Metadata:
  Identification_Information:
 
    Citation:
      Citation_Information:
        Originator: Alan Rea and Robert L. Tortorelli
        Publication_Date: 1999
        Title: Precipitation depth-duration-frequency surfaces for Oklahoma 
        Edition: Version 1.0, July 23, 1999
        Geospatial_Data_Presentation_Form: map
        Series_Information:
          Series_Name: USGS Open-File Report
          Issue_Identification: USGS OFR 99-463
        Publication_Information:
          Publication_Place: Oklahoma City, Oklahoma
          Publisher: U.S. Geological Survey
        Online_Linkage:
          <URL:http://water.usgs.gov/lookup/get?OFR99-463>
 
    Description:
 
      Abstract:
 
        These geospatial data sets consist of surface grids
        of precipitation depths for seven frequencies at 12
        durations used in a regional precipitation frequency analysis
        for Oklahoma. Eighty-four depth-duration-frequency surfaces
        were interpolated from precipitation-station data. The grid-cell
        spacing is 2,000 meters. Each depth-duration-frequency surface
        was interpolated using the same interpolation function and
        parameters. 

        Data were used from precipitation gage stations with at least 10
        years of record within Oklahoma and about 50 kilometers into
        bordering states. Data were analyzed for seven frequencies
        (expressed as recurrence intervals of 2-, 5-, 10-, 25-, 50-, 100-,
        and 500-years) and 12 durations (15-, 30-, and 60-minutes; 1-,
        2-, 3-, 6-, 12-, and 24-hours; and 1-, 3-, and 7-days). 

        Statistical methods were used to estimate precipitation depths
        for each duration-frequency at each station. These station
        depth-duration-frequency estimates were interpolated to produce
        continuous grids with grid-cell spacing of 2,000 meters. Contour
        lines derived from these surfaces (grids) were used to produce
        the maps in the "Depth-Duration Frequency of Precipitation for
        Oklahoma," by R.L. Tortorelli, Alan Rea, and W.H. Asquith, U.S.
        Geological Survey Water-Resources Investigations Report 99-4232. 

        These geospatial data sets may be used to determine an
        interpolated value of depth-duration-frequency of precipitation
        for any point in Oklahoma.

      Purpose:
 
        These data sets were used to produce the maps in the
        Depth-Duration Frequency of Precipitation for Oklahoma
        (Tortorelli, Rea, and Asquith, 1999). 
 
      Supplemental_Information:
 
        L-moment statistical methods were used to estimate
        depth-duration-frequency relations for each precipitation
        station. See Tortorelli, Rea, and Asquith, (1999) for discussion
        of the statistical methods. 

        L-coefficient of variation and L-skew were averaged for each
        station with its four nearest neighbors. The values were
        weighted by the number of years of record at each station. The
        means were not averaged. The IDW function in ARC/INFO was used
        with an exponent of zero to average the values for each point
        and its four nearest neighbors. The effect was to spatially
        smooth the frequency distribution parameters while leaving the
        mean values unchanged. The frequency distribution parameters
        were used to calculate the depth-duration-frequency (DDF)
        relation for each precipitation station. 

        The ARC/INFO GRID function, POINTINTERP (ESRI, 1997) was used to
        interpolate surface grids of each DDF on a 2-km cell size. The
        exponential smoothing option was used for POINTINTERP, with a
        decay radius of 50 km and a neighborhood radius of 100 km. The
        POINTINTERP function calculates distance-weighted averages for
        each grid cell of all observed values within the neighborhood
        radius of 100 km. The weight function is an exponential decay
        function that goes to 0 at the neighborhood radius of 100 km. 

        Length-of-record weighting for the grid interpolation was
        accomplished by multiplying the DDF values by the number of
        years of record for each station, then interpolating a surface
        of the multiplied values using the POINTINTERP function. Then
        POINTINTERP was used to interpolate a surface of the years of
        record using the same decay and neighborhood parameters. Then
        for each grid cell the values of the first surface were
        divided by the values of the second surface. This had the
        effect of weighting the final surface by the period of
        record. The result of this length-of-record weighting is that
        the final interpolated surface is closer to the observed
        values at stations having longer periods of record than at
        stations with shorter periods of record. 
 
        After interpolation the DDF grids were masked to include just
        the area of Oklahoma plus a 20-km buffer. This was done to (1)
        minimize the effects of poorer interpolations near the edges
        of the data (at 50 km outside Oklahoma), and (2) assist in
        interpolating depth values at the State border. Contour
        lines were then derived from each surface, choosing an
        appropriate contour interval. It is intended that the maps be
        used to estimate the precipitation depth values within the
        State. 
 
        The DDF grids were checked to ensure that the DDF values
        increase monotonically (are continuously increasing) with
        increasing duration and increasing frequency. For example,
        (1) within the 12-hour duration, the 100-year DDF is larger
        than the 50-year DDF for all grid cells, and (2) within the
        100-year frequency, the 24-hour DDF is larger than the 12-hour
        DDF for all grid cells. Only a few very small deviations from
        monotonically increasing DDFs were observed. All were near the
        edges of the grids at the 500-year frequency at durations
        6-hours and above and all were outside Oklahoma.   
 
        An excerpt from the Arc Macro Language (AML) program used to
        interpolate the surfaces follows:

        >/* Add items and calc products for period-of-record weighting
        >tables
        >additem sites%dur%.pat porw2yr 9 9 n 4
        >additem sites%dur%.pat porw5yr 9 9 n 4
        >additem sites%dur%.pat porw10yr 9 9 n 4
        >additem sites%dur%.pat porw25yr 9 9 n 4
        >additem sites%dur%.pat porw50yr 9 9 n 4
        >additem sites%dur%.pat porw100yr 9 9 n 4
        >additem sites%dur%.pat porw500yr 9 9 n 4
        >sel  sites%dur%.pat
        >calc porw2yr = yrsrec * freq2yr
        >calc porw5yr = yrsrec * freq5yr
        >calc porw10yr = yrsrec * freq10yr
        >calc porw25yr = yrsrec * freq25yr
        >calc porw50yr = yrsrec * freq50yr
        >calc porw100yr = yrsrec * freq100yr
        >calc porw500yr = yrsrec * freq500yr
        >quit
        >
        >/* run pointinterp() to make Depth-Duration-Frequency surfaces
        >/* First set cell size and snap to snap grid
        >
        >grid
        >setwindow -700000 1120000 190000 1640000
        >setcell 2000
        >
        >ayrs = pointinterp ( sites%dur%, yrsrec, #, EXP_SMOOTH, 50000, 0, 100000 )
        >
        >&do freq &list 2yr 5yr 10yr 25yr 50yr 100yr 500yr
        >
        >porw%freq% = pointinterp(sites%dur%,porw%freq%,#,EXP_SMOOTH,50000,0,100000)
        >setmask /proj/temp1/okrain/sitemaps/okbuff20g
        >surf%freq% = porw%freq% / ayrs
        >setmask off
        >
        >&end /* of do-freq-list

        Related Data sets--

        The precipitation stations data sets and contour data sets
        derived from each surface are being published with these data
        sets. 

        Other References Cited--

        Asquith, W.H., 1998, Depth-duration frequency of precipitation
        for Texas: U.S. Geological Survey Water-Resources Investigations
        Report 98-4044, 107 p. 

        Environmental Systems Research Institute, Inc. (ESRI), 1997,
        ARC/INFO Command Reference, ARC/INFO Version 7.1.1 on-line
        help: Redlands, CA. 

        Tortorelli, R.L., Rea, Alan, and Asquith, W.H., 1999,
        Depth-duration frequency of precipitation for Oklahoma:
        U.S. Geological Survey Water-Resources Investigation Report
        (WRIR) 99-4232, 113 p.

        Notes--

        Any use of trade, product, or firm names is for descriptive purposes
        only and does not imply endorsement by the
        U.S. Government.
 
        Although this Federal Geographic Data Committee-compliant metadata
        file is intended to document the data set in nonproprietary form,
        as well as in ARC/INFO format, this metadata file may include some
        ARC/INFO-specific terminology.
 
    Time_Period_of_Content:
      Time_Period_Information:
        Single_Date/Time:
          Calendar_Date: 1999
      Currentness_Reference: Publication date
 
    Status:
      Progress: Complete
      Maintenance_and_Update_Frequency: None Planned
 
    Spatial_Domain:
      Bounding_Coordinates:
        West_Bounding_Coordinate: -104.00654778
        East_Bounding_Coordinate: -93.82201908
        North_Bounding_Coordinate: 37.77692744
        South_Bounding_Coordinate: 32.90086456
 
    Keywords:
      Theme:
        Theme_Keyword_Thesaurus: none
        Theme_Keyword: Precipitation Frequency Analysis
        Theme_Keyword: Precipitation Intensity
      Place:
        Place_Keyword_Thesaurus: none
        Place_Keyword: Oklahoma

    Access_Constraints: none
 
    Use_Constraints:
      These data are designed for regional analyses appropriate to the
      2,000-meter grid cell size.
 
    Point_of_Contact:
      Contact_Information:
        Contact_Person_Primary:
          Contact_Person: Alan Rea
          Contact_Organization: U.S. Geological Survey
        Contact_Position: Hydrologist
        Contact_Address:
          Address_Type: mailing and physical address
          Address: 230 Collins Road
          City: Boise
          State_or_Province: Idaho
          Postal_Code: 83702
          Country: USA
        Contact_Voice_Telephone: (208)387-1323
        Contact_Facsimile_Telephone: (208)387-1372
        Contact_Electronic_Mail_Address: (ahrea@usgs.gov)
 
    Data_Set_Credit:
      This data set was created under a cooperative agreement with the
      Oklahoma Department of Transportation.
 
    Native_Data_Set_Environment:
      SunOS, 5.5.1, sun4u UNIX
      ARC/INFO version 7.2.1
 
  Data_Quality_Information:
    Attribute_Accuracy:
      Attribute_Accuracy_Report:
        The error associated with each surface was analyzed using
        methods described by Asquith (1998). The analysis was only
        conducted for stations within Oklahoma (statewide). The error
        associated with a station is defined as

        >                        E(i) = X(i) - S(i) 
        >
        >where
        >
        >E(i) = error associated with precipitation depth for a duration
        >       frequency at station i;
        >
        >X(i) = value of the precipitation depth for a duration-frequency
        >       at the station (control point);
        >and 
        >S(i) = value of the precipitation depth for a duration-frequency
        >       at the station from the surface.

        The error defined is only indicative of the true error because
        each station was used in the surface development. An
        independent means to measure error is not available. 

        The error analysis results for precipitation depth are
        listed in the file "table1.htm" accompanying this metadata. The
        table shows the comparison of the statewide mean, the standard
        deviation of differences between the statewide mean and each
        control point (station) value, and the coefficient of variation.
        The mean surface (contour) error, mean absolute surface
        (contour) error, maximum and minimum surface errors, root mean
        square error, and percent change from standard deviation to root
        mean square error for each depth-duration frequency also are
        listed. Each statistic is a weighted value, except for the
        maximum and minimum surface errors, which means the length of
        record of each station was considered in the computation.
        Maximum and minimum surface errors are listed to show the range
        of errors found in each depth-duration frequency of
        precipitation surface. 

        The weighted mean surface error is the mean difference between
        values of the control points and values from the surface. Mean
        surface errors near zero are desirable, because this indicates
        how well the interpolation routine worked. This error reflects
        any potential bias in the surface. 

        Root mean square error (RMSE) is analogous to the standard
        deviation (SD), and, therefore, the RMSE is comparable to the SD
        of the control points (differences between the statewide mean
        value and individual station values). The percent change from SD
        to RMSE, 100*[(RMSE  SD) / SD], indicates the improvement in
        the precipitation depth estimate by using the surfaces (or
        contour maps) rather than simply using the corresponding
        statewide mean. If the percent decrease from the statewide
        standard deviation to the root mean square error is not at least
        15 percent (Asquith, 1998), then a contour map is not better
        than a single statewide mean. 

        Percent changes from standard deviation to root mean square
        error for the (1) 15-minute to 60-minute durations ranged
        between -34.7 to -52.0 percent; (2) 1-hour to 24-hour durations
        ranged between -42.1 and -70.4 percent; and (3) 1-day and 7-day
        durations ranged between -51.8 and -70.3 percent. Therefore, the
        use of the surfaces (contour maps) results in more accurate
        estimates of the precipitation depth than simply using a
        statewide mean. 

    Logical_Consistency_Report:
      Not applicable for raster data.
 
    Completeness_Report: 
      For more information see Tortorelli, Rea, and Asquith, 1999.
 
    Positional_Accuracy:
      Horizontal_Positional_Accuracy:
        Horizontal_Positional_Accuracy_Report:
          Locations of weather stations used in the depth-duration
          frequency analyses were recorded to the nearest 1 minute of
          latitude and longitude. 

    Lineage:
      Process_Step:
        Process_Description:
          First draft of metadata created by ahrea using
          FGDCMETA.AML ver. 1.33 07/15/99 on ARC/INFO data set
          /proj/temp1/okrain/cdproto/arc_data/7day/surf500y
        Process_Date: 19990723
 
  Spatial_Data_Organization_Information:
    Direct_Spatial_Reference_Method: Raster
    Raster_Object_Information:
      Raster_Object_Type: Grid Cell
      Row_Count: 260
      Column_Count: 445
 
  Spatial_Reference_Information:
    Horizontal_Coordinate_System_Definition:
      Planar:
        Map_Projection:
          Map_Projection_Name:           Albers Conical Equal Area
          Albers_Conical_Equal_Area:
            Standard_Parallel:             29.5
            Standard_Parallel:             45.5
            Longitude_of_Central_Meridian: -96
            Latitude_of_Projection_Origin:             23
            False_Easting: 0.00000
            False_Northing: 0.00000
        Planar_Coordinate_Information:
          Planar_Coordinate_Encoding_Method: coordinate pair
          Coordinate_Representation:
            Abscissa_Resolution: 2000.0
            Ordinate_Resolution: 2000.0
          Planar_Distance_Units: Meters
      Geodetic_Model:
        Horizontal_Datum_Name: North American Datum of 1983
        Ellipsoid_Name: GRS1980
        Semi-major_Axis: 6378206.4
        Denominator_of_Flattening_Ratio: 294.98
 
  Entity_and_Attribute_Information:
    Overview_Description:
      Entity_and_Attribute_Overview:
 
        Each depth-duration-frequency surface is identified by its
        name and position in the directory structure. For example, the
        SURF100Y data set in the 7day subdirectory contains
        precipitation depths for the 100-year 7-day precipitation
        event. All depths are in inches.

      Entity_and_Attribute_Detail_Citation: none
 
  Distribution_Information:
    Distributor:
      Contact_Information:
        Contact_Organization_Primary:
          Contact_Organization: U.S. Geological Survey
        Contact_Position: Water Webserver Team
        Contact_Address:
          Address_Type: mailing and physical address
          Address: 12201 Sunrise Valley Drive, MS 440
          City: Reston
          State_or_Province: VA
          Postal_Code: 20192
          Country: USA
        Contact_Voice_Telephone: (800) 426-9000
        Contact_Instructions: Contact via email
        Contact_Electronic_Mail_Address: h2oteam@usgs.gov
 
    Distribution_Liability:
 
      Although these data have been used by the U.S. Geological Survey,
      U.S. Department of the Interior, no warranty expressed or implied
      is made by the U.S. Geological Survey as to the accuracy of the
      data.
 
      The act of distribution shall not constitute any such warranty,
      and no responsibility is assumed by the U.S. Geological Survey in
      the use of this data, software, or related materials.
 
  Metadata_Reference_Information:
    Metadata_Date: 19990723
    Metadata_Review_Date: 19990901
    Metadata_Contact:
      Contact_Information:
        Contact_Organization_Primary:
          Contact_Organization: U.S. Geological Survey
          Contact_Person: Alan Rea
        Contact_Position: Hydrologist
        Contact_Address:
          Address_Type: mailing and physical address
          Address: 230 Collins Road
          City: Boise
          State_or_Province: Idaho
          Postal_Code: 83702
          Country: USA
        Contact_Voice_Telephone: (208)387-1323
        Contact_Facsimile_Telephone: (208)387-1372
        Contact_Electronic_Mail_Address: (ahrea@usgs.gov)
    Metadata_Standard_Name:
      FGDC Content Standards for Digital Geospatial Metadata ("CSDGM version 2")
    Metadata_Standard_Version: FGDC-STD-001-1998
    Metadata_Access_Constraints: none
    Metadata_Use_Constraints: none