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

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”

What is Load?  Concentration x Flow => pounds P  Daily: mg/L x ft 3 /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)

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

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

South Platte – no connection to flow

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

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

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 ( ) 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

Flow Categories for South Platte (20-y) 25 th percentile: 32 cfs 75 th percentile: 72 cfs LowHighIntermediate

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 ( ) Assign to low flow category (<32 cfs)

South Platte Phosphorus Measurements Flow’87-’91’92-’96’97-’01’02-’06Total Low Mid High Total

Compare Years using Phosphorus Measured during Intermediate Flows

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?

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)

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

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

Plum Creek TP over Time  Is there a trend? Not obvious

Flow Categories in Plum Creek (20-y) 25 th percentile: 4.4 cfs 75 th percentile: 24 cfs LowHighIntermediate

Plum Creek Phosphorus Data Flow’87-’91’92-’96’97-’01’02-’06Total Low Mid High Total

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

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

Plum Creek Annual Loads with Reps  Note magnitude comparable to Platte

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

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

Total Phosphorus Load

Compare Loads  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 South Platte Plum Creek

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