![]() The map on the right shows the major hydrologic regions in the United States that the USGS derived by using digital elevation models and terrain information. The Tulare basin and the San Joaquin Valley is well defined, but there are other areas like on the Central or North Coast that consist of many, many small watersheds that drain water to the Pacific Ocean. The largest watershed is the Sacramento and San Joaquin River watershed there are parts of the watershed that do reach into Oregon. The map on the left shows the hydrologic regions in the state that more or less fit the hydrologic watershed boundaries that are derived from the terrain. One can use topographic maps, analyze digital elevation data with a geographic information system (GIS), or use established hydrologic unit maps. There are many different ways to define watersheds generally topography is used because water follows gravity and flows downhill. “ What we often do is choose a point such as a stream gauge on the river because this would then allow us to relate measurements of flow that we take at that stream gauge to whatever processes that happen upstream in the watershed.” “ When you want to define a watershed on the landscape, it’s up to you as the user to define the point that you set as the outlet for the watershed,” said Dr. And some of that water will percolate into the ground and accumulate in the groundwater basin. ![]() Some of the rainfall can drain off the land surface and end up in lakes and rivers where it might even flow out to the ocean. When it rains, much of the rainfall is absorbed by the ground or the plants and then evaporates back into the atmosphere. Watersheds are the source of surface water. Smaller watersheds are sometimes referred to as catchments large river basins are sometimes called drainage basins. A watershed can be a small area or as large as the Sacramento-San Joaquin basin. NOAA defines a watershed as the land that channels rainfall or snowmelt to creeks, rivers, streams, lakes, and eventually to outflow points such as surface reservoirs, bays, estuaries, and the ocean.Ī watershed includes all the natural and man-made features, including surface and subsurface features, climate and weather patterns, geologic and topographic history, soils and vegetation, and land use. Historically, watersheds were defined as the divide between two drainage basins that separates the rainfall runoff into one of the basins, but in recent years, the term has been come to be applied to the entirety of the basin and not just the divide. Helen Dahlke at UC Davis as part of the UC Davis Shortcourse on Groundwater and supplemented with additional research. This post is based in part on a lecture given by Dr. This post provides an overview of watersheds, the components of a watershed, water budgets, how to estimate runoff, protecting water quality, and watershed management. ![]() However, managing land and water for selected products or services has altered the conditions and functions in many of California’s watersheds, and so protecting and restoring watersheds is important for maintaining the resources that residents within the watershed depend on. A well-functioning watershed provides clean water for drinking and irrigation, healthy soils, flood protection, habitat for wildlife, and outdoor recreation opportunities. The water flowing over the land and through the soil creates a complex network that connects the uplands, springs, wetlands, streams, and rivers of the watershed. No matter where you are in the world, you are in a watershed.
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