1. The Emerging Global Water Crisis
Over 75% of Earth’s land surface has now been impacted by human development [1
], which exerts an expanding footprint on water resources. There are distinct patterns in the evolution of land use change [2
], and land use is a template on which climate interacts to influence the quantity and quality of Earth’s water [4
]. In order to support human-dominated land use, ground water in semi-arid and arid landscapes has been decreasing by approximately 150 cubic km per year due to increasing extraction [5
] (and references there in). Water stored as ice on land, which is critical for major drinking water supplies, has decreased at a rate of approximately 300 cubic km per year due to warming and changes in regional precipitation patterns [5
] (and references there in). In fact, access to clean water is considered one of the contemporary grand challenges for engineering by the U.S. National Academy of Engineering (http://www.engineeringchallenges.org/challenges/water.aspx
). Globally, major impacts are being caused by the increasing irrigation and accompanying dam construction and groundwater extraction [6
]. Thus, a water crisis has been proposed as a major global issue [7
], and much work has described impairments and alterations of water systems [9
]. However, critical questions remain regarding our current state of the world’s water and how water impairments have evolved over time [11
]. Can we alter our future course to avoid a global water crisis and related pitfalls [7
]? How will we manage water in the future [12
]. The papers in this special issue on Land Use, Climate, and Water Resources provide practical information and, along with case studies from different regions of the world, help address some of the most important water management questions facing us now.
2. Human Domination of Water-from Degradation to Restoration Cycles
Humans have dominated Earth’s ecosystems over millennia [14
]. Since recorded history, humans have influenced every major hydrologic process of the water cycle including: altered rainfall regimes (e.g., modified by urban areas), accelerated runoff and overland flow from impervious surfaces and tile drains, reduced infiltration from soil compaction and impervious surfaces [15
], and induced changes in evapotranspiration from irrigated agriculture and trees in urban areas. Recharge has also changed due to consumptive processes in ground water, and there is also less recharge due to melting snowpacks and shifts in regional rainfall patterns [16
]. Water systems also evolve over time due to adaptations in response to selective pressures and decisions by humans [2
]. Furthermore, there are coinciding changes in aquatic ecosystem structure, function, and associated services, coupled with vulnerability to climate at each step of human domination of the water cycle from degradation to restoration (Figure 1
). It is now well known that human activities have contributed to increasing emissions of greenhouse gases, which is responsible for regional climate change and variability—this can spur regional and local adaptation and mitigation strategies [17
]. However, restoration efforts to regain losses in structure, function, and services are unrealistic unless they consider the path of degradation of water systems (Figure 1
). Thus, restoration and water management are “ex post facto” responses with an “impair and then repair” approach [11
]. Instead, we need a predictive and proactive approach based on an understanding of both root causes and trends of water degradation and an appreciation for nonstationarity and uncertainty in climate trends—e.g., historical trends in the past do not always help us predict future changes in water management [12
]. Overall, the losses in structure, function, and services due to land use change have contributed to a global water crisis related to the resiliency and resistance of water to climate change.
Although there is interest in ecosystem restoration globally, there is widespread recognition that it is not possible to return to historical pre-disturbance conditions prior to human activities. As land use change proceeds globally, it is inevitable that water systems and their associated ecosystem services will progress through a series of stages. Tracking the evolution of the global water crisis can not only help diagnose a global syndrome impairing water quantity and quality, e.g., [10
], but it can also inform realistic management and ecosystem restoration. Specifically, it can help to (1) evaluate the severity of impairments across stages, (2) anticipate and/or predict the prognosis of water quantity and quality over time, (3) improve and inform monitoring of water quantity and quality over time, (4) identify and detect stability, resistance, and resilience of water resources over time, and (5) select appropriate infrastructure management and/or ecosystem restoration interventions over the course of time.
4. The Future of Water: Stepping into the Unknown
Over time, human actions change hydrology and water movement through ecosystems. Management may be necessary to address excess water, e.g., storm water runoff, or water scarcity and decreased water quality e.g., access to clean drinking water. In addition, extremes in climate are increasing regionally and there is nonstationarity in climate and water [12
]. Regions that currently have favorable hydrology may not in the future. Regions that do not have favorable hydrology must adapt by investing in infrastructure to manage and store water. As illustrated in this special issue, fresh water is not distributed equally globally, with many regions receiving too little or too much water, and this will impact regional differences in stages of the global water crisis.
There are many research frontiers and unknowns related to the interactive effects of land use and climate on water. We focus on four specific research needs related to intersections between (1) hydrogeology and land use, (2) remote sensing and hydrology (3) ecology and engineering, and (4) translating watershed research into management. First of all, land use has a vulnerability to climate based on geologic setting and topography. Geologic setting is important to water quantity and quality because of its fundamental influence on water movement, infiltration, erosion rates, and chemistry. However, it is still underappreciated that urbanization creates its own distinct geology, which can dramatically influence the transport and chemistry of water in the built environment [2
]. Stormdrain and ditch networks dramatically alter drainage networks and geologic materials in close contact with water [28
]. These drainage structures and geologic materials significantly affect water chemistry [32
]. Land use change can alter geological processes such as human-accelerated weathering of impervious surfaces, which can increase many major ions in streams and rivers [33
]. Thus, a research frontier is elucidating the impacts of impervious surfaces (as geologic materials subject to degradation and weathering) on the chemistry and quality of fresh water.
Secondly, remote sensing can provide new information related to land use and climate change and corresponding feedbacks on changes in hydrologic processes that would be difficult or impossible to quantify [47
]. For example, we can now characterize changes in land use and land cover and conversion to agricultural lands, which can influence water recycling rates [48
] evapotranspiration, and runoff processes [48
]. High-resolution satellite date can allow us to determine finer scale variations in the energy balance of human-dominated watersheds as well as changes in regional water balances due to land use, land cover, and management decisions [50
]. This is important in urbanizing regions given the expanding impacts of urban heat islands [51
], and their contribution along with climate change on rising river temperatures and related impacts on water quantity and quality [24
Thirdly, there is often a disconnection between ecology and engineering in managing water resources, although access to clean water is a grand challenge for engineering, as mentioned earlier. Government agencies sometimes seek engineering solutions as opposed to ecological ones, likely because ecology has many complexities, uncertainties, and it takes time to collect enough monitoring data to comprehend all the interactions in an abiotic and biotic system. Engineering solutions are often sought to meet a specific regulation and permit without fully considering ecological interactions. As studies in this special issue point out, there are many interactive effects associated with land use and climate change, which should be considered. For example, the interactive effects of urbanization and climate variability can be managed by regenerative stormwater conveyance in some urban areas [54
]. Regenerative stormwater conveyance is an example of an engineering approach that can enhance denitrification by creating favorable redox conditions. Yet, regenerative stormwater conveyance can also trigger release of iron during these redox conditions, which is an unintended consequence and can impact aquatic habitat by creating dense mats of iron flocculate [54
]. Sometimes engineering solutions are implemented without a full understanding of the ecosystem consequences. As an initial step, ecohydrological modeling can be used to better incorporate ecology and biogeochemistry into engineering solutions [55
]. These models can use easily accessible environmental data that was not available in the past such as soil moisture, temperature, topography, irradiance, and remotely sensed data that can be used as model inputs. Integrated modeling encompasses multiple processes [56
] and preferably uses standard interfaces [57
]. Data related to environmental monitoring is not always readily available, and there is a need for automated sampling stations, as highlighted by previous work [58
]; this can pose a problem in integrated eco-hydrological modeling studies [24
]. Ultimately, monitoring is still critical for optimizing better ecological and engineering solutions and detecting unintended consequences that cannot be easily anticipated (please see below).
Fourthly, translating watershed research into management also remains a frontier for the future of water resources [60
]. In many instances, regulations cannot keep pace with the science—there can be a lag time between dissemination of monitoring results and the development and implementation of management and policy. Watershed managers, stakeholders, and decision-makers are often left with uncertainties with which strategies or technologies they should pursue or how they should address an emerging problem in water resources management [61
]. Thus, science and technology transfer often falls short because it is either difficult to communicate messages and/or there needs to be strong incentives for scientists and managers to work together [62
]. In some cases, one scientist may have a disproportionate influence on policymakers due to their communication approaches, personality, style, and/or marketing of research, where different viewpoints may be overlooked. One way to integrate multiple viewpoints is through the establishment of expert panels, which can bring together broader groups of watershed managers and scientists to influence the evolution of water and biogeochemical cycles [2
]. Scientific and engineering information can then be synthesized by communications specialists via webinars, workshops, technical reports, interactive websites, and training programs.
Across the world, the studies in this special issue and growing work suggest that future trends in water management will not be static in the future [63
] (Figure 1
). Instead, they will need to be dynamic processes based on adaptive management. Thus, we will need to keep evaluating whether management approaches are still effective in response to the increasing interaction between land use and climate change on a global stage. This brief review and studies in this special issue suggest that conservation of natural lands (in addition to ex post facto
restoration) is critical to slow down and/or reverse the interactive effects of land use and climate on water resources [64
]. Overall, global water security cannot be adequately restored without considering an increasing interaction between land use and climate change across progressive stages and our ever-increasing human domination of the water cycle from degradation to ecosystem restoration.