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100 percent. The impervious soils produce surface runoff much greater in amount and rate of flow than that which would have occurred on undisturbed lands. Land slope in a watershed also influences the rate and quantities of runoff. Steep slopes produce higher rates and greater volumes of runoff. Moderate slopes allow infiltration. Wetlands enhance temporary storage and infiltration. The elimination of wetlands will lead to increased flooding and, possibly, retard groundwater recharge. Terracing and contouring will reduce runoff and generally increase infiltration and groundwater recharge. Vegetation influences the amount of water reaching the land surface, the amount lost through transpiration, and the rate of overland flow. Trees, bushes, and grasses tend to trap water on their leaves, reduce the velocity of flow, and deter erosion. Where vegetation is stripped from land surfaces, runoff rates and erosion rates increase appreciably. Although surface water is more visible, its total quantity is small in comparison to that of groundwater, a major source of water supply in this country. It is usually found far below ground surfaces, often several hundred feet. Surface soils such as clays and silts provide little water storage. Water Resources Concerns, and Techniques for Solving or Preventing Problems The nature and degree of public concern relating to water resources is highly dependent upon whose viewpoints are considered. A farmer may be worried that excessive rainfall accumulations on fields and high groundwater tables will inter- fere with plowing and planting; whereas, officials of urban public agencies may view these same conditions as attributes for controlling flood damage to property and replenishing depleted underground municipal water supplies. Because this study is oriented to urban and metropolitan areas, the needs, concerns and viewpoints of people located in such places are basic to this report. These and the social, eco- nomic, environmental and political impacts are the subject of this study. Urban and Metropolitan Concerns and Approaches Conversion, by man, of pervious surface soils to non-pervious, man-made surfaces produces serious and costly problems on the earths surface caused by increased sur~ face runoff quantities and flows. Real-world examples were given in the previous chapter. The "physical" problems can ordinarily be grouped in four categories: (1) flooding, (2) soil erosion, (3) sedimentation, and (4) pollution of lands and water bodies. In places where groundwater constitutes the principal source of muni- cipal water supply, "interference with groundwater supplies" should be added as a fifth type of physical problem. Flooding. On a nationwide basis, flooding and the related impacts are of major concern to people in highly-developed municipalities and urban counties. The approaches available to control flooding and flood damages can be characterized as either "structural" or "non-structural". Structural measures including storage reservoirs, channelization, levees and dikes, have dominated flood control programs in past years. Many persons feel there are better solutions available which would not present the hazards to local water resource ecosystems as do structural solu- tions. Non-structural measures include land treatment (contour plowing, grading, forest management, Etc.), flood-proofing, land use control, and adoption of various standards and requirements for drainage system design and construction to limit peak runoff rates. SoLI Erosion and Sedimentation. Soil erosion and the accompanying sedi- mentation are major concerns in many urbanized areas, particularly where highly- erosive, silty, surface soils are prevalent. Steep topography and sparse or poor 36 I I I I I I I I I I I I I I I I I I I