The NRCS PRISM Climate Mapping Project

The Need for a New Technology to Map Climate

The most common lament of climate analysts, hydrologists and many natural resource managers is the lack of climate observations just where they are needed most--often in mountainous regions, or rural areas. An irony of climate measurement is that most observations are made in lower elevations where people live, while the greatest precipitation falls in higher elevations. Spatially representative climate depiction in these mountainous areas using traditional methods produces unsatisfactory results because of the complex nature of climate in those regions. Even in non-mountainous regions, climate may be quite spatially variable, and difficult to estimate. These realities, and the thirst for spatial climate data, provided the impetus for the development of a modeling system capable of producing representative climate fields.

Maps Developed Using PRISM

PRISM (Parameter-elevation Regressions on Independent Slopes Model) was developed by Dr. Christopher Daly of Oregon State University, and is a hybrid statistical-geographic approach to mapping climate. PRISM uses point measurements of climate data and a digital elevation model (DEM, a digital, gridded version of a topographic map) to generate estimates of annual, monthly and event-based climatic elements. These estimates are derived for a horizontal grid, and are compatible for use on Geographic Information Systems (GIS). PRISM is not a static system of equations; rather, it is a coordinated set of rules, decisions and calculations designed to mimic the decision-making process an expert climatologist would invoke when creating a climate map. PRISM was originally developed in 1991 for precipitation estimation, but more recently has been generalized and successfully applied to other climate elements and derived variables, including temperature, snowfall, degree-days (heat units) and frost dates.

Map, GIS Issues

Each PRISM layer is available in a variety of formats to suit user's needs. First, gridded fields of values are derived over the domain of interest using the PRISM system, typically at a horizontal resolution of approximately 4 km. The 4 km horizontal resolution data are then filtered to a 2 km resolution, which are made available to users for both ARC/INFO and GRASS GIS systems in latitude-longitude (GEO) format. Additionally, contour lines (in both GEO and Albers Equal Area (AEA) projections) were produced for each layer. For selected layers, such as mean annual precipitation, an ARC/INFO process has been run to produce GEO and AEA-projected polygons. These polygons are then colorized and result in the relatively smoothed, colored areas on the mean annual precipitation map.

Each printed state mean annual precipitation map is presented in an AEA projection, with a central meridian centered on the reference longitude of each state. Overlays of state and county boundaries, roads and streams were derived from the 1:1 million scale Digital Chart of the World (DCW). Final cartographic production of the hardcopy mean annual precipitation maps was performed at the NRCS National Cartography and Geospatial Center (NCG) in Ft. Worth, Texas.

PRISM Project Structure, and Contact Personnel

Project oversight and funding have been and continue to be provided by the NRCS National Water and Climate Center (NWCC) in Portland, Oregon. General questions about PRISM products for USDA use, and current PRISM projects should be directed to Greg Johnson, PRISM project manager for the NRCS. Contact information is listed on the back of this sheet. Specific requests for additional, hardcopy mean annual precipitation maps should be directed to the NRCS Climate Data Liaison (CDL) in each state.

PRISM Products: On the web, on CD-ROM, and Hardcopy

Much more information about technical aspects of PRISM, as well as the capability to directly download PRISM data layers in a variety of formats can be obtained from the PRISM Internet web page at:

http://www.ocs.orst.edu/prism/prism_new.html

This web site is maintained by Oregon State University's Oregon Climate Service (OCS), which serves as the primary contact center for all PRISM products worldwide. At this site users also may view and download PRISM images (mostly in .gif format) of various maps. OCS maintains a complete listing of appropriate metadata for each layer. Users also can link to this site through an NRCS climate web page (http://www.ftw.nrcs.usda.gov/climate_data.html). Eventually, all PRISM layers will be reposited at NCGC's ftp site.

CD-ROMs containing precipitation layers for all 50 states, in all the formats described above (GEO projection only), will be available from the NCGC in the summer of 1998. These CDs also will provide more complete documentation of the PRISM process, and will include all metadata. The CDs will contain ArcExplorer software, a free data browser for PC's from Environmental Systems Research Institute (ESRI), producers of ARC/INFO. Users should contact the NCGC to order CDs (phone: 800-672-5559; Fax: 817-334-5469; or email: data@ftw.nrcs.usda.gov).

Hardcopy state maps are available to NRCS personnel through their Climate Data Liaison (CDL). In general, a repository of specialized hardcopy maps (i.e. county, watershed, other climate element) will not be kept at any NRCS national, state or local office. In most cases, maps will be prepared by GIS specialists on demand, or for special projects. NCGC is developing procedures to create county mean annual precipitation maps that will be provided to NRCS GIS personnel. Users outside the USDA should contact the Oregon Climate Service at Oregon State University for hardcopy map ordering information (phone 541-737-5705).

How PRISM works--An Example using Mean Annual Precipitation

The PRISM system determines climate at grid cells by calculating linear relationships between the climate element in question (like precipitation) and elevation. The slope of these linear regression lines changes locally with elevation, as dictated by the available point climate data. Each grid cell estimate is then achieved by determining a separate regression function using data from many nearby climate stations. Each station in the multiple regression is weighted based on five factors: Distance, elevation, vertical layer, topographic facet, and coastal proximity. In short, the closer a given station is to a target grid cell in distance and elevation, and the more similar that station is in its climatology to the cell (given by the other three factors), the higher the weight the station will have on the final, predicted value for that cell. A technique within PRISM determines the lowest possible prediction error for the map as a whole (all cells). PRISM typically is configured to predict values approximately every 4 km horizontally.

Mean annual precipitation maps were developed by first using PRISM to calculate mean monthly precipitation layers, and then summing these 12 layers. Each monthly precipitation layer was derived using all available and appropriate climate station data. Station data sources included the National Weather Service Cooperative Observing Network (NWS Coop), the Natural Resources Conservation Service's SNOTEL network, and local, state, regional and Federal networks.

PRISM Contacts

For more information about PRISM and access to these climate data layers and maps users within the USDA should contact:

Jan Curtis, Applied Climatologist
NRCS - National Water & Climate Center
1201 NE Lloyd Blvd., Suite 802
Portland 0R 97232
email: jan.curtis@por.usda.gov
phone: (503)414-3017 fax: (503)414-3101

Users outside the USDA community should contact:

George Taylor
Oregon Climate Service
326 Strand Ag. Hall
Oregon State University
Corvallis OR 97331
email: taylor@oce.orst.edu
phone: (541)737-5705 fax: (541)737-2540

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