The hydrologic cycle within urban areas can be quite different from more natural areas, and surface water runoff is one of the most representative examples of this altered cycle. In urban and suburban areas, water from precipitation and irrigation moves through the topography must faster. Urban areas contain more impervious surface, including sidewalks, roads, and buildings, which results in less groundwater infiltration as it and instead collects and flows along the surface. While urban areas are designed to channel this runoff into storm drains that flow to treatment plants and rivers, during large events, the runoff system may be overwhelmed and cause localized flooding when storm drains are inadequate to carry the amount of water.
In addition to sheer amount of water, storm drains can also become clogged when blocked by debris that collects along the runoff channels. In midtown Sacramento, the middle of a street is often significantly higher than the curb areas, creating a catchment that becomes filled with such litter and debris. The Sacramento Department of Utilities regularly cleans the street in the downtown area (a weekly curse to any street parkers who forget to move their cars), but this debris is also swept away by moving water during large events. Storm drains can quickly become clogged during such times. In contrast, the biogeophysical characteristics of natural areas tend to slow down runoff through infiltration, which is approximately 35% higher in natural areas, uptake, and varied topography with multiple impediments.
Water is a powerful force of change, which can have significant impact on the pervious surfaces in urban areas. The photo below of Sutter Fort in Sacramento shows how during moderate and heavy rain events, water collects and channelizes at a local scale. In some areas, runoff can combine with lack of vegetative cover, heavy foot traffic, and soil characteristics to cause significant erosion. This is often evident near tree roots, which show where past soil levels once stood. This photograph of a tree underneath the Capitol City Freeway in Sacramento shows how disturbed soils with little vegetation loose the capability to keep soil intact. Various scales of plant roots (small and large) help to hold soil together, but constructed areas have often lost the valuable top soil that breeds vegetation. Fast-moving water can collect fine and coarse material, such as the sand outside Sutter Fort, as well as pollutants lying on impervious surfaces. The concept of Low Impact Development (LID) seeks to reduce urban runoff by creating infrastructure that increases infiltration and reduce volume and velocity. Through technologies and design strategies such as pervious road and sidewalk materials, rain gardens, rainwater harvesting, and elimination of gutters, urban areas can better emulate hydrologic cycles in natural areas.
Urban areas also have distinct ecological processes related to solar absorption and photosynthesis. Both urbanized and natural areas show localized variation in climate and structure within a larger habitat or boundary area. In natural areas, this results in variations amongst species, water availability, and nutrients, among others. A good example would be the change in temperature found in the lower part of a small ravine, which can affect the types and numbers of plant species. In the urban setting, biogeophysical characteristics such as topography and soil content combine with the design of built infrastructure to create localized variation. The size and shape of buildings and roadways, for instance, can cause localized differences in the same indicators of temperature, water availability, and nutrients.
Solar absorption is greatly affected by the physical characteristics of the urban structure. For instance, 28th street in Sacramento is shaded at 10AM on a sunny morning by Sutter Hospital during the winter season. The nearby buildings and streets can be significantly affected by this. Solar radiation in the winter can be quite useful in the morning in order to warm cold surfaces, and the shadow cast by a large building may necessitate more heat use. In the summer time, however, this could be beneficial, as cooler temperatures would be more desirable. The orientation of a building, as well as the characteristics of surrounding buildings, significantly influences solar absorption.
The amount of solar radiation absorbed is also affected by the surface composition. In urban areas seeking to reduce heat absorption, painting surfaces light colors increases reflectivity and reduces heat absorption. The effect of this is to prevent later heat radiation from the surfaces during the evening and night hours, part of what is generally referred to as the “urban heat island.” Infrastructure can be engineered to increase or decrease albedo in order to influence the microclimate.
In addition to infrastructure, plants also absorb heat. Plants uptake carbon dioxide and use light in order to stimulate photosynthesis and growth. Trees are very effective at this, but smaller shrubs and grasses also contribute. The role of vegetative cover in absorbing solar radiation is significant. The amount of solar energy absorbed is affected by natural cycles such as the seasons, as well as human-influenced decisions such as landscaping. In the winter in many urban areas, deciduous trees that have lost their leaves uptake less carbon dioxide, causing changes in daily cycles related to air quality.
Urban plants, especially trees, can also greatly increase livability by providing needed shade during hot summer months. This regulates localized temperatures very effectively and, when combined with other vegetation such as grasses or gardens, can reduce re-radiation of solar heat, creating pleasant microclimates that offer refuge from hot temperatures. Many cities around the world without vegetative cover, especially those in lesser-industrialized countries, experience dramatic spikes in heat absorption during the day, and the lack of trees can also contribute to reduced air quality.