Monte Carlo analysis of the Copano Bay fecal coliform model Prepared by, Ernest To

Copano Bay model domain

Copano Bay schematic network

The concept of Monte Carlo Analysis To use uncertainties in the inputs and parameters to estimate uncertainties in the model output. β α θ λ Parameters Inputs Output 10% of population < 43 cfu/100 ml median of population < 14 cfu/100 ml EMCs Flows, Q Decay rate, Kd

The goal of Monte Carlo Analysis To match the variation in actual fecal coliform monitoring data Cumulative Density Function (CDF) of Fecal Coliform Concentration (CFU/100mL) at Schemanode 75

What is Monte Carlo? Monte-Carlo analysis uses random numbers in a probability distribution to simulate random phenomena. For each uncertain variable (whether inputs or parameters), possible values are defined with a probability distribution. Distribution types include: Beta

Variables of the Copano Bay Fecal Coliform model Schema link for river Schema link for watershed Kd = decay rate Tau = residence time in river Kd = decay rate Tau_w = residence time in watershed L downstream L upstream L watershed = EMC watershed * Q watershed L downstream = L upstream *exp(-Kd*Tau) + L watershed *exp(-Kd*Tau_w) Inputs: EMC watershed’ Q watershed Parameters: Kd, Tau, Tau_w

Flow (Q) Matched flow distributions at USGS gages using lognormal distributions. Applied matched distribution (with adjustments) to other schemanodes along the river. Lognormal Measured and simulated cumulative distributions for flow at USGS gage

Event mean concentrations (EMCs) Defined as total storm load (mass)/ divided by the total runoff volume. According Handbook of Hydrology by Maidment et al., EMC for fecal coliform in combined sewer outfalls follows a lognormal distribution with a coefficient of variation of 1.5. (where coefficient of variation = standard deviation/mean) Lognormal

Decay rate (Kd) Decay rate is an experimentally derived property Difficult to determine the distribution of Kd Most likely within a finite range and has a central tendency. Therefore assume beta distribution, with parameters A=2 and B=2. Beta

Program concept Random number generators New EMCs New flow and decay rates Process Schematic SchemaNode SchemaLink Success Abort Results Table Loop for N times (where N = integer specified by user) Schematic Processor

Implementation Wrote simple program that performs a similar function as Schematic Processor in Excel Imported schemalink and schemanode tables into Excel Programmed random number generators for Kd, Q and EMCs. Programmed a simple “for” loop to execute function multiple times. Created a simple user-interface

On to the demo…..

Remaining tasks Complete calibration of model to Fecal Coliform monitoring data. Perform kriging on bay fecal coliform data (challenging because of fluctuation of data)

Acknowledgements Dr. David Maidment Carrie Gibson

Questions?