8: Water, Flooding, and Drought
- Page ID
- 34062
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This chapter addresses the basics behind where water is naturally stored on Earth, namely streams and the ground. It also talks about what happens when streams get full - it floods. Understanding what natural flooding entails helps to understand how humans increase the size and frequency of flood events. And finally, related to water is also the concept of drought - or a lack of water. This also happens naturally but can be exasperated by human activity.
- 8.1: The Hydrologic Cycle
- Water is constantly being moved from one place to another on Earth, and that motion is powered by the sun and gravity. The important reservoirs of the cycle are the oceans, glaciers, groundwater, lakes and streams and the atmosphere. We can extract water from the hydrologic cycle for our use, but we must be careful to ensure that has only minimal effects on other parts of the cycle.
- 8.2: Streams and Stream Flow
- Water flowing through a channel at surface is a stream, and a drainage basin is the area within which all of the water eventually flows into a main stream. Stream flow varies through a year depending on rates of evapotranspiration, precipitation and melting. It is important to understand those variations if we intend to use a stream as a water source.
- 8.3: Groundwater
- The water stored within the materials beneath us is groundwater. It represents a much larger source of water than surface water, and it forms the basis of the hydrologic cycle. An aquifer is a body of rock or sediment that is porous–so that it has some room for water storage–and permeable–so that water will flow through it. The rate of that flow will depend on the hydraulic gradient and the permeability. There is continuous exchange between surface water and groundwater.
- 8.4: Natural Effects on Water Quality
- Groundwater is in close contact with rock or sediment, and can naturally be chemically affected by those materials. Some of the problematic issues with water chemistry include elevated levels of hardness, iron, fluoride and arsenic. In areas near to the ocean groundwater can also become contaminated with salt. One of the key issues with surface water is an elevated level of suspended matter, which can be a problem as it inhibits disinfection measures.
- 8.5: Flooding
- Most flooding results from heavy rainfall. Flooding also results from rapid snow melt, and in some cases rain and snowmelt are coincident. To understand flooding we need to understand overland flow (water flowing over the surface) and base-flow (from groundwater discharge). Infiltration of water into the ground is one of the keys to reducing flood potential, but most of what we do in urban areas results in hardened surfaces that prevent infiltration.
- 8.6: Drought
- Drought is a period of reduced rainfall and/or snowfall that results in water-related problems. While precipitation (rainfall or snowfall) at a particular location varies from year to year, the average amount is fairly constant over a period of years. This relates to the difference between weather which describes conditions at a point in time versus climate which is an average of those conditions.