This article introduces a GoldSim model "PrecipGen" that simulates daily precipitation using a first-order, 2-state Markov chain-gamma model with long-term adjustments. A Markov process is used to model the change between states randomly over time, where the probability of a state change depends on the previous state. The precipitation rate is modeled by sampling from a gamma probability distribution on wet days. The model leverages the foundational work of Dee Allen Wright and the WGEN model from 1983, which was implemented in FORTRAN. This GoldSim iteration inherits a legacy of reliability in precipitation simulation and introduces some advancements.

One of the key enhancements is the capability to simulate long-term variations in precipitation patterns. This feature enables the model to represent multi-decadal oscillations, such as prolonged droughts and periods of heavy rainfall, providing a more comprehensive simulation of climatic patterns over extended periods. This added functionality enhances the model’s utility for understanding and predicting precipitation behavior, making it a valuable tool for those building water balance models in GoldSim.

The model includes two components:

1. PrecipGen PAR
2. PrecipGen

Screen captures of both components are provided below for reference.

Figure 1 - Screen capture of PrecipGen PAR.

Figure 2 - Screen capture of PrecipGen showing the daily precipitation rate with annual totals.

Before calculating daily precipitation rates, PrecipGen adjusts the input parameters (PWW, PWD, Mean, SD) annually by sampling from a gamma distribution with correlations.

Correlations are established for the following value pairs:

• PWW with time (autocorrelation)
• PWW and PWD 
• PWW and Mean 

Using the Model

To generate the input parameters for PrecipGen, use the model PrecipGen PAR. Instructions are provided in the Note Pane of the models.

Download the Latest Version

Click the links below to download the latest version of PrecipGen and PrecipGen PAR. We welcome your feedback and questions about how to use this model. Please let us know if you encounter any problems.