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Chatfield Reservoir Phosphorus Budget
Jim Saunders and Jamie AnthonyWQCD, Standards Unit13 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 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)
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?
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
0.0
0.2
0.4
0.6
0.8
1.0
1.2
10 100 1000 10000
Waterton Flow, cfs
To
tal
Ph
osp
ho
rus,
mg
/L
Measured Nondetect
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 (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
Flow Categories for South Platte (20-y)
1
10
100
1000
10000
0% 25% 50% 75% 100%
Flow Percentile
Dai
ly F
low
, cf
s
25th percentile: 32 cfs
75th percentile: 72 cfs
Low HighIntermediate
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)
South Platte Phosphorus Measurements
Flow ’87-’91 ’92-’96 ’97-’01 ’02-’06 Total
Low 19 17 12 27 75
Mid 35 53 39 37 164
High 39 40 33 19 131
Total 93 110 84 83 370
Compare Years using Phosphorus Measured during Intermediate Flows
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.050
2002 2003 2004 2005 2006
Year
Ph
os
ph
oru
s C
on
ce
ntr
ati
on
(m
g/L
)
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 1994
1994 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
0
0.1
0.2
0.3
0.4
0.5
0.6
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
To
tal P
ho
sph
oru
s (m
g/L
)
Flow Categories in Plum Creek (20-y)
25th percentile: 4.4 cfs
75th percentile: 24 cfs
Low HighIntermediate
Plum Creek Phosphorus Data
Flow ’87-’91 ’92-’96 ’97-’01 ’02-’06 Total
Low 14 4 10 10 38
Mid 35 35 34 43 147
High 36 46 31 10 123
Total 85 85 75 63 308
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 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
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
45%
45%
1%
7%2%
South Platte River
Plum Creek
Alluvium
Ungaged Runoff
Precipitation
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