Chatfield Reservoir Phosphorus Budget

1. Chatfield Reservoir Phosphorus Budget Jim Saunders and Jamie Anthony WQCD, Standards Unit 13 Dec 2007

2. Purpose of Phosphorus Budget • Quantify annual loads associated with inflow sources • Assess relative importance of sources (typical as well as variation) • Determine annual loads as precursor to development of “load translator”

3. What is Load? • Concentration x Flow => pounds P • Daily: mg/L x ft3/s x units factor; get lbs/day • Annual: Σ(daily loads) • Can simplify if concentration does not vary with flow or over time: concentration x Σ(daily flows)

4. Sampling Frequency Problem • Flow is reported daily for major tribs (South Platte and Plum) • Concentration is measured 15-20 times per year on average (~5% of days) • What is best way to assign concentrations to every day?

5. Characterize Variation in Concentration • Common for concentration to vary with flow; often lower conc at higher flow • If the linkage is strong, can predict concentration for any flow (regression analysis) • Trend over time might be expected in case of development (or wildfires?) • Overlay of patterns associated with flow and time harder to deal with

6. South Platte – no connection to flow

7. More about South Platte • Absence of flow concentration linkage reflects role of upstream reservoirs and intensive flow management • Why not use average? Large range of concentrations; don’t want to eliminate observed variability • Could have subtle flow-concentration link obscured by temporal trend

8. South Platte TP over Time • (MDL problems in some years) • Is there a trend over time?

9. Disentangling Time and Flow • Assume there are underlying links between concentration and both flow and time for the South Platte • Define categories of flow based on quartiles for period of record • Define consecutive blocks of time • Assign each day (1987-2006) to a time block and a flow category based on daily flow • Each phosphorus measurement can be classified in the time-flow matrix based on flow observed on sampling date

10. Flow Categories for South Platte (20-y) 75th percentile: 72 cfs 25th percentile: 32 cfs Low Intermediate High

11. Classify Phosphorus Measurements by Time and Flow • Divide sampling record into consecutive 5-y time blocks beginning with 1987 • Based on sampling date, classify each measured phosphorus concentration according to one of 3 flow categories • Example: TP=0.024 mg/L on 3/29/05; flow was 30 cfs • Assign to time block 4 (2002-2006) • Assign to low flow category (<32 cfs)

12. South Platte Phosphorus Measurements

13. Compare Years using Phosphorus Measured during Intermediate Flows

14. Overview of Phosphorus Comparisons • No difference in concentration across years within intermediate or high flow categories in any of the 5-y blocks • Sample size too small for same comparison in low flow category, but will assume no difference • Safe to lump phosphorus data across years within flow categories within time blocks • Are there patterns over time within flow categories?

15. Random Sampling Methodology • Flow and time disentangled as much as practicable (3 flow x 4 time units) • Load strategy: assign concentration to every day based on flow in South Platte • Preserve variation observed in concentration data (i.e., don’t use avg) • In each time block, each measured concentration in a flow category is equally valid for every date that falls in that category (i.e., can select at random)

16. Random Sampling Example 1994 1994 flows; 31%:49%:21% 1994 TP; 7:8:7

17. Annual Loads with Replication • 365 daily loads, summed for annual load • Can repeat as often as you want; we did 100 reps

18. Plum Creek TP over Time • Is there a trend? Not obvious

19. Flow Categories in Plum Creek (20-y) 75th percentile: 24 cfs 25th percentile: 4.4 cfs Low Intermediate High

20. Plum Creek Phosphorus Data

21. Flow Patterns for Plum Creek?Yes, but can ignore time

22. Plum Creek: Use Flow Deciles • No apparent temporal trend • Collapse time blocks into one • “Noisy” linkage to flow • Subdivide flows into 10 flow categories (deciles) • All deciles (except lowest flow) have at least 24 concentration measurements • Lowest decile – only one concentration; all zero flow days

23. Plum Creek Annual Loads with Reps • Note magnitude comparable to Platte

24. More Phosphorus Sources • Direct Precipitation • Monthly avg lake area * Kassler gage • …monthly AF * [TP] • Set [TP] to 0.087 mg/L (Clean Lakes study) in all months • Alluvial • Estimated annual inflow * [TP] • Set [TP] to 0.010 mg/L (monitoring data) in all years

25. Phosphorus from Ungaged Areas • Limited data for ungaged tribs (1.4% of basin area); no data for direct runoff (0.4%) • Assume TP yield similar to Plum Creek watershed; i.e., scale up by water yield

26. Total Phosphorus Load

27. Compare Loads South Platte • Old vs. new load method; compare to equivalence line • SP shows strong bias (new>old) • Plum similar • Assumptions behind methods likely different • New method benefits from review of 20-y record Plum Creek

28. Conclusions and Comments • Random sampling methodology provides robust and flexible approach to load estimates for gaged inflows • Gaged inflows contribute about 90% of annual load; even split between SP and Plum (but much variation among years) • Small contribution likely for precip and alluvium; apply simple method • Some concern about under-estimating Plum due to issues with water budget • Expect to make spreadsheets available next month