This example model simulates water resources in a shared reservoir system like those in regions governed by water rights doctrines like the "prior appropriation" system prevalent in the Western United States. Legal entitlements and dynamic hydrological conditions all interact to determine who gets how much water, and when.

This model simulates water rights principles like storage priority, ownership tracking, and evaporation sharing within a dynamic simulation framework. The model represents a single physical body of water, but its total volume is divided among multiple storage accounts, each representing a distinct water right holder. For this example, I've defined four accounts:

• Dead Pool: This account represents the unusable (minimum operational) storage, ensuring a base level of water is always maintained. It holds the highest priority for receiving and retaining water.
• Senior Account: Representing an early and high-priority water right, typically for established agricultural uses.
• Junior Account: Representing a more recent water right, often for growing municipal needs.
• Conservation Account: Representing dedicated flows for environmental benefits, flood control, recreation, and other non-consumptive purposes. This account serves a dual purpose: it holds the most junior conservation water right for its target volume, but also acts as the reservoir's surcharge pool, accepting surplus water when other accounts are full, up to the physical limits of the reservoir.

Each of these accounts tracks its current volume and maximum capacity, allowing for accurate ownership tracking within the shared physical reservoir. The above schematic diagram illustrates this tiered structure, visually demonstrating how higher-priority accounts are more protected, being the first to receive water and the last to experience curtailment.

Priority of Storage: The Cascading Bucket System

An important concept of Western water law is the "first in time, first in right" principle, which extends to the right to store water (Getches, 2020). This model simulates this principle using a cascading bucket approach for allocating incoming water.

On each simulated day, inflows to the reservoir are allocated sequentially, based on a strict priority order. The water first attempts to fill the Dead Pool. Once the Dead Pool is full to its capacity, any remaining inflow "spills over" to the next higher priority account. This process continues down the line: Senior to Junior, and Junior to Conservation.

The Conservation Account, being the most junior, is the last to receive water to meet its specific target volume. If all other accounts are full, any additional incoming water flows into the Conservation Account's "flood control" portion. This effectively makes the Conservation Account the reservoir's primary buffer for managing excess water before it's released as flood flow. Any water that cannot be held even within the Conservation Account's flood control capacity is considered "flood flow" and is released downstream.

This cascading system reflects how senior water rights are prioritized for filling their allotted storage space.

Managing Losses: Evaporation Deduction

Evaporation is an unavoidable loss in reservoir operations. Our model accounts for this by:

1. Calculating Total Evaporation: At each time step, the total volume is used in conjunction with a Volume-Area lookup table to calculate the reservoir's current water surface area. This area, multiplied by a daily evaporation rate, provides the total evaporation volume lost from the reservoir's surface for that time step.
2. Proportional Deduction: Once the total evaporation volume for the reservoir is known, this loss is distributed proportionally among all active Storage Accounts (Dead Pool, Senior, Junior, and Environmental). Each account's share of the evaporation is determined based on its current volume relative to the total current volume of all active accounts in the reservoir. This standard method ensures that accounts with more water bear a larger share of the evaporation burden, maintaining overall mass balance within the system (e.g., Loucks & Van Beek, 2017).

Water Delivery and Demand Management

The model also simulates the delivery of water to meet demands, demonstrating how release priorities are managed:

• Demand Fulfillment from Owned Water: Requests for water are made by each account holder based on their demand rates. Each account can only draw water from its own currently stored volume.
• Operational Sequence within Accounts: Within each individual Storage Account, outflows are processed in a specific order: first, evaporation loss, then transfers to fulfill senior water rights (if applicable), followed by the controlled deliveries to meet the account's demand.
• Shortage Impacts (Prioritized implicitly): When water is scarce, the effect of the hierarchical filling (cascading bucket) and the senior right fulfillment methods means that junior accounts will be the first to face limitations in meeting their demands. 

As seen in the below plots, even though the Conservation account can store a significant volume, its junior priority means it must transfer water to more senior accounts when they are short. Consequently, looking at the outflows (on right), Conservation flows are consistently the last to begin and the first to cease. It is also notable that the Conservation account may have an outflow that exceeds that of a more senior account. This behavior is allowed under the prior appropriation doctrine due to the 'use it or lose it' principle (formally, forfeiture or abandonment for non-use), which incentivizes right holders to exercise their full water right for a beneficial use, including specific flow requirements for environmental or conservation purposes (Getches, 2020).

This prototype model, though simplified, offers a tool for visualizing and understanding the dynamic system of inflows, losses, storage priorities, and delivery rules under the Western USA's prior appropriation doctrine. By explicitly tracking individual account ownership within a shared physical reservoir, it provides a demonstration of how legal entitlements translate into operational decisions within a GoldSim model.

Note about the author: While I don’t specialize in water rights, I’ve encountered these challenges in several projects and developed modeling strategies that may be useful to others working in similar contexts. My goal is to share what I’ve learned and offer practical tools that can help you build more flexible and transparent models. The modeling approaches demonstrated are consistent with practices used in tools like WRAP and RiverWare, and reflect common strategies for simulating water rights in reservoir systems.

References

Getches, D. H. (2020). Water Law in a Nutshell (6th ed.). West Academic Publishing.

Loucks, D. P., & Van Beek, E. (2017). Water Resources Systems Planning and Management: An Introduction to Methods, Models, and Applications. Springer.

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