Chatfield Reservoir Hydrologic Scenario Development Jim Saunders WQCD Standards Unit 13 March 2008

Roadmap for Technical Review MonthTopic Sep-07Technical comparison of existing control regulations Oct-07Existing chlorophyll target, incl magnitude, frequency, duration Nov-07Evaluation and discussion of concentration translator Dec-07Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads Jan-08Phosphorus load estimates; produce common set by source Feb-08Evaluation and discussion of load translator Mar-08Hydrologic considerations for TMAL Apr-08Discuss chlorophyll-phosphorus-load linkages as basis for proposal Jun-08WQCD to finalize proposal and circulate Jul-08Notice due Nov-08WQCC RMH

For Today…  Explain purpose served by hydrologic scenario  Review examples  Outline issues for Chatfield Problems with existing scenario Options for new scenario  Make a recommendation

What Purpose Does the Hydrologic Scenario Serve?  Part of logical basis for linking implementation of controls to attainment of standard  Necessary for defining allowable load in terms of pounds (=flow x concentration)  Control regulations define allocations in pounds

Hydrologic Scenarios in Existing Control Regulations  Dillon 1982 (212,000 AF); return period ~3y Index future P loads to base year (1982)  Cherry Creek 1982 (2245 AF); return period ~1.2y Index to 1982 base year  Chatfield Original: 1982 (93,000 AF); return period ~3y Revised: Q 10 (261,000 AF); actual return period ~5y  Bear Creek – not specified

Comments on Chatfield Scenario  Rationale for Q10 is based on exceedance probability for load rather than in-lake concentration Concentration threshold could be exceeded at any flow if load is high enough  Assumes implicitly that higher load means poorer WQ; not necessarily true

Conceptual Basis for New Scenario  How is the allowable phosphorus load influenced by hydrologic conditions? Is the chl-TP relationship affected by flow? – depends (in concept); flow may control of TP Is the TP conc-load relationship affected by flow? – depends (in concept) on P retention Logical basis: highest inflow concentration is most likely to yield highest in-lake concentration  What determines highest inflow TP concentration? Not necessarily a low flow scenario Depends on mix of two sources: SP and Plum

Starting Point for Hydrologic Scenario Development  Select median total inflow WQCD often uses median flow in TMDL development for streams Median computed inflow: 100,860 AF  Determine relative importance of the two main tributaries for setting the inflow concentration Inflow concentration is total load/total inflow Does each tributary represent a constant proportion of total inflow? Does concentration vary with flow in either tributary?

Phosphorus Annual Average Concentration and Tributary Flow  South Platte – conc not related to flow  Plum Cr – higher conc at higher flow  Which influence is stronger in mixed flow?

Flows Largely Independent

Relative Importance of Plum Creek  TP concentration in Plum Cr >> South Platte  When is %Plum highest?; not at highest flows  Median %Plum = 16%

Expanding the Scenario  Started with median total inflow  Set proportion from Plum Creek Median (16%) High end (>30%) Return period?  What determines Plum Creek contribution to inflow TP concentration? Dependence of concentration on flow Relative importance of flow

Concentration and Flow in Plum  Annual avg concentration is load/inflow  Plateau abv 20,000 AF/y (TP~0.175 mg/L)

Influence of Plum Creek on Inflow TP  Realistic range of inflow % (backdrop of median total inflow)  More Plum Cr flow (as %) means higher inflow phosphorus concentration for reservoir

Defining a Return Period  Plum Cr > 20,000 AF/y in 11/31 yrs  Plum Cr > 20% of inflow in 11/31 yrs  Both criteria met in 6/31 yrs (19%); return period about 5 y

WQCD Recommendation for Hydrologic Scenario  Median total inflow – 100,860 AF/y  Plum Creek; set % contribution Option 1: median (16%)  About 16,000 AF/y; TP conc below plateau Option 2: 20%  About 20,000 AF/y; TP conc on plateau  Exceedance frequency about once-in-5 yrs

TMAL Development Issues not included in Technical Review  Partitioning of load between South Platte and Plum Creek basins  Allocations to sources within each basin  Define margin of safety

What’s Next?  Next month – technical review as basis for proposal; connecting the dots Hydrologic scenario Load translator Concentration translator Standards, goals, and attainment  Tracking memo