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Hydrologic Condition Calculations
 
Current Calculations
December 2016 through February 2017
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 2,087 cubic feet per second, equivalent to the 91.1 percentile level of 1947-1994 November conditions.
  2. The November end-of-month content in Lake McConaughy was approximately 1,489,800 acre-feet, equivalent to 85.5 percent of capacity and the 64.6 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 2,064,305 acre-feet, equivalent to the 95.7 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 179,482 acre-feet, equivalent to the 64.8 percentile level of 1947-1994 end-of-month November conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = min[(0.579 * 0.911) + (0.138 * 0.646) + (0.317 * 0.957) + (0.236 * 0.648) - 0.129, 0.990]
Hydrologic Conditions Index = 0.944
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 December through February period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.
 
Previous 2016/2017 Calculations (starting with the most recent)
October and November 2016
​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 1,804 cfs, equivalent to the 87.5 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 2.173, estimated as the 78.2 percentile level of 1947-1994 September conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.658 * 0.875) + (0.342 * 0.78.2) - 0.048
Hydrologic Conditions Index = 0.795
For this period, the threshold index value is 0.25 for "normal" conditions and 0.67 for "wet" conditions. The current index value is greater than 0.25 and less than 0.67 and constitutes "WET" conditions.

Upcoming 2017 Calculations
March and April 2017
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 ____ cubic feet per second, equivalent to the ___ 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 ___, equivalent to the ___ 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 _________ acre-feet, equivalent to the ___ percentile level of 1947-1994 end-of-month February conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.120 * ___) + (0.662 * ___) + (0.198 * ___) - 0.011
Hydrologic Conditions Index = ____
For this period, the threshold index value is 0.25 for "normal" conditions. The current index value is less than/greater than this and constitutes "???" 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.

May 2017
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 ____ cfs (provisional USGS data), or at the ___ percentile level of 1947-1994 April flows.
  2. The April end-of-month content of Lake McConaughy was ________ acre-feet, equivalent to ___% of maximum capacity and ___ 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 ______ acre-feet, equivalent to the ___ 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 ____% of normal, equivalent to the ____ percentile level of 1947-1994 conditions.
The Index was therefore calculated as follows: 
Hydrologic Conditions Index = (0.601 * ___) + (0.271 * ___) + (0.031 * ___) + (0.252 * ___) - 0.065
Hydrologic Conditions Index ____ 
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/greater than 0.30 and less than 0.70 and constitutes "???" condition.

June 2017
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 ____ cfs (provisional USGS data), or at the ___ 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 ___ equivalent to the ___ percentile level of 1947-1994 May conditions.
  3. The mean May streamflow in the South Platte River at Julesburg (CO-NE stateline) was ____ cfs (provisional data), equivalent to ___ 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 ____% of normal, equivalent to the ___ percentile level of 1947-1994 conditions. 
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.648 * ___) + (0.121 * ___) + (0.023 * ___) + (0.082 * ___) + 0.097
Hydrologic Conditions Index ___
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/greater than 0.30 and less than/greater than 0.70 and constitutes "???" conditions.

July 2017
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 ___ cfs, or at the ___ 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 ___, equivalent to the ___ percentile level of 1947-1994 June conditions.
  3. The mean June streamflow in the South Platte River at Julesburg (provisional data) was ____ cfs, or at the ___ percentile level of the 1947-1994 June flows.
  4. The Lake McConaughy June end-of-month content was ________ acre-feet or ___% of capacity, equivalent to the ___ percentile level of 1947-1994 June conditions.
  5. The North Platte River Reservoirs in Wyoming June content was ______ acre-feet, or at the ___ percentile level of 1947-1994 June conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.237 * ___) + (0.441* ___) + (0.218 * ___) + (0.109 * ___) + (0.105 * ___) - 0.071
Hydrologic Conditions Index = ___
For this period, the threshold index value is 0.31 for "normal" conditions. The current index value is less than/greater than this and constitutes "???" 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 2017
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 ___, equivalent to the ___ percentile level of 1947-1994 July conditions.
  2. The Lake McConaughy July end-of-month content was ______ acre-feet, equivalent to ___% of capacity and the ___ percentile level of 1947-1994 July conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.464 * ___) + (0.404 * ___) + 0.061
Hydrologic Conditions Index = ___
For this period, the threshold index value is 0.29 for "normal" conditions. The current index value is less than/greater than this and constitutes "???" conditions. 
 
Note that for the August-September period, there are only "dry" and "normal" designations because "wet" targets are the same as "normal" targets for this period.

October and November 2017
​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 ____ cfs, equivalent to the ___ 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 ___, estimated as the ___ percentile level of 1947-1994 September conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.658 * ___) + (0.342 * ___) - 0.048
Hydrologic Conditions Index = ___
For this period, the threshold index value is 0.25 for "normal" conditions and 0.67 for "wet" conditions. The current index value is less than/greater than 0.25 and less than/greater than 0.67 and constitutes "???" conditions.

* 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.