Urbanization and water resources
Urbanization and water resources
Done by
Muayad Jumah Ismail Rimawi
05/12/2019
Introduction
Urban water agencies have great difficulties to provide proper water supply access
and sanitation due to rapid urbanization and insufficient capacity and resources (UN
HABITAT 2001).The challenges to meet water demand are much bigger, as compared
to cities in the developing world, due to a much higher urban growth rate and lower
average income .(BISWAS 2006) Urban growth will lead to considerable changes in
irrigated agriculture in the future, in terms of wastewater irrigated area and water
resource availability of existing systems. (Rooijen, 2011) Today, more than half of the
world’s population resides in urban areas, a total projected to reach almost 70%.
(Seto, 2011)
These population concentrations are marked by built-up landscapes that transform
portions of the Earth’s natural surface into impervious surfaces that are rougher in
texture and far more heterogeneous than those in surrounding rural areas. Such
changes have serious ecological and environmental consequences (Grimm et al.,
2008)
The hydrological impacts of urbanization and heat island formation have been of
particular concern. The effect of urbanization on precipitation and the consensus view
is that the key factors are the urban-rural land surface discontinuity and the
concentration of urban aerosols. (Han and Baik, 2008) As one of the world’s largest
metropolises, Beijing has experienced accelerated urban expansion over the past four
decades. The built-up area has increased from 184 km2 to 1350 km2 between 1973
and 2012, with the metropolitan population approaching more than 20 million (Wu,
2012; Yang et al., 2014).
Urbanization introduced myriad new challenges, most notably air quality issues, water
scarcity crises, and urban flooding problems. Severe rainstorms and flood events in
Beijing have become more frequent in recent years. Rapid urbanization also causes
the imbalance between water supply and demand to become even more serious. One
alarming sign is that aridity and water shortages have become increasingly critical.
Recorded observations reveal steadily decreasing precipitation.
Investigated the influences of urban expansion on summer heavy precipitation using
observations and a mesoscale weather/land surface/urban-coupled model and
showed that the urban expansion can alter the water vapor conditions and lead to a
reduction in precipitation. (Zhang et al, 2009)
Miao [2011] analyzed the impacts of urbanization on summer precipitation using the
Weather Research and Forecasting model and concluded that changes in precipitation
depend on the degree of urbanization.
A decrease in mean annual precipitation is observed for Beijing after 1960. Although
the mean annual precipitation exhibits large inter annual variability, it has decreased
overall by almost 32% or Within the Beijing metropolis, annual precipitation has
significantly decreased from 1950 to 2012 (by almost 32%). (Song and others, 2014)
Figure 1: (Song and others, 2014) .
(a) Percentage contributions of monthly precipitation to the annual precipitation
amount and (b) annual cycle of mean precipitation for different periods over the
Beijing area.
Monthly precipitation amounts and their percentage shares of annual precipitation
are displayed in Figure.
Results indicate that monthly precipitation patterns do not change significantly. It is
clear that a large proportion of annual precipitation amount occurs in the warm
season (76.4–82.6%), especially during July (27.5–36.8%) and August (21–31.8%).
(Song and others, 2014)
It should also be noted that urban heat island effects also constitute an important
factor. In urbanized areas, sizeable quantities of anthropogenic heat are generated by
human activities (Zhang et al., 2013b).
Moreover, growing energy consumption exacerbates local environmental problems,
as well as reinforcing temperature increases in the urban atmosphere. Furthermore,
the radioactive properties of the urban environment are distinctly different, allowing
the absorption of additional radiation due to the nature of the urban canopy [Aikawa
et al., 2009].
Such changes in the surface heat budget produce atmospheric conditions in urbanized
areas that are quite different from those in rural areas and significantly impact local
air circulation and patterns of precipitation (Huong and Pathirana, 2013).
Zhang et al. [2009] found that urban expansion produces less evaporation, higher
surface temperatures, larger sensible heat fluxes, and a deeper boundary layer, which
leads to less water vapor, more mixing of water vapor in the boundary layer, and
reduces precipitation in Beijing.
In our analysis, the effect of urbanization on precipitation intensity has manifested
itself in a slightly increasing trend in the mean hourly precipitation intensity and
maximum 1 h precipitation intensity in the urban areas. Several other causal factors
are known to exist such as large surface roughness and higher aerosol concentration .
The amount and intensity of precipitation in the plain areas is greater than in the
mountainous areas, and precipitation in the urban areas is relatively greater than in
the suburb areas. Higher precipitation intensity elevates the risk of urban flooding.
The rapid growth of a low density city has not only changed the balance of urban-rural
population but has consumed land which is capable of sustaining rain fed agriculture,
which is a rare commodity in Jordan. Water demand has increased with population
and has required groundwater to be exploited and pumped to Amman.
Water demand has risen both from increased domestic use and the need for irrigation
for agricultural production. The availability of water resources and the demand for its
use, as a component of the interaction between urbanisation and agriculture. ( Higgitt
D.L 2001)
There are eleven principal groundwater basins in Jordan and these have become
important sources in addition to transfer schemes form the Jordan Valley. In the mid
1970s, transfer began from the Azraq Basin (Haddadin, 1996) which partly underlies
the BRDP area. Since 1980, abstraction rates have increased rapidly to an estimated
43 MCM (Dutton, 1998), with 15-20 MCM supplied to Amman as drinking water from
the well field at Azraq. The ecologically important Azraq oasis dried up in 1992 as a
consequence of continued abstraction exceeding recharge. (Higgitt D.L 2001)
Options for improving water supply nationally include further transfer schemes, reuse
of urban waste water for irrigation, use of brackish water and desalinization.
(Higgitt D.L 2001) Possible international water transfers from the Euphrates or
southern Turkey have not proceeded because of the cost. (Higgitt D.L 2001)
The effects of urbanization on water resources can be organized into four categories:
?Water movement (hydrology).
? Stream channel shape and function (fluvial geomorphology)
?Water quality
? Habitat
How does urbanization affect stream hydrology?
? Disrupts natural water balance ( O’Driscoll and others ,2010).
? Increases flood peaks, storm water
? Runoff, and bank full flows
? More frequent flooding
? Lower base flow to streams
? Less water in the stream
Stream hydrology is defined as the study of the movement or flow of water in streams.
Understanding water movement is essential to understanding the impact of
development on urban streams. (Hardison and others,2009)
Figure 2: Source: State of Maryland: Dept. of the Environment.
Water balance is a measure of the amount of water entering and leaving a system.
As rain falls to earth, some of it is infiltrated, absorbed, and evapotranspired, and
some becomes runoff.
As you learned earlier, in a pre-developed setting, much of the rainfall is absorbed by
the surrounding vegetation, soil and ground cover. This diagram shows how
development and its corresponding increase in impervious cover disrupt the natural
water balance. In the post-development setting, the amount of water running off the
site is dramatically increased and the amount of water infiltration is decreased. The
changes in the water balance in urban streams are exemplified by changes in the
volume of runoff, increased flood peaks, increased and earlier peak flows and
frequency of bank full flows, floodplain widening, and decreased dry weather flows.
(Wang and other, 2008)
Flooding occurs when there are excessive runoff volumes. These excessive volumes
are caused by both the total amount of impervious cover as well as the rate at which
the runoff is delivered to the stream. Curbs and gutters, storm drains, storm drain
pipes, ditches, catch basins and other drainage systems quickly speed the runoff to a
storm water detention/retention facility or directly into the nearest water body.(
Leopold,1968)
Curbs and gutters are designed to deliver storm water away from the road surface in
an efficient and timely manner. Catch basins or inlets collect storm water and direct it
through pipes to a downstream storm Figure 1: /retention facility or to the nearest
water body.( Lee,2003) The large amount of runoff entering an urban stream can
frequently result in flooding, such as this moderate overbank flooding event.
Figure 3: water detention.
Figure 4: Source : US Environmental Protection Agency .
This slide illustrates the effect of urbanization on hydrograph peak discharge. A
hydrograph is a graph showing the changes in stream flow with respect to time. During
storms, in pre-developed “natural” conditions (the solid line), the stream flow
gradually increases to a relatively flat, prolonged peak that is about twice the prestorm
flow rate, and gradually descends to a low-flow condition (“gradual
recession”).( Bosch,2003).
During storms, in the urbanized condition (dashed line), the flow rapidly increases to
a peak that occurs earlier in time due to the rapid delivery of water from storm drains
and pavement. You’ll notice that the peak flows are much higher than occurred prior
to development (more than double) which means that flows in the stream will be
much higher than occurred previously, and flooding may increase. The flow then
sharply decreases, often to a low-flow condition that is lower than occurred prior to
development. This means that during dry periods, the flow in streams is decreased
and impacts to water users or aquatic habitat may occur. (Bosch, 2003)
In addition to the increase in flood peaks and peak discharge, the frequency of bank
full flows also increases with increased urbanization. Bank full flows are simply runoff
events that fill the normal channel of a stream to the top of the banks. Bank full flows
are significant because they are the channel-forming flows in streams, and they are
highly erosive, turbid, and damaging to the natural morphology of the stream. (Bosch,
2003)
Figure 5: Decline in stream flow due to diminished groundwater recharge.
An increase in impervious surface often decreases the amount of rainfall available for
infiltration. Without infiltration, the groundwater will not be recharged and the
stream will lose this potential source of water. Thus low flows tend to be lower in
urbanized watersheds. (Leopold, 1968)
How does urbanization affect the stream form and function?
? Stream widening & erosion
? Reduced fish passage
? Degradation of habitat structure
? Decreased channel stability
? Loss of pool-riffle structure needed by fish
Figure 6: channel degradation .
This diagram shows the progression of stream channel degradation. Many functioning
channels are narrow and deep, with overhanging banks. Vegetation shades the water
surface, keeping temperatures cool. Stored ground water sustains grass, shrub, and/or
tree communities. The riparian resources of vegetation, water, and soil are working
together.
As the riparian area loses its vegetation, erosion increases, and the channel becomes
very wide and very shallow before it creates a new floodplain down along the sides of
the channel. (Wolman, 1967)
How does urbanization affect water quality?
Increased stream temperature (thermal stress)
? Increased pollutants
? Increased risk of fishing
? advisories/beach closures
? Increase in costs!
In addition to hydrologic and geomorphologic changes to the stream, urbanization
directly impacts the quality of the receiving water. Some of the indicators of the
impact of urbanization on water quality include increased stream temperature and
pollutants. (Poole and others, 2001)
Stream temperature is a very important habitat parameter for salmon, trout, and
other fish and insects. Temperature variability can dictate the growth of aquatic
insects and timing of migration and molts. Impervious cover increases air and soil
temperatures and can create an increase of 5 to 10 degrees Fahrenheit in urban
streams. (Gardiner and others, 2009)
An increase in pollutants can adversely affect aquatic organisms and can result in
beach closure, safety concerns, and/or activity limitations for people. High levels of
bacteria can create unsafe conditions, leading to warnings and use restrictions.
Another common pollutant in urban storm water is sediment. Sediment can smother
bottom organisms or it can clog gills of fish and aquatic insects when it is in the water
column. Sources of sediment include stream bank erosion, construction sites, and the
runoff from paved surfaces. . (Leant and other, 1994)
All vegetation is not equal! Trees are useful for providing stability, shade, and habitat
benefits. This stream, while it is well-vegetated with grass and has good floodplain
access, could have been planted with a variety of types of vegetation to improve
habitat. Also, the intensive use of turf grass is likely associated with fertilizer and
pesticide inputs. . (Sun t and others, 2005)
The creation of fish barriers is another impact of urban development. Culverts and
dams like this one at Marble Bluff act as barriers to fish migration unless they are
carefully designed. Barriers can prevent the movement of both anadramous
(migrating) and resident fish. In some cases, the fish barriers are created by culverts
that are put in stream crossings for roads and other urban infrastructure. As the
stream erodes down, vertical barriers to fish movement are created that cut off
spawning areas. In many areas of the West Coast, where salmon spawn, fish barriers
are a major biological problem associated with urban development. (Lenat, 1994)
In an enlarged urban stream, habitat structure is lost and flows become shallower,
slow moving and indistinct. The channel enlargement process sharply degrades
stream habitat structure and flows become shallower, slow-moving and indistinct.
High-quality streams have a series of pools, riffles, and glide that provide unique and
stable habitat areas for different fish and aquatic insects. Urban streams are
characterized by highly scattered and poor-quality stream beds. . (Bernhardt and
others, 2007)
Figure 6: Riparian-wetland .
Riparian-wetland areas are functioning properly
When adequate vegetation, landform, or large woody debris is present to dissipate
stream energy associated with high water flows, thereby reducing erosion and
improving water quality; filter sediment, capture bed load, and aid floodplain
development; improve flood-water retention and groundwater recharge; develop
root masses that stabilize stream banks against cutting action; develop diverse
ponding, and channel characteristics to provide the habitat and the water depth,
duration, and temperature necessary for fish production, waterfowl breeding, and
other uses; and support greater biodiversity. The functioning condition of riparianwetland
areas is a result of interaction among geology, soil, water, and vegetation.
(Leopold and other, 2005)
Impervious surfaces vs. runoff Increased:
? Runoff speed
? Bank full flows
? Peak flows
? Volume
? Flood heights Decreased:
? Infiltration
? Base flow
? Groundwater recharge
? Pollutant filtration
? Runoff quality
We’re faced with evidence daily that growth has steadily increased since 1992 in the
Truckee Meadows. Growth is a fact of life, but we must ensure that our development
plans and methods work towards decreasing runoff, promoting infiltration, and
protecting waterways. This will help us meet our ultimate goal - protecting water
quality. (Shepherd, 2005)
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