Chatfield Reservoir Water Budget

Jim Saunders and Jamie AnthonyWQCD, Standards Unit13 Dec 2007

Roadmap for Technical Review

Month Topic

Sep-07 Technical comparison of existing control regulations

Oct-07 Existing chlorophyll target, incl magnitude, frequency, duration

Nov-07 Evaluation and discussion of concentration translator

Dec-07 Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads

Jan-08 Phosphorus load estimates; produce common set by source

Feb-08 Evaluation and discussion of load translator

Mar-08 Hydrologic considerations for TMAL

Apr-08 Discuss chlorophyll-phosphorus-load linkages as basis for proposal

Jun-08 WQCD to finalize proposal and circulate

Jul-08 Notice due

Nov-08 WQCC RMH

Purpose of Water Budget

Identify and quantify flow sources Rank sources in terms of

importance (for determining phosphorus concentrations and loads)

Aim is to understand hydrology well enough to support development of a phosphorus budget

Inflows

USACE computed inflow is “gold standard”

Surface inflows Gaged (South Platte, Plum Creek) Ungaged (Deer, Massey) Direct runoff

Alluvial inflow (chiefly Plum Creek) Direct Precipitation

Gaged Surface Inflows

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An

nu

al I

nfl

ow

, A

F

South Platte Plum Creek Residual

Add Precip (directly to surface)

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South Platte Plum Creek Precip Residual

What’s Left?...~7%

Ungaged surface flow, mostly from low elevation

Alluvium Both are more likely to be controlled

by factors in common with Plum Creek than with the South Platte

Further Parsing of Flows Strong association between Plum Creek

and residuals; slope and intercept useful

y = 1.2274x + 3786.9

R2 = 0.9631

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Plum Creek Gaged Flow, AF/y

Plu

m C

reek

+ R

esid

ual

s, A

F/y

Interpreting Graphs1992-1996

y = 1.3398x + 6269

R2 = 0.9801

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Plum Creek Gaged Inflow, AF/y

Plu

m C

reek

+ R

esid

ual

, A

F/y

Intercept is residual when no flow in Plum; =alluvium

Slope is proportional increase in Plum Cr runoff; =ungaged area

Overview of Graphical Analysis

Years Slope Intercept R2

1976-1981 1.337 1804 0.966

1982-1986 1.242 6550 0.992

1987-1991 1.234 4656 0.983

1992-1996 1.340 6269 0.980

1997-2001 1.023 6228 0.950

2002-2006 0.759 6794 0.844

Take Home from Graphs

Parsing a very small % of inflow (~7%) Alluvial contribution (intercept) relatively

stable; use constant for each 5-y block Added runoff (slope) similar to ungaged

area (24% of Plum Creek area) for first 20 years

Trend in slope over last decade is puzzling; we’re still seeking an explanation

Chatfield Annual Water Budget

Conclusions and Comments

Water budget provides a solid basis for estimating phosphorus loads Some snags with high resolution approach

(measured inflows tended to exceed computed inflows in last 10 years), but still a solid basis

Alternative could be developed on basis of two gages and precip (>90% of computed inflow)

Fortunately, uncertainty affects only very small components of inflow

Open to ideas about approach