Monte Carlo Realizations & Scenario Modeling

First published: October 2026
Last updated: October 23, 2026

[!NOTE] The primary benefit of using CligenWeather.dll in GoldSim is the power of true Monte Carlo simulation: evaluating hundreds of distinct, plausible 30-year futures to quantify environmental and operational risk.

This guide explains how Monte Carlo realizations operate with the DLL and how to interpret probabilistic results.

How Realizations Work with the DLL

In GoldSim, when you select Monte Carlo simulation mode and specify (for example) 100 Realizations, GoldSim loops through 100 complete iterations of your simulation timeframe (e.g., from Year 0 to Year 30).

At the start of each realization: 1. GoldSim updates its internal run property Realization (incrementing from $1$ to $2, 3, \dots, 100$). 2. The DLL receives the current realization index. 3. The DLL automatically initializes its pseudorandom number generator with an independent, repeatable seed derived from the realization index. 4. The result: Every single realization receives an entirely unique, independent 30-year weather time series, while maintaining identical statistical climate properties (identical seasonal rainfall means, storm clustering, and temperature envelopes).

graph TD
    A["GoldSim Monte Carlo Run<br/>(100 Realizations)"] --> R1["Realization 1<br/>Seed 1001"]
    A --> R2["Realization 2<br/>Seed 1002"]
    A --> R3["Realization ..."]
    A --> R100["Realization 100<br/>Seed 1100"]

    R1 --> W1["Generates 30-Yr Sequence A<br/>(Contains 1997-style wet spell)"]
    R2 --> W2["Generates 30-Yr Sequence B<br/>(Contains 3-yr dry drought)"]
    R3 --> W3["Generates 30-Yr Sequence C"]
    R100 --> W100["Generates 30-Yr Sequence N"]

    W1 --> P["GoldSim Probabilistic Results<br/>(P10, P50, P90 Envelopes, Spill Risk)"]
    W2 --> P
    W3 --> P
    W100 --> P

Analyzing Probabilistic Results in GoldSim

Once the 100-realization run completes, open a Multi-Realization Time History Result element in GoldSim:

1. Probabilities of Spill or Overflow

If modeling a water storage reservoir or tailings impoundment: - Plot the water surface elevation over time. - Switch the display view from All Realizations to Probabilities. - Set the threshold to your spillway crest elevation. GoldSim will report the exact percentage of realizations that reached or exceeded the spillway (e.g., "There is a 3.2% probability of a spill event over the 30-year operational life of the facility").

2. Multi-Year Drought Vulnerability

• View cumulative water storage at the 10th percentile (P10).
• If your system maintains adequate storage even along the lower P10 trajectory, your water rights and storage design demonstrate high drought resilience.

3. Peak Storm Sizing

• View maximum daily runoff rates across all 100 realizations.
• Identify the worst-case storm event across the entire 3,000 synthetic years ($100 \text{ realizations} \times 30 \text{ years}$) to test emergency overflow spillway capacities.

Reproducibility & Seed Integrity

A critical requirement in engineering consultancy and regulatory hearings is reproducibility: - Because the DLL uses deterministic seeding based on GoldSim's realization index, re-running the GoldSim model with the same settings will produce the exact same sequence of weather numbers every time. - If an auditor or client opens your .gsm file and clicks Run, their results will match yours down to the last decimal place.

Troubleshooting Common GoldSim Questions

Q: Why do all realizations show the same weather?

A: Ensure that GoldSim's built-in run property Realization is properly linked to the DLL's realization input argument in the External element dialog. If this input is hardcoded to a static number (like 1), the DLL will generate the same seed every time.

Q: How many realizations should I run?

A: - For rapid testing and model debugging: 5 to 10 realizations. - For typical water balance sizing and environmental permitting: 50 to 100 realizations. - For rare tail-risk assessment (e.g., 1-in-1,000 year extreme containment risk): 500+ realizations.

With the native 64-bit DLL, 100 realizations of a 30-year daily model typically runs in just a few seconds.