, snowmelt, or spring flow), and necessity of surface water. What is the role of water in creating specific landforms? Although scientists have the advantage at times of seeing geomorphic processes oc- curring on Earth, they more often have only the resulting morphology to interpret. Hence this research, while motivated by planetary questions, also has bearing on the understanding of landforms and what they reveal about processes. What are the distinguishing metrics and mechanistic theory that can yield these insights? Is it possible to estimate the magnitude, duration, and frequency of surface waters (river channel flow, springs, lakes, and oceans) from morphologic evidence alone? This question, which emerged with regard to Mars after the discovery of so many compelling, potentially water-driven features, applies equally to Earth and other planets. Perhaps the clearest example is the problem of how to extract such information about flow from a simple river channel. If scientists had data on channel plan form, cross-section, slope, and even bed material grain size (much more difficult to obtain on other planets), then what can be said about the flows that the channel experiences? Sedi- mentologists viewing preserved channels in the stratigraphic record push even further and ask what can be said about the drainage area and sedi- ment supply the channel carried. These questions are central to the field of
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THE WATER CYCLE: AN AGENT OF CHANGE 77 paleohydrology and, in general, to the understanding of the relationship between flow characteristics and channel morphology. With sufficient topographic and grain size information, a calculation of the flow that just fills a channel (bankfull flow) can be made with reason- able accuracy. This is widely practiced on terrestrial channels and on Mars. But what clues are there about how long the bankfull flow lasts, how often it occurs, how often much larger flows occur, and whether there could be sustained low flow? Empirical studies of direct measurements of terrestrial channels provide some data. How can such findings be extrapolated to other planets and, importantly, to other ungauged channels on Earth? The prediction of flows in ungauged basins has generally relied on some mixture of empirical characterization of regional runoff characteristics and quanti- tative measures of basin properties (e.g., channel network structure). These methods typically require data on precipitation, whereas on other planets, the question being asked is, from channel morphology (and perhaps basin characteristics) can scientists estimate the amount of precipitation needed to create that morphology? These questions point to another gap in the knowledge of terrestrial hydrogeomorphic processes. Is there a climatic signature in river morphol- ogy? For example, other factors being equal, will channels primarily fed by snowmelt differ significantly from those experiencing only rare monsoonal runoff events? This question has received little attention, yet lies at the heart of understanding of how river morphology records hydrologic processes. Progress on these questions about terrestrial hydrology and morphology will greatly inform exohydrology studies. CONCLUDING REMARKS The aspects of the water cycle highlighted in this chapter present nu- merous 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 wa- ter cycle topics. Recent advances in observing, measuring, analysis methods, and modeling make addressing many of these challenges attainable. Pair- ing these new advances with scientific challenges is a critical component of accounting for and predicting the human footprint on Earth's water cycle. For example, the coupling of modeling and observational advances could allow for continuous, regionally detailed monitoring and prediction of water-cycle dynamics, with forecasting times and accuracy extending ever further into the future, potentially on interannual and longer time scales. Modeling advances could also allow for simulation of alternative scenarios
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78 CHALLENGES AND OPPORTUNITIES IN THE HYDROLOGIC SCIENCES for water's future in a climate system that includes socioeconomic as well as natural controls on its movement. Finding solutions to the questions presented here requires research along the traditional lines of hydrologic sciences to, for example, promote scaling theories. Yet this effort also requires interdisciplinary research in relatively new disciplines such as hydroclimatology and paleohydrology. Entirely new disciplines, for example, exohydrology, will also generate new thinking and activity to further understanding. Field studies, whenever feasible and appropriate, are important. As the committee noted in the beginning of this chapter, water does not respond passively to physical processes governing Earth: it is a dynamic agent whose influence is central to processes that produced the world as society knows it and that will affect its evolution into the future. Water is locally exhaustible, which is why it is critical to understand its dynamics. Moreover, human intervention in the water cycle alters water's role on the planet. All of the phases and states of the water cycle are linked, and impacts of human activities on one component of the hydrologic cycle are consequently circulated to other components. This chapter focuses on the water cycle, but the next two chapters will revisit the water cycle as the component that integrates water throughout all biological and Earth systems. Chapter 3 extends this discussion beyond the processes that were addressed above into a host of ecohydrological topics. REFERENCES Andrews-Hanna, J. C., and K. W. Lewis 2011. Early Mars hydrology: 2. Hydrological evo- lution in the Noachian and Hesperian epochs. Journal of Geophysical Research 116: E02007. doi: 10.1029/2010JE003709. Chiu, Y., B. Walseth, and S. Suh. 2009. Water embodies in bioethanol in the United States. Environmental Science and Technology 43(8):2688-2692. doi: 10.1021/es8031067. Christensen, N., and D. P. Lettenmaier. 2006. A multimodel ensemble approach to assessment of climate change impacts on the hydrology and water resources of the Colorado River basin. Hydrology and Earth System Sciences Discussions 3:3727-3770. Coleman, J. S. M., and D. Budikova. 2010. Atmospheric aspects of the 2008 Midwest floods: A repeat of 1993? International Journal of Climatology 30(11):1645-1667. doi: 10.1002/ joc.2009. Crowley, W., J. X., Mitrovica, R. C. Bailey, M. E. Tamisiea, and J. L. Davis. 2008. Annual variations in water storage and precipitation in the Amazon Basin: Bounding sink terms in the terrestrial hydrological balance using GRACE satellite gravity data. Journal of Geodesy 82(1):9-13. doi: 10.1007/s00190-007-0153-1. EPA (U.S. Environmental Protection Agency). 2008. Combined Heat and Power Partnership: Dry Mill Ethanol. Available online at http://epa.gov/chp/markets/ethanol.html [accessed December 6, 2011]. Falkenmark, M., J. Lundquist, and C. Widstrand. 1989. Macro-scale water scarcity requires micro-scale approaches: Aspects of vulnerability in semi-arid development. Natural Re- sources Forum 13(4):258-267. doi: 10.1111/j.1477-8947.1989.tb00348.x.
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THE WATER CYCLE: AN AGENT OF CHANGE 81 SUGGESTED READING Averyt, K., J. Fisher, A. Huber-Lee, A. Lewis, J. Macknick, N. Madden, J. Rogers, and S. Tellinghuisen. 2011. Freshwater use by U.S. power plants: Electricity's thirst for a pre- cious resource. A report of the Energy and Water in a Warming World Initiative. Cam- bridge, MA: Union of Concerned Scientists. Baker, V. R., J. J. Dohm, A. G. Fairen, T. P. A. Ferre, J. C. Ferris, H. Miyamoto, and D. Schulze-Makuch. 2005. Extraterrestrial hydrogeology. Hydrogeology 13(1):51-68. doi: 10.1007/s10040-004-0433-2. Bales, R. C., N. P. Molotch, T. H. Painter, M. D. Dettinger, R. Rocei, and J. Dozier. 2006. Mountain hydrology of the western United States. Water Resources Research 42:W08432. doi: 10.1029/2005WR004387. Blöschl, G., and M. Sivapalan.1995. Scale issues in hydrological modelling: A review. Hydro- logical Processes 9(3-4):251-290. doi: 10.1002/hyp.3360090305. Bottger, H. M., S. R. Lewis, P. L. Read, and F. Forget. 2005. The effects of the martian regolith on GCM water cycle simulations. Icarus 177(1):174-189. doi: 10.1016/j.icarus.2005.02.024. Cabrol, N. A., and E. A. Grin. 2001. The evolution of lacustrine environments on Mars: Is Mars only hydrologically dormant? Icarus 149(2):291-328. doi: 10.1006/icar.2000.6530. Crutzen, P. J. 2002. Geology of mankind. Nature 415(3):23. Gleick, P. H. 2010. Roadmap for sustainable water resources in southwestern North America. Proceedings of the National Academy of Sciences 107(50):21300-21305. doi: 10.1073/ pnas.1005473107. Gleick, P. H., and M. Palaniappan. 2010. Peak water limits to freshwater withdrawal and use. Proceedings of the National Academy of Sciences 107(25):11155-11162. doi: 10.1073/ pnas.1004812107. Herwiejer, C., and R. Seager. 2008. The global footprint of persistent extra-tropical drought in the instrumental era. International Journal of Climatology 28(13):1761-1774, doi: 10.1002/joc.1590. Hirsch, R. M., and K. R. Ryberg. 2011. Has the magnitude of floods across the USA changed with global CO2 levels? Hydrologic Sciences Journal 57(1):1-9. doi: 10.1080/02626667.2011.621895. Linton, J. 2008. Is the hydrologic cycle sustainable? A historical-geographical critique of a modern concept. Annals of the Association of American Geographers 98(3):630-649. doi: 10.1080/00045600802046619. Nykannen D. K., and E. Foufoula-Georgiou. 2001. Soil moisture variability and scale depen- dency of nonlinear parameterizations in coupled land-atmosphere models. Advances in Water Resources 24(9-10):1143-1157. doi: 10.1016/S0309-1708(01)00046-X. Perrone, D., J. Murphy, and G. M. Hornberger. 2 011. Gaining perspective on the water-en- ergy nexus at the community scale. Environmental Science and Technology 45(10):4228- 4234. doi: 10.1021/es103230n. Samain, B., G. W. H. Simons, M. P. Voogt, W. Dofloor, N. J. Bink, and V. R. N. Pauwels. 2011. Consistency between hydrological model, large aperture scintillometer and remote sensing based evapotranspiration estimates for a heterogeneous catchment. Hydrology and Earth System Science 8:10863-10894. doi: 10.5194/hessd-8-10863-2011. Schubert, S., D. Gutzler, H. Wang, A., Dai, T. Delworth, C. Deser, K. Findell, R. Fu, W. Higgins, M. Hoerling, B. Kirtman, R. Koster, A. Kumar, D. Legler, D. Lettenmaier, B. Lyon, V. Magana, K. Mo, S. Nigam, P. Pegion, A. Phillips, R. Pulwarty, D. Rind, A. Ruiz- Barradas, J. Schemm, R. Seager, R. Stewart, M. Suarez, J. Syktus, M. Ting, C. Wang, S. Weaver, and N. Zengnand. 2009. A U.S. CLIVAR project to assess and compare the responses of global climate models to drought-related SST forcing patterns: Overview and results. Journal of Climate 22:5251-5272. doi: 10.1175/2009JCLI3060.1.
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