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Hydrologic Condition Calculations
 
Current Calculations
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 -0.67, equivalent to the 20.6 percentile level of 1947-1994 July conditions.
  2. The Lake McConaughy July end-of-month content was 1,351,600 acre-feet, equivalent to 77.5% of capacity and the 29.0 percentile level of 1947-1994 July conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.464 * 0.206) + (0.404 * 0.290) + 0.061
Hydrologic Conditions Index = 0.274
For this period, the threshold index value is 0.29 for "normal" conditions. The current index value is less than this and constitutes "DRY" 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.
 
Previous 2016/2017 Calculations (starting with the most recent)
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 2,200 cfs, or at the 59.2 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 -0.81, equivalent to the 33.3 percentile level of 1947-1994 June conditions.
  3. The mean June streamflow in the South Platte River at Julesburg (provisional data) was 1,193 cfs, or at the 64.0 percentile level of the 1947-1994 June flows.
  4. The Lake McConaughy June end-of-month content was 1,498,100 acre-feet or 85.9% of capacity, equivalent to the 37.1 percentile level of 1947-1994 June conditions.
  5. The North Platte River Reservoirs in Wyoming June content was 1,657,748 acre-feet, or at the 34.3 percentile level of 1947-1994 June conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.237 * 0.592) + (0.441* 0.333) + (0.218 * 0.640) + (0.109 * 0.371) + (0.105 * 0.343) - 0.071
Hydrologic Conditions Index = 0.432
For this period, the threshold index value is 0.31 for "normal" conditions. The current index value is greater than this and constitutes "NORMAL" 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
 
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 2,608 cfs (provisional USGS data), or at the 81.0 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 0.63 equivalent to the 64.4 percentile level of 1947-1994 May conditions.
  3. The mean May streamflow in the South Platte River at Julesburg (CO-NE stateline) was 1,255 cfs (provisional data), equivalent to 72.2 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 91.31% of normal, equivalent to the 38.0 percentile level of 1947-1994 conditions. 
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.648 * 0.810) + (0.121 * 0.644) + (0.023 * 0.630) + (0.082 * 0.722) + 0.097
Hydrologic Conditions Index = 0.749
For this period, the threshold index value is 0.30 for "normal" conditions and 0.70 for "wet" conditions. The current index value is greater than 0.30 and greater than 0.70 and constitutes "WET" conditions.
 
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 1,798 cfs (provisional USGS data), or at the 50.8 percentile level of 1947-1994 April flows.
  2. The April end-of-month content of Lake McConaughy was 1,448,900 acre-feet, equivalent to 83.1% of maximum capacity and 24.7 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 189,338 acre-feet, equivalent to the 71.0 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 91.31% of normal, equivalent to the 34.1 percentile level of 1947-1994 conditions.
The Index was therefore calculated as follows: 
Hydrologic Conditions Index = (0.601 * 0.508) + (0.271 * 0.247) + (0.031 * 0.710) + (0.252 * 0.341) - 0.065
Hydrologic Conditions Index 0.415 
For this period, the threshold index value is 0.30 for "normal" conditions and 0.70 for "wet" conditions. The current index value is greater than 0.30 and less than 0.70 and constitutes "NORMAL" condition.
 
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 2,039 cubic feet per second, equivalent to the 66.2 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 -0.02, equivalent to the 50.0 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 2,208,953 acre-feet, equivalent to the 99.0 percentile level of 1947-1994 end-of-month February conditions.
The Index was therefore calculated as follows:
Hydrologic Conditions Index = (0.120 * 0.662) + (0.662 * 0.500) + (0.198 * 0.99) - 0.011
Hydrologic Conditions Index = 0.596
For this period, the threshold index value is 0.25 for "normal" conditions. The current index value is greater than this and 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.
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. 

 
Upcoming 2017 Calculations
 
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.