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Hydrologic Condition Calculations

 

Current Calculations 

May 2012

To set the hydrologic condition for May, an index of conditions is computed using four Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean April streamflow in the Platte River at Grand Island was 1,614 cfs (provisional USGS data), or at the 38.3 percentile level of 1947-1994 April flows.
  2. The April end-of-month content of Lake McConaughy was 1,432,800 acre-feet, equivalent to 82.2% of maximum capacity and 21.6 percentile level of the 1947-1994 April percent-of-capacity.
  3. The April end-of-month content of three upper South Platte reservoirs (Antero, Eleven-Mile, and Cheesman) was 185,509 acre-feet, equivalent to the 67.1 percentile level of their 1947-1994 April contents.
  4. The April 1 percent-of-normal snowpack in the North Platte Basin of Wyoming* (seven specific NRCS locations) was 51.5% of normal, equivalent to the 2.0 percentile level of 1947-1994 conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.601 * 0.383) + (0.271 * 0.216) + (0.031 * 0.671) + (0.252 * 0.020) - 0.065

Hydrologic Conditions Index = 0.250

For this period, the threshold index value is 0.30 for "normal" conditions and 0.70 for "wet" conditions. The current index value is less than 0.30 so constitutes "Dry" conditions.

 

Previous 2011/2012 Calculations (starting with the most recent) 

March and April 2012

To set the hydrologic condition for March-April, an index of conditions is computed using three Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean February streamflow for the Platte River near Grand Island gage (provisional USGS data) was 2,638 cubic feet per second, equivalent to the 77.5 percentile level of 1947-1994 February conditions.
  2. The mean February Palmer Drought Severity Index for four climate zones in three states (NCDC divisions Colorado #4, Wyoming #8, and Nebraska #1 and #7) was 3.31, equivalent to the 94.4 percentile level of 1947-1994 February conditions.
  3. The February end-of-month content of seven North Platte reservoirs (Alcova, Glendo, Gray Reef, Guernsey, Kortes, Pathfinder, and Seminoe) was approximately 2,243,815 acre-feet, equivalent to the 98.0 percentile level of 1947-1994 end-of-month February conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.120 * 0.775) + (0.662 * 0.944) + (0.198 * 0.980) - 0.011

Hydrologic Conditions Index = 0.901

For this period, the threshold index value is 0.25 for "normal" conditions. The current index value is greater than this so constitutes "Normal" conditions.

Note that for the March - April period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period. If ice is present at the Grand Island gage, the Kearney gage streamflow data are used.

 

December 2011 through February 2012

To set the hydrologic condition for December-February, an index of conditions is computed using four Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean November streamflow for the Platte River near Grand Island gage (provisional USGS data) was 3,086 cubic feet per second, equivalent to the 94.4 percentile level of 1947-1994 November conditions.
  2. The November end-of-month content in Lake McConaughy was approximately 1,484,300 acre-feet, equivalent to 85.2 percent of capacity and the 62.9 percentile level of 1947-1994 November end-of-month conditions.
  3. The November end-of-month content of seven North Platte reservoirs (Alcova, Glendo, Gray Reef, Guernsey, Kortes, Pathfinder, and Seminoe) was approximately 2,110,175 acre-feet, equivalent to the 98.0 percentile level of 1947-1994 end-of-month November conditions.
  4. The November end-of-month content of four upper South Platte reservoirs (Antero, Elevenmile, and Cheesman) was approximately 190,447 acre-feet, equivalent to the 86.8 percentile level of 1947-1994 end-of-month November conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = max[(0.579 * 0.944) + (0.138 * 0.629) + (0.317 * 0.980) + (0.236 * 0.868) - 0.129, 0.990]

Hydrologic Conditions Index = 0.990

For this period, the threshold index value is 0.25 for "normal" conditions. The current index value is greater than or equal to this so constitutes "Normal" conditions.

Note that for the December through February period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.

 

October and November 2011

To set the hydrologic condition for October-November, an index of conditions is computed using two Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean September streamflow at the Platte River near Grand Island gage (provisional USGS data) was 4,235 cfs, equivalent to the 95.8 percentile level of 1947-1994 September conditions.
  2. The mean September Palmer Drought Severity Index for four zones in Colorado, Wyoming, and Nebraska (NCDC divisions Colorado #4, Wyoming #8, and Nebraska #1 and #7) was 3.93, equivalent to the 93.8 percentile level of 1947-1994 September conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.658 * 0.958) + (0.342 * 0.938) - 0.048

Hydrologic Conditions Index = 0.903

For this period, the threshold index value is 0.25 for "normal" conditions and 0.67 for "wet" conditions. The current index value is above 0.67 so constitutes "Wet" conditions.

 

Upcoming 2012 Calculations  

 June 2012

To set the hydrologic condition for June, an index of conditions is computed using four Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean May streamflow in the Platte River at Grand Island was X,XXX cfs (provisional USGS data), or at the XX.X percentile level of 1947-1994 April flows.
  2. The mean May Palmer Drought Severity Index for four zones (NCDC climate divisions Colorado #1, Wyoming #8, and Nebraska #1 and #7) was X.XX, equivalent to the XX.X percentile level of 1947-1994 May conditions.
  3. The mean May streamflow in the South Platte River at Julesburg (CO-NE stateline) was XXX cfs (provisional data), equivalent to XX.X percentile of 1947-1994 May flows.
  4. The April 1 percent-of-normal snowpack in the North Platte Basin of Wyoming* (seven specific NRCS locations) was XX.X% of normal, equivalent to the XX.X percentile level of 1947-1994 conditions.

The Index was therefore calculated as follows

Hydrologic Conditions Index = (0.648 * 0.XXX) + (0.121 * 0.XXX) + (0.023 * 0.XXX) + (0.082 * 0.XXX) + 0.097

Hydrologic Conditions Index = 0.XXX

For this period, the threshold index value is 0.30 for "normal" conditions and 0.70 for "wet" conditions. The current index value is above/below 0.70 so constitutes "XXX" conditions.

 

July 2012

To set the hydrologic condition for July, an index of conditions is computed using five Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean June streamflow in the Platte River at Grand Island (USGS provisional data) was X,XXX cfs, or at the XX.X percentile level of 1947-1994 June flows.
  2. The mean June Palmer Drought Severity Index for four zones (NCDC divisions Colorado #1, Wyoming #8, and Nebraska #1 and #7) was X.XX, equivalent to the XX.X percentile level of 1947-1994 June conditions.
  3. The mean June streamflow in the South Platte River at Julesburg (provisional data) was X,XXX cfs, or at the XX.X percentile level of the 1947-1994 June flows.
  4. The Lake McConaughy June end-of-month content was X,XXX,XXX acre-feet or XX.X% of capacity, equivalent to the XX.X percentile level of 1947-1994 June conditions.
  5. The North Platte River Reservoirs in Wyoming June content was X,XXX,XXX acre-feet, or at the XX.X percentile level of 1947-1994 June conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.237 * 0.XXX) + (0.441* 0.XXX) + (0.218 * 0.XXX) + (0.109 * 0.XXX) + (0.105 * 0.XXX) - 0.071

Hydrologic Conditions Index = 0.XXX

For this period, the threshold index value is 0.31 for "normal" conditions. The current index value is greater than/less than this so constitutes "XXX" conditions.

Note that for the July period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.

 

August and September 2012

To set the hydrologic condition for August-September, an index of conditions is computed using two Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean July Palmer Drought Severity Index for two zones in central Nebraska (NCDC divisions Nebraska #5 and #8) was X.XX, equivalent to the XX.X percentile level of 1947-1994 July conditions.
  2. The Lake McConaughy July end-of-month content was X,XXX,XXX acre-feet, equivalent to XX.X% of capacity and the XX.X percentile level of 1947-1994 July conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.464 * 0.XXX) + (0.404 * 0.XXX) + 0.061

Hydrologic Conditions Index = 0.XXX

For this period, the threshold index value is 0.29 for "normal" conditions. The current index value is greater than/less than this so constitutes "XXX" conditions.

Note that for the August through September period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.

 

October and November 2012

To set the hydrologic condition for October-November, an index of conditions is computed using two Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean September streamflow at the Platte River near Grand Island gage (provisional USGS data) was X,XXX cfs, equivalent to the XX.X percentile level of 1947-1994 September conditions.
  2. The mean September Palmer Drought Severity Index for four zones in Colorado, Wyoming, and Nebraska (NCDC divisions Colorado #4, Wyoming #8, and Nebraska #1 and #7) was X.XX, equivalent to the XX.X percentile level of 1947-1994 September conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.658 * 0.XXX) + (0.342 * 0.XXX) - 0.048

Hydrologic Conditions Index = 0.XXX

For this period, the threshold index value is 0.25 for "normal" conditions and 0.67 for "wet" conditions. The current index value is above/below 0.67 so constitutes "XXX" conditions.

 

December 2012 through February 2013

To set the hydrologic condition for December-February, an index of conditions is computed using four Platte basin variables. These variables and their corresponding values were as follows:

  1. The mean November streamflow for the Platte River near Grand Island gage (provisional USGS data) was X,XXX cubic feet per second, equivalent to the XX.X percentile level of 1947-1994 November conditions.
  2. The November end-of-month content in Lake McConaughy was approximately X,XXX,XXX acre-feet, equivalent to XX.X percent of capacity and the XX.X percentile level of 1947-1994 November end-of-month conditions.
  3. The November end-of-month content of seven North Platte reservoirs (Alcova, Glendo, Gray Reef, Guernsey, Kortes, Pathfinder, and Seminoe) was approximately X,XXX,XXX acre-feet, estimated to the XX.X percentile level of 1947-1994 end-of-month November conditions.
  4. The November end-of-month content of four upper South Platte reservoirs (Antero, Elevenmile, and Cheesman) was approximately XXX,XXX acre-feet, estimated to the XX.X percentile level of 1947-1994 end-of-month November conditions.

The Index was therefore calculated as follows:

Hydrologic Conditions Index = (0.579 * 0.XXX) + (0.138 * 0.XXX) + (0.317 * 0.XXX) + (0.236 * 0.XXX) - 0.129

Hydrologic Conditions Index = 0.XXX

For this period, the threshold index value is 0.25 for "normal" conditions. The current index value is greater than or equal to/less than this so constitutes "XXX" conditions.

Note that for the December through February period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.

 

* Both May and June calculations use April 1 percent-of-normal snowpack in the North Platte Basin of Wyoming.  However, percentiles may vary between these months for the same snowpack value because several months with unusually high localized precipitation events were removed from the period of record used in calculations (eight months were removed for May calculations and nine months were removed for June calculations).  This is described in more detail in Characterization of Hydrologic Conditions to Support Platte River Species Recovery Efforts, Journal of the American Water Resources Association, 42(5):1391-1403.

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