California Interagency Watershed Mapping
Committee
|
|
CalWater Frequently Asked Questions -- FAQs
1. Is CalWater a "true" hydrologic dataset, following ridgelines?CalWater 2.0 and 2.2 are not true hydrologic datasets following ridgelines. However, this is only because the original digital linework used to create these maps was not. The intent of the IWMC is to continue to edit this linework and make it a more accurate reflection of surface water drainage. The role of the IWMC is to guide this process. It is a work in progress. In late 1995 the USFS, USGS and NRCS presented the IWMC group with the NRCS NI 170-304 standards. The committee agreed to include these guidelines in the CalWater mapping standards, and the three Federal agencies agreed to support this State and Federal interagency mapping effort. An MOU was drafted in March 1997, formalizing this agreement. However, the current CalWater metadata states: The California Watershed Map (CalWater version 2.2) is a set of standardized watershed boundaries meeting standardized delineation criteria. The following are subjective comments regarding this data: CalWater boundaries were digitized on a 1:24,000-scale base and thus very accurately divide surface water features depicted on 1:100,000-scale Digital Line Graph hydrography. However, CalWater delineations are primarily designed to be administrative reporting units, and the boundaries should not be used to define authoritative drainage area above a given point as a portion of their definition includes non-physical boundaries, particularly in valley floor and urbanized coastal regions. Attribute completeness is good. Compatibility with existing state and federal watershed delineations is good, except where explicitly different boundary configurations are applied. Over the last several years many federal and state agencies have realized current 8-digit hydrologic unit (HU) maps are unsatisfactory for many purposes, because of inadequate bases or scales. Because of this, the NRCS has continued to work with other federal and state agencies and with the Subcommittee on Spatial Water Data Federal Geographic Data Committee (FGDC) to establish a Federal interagency standard covering mapping and delineation of hydrologic units that would be suitable for all agencies. In cooperation with the FGDC and the Advisory Committee on Water Information (ACWI), a new interagency guideline has been written. During December of 2000, this document was presented to the FGDC for their review. Under the new guidelines, hydrologic unit boundaries define the areal extent of surface water drainage to a point. The goal of this initiative is to provide a hydrologically correct, seamless and consistent national Geographic Information System (GIS) database at a scale of 1:24,000, that has been extensively reviewed and matches the USGS topographical 7.5 minute quads. The new levels are called watershed (5th level, 10-digit) and subwatershed (6th level, 12-digit). The watershed level is typically 40,000 to 250,000 acres and subwatershed level is typically 10,000 to 40,000 acres with some as small as 3,000 acres. An estimated 22,000 watersheds and 160,000 subwatersheds will be mapped to the 5th and 6th level. The GIS coverages will be available by the Internet to any person, including federal, state, local government agencies, researchers, private companies, utilities, environmental groups, and concerned citizens. The database will assist in planning and describing water use and related land use activities. So in terms of CalWater 2.2, the final answer is that the current data does not meet these standards, but they will soon, through the efforts and guidance of the IWMC, locally held delineation workshops, and the new National (FGDC) Watershed Mapping Standards we develop a more accurate CalWater Version 3.0! If you are currently trying to work with CalWater 2.2 as the "best available data" please see "Working with CalWater 2.2" for more information. 2. What is the difference between CalWater 2.0 and 2.2?Version 1.2 was completed in 1995 by Tierra Data Systems (Jim Kellogg). Linework was captured by overlaying the Basin Plan Maps on 1:24,000-scale USGS quad sheets, redrawing and digitizing lines to match 1:24,000-scale watershed boundaries, and subdividing the 4th level Hydrologic Sub areas (HSAs) into 5th level Super Planning Watersheds (SPWS) and 6th level Planning Watersheds (PWS). Version 2.0 called for the removal of the 5th level Super Planning Watersheds and 6th level Planning Watersheds, introduction of the groundwater line around the Central and Salinas valleys, and was subject to an extensive cooperative planning and review effort by the California Interagency Watershed Mapping Committee (IWMC). CalWater version 2.2 is the third version of CalWater (after versions 1.2 and 2.0). CalWater 2.2 adds the 5th and 6th level watershed units back in, incorporates the State Water Resources Control Boards watershed system, and contains the crosswalk to the USGS system of hydrologic unit codes (HUCs). The watershed name fields were also fully populated. Work on CalWater version 2.5 was suspended when work on the WBD (Watershed Boundary Dataset) began, but was intended to add the Federal Hydrologic Unit codes for the 5th field (10 digit) and 6th field (12 digit) extensions of the USGS 8 digit HUCs, with detailed editing in the National Forests. The DWR's DAU (digital accounting unit) linework are being added to the Central Valley area. 3. What do the "codes" mean?
[**NB: The NRCS standards require three digits in the 5th and 6th fields (called 11-digit and 14-digit by the NRCS literature), while the other Federal agencies continue with 2 digits structure creating 10-digit and 12-digit codes. In California, the NRCS will use the 2 digit coding system, but create a separate 3 digit number column with the 2 digit code, plus a following zero.] 4. How do the State and Federal system compare?The state HAS (Hydrologic Sub-Area) approximates the USGS 4th field, 8 digit HUC "Cataloguing Unit" The state SPW (Super Planning Watersheds) approximate the USGS 5th field, 10 digit "Watersheds". 5. Why are there duplicate names?In both the State and Federal system, the numbers are unique identifiers (each number only occurs once for a given unit), while many streams may share the same name. For example, in California their are many "Redwood," "Deer," and "Clear" creeks. 6. What is the HUC crosswalk?CalWater maps fairly neatly to the federal Hydrologic Unit Codes (HUCS). Generally CalWater watersheds fit within HUCs, though there are some exceptions. The IDNUM to HUC cross-reference was developed with the following rules: For a given [state] CalWater watershed, overlapping [federal] Hydrologic Unit Codes (at the 8-digit Cataloging Unit [CU] level) are listed up to a maximum of 80% of the CalWater watershed OR three CUs, whichever comes first. For example: Where there is only one CU code listed for a given CalWater watershed code, that CU overlaps at least 80% of the CalWater watershed. Where there are two CU codes (CU and CU2) listed for a given CalWater code (only 18 watersheds fit this criterion), the largest CU alone overlaps less than 80% of the CalWater watershed, but combining CU2 then provides overlap of at least 80% of the CalWater watershed. Where there are three CU codes (CU, CU2, and CU3) listed for a given CalWater watershed (only 1 watershed fits this criterion), the largest 2 CUs together overlap less than 80% of the CalWater watershed; combining a third CU code produces additional overlap. Complete overlap statistics are left for calculation by the user. If a watershed consists of multiple polygons (because it is split by the groundwater line, the state boundary, or it is comprised of multiple islands), the total area of the watershed (the sum of all the polygons) is the value entered in the ACRES item. The individual polygons of a multi-polygon watershed all have the same watershed code. These cases are itemized below: Legitimate Duplicate Codes (Multiple Polygons for a Single Watershed)
From the CalWater 2.2 Metadata. 7. Common Federal TermsCataloging UnitThe 4th level (and smallest in typical area) of drainage delineation of the 1974 Water Resources Council / USGS hierarchy of national HU's. This HU is coded with 8-digits. Minimum size is about 700 sq. miles (448,000 acres). Hydrologic Unit (HU)An area of land upstream from a specific point on a stream (designated as the mouth or outlet), which is defined by a hydrologic boundary that includes all of the source area that could contribute surface water runoff directly and indirectly to the designated outlet point. The HU may also contain associated surface areas such as related contributing drainage, non-contributing, and trans-basin diverted areas to form a landform associated with the certain designated outlet point. It may also have one or more of the same level hydrologic units completely contained within the upstream drainage area. Earlier versions of the Watershed and Subwatershed delineations may have utilized administrative boundaries such as state and county or project lines to define HU's. The HU boundaries must be determined solely upon hydrologic principles to obtain a homogeneous national seamless digital data layer. Hydrologic Unit Code (HUC)The identifier of a specific HU. It is determined by an established system of assigning numbers and alpha characters in a specified order and in different fields of a composite code (Watershed and Subwatershed, 10/11 and 12/14) that represents specific subdivisions of the nations drainage at different levels of detail or size. River BasinA drainage unit defined by the hydrologic boundary that feeds a river. This is not a standard HU designation. SubwatershedA delineated HU depicting the location of a drainage area that is typically 10,000 to 40,000 acres in size. The 6th division level of the nation's drainage. Represented by extending the 11-digit HUC to 14-digits. WatershedThe 5th level of HU delineation. Continues the 1974 Water Resources Council /
USGS fourth level hierarchy of HU's to the next smaller useful size. This is
represented by extending the 8-digit HUC to 11-digits. Typical size is 40,000 to
250,00 acres. |
|
|
