Wet Years / Dry Years: Adjusting Nutrient Loads to Monitor Progress Toward TMDL Reductions

Transcription

1 Wet Years / Dry Years: Adjusting Nutrient Loads to Monitor Progress Toward TMDL Reductions Florida Stormwater Association December, 2016 Mike Wessel, Janicki Environmental Co-authors: Robert Burnes, Sarah Malone, Kelly Levy, Pinellas County Tony Janicki, Ray Pribble, Steve West, Janicki Environmental, Inc.

2 Problem Definition The Florida Department of Environmental Protection (FDEP) has recently requested that Non-Point Source Discharge Elimination System (NPDES) permittees: include a mechanism by which to account for the effects of climatological variation in rainfall on pollutant loading. MS4 Excerpt

3 Objective Using existing information, identify a mechanism by which Pinellas County could: adjust ( Normalize ) their annual pollutant loading estimates in order to comply with the new FDEP directive, and track progress of their Stormwater Master Plan actions over time.

4 Pinellas County and Tampa Bay

5 Pinellas County Eighteen nutrient TMDL waterbodies that receive discharge from the Pinellas County MS4. Twelve load-based TMDLs and 6 concentration-based TMDLs. County has prioritized the list of TMDLs for which they are responsible and developed a schedule for addressing load reductions under the TMDLs (Pinellas County 2013).

6 This Presentation Details the development of a procedure used to: 1. Establish the Baseline Loading Estimate. 2. Evaluate future data against this Baseline Load. 3. Evaluate progress towards pollutant reduction goals after accounting for the effects of interannual variability in rainfall.

7 Existing Information (Data) Gaged Rainfall NEXRAD Rainfall Pinellas Water Quality Data IWMRP NPS Model Nutrient and Hydrologic Loads Pinellas Streamflow Pinellas County Nutrient and Hydrologic Loads

8 Spatial Summary Major Basins IWRMP Basins

9 Pinellas Basins IWRMP Basins with NCDC Rain Gages

10 WBIDs within Basins

11 TMDL Basins with WQ Station Locations

12 Conceptual Model Natural Log TN Load Hydrologic Condition

13 Procedure Generate long-term Interpolated basin-specific rainfall using NPS model interpolation subroutine. Generate Baseline hydrologic condition using long term interpolated rainfall index. Calculate hydrologic loads for using NPS model. Multiply ambient WQ concentrations and hydrologic loads to generate empirically-based nutrient loads. Standardize loads to rainfall index to account for variation in rainfall.

14 It All Starts with Rainfall Long Term Gages Whitted and Tarpon. Interpolated rain gage data from NPS model subroutine: NEXRAD 20 years:

15 NEXRAD GRID and Example from July 2009 Day Month

16 Grand Median Rainfall

17 NEXRAD

18 NEXRAD Same Scale

19 Comparing Gaged Rain and NEXRAD in Gaged Rain Basins

20 McKay Creek

21 Three types: Drought Indices Pinellas County Precipitation Index (PCPI). Standardized Precipitation Index (SPI). Standardized Precipitation and Evaporation Index (SPEI).

22 Pinellas County Precipitation Index PCPI = x mi m m Where: x m i m m = monthly rain at rainfall Gage X for month i = the long term monthly mean value for Gage X = standard deviation of the long term monthly mean values

23

24 Standardized Precipitation Index McKee, T.B., N.J. Doesken, and J. Kliest Drought monitoring with multiple time scales. In Proceedings of the 9th Conference of Applied Climatology, January, Dallas TX. American Meteorological Society, Boston, MA

25 SPI Calculation Calculate probability density function using Gamma distribution for rainfall distribution. Calculate deviations from expected distribution for given time period and standardize (0,1). Calculate over various time scales to 60 months.

26

27 SPI Strengths and Weaknesses KEY STRENGTHS: Flexible Short time scales provide early warning of drought Comparable over different locations Relevance to historical context aids decision making KEY LIMITATIONS: Based only on precipitation No soil water balance component No ET

28

29 Standardized Precipitation Evapotranspiration Index Vincente Serrano, S. M., S. Begueria, and J. I. Lopez- Moreno A multiscaler drought index sensitive to global warming: The Standardized Precipitation Evaporation Index. Journal of Climate. V23:

30 SPEI Calculation The procedure for calculating the SPEI is similar to that for the SPI. SPEI uses Thornthwaite estimator to calculate PET using temperature, latitude and a reference ET (Eto). ETo represents evaporating power of the atmosphere at a specific location and time of the year. The difference between precipitation and the reference evapotranspiration (D i = P i - PET) is then used as the input rather than precipitation (P). D values are fit to a probability distribution to transform the original values to standardized units that are comparable in space and time and at different SPEI time scales, following the same procedure as that for the SPI.

31 Tarpon Springs SPEI at Various Timescales SPEI-1 SPEI-12 SPEI-24 SPEI-36 SPEI-48 SPEI-60

32 Tarpon Springs Twelve Month SPI vs SPEI SPI-12 SPEI-12

33 The Baseline Load

34

35 Existing Methods

36 Delivery Ratio Rain/Stream Hyd. Anomalies

37 Important Aspects of Two Methods Delivery Ratios Apples to Apples Comparison (same model) Adjustment performed for every single year irrespective of magnitude of difference Model needed to derive numbers for adjustment Hydrologic Anomalies Relates streamflow/rainfall and model hydrologic load using regression Adjustment anticipated based on confidence intervals Nutrient model not needed to predict when adjustment is likely

38 Normalization Procedure 1. Generate Interpolated long-term rainfall using NPS model interpolation procedure ( Normal ). 2. Generate Baseline hydrologic condition using long term interpolated rainfall index (SPI). 3. Calculate hydrologic loads for using NPS model. 4. Multiply ambient WQ concentrations and hydrologic loads to generate empirically-based nutrient loads. 5. Standardize loads to SPI values.

39 Step 1. Generate Interpolated long term rainfall using NPS model interpolation procedure

40 Step 2. Generate Basin-Specific SPI Values Basin 6

41 Step 3. Calculate hydrologic loads for using NPS model and 2011 Landuse

42 Step 4. Multiply ambient WQ concentrations and hydrologic loads to generate empiricallybased nutrient loads

43 Step 5. Standardize loads to SPI values Predicted Load= *SPI Adjusted Load = Observed *SPI Baseline Load = 9695 kg/yr

44 Total Nitrogen (mg/l) Total Phosphorus (mg/l) R

45 TMDL Basins with non-significant regression relationship between nutrient Loads and SPI

46 Summary The use of the SPI (based on long-term rainfall data) allowed for the estimation of both a Baseline Load and the development of a method to adjust annual loading estimates to the Baseline. The Baseline Load is established using available, ambient water quality concentrations and model-based hydrologic loads. In this way, any improvements resulting in improved water quality should be reflected in the calculated load. The majority of Basins with active water quality sampling stations displayed positive relationships between rainfall and pollutant loading.

47 Recommendations Those Basins without a quantitative relationship should be investigated for potential point source discharges. In such cases, the Baseline Load is currently defined as the annual (geometric) average load (or concentration) over the period of record between 2003 and 2015 until such time as a relationship can be established.

48 Future Efforts Exploration of the methodology described above using the shorter timeseries of NEXRAD rainfall and basin-specific NEXRAD-based hydrologic loads would be worthwhile to see if it improves the relationships, especially in those basins where the regressions achieved an R squared value was less than The use of the SPEI index may improve the long term estimates of changes in effective rainfall if ET can be effectively estimated on a basis-specific level in Pinellas County. With more data, statistical certainty could be included in the determination of whether or not the loads have decreased over time after accounting for variation in rainfall.

49 SPI<-1 SPI>1

50 Questions?