Keywords

1 Introduction

The primary purpose of this chapter is to establish understanding of how urban systems intersect with the water cycle, also called the hydrological cycle. We will begin with an overview of how the cycle functions naturally and how the components or the physical processes that make up the water cycle interact with each other to drive the cycle. Then we will examine the principal factors that influence water movement and distributions in the context of the urban environment, followed by a review of the implications of climate change as well as anthropogenic changes such as development (e.g., urbanization) on water quantity (availability) and quality in an urban hydrological system. Having this basic knowledge is vital to creating more sustainable urban living environments where people need access to fresh water, flooding can be detrimental to the urban economy and human health, and ecosystem integrity of downstream waterways must be maintained.

2 The Hydrological Cycle

2.1 Definition

The hydrologic cycle can be described as the continuous circulation of the Earth’s water through the hydrosphere, atmosphere, lithosphere, and biosphere. It is a dynamic and integrated system of stocks and flows driven by a multiplicity of interacting processes including precipitation, infiltration and percolation, evapotranspiration, snow accumulation and melt, canopy interception, sublimation/condensation, etc. [1,2,3]. To comprehend this system as a whole, we have to understand not only the components that comprise the system but also the dynamic interactions and feedback mechanisms between the processes.

The hydrologic cycle has no starting point. However, let us say that the cycle starts from sun-powered evaporation from the ocean (Fig. 6.1). Liquid water changes into water vapor due to heat energy produced by solar radiation and forms clouds in the atmosphere through the process of condensation. With suitable temperature and atmospheric pressure, the vapor becomes liquid, and precipitation occurs in the form of rain, snow, hail, etc. With liquid precipitation a portion returns to the ocean where it evaporates back to the atmosphere. Initially, however, a portion of rainwater may be intercepted (interception) by vegetation, buildings, and other objects from which the rainwater may be either evaporated back to the air or move down from these surfaces to the ground surface. Once the water reaches the ground surface, one of following three processes occurs: (1) Water is stored temporarily in natural depressions, e.g., pits and trenches, and will be lost largely by evaporation. (2) Water soaks into the soil, i.e., infiltration . Some of the infiltrated water is lost via soil evaporation and/or is transpired by plants. Combined, these processes are called evapotranspiration . The remaining water continuously moves downward through the soil profile following infiltration in a process called percolation and recharges the groundwater that will eventually discharge into streams and return to the ocean (groundwater flow) where the cycle begins again. (3) When the water holding capacity of soil is exceeded, rainwater may flow over the surface (surface runoff ) and will eventually reach the ocean. When precipitation falls as snow, much of it, especially in high-altitude mountain regions, is temporarily stored as snowpack (snow accumulation) although some water returns to the atmosphere through the process of sublimation, which is the process of snow changing into water vapor without melting first. Long-term storage of water can occur in glaciers and ice caps. The hydrologic processes described above for liquid precipitation begin as snow starts to melt (snowmelt).

Fig. 6.1
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The hydrologic cycle. Graphic courtesy of the National Oceanic and Atmospheric Administration National Weather Service, released into the Public Domain. https://www.noaa.gov/resource-collections/water-cycle

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2.2 Physical Factors that Influence the Hydrological Cycle

Hydrologic processes are highly variable both spatially and temporally as a result of complex interactions between climate, terrain, soil, and vegetation. The global hydrologic cycle, however, is less variable compared to that that occurs at smaller geographic scales (such as regionally or even in a small watershed) because variations in some regions can be counteracted by opposite variations elsewhere. Here we focus on the physical control factors that govern regional to local hydrologic cycles. The physical factors can be grouped into two categories: (1) meteorological factors including precipitation characteristics, air temperature, solar radiation, humidity, wind speed, etc. and (2) physiographic factors such as topographic characteristics, geological factors that have created the hydrological characteristics of surface and subsurface soil layers, and land use and land cover.

2.2.1 Meteorological Factors

At a given geographical location, precipitation, which determines the water availability circulating in the water cycle, is one of the primary drivers of the hydrologic cycle over time and space [2, 4, 5]. Under the rain-only situation, the rainfall characteristics of volume, timing, intensity, and duration have direct impacts on the water available for surface, subsurface and groundwater flow, as well as soil moisture. In the case of small rainfall events, before runoff begins, the majority of water is lost to what is called the initial abstraction, which includes water retained in surface depressions, water taken up by vegetation, soil evaporation, infiltration, percolation, etc. No runoff will occur if the remaining rainwater does not exceed soil capacity. (“Lost” is used by hydrologists to account for what happens to the quantity of precipitation once it falls on the Earth, as in a mathematical equation.) Storm events of short duration and high intensity tend to produce higher peaks in flow volume, while long-duration moderately large rainfall events often generate less runoff as a large proportion of rainwater is lost to soil. In high-latitude mountainous regions, where snow represents a considerable proportion of precipitation, the hydrologic cycle is affected in many ways by winter meteorological conditions. The hydrological processes in snow-dominated or transient rain-snow zones at intermediate elevations are often much more complex than those of low-altitude rain-dominated zones [6]. The magnitude and timing of snow accumulation and melt can vary considerably from point to point as evidenced by the long-term point observations of snow water equivalent (SWE) from the Natural Resources Conservation Service Snow Telemetry stations (NRCS SNOTEL) across the mountain regions of the western United States [7, 8]. The marked spatial heterogeneity can be attributed largely to the complex interplay between local meteorology (e.g., precipitation, air temperature, radiation, wind speed, and direction), terrain (slope and orientation), and forest canopy and structure [9,10,11].

Another hydrological process that can be very responsive to meteorological conditions is evapotranspiration. Soil evaporation and plant evapotranspiration account for a great proportion of water loss from plants and soils especially in arid areas. In humid regions, the evapotranspiration rate is generally slower than in arid regions because the water vapor content in the atmosphere tends to be close to saturation and less water is able to evaporate into the air. Evapotranspiration is also noticeably reduced during cold seasons when plant growth and metabolism are generally diminished [2, 3]. When we discuss the evapotranspiration process in winter conditions, we must take into account the additional reduction in evaporation as a result of frozen soil and the lack of leaves on deciduous plants. When soil temperature drops below 0o C, water in the soil may freeze and thus dramatically hinder water infiltration into the soil [12].

2.2.2 Physiographic Factors

There are a number of factors we call physiographic that play a key role in land surface hydrological processes principally that of runoff. We emphasize runoff of surface water as this is the part of the hydrological cycle that can be most detrimental to waterways, both urban and rural. Flooding, erosion, stream sedimentation, and pollution of waterways are all related to runoff. Topographic (landform) features that influence runoff are altitude/elevation, slope angle, and solar orientation or aspect (N, S, E, W). These, along with land cover characteristics such as type of vegetation, amount of vegetation, amount of paving, etc., are the physical features of an area. Elevation and slope play significant roles in determining the surface and subsurface runoff direction, travel time, and volume. The upslope contributing area determines how much precipitation is captured and potentially drains to any point in a watershed, which can be thought of as the outlet of the watershed [13,14,15]. Slope orientation has a direct impact on the amount of sunlight or incident radiation reaching the surface and thus is very important to the distribution of soil moisture and vegetation [13,14,15,16]. Land use and land cover characteristics play a critical role in the hydrologic cycle as well. Natural vegetated cover and impervious land cover produce quite different effects. In densely forested environments, canopy cover significantly alters the surface mass and energy balance through evapotranspiration, canopy shading, and canopy interception, which control the availability and seasonality of river flow especially in snow-fed rivers [10, 11]. Deep-rooted trees and plants can improve soil structure and increase soil infiltration capability, thus reducing runoff potential [17]. Vegetation cover also has been found effective in trapping pollutants borne by runoff, acting as a natural filter protecting groundwater and surface waters from contamination [18,19,20].

Another physiographic factor in hydrology that influences the hydrological cycle is geology. Surface and subsurface soils are the product of the rocks that were laid down long ago in an area and acted upon by weather over centuries. Clay soils versus sandy soils, for instance, have quite different properties that affect how much precipitation is absorbed versus how much becomes surface runoff. Geology thus accounts for the hydrologic properties of soils, such as hydraulic conductivity, permeability, and the hydraulic gradient [15, 16]. These properties together determine the velocity at which groundwater moves. (Saturated) hydraulic conductivity is a measure of the water-transmitting capability of an aquifer. A high hydraulic conductivity value indicates that an aquifer can readily transmit water through a saturated zone. In general, coarse-grained sands and gravels have high hydraulic conductivity values which means that water can move through them between 50 and 1000 m/day, while grained silts and clays have low values in the range of 0.001–0.1 m/day. Permeability is a measure of the ease with which fluid will flow through a porous medium under a specified hydraulic gradient and is directly associated with infiltration capacity , but not with the fluid itself. Permeability is determined by the amount and size of pore space in the soil. Gravels typically have high porosities meaning that water flows easily through them, while silts and clays tend to have low porosities. Groundwater flow can also deliver contaminants to waterways. The hydraulic gradient is what causes water to move underground. It depends on the difference in hydraulic head (pressure and gravitational energy) between two different points. It is calculated as the change in head between those two points divided by the distance between them over the flow path. Of these three factors, hydraulic conductivity generally has the most effect on groundwater velocity [15].

3 Water Quality Dynamics

3.1 Stream Metabolism

Lotic (flowing) ecosystems such as streams, rivers, and estuaries are composed of abiotic and biotic parts that interact to perform ecological work [21]. This can be quantified in a holistic way by measuring the stream metabolism commonly done by monitoring changes in dissolved oxygen (DO) over a diurnal (daily) cycle [22,23,24]. Over the course of a day, DO increases during daylight due to primary production (photosynthesis by plants) and declines at night due to respiration (by the entire community of plants, animals, bacteria, fungi, etc.). Diffusion of oxygen between the water surface and atmosphere must be taken into account; this can be done either by direct measurements of gas exchange or by use of standard models that require information on stream channel dimensions and discharge (how much water over a time period passes a given point in the stream). Oxygen and other water quality parameters (e.g., temperature, conductivity, and turbidity) can be monitored at a high temporal resolution with data-logging sondes.

3.2 Water Quality Issues

Nutrients and suspended sediments have been identified as major sources of water quality degradation and are often monitored and evaluated as primary indicators of biochemical characteristics of water [20, 25,26,27]. Agricultural runoff from pastures and farmlands as well as human and industrial wastewater are common sources of nutrient pollution. Nutrient pollutants, primarily nitrogen and phosphorous, can stimulate the aquatic primary productivity rate, resulting in eutrophication of surface waters and consequent algae blooms. Increased aquatic productivity can ultimately result in depletion of DO levels in water, posing threats to aquatic organisms and adversely impacting stream metabolic processes [20]. Sedimentation can be defined as the process of inorganic or organic particles transported, suspended, and deposited in streams or lakes by gravity. The principal sources of sediments are upland erosion from overland flow and weathering. Excessive amounts of suspended sediment can reduce water clarity (a measure of the amount of light penetration in water) and hence affect plant photosynthesis. Deposited sediment can clog fish spawning beds and damage aquatic habitats [28,29,30]. Sediments transported through runoff often carry a significant loading of nutrients and toxic chemicals to water bodies and consequently impair their water quality [27].

4 Urbanization and Climate Change Implications

4.1 Urban Hydrologic Systems

There is a long history of building cities on or near rivers in order to have access to clean drinking water, and navigable waterways that provided a means for transportation/trade, and conveyed wastes away from the city. Urban development in the United States, which proceeded rapidly following the World War II and accelerated over the last three decades, has induced an expansion of impervious areas (cement and asphalt through which water does not flow) and a rapid reduction in natural vegetated areas. As a result, the natural hydrologic cycle shifts from an infiltration- and evapotranspiration-based system to a surface runoff-dominated system [30,31,32,33]. During the past half century, to accommodate the growing water demand in the face of increasingly limited freshwater resources, more than 30,000 large dams have been built globally as a common practice for securing water for municipal water supply, hydropower, irrigation, etc. [34, 35]. Some of these hydraulically modifying structures can alter the natural streamflow pattern substantially by following various management protocols such as a drought response plan that may hold back water, allow minimum release, and/or manage for downstream ecological flow requirements [34, 36, 37].

In urban hydrologic systems, instead of water running its natural course, streams that were once natural may now be channelized into artificially modified or constructed stream beds. Runoff from roofs and street surfaces may be diverted to the storm sewer system through storm drains, pipes, and ditches, creating various types of storm sewer systems, including combined sewer system (CSS), sanitary sewer system (SSS), and municipal separate stormwater sewer system (MS4). Storm and sewer systems were constructed as a means of dealing with two consistent problems in cities: (1) how to get rid of large amounts of wastewater and (2) how to protect property and lives from flooding particularly during large storm events. Most of today’s sewers were constructed in the late nineteenth and early twentieth centuries. CSS pipes collect sanitary sewer and stormwater flows together and route them to a wastewater treatment facility. SSS is designed to collect and transport only sewage from domestic, commercial, and industrial buildings to a wastewater treatment system. MS4 refers to a system that collects and discharges stormwater directly to surface waters. During low-flow periods, sewage from CSS and SSS is routed to a wastewater treatment facility. However, during large magnitude rainfall or snowmelt events, when stormwater runoff exceeds the capacity of the wastewater treatment system and/or the pipes, the surplus volume is released into neighboring surface waters, through large doors that open under pressure. These are often referred to as combined sewer overflow (CSOs ) (Fig. 6.2) and sanitary sewer overflow (SSOs).

Fig. 6.2
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In a combined sewer overflow (CSO), where during high-volume precipitation or snowmelt events, a gate opens automatically and releases stormwater mixed with sanitary effluent from the combined sewer system (CSS) and sanitary sewer system (SSS) trunk lines directly to urban lakes, rivers, and streams (Source: www.moundsvillewwtp.com/CSOs.html, with permission of Superintendent Larry Bonar)

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Water quality as well as quantity along a stream can be significantly affected by the location and timing of return flow through sewage effluent. Elevated pollutant concentrations found in urban runoff are directly related to the degree of urbanization, which increases runoff volume and peak flow rates [38, 39]. As stormwater flows over impervious surfaces, it often picks up a variety of pollutants produced from diverse human and industrial activities, e.g., pet waste, pesticide used for controlling insects and weeds, and petroleum pollutants from parking lots. These pollutants along with the stormwater runoff enter storm sewers and nearby water bodies contributing to elevated concentrations of pollutants in the water. Stormwater runoff of large magnitude contributes greatly to another major source of water pollution: sediment pollution. After a major rainfall event, runoff of large volume may wash loose soil off impervious surfaces into the lakes and streams posing a threat to aquatic organisms and plants [40, 41].

4.2 Urban Stream Metabolism

Because many cities are located along flowing waters, sewer overflows following high-flow events can have adverse impacts upon these ecosystems and their biota because these waters are used to receive both pre- and post-treatment effluent. Typically the sewage water delivers large amounts of organic matter and nutrients into the recipient water. Downstream, bacteria decompose the organic matter and deplete the oxygen, often severely. If one were to measure DO upstream of a CSO, and then downstream at various distances, one would observe a marked depression in DO just downstream of the CSO, followed by a gradual recovery of DO. This is referred to as a “dissolved oxygen sag” and is part of the classic urban stream pathology.

A good example of the effects of urban impervious surfaces and the impacts of CSOs can be seen by comparing two sites along a typical urban stream in a small- to mid-sized temperate city: Onondaga Creek in the city of Syracuse, NY. The upstream site is channelized, but the surrounding region is relatively open with forests and suburban lawns. The downstream site is within downtown Syracuse with almost complete impervious cover. The differences are greatest during storm events. During low-flow (dry) summer conditions, DO shows typical cycles of peaks in the daytime and minima at night (Fig. 6.3a). However, during a period of rainstorms, the peak/trough pattern of DO is greatly reduced, and in the CSO region of the downtown, it is essentially extinguished (Fig. 6.3b).

Fig. 6.3
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Comparison of DO curves, between CSO and non-CSO sites along the Onondaga Creek during (a) dry period and (b) rainy period of Summer, 2011

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Ecosystem metabolism is a rate measurement, and so we need to integrate the rate of change of DO after accounting for diffusion, as in Fig. 6.4. Doing that, we see that the daytime peak in dry weather productivity is slightly shifted in the city vs. upstream (rural) site (Fig. 6.4a), but otherwise the dynamics are similar. During the storm period, oxygen changes quickly both upstream and downstream (left side of Fig. 6.4b), but the variations are much exaggerated in the more urban part of the stream. Both sites were affected by the clouds and rain, but the runoff response of the city area is truly “flashy.” Thus, we can conclude that during dry periods, the stream behaves in a similar way regardless of surface cover, but in wet periods the concrete urban surfaces and enhanced runoff greatly affect the stream.

Fig. 6.4
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Rate of DO change in CSO vs. non-CSO sites along the Onondaga Creek during (a) dry period and (b) rainy period of Summer, 2011

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Stream turbidity (cloudiness) is an interesting corollary to the dissolved oxygen dynamics in this example. Turbidity is correlated to sediment load, as it is partly caused by the particles suspended in the water column. The Onondaga Creek is a turbid stream due to groundwater upwelling upstream. In low-flow conditions, the upstream, relatively rural site is more turbid, but we also see a diurnal pattern of rise and fall in turbidity (Fig. 6.5a). The peak in turbidity at night when DO concentration is lowest might be accounted for by animal activity. During storms, the impervious surfaces of the city wash all the surface grit down into the CSOs from where they are released to the creek along with accumulated silts, which have been accumulating behind the CSO door, and are also resuspended (Fig. 6.5b) because of the high flows, creating “turbidity spikes.” The ecological effect of such spikes is unclear but is likely stressful to fish and other aquatic life.

Fig. 6.5
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Comparison of turbidity in CSO vs. non-CSO sites along the Onondaga Creek during (a) dry period and (b) rainy period of Summer, 2011

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4.3 Climate Impact on Urban Hydrologic Systems

Water resources are under increasing stress in many regions across the world from altered hydrologic, thermal, and water quality conditions resulting from changing climate [40, 42,43,44]. The frequency of heavy precipitation events has increased by about 20% on average in the United States since 1958. The increasing trend of extreme precipitation events is expected to continue in the future, with likely intensification over much of the United States [45, 46]. The heavy rainfall events have posed substantial risk to urban drainage infrastructures that are mostly designed to handle the design storms of 50 years and more ago. While extreme rain events are often the cause of extreme flood events, an increasing number of large flood events are associated with snowmelt from deep snowpack especially during rain-on-snow (ROS) events in many snow-fed rivers across the western United States [8, 47]. The ROS events, typically occurring with warm temperature and high winds, significantly accelerate snowmelt and can produce severe flood events both in the mountainous regions and lowlands. The warming trend of air temperature over the last century has led to changes in mountain snowpacks that are well documented in many parts of the United States [6, 7, 40, 42, 43, 47,48,49]. The projected increase in average temperatures across the country will reduce the proportion of precipitation falling as snow and lead to a shorter duration of snow cover causing a shift of snow regime. Early occurrence of snow melt and lack of adequate spring melt would lead to declined stream flows in late spring and summer and impose profound stress for both cities and the ecosystems in which they are embedded. This is especially true for cities in snowmelt-dominated regions that are subject to dry summers when water demand is also greatest, such as in much of the western United States.

Rapid urbanization will exacerbate impacts on urban streams from climate change in a number of ways. (1) Hurricanes fed by warmer oceans will bring heavier rainfall. Urban development on water-absorbing wetlands as was done in Houston, TX, or expanding development on mountain slopes above cities like San Juan, Puerto Rico, exacerbates the flooding associated with the extremely heavy rainfall accompanying 2017 hurricanes Irma and Maria. Coastal cities will see more impactful storm surges that flood streets, homes, and drainage systems. (2) Temperature in urban small streams, in comparison to large rivers, is more susceptible to air temperature increases due to their relatively low heat capacity, particularly during low-flow summer conditions [40, 44]. Elevated water temperature can negatively impact the stream water quality, as well as the physical, chemical, and biological health of aquatic ecosystems. It is worth mentioning that the thermal input from paved surfaces is a main contributor to abruptly increased water temperature in urban streams during rain storms [44]. Finally, reduced surface water resources due to drought or timing of snowmelt will encourage human intervention in water use, for example, reservoir construction and regulation, water conservation measures, and statewide emergency water shortages such as in Los Angeles during California’s long drought of 2011–2017. Lastly, changing climate is anticipated to have substantial impacts for energy production. About 66% of electricity generation in the United States requires water for cooling, and about 40% of freshwater withdrawal is required for thermoelectric production in the United States. Due to environmental restrictions on availability of cooling water and on warm water discharges back into rivers, the projected lower water availability and higher surface water temperature in the future would have great implication for the electricity sector across the world including the United States [50].

5 Remediation Technologies

5.1 Green Infrastructure

As discussed earlier, CSOs are a major problem for urban systems during storm events. Once the sewage effluent and street runoff are combined, the contaminants in the sewage will pollute the receiving water. This poses a considerable threat to both the ecological metabolism of the city aquatic life, human health, and groundwater quality and to the sociological quality of neighborhood life and economic activity that may rely on receiving waters for tourism, fishing, recreation, and real estate values, hence greatly affecting the city’s overall metabolism. The traditional solution to CSOs is installation of grey infrastructure (e.g., sewage treatment plants and underground storage tanks) or enlargement of current treatment facilities to enhance their storage and conveyance capacities. The cost of conventional approaches, however, often exceeds the affordability standard for stormwater management or is limited by land availability in the already tightly structured urban landscape. More importantly, the conventional solution cannot be considered sustainable because it does not treat the runoff volume or stormwater-carried pollutants at the source nor address the problem of groundwater depletion that occurs because precipitation cannot infiltrate urban soils that lie under the buildings and pavement [39, 51].

Green infrastructure (GI), on the other hand, offers cities and communities a more cost-effective opportunity to achieve CSO mitigation requirements. GI technologies consist of decentralized structures that have the potential to capture, retain, infiltrate, evapotranspire, and reutilize the stormwater runoff. GI structures at the parcel scale (meaning an individual household, apartment building or commercial lot) include rain gardens and bioretention cells, rain barrels, green roofs, and porous paving. Rain gardens and bioretention cells are vegetated depressions receiving stormwater runoff water from rooftops or other impervious surfaces such as driveways along the flow path (see Fig. 15.11). Soils in the rain gardens and bioretention cells often have a higher infiltration capacity and a saturated hydraulic conductivity rate that allows for water retention. Instead of flowing to a street or into the storm drains, the detained stormwater is either evaporated and transpired by plants or percolates into the soil’s unsaturated zone where it will contribute to groundwater recharge. The plants and mulch in rain gardens and bioretention installations can also trap and even remove the pollutants from runoff through physical, biological, and chemical processes. Rain barrels operate as detention tanks, which are usually placed underneath rooftop downspouts. They collect rainwater from rooftops for reuse such as irrigation. Green roofs are rooftops which are planted with a vegetation layer (see Fig. 15.10). There are two types of green roofs: (1) extensive roofs, which are constructed with a minimal soil layer of less than 6-inch thickness and support primarily dense, low-growing and drought-resistant vegetation, and (2) intensive roofs, which have a thick layer of soil of greater than 6-inch and can support all types of vegetation. Green roofs have many benefits. They can reduce the stormwater runoff and enhance water quality by capturing rainwater and filtering the pollutants in rainwater. Green roofs can also cool the rooftop and surrounding air through both less retention of solar radiation (see Chap. 7) and evapotranspiration, thereby reducing cooling demand in summer. Porous paving is an alternative to conventional impervious asphalt or concrete surfaces. It allows stormwater to infiltrate through the pavement and percolate into the underlying subsoil promoting groundwater recharge. Compared to CSO storage and conveyance systems, GI may cost less to implement than grey infrastructure solutions (comparisons should be conducted before a decision is made (see Chaps. 1 and 15)) and has lower operation and maintenance (O&M) costs since plants, if well chosen (see Chap. 11), do the work. GI also helps reduce CSO events and water pollution potential by capturing pollutants and treating stormwater onsite before it enters the storm drains. Additional benefits include promoting infiltration and groundwater recharge, restoring water bodies, improving air quality, mitigating the heat island effect, sequestering carbon, and contributing to pleasant landscapes for the living environment.

5.2 Phytotechnology

Lastly, we would like to briefly introduce an emerging technology, phytotechnology, which consists of a set of technologies using plants to remediate or take up contaminants in soil, groundwater, surface water, or sediments. As plants play a significant role in the regional and local water cycle, phytotechnology is often operated on a catchment scale. Most common and widely applied phytotechnologies include phytostabilization, phytohydraulics, phytoextraction, and phytovolatilization [52, 53]. Phytostabilization controls infiltration by promoting interception and evapotranspiration by plants and subsequently lowers the groundwater table locally and limits contact of shallow-contaminated soils with groundwater. Phytohydraulics uses the ability of plants to evapotranspire surface water and groundwater. Phytoextraction refers to the use of pollutant-accumulating plants that can extract and translocate contaminants to the harvestable parts. Translocation of contaminants from the roots to the shoots is powered by leaf transpiration. Phytovolatilization refers to the process whereby plants extract and transpire volatile organic contaminants from media such as soil, groundwater, and sediment into the atmosphere.

Generally speaking, phytotechnologies have the following advantages: (1) cost-effectiveness compared to conventional techniques, e.g., off-site clean up, digging, and chemical treatment; (2) sustainability, because as opposed to conventional techniques, phytotechnologies provide continuous, long-lasting, and “free” (provided by nature) treatment to a wide range of contaminants, e.g., volatile organic compounds (VOC), heavy metals, and petroleum hydrocarbons; and (3) protection from soil erosion [54]. On the other hand, the phytotechnology applications are subject to the following limitations: (1) They can treat only sites with lower contaminant concentrations and contamination in shallow soils and groundwater. (2) The choice of plants is limited by local weather features. (3) Some applications based on plant evapotranspiration may be seasonal and not applicable during the winter period. (4) Some phytotechnology measures, e.g., constructed wetland and tree barriers, require a large surface area of land and may take longer than traditional methods to reach final cleanup levels.

6 Conclusion

A comprehensive understanding of the urban hydrological system is vital for developing sustainable and reliable water management/adaptation practices in urban areas that are vulnerable hotspots of climate change. This chapter provides an overview of the hydrological cycle, including the influence of meteorological, physiographic, and human factors on key aspects of the cycle, as well as water quality, in the urban context. Urbanization, coupled with projected warming air temperatures and more frequent extreme precipitation events due to climate change, is anticipated to impose greater challenges to water resources across multiple water uses including municipal water supply, agricultural demand, energy production, and environmental flow for sustainable ecosystem functioning. We discuss the common and emerging remediation practices for accommodating the potential changes in urban hydrological systems. Lastly, it should be emphasized that a systematic and interdisciplinary approach is required to study the urban hydrological system.