Chatfield Reservoir Water Budget Jim Saunders and Jamie Anthony WQCD, Standards Unit 13 Dec 2007

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

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

Add Precip (directly to surface)

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

Interpreting Graphs Intercept is residual when no flow in Plum; =alluvium Slope is proportional increase in Plum Cr runoff; =ungaged area

Overview of Graphical Analysis YearsSlopeInterceptR2R

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