Operation Rules represent the operational goals and constraints for each zone of the operation set. Each zone can contain a different set of rules depending on how the regulation plan describes the flow limits and requirements for that operating zone.

As previously described in "Defining Operation Zones" and illustrated in "Figure: Reservoir Editor - Operations Tab - Zone Editor", as you create and arrange your operating zones and rules in the current operation set, they are displayed in the Zone-Rules Tree on the left side of the Operations tab of the Reservoir Editor. The order of the rules in each zone indicates their relative priority.

In "Reservoir Operations - The Basics", you learned about guide curve operation and operation zones. In this chapter, you'll learn about the variety of Operation Rules you can create to describe the goals and constraints on the operation of your reservoirs. But, to understand how rules work, it is important to understand the release decision process and how it uses the rules and their relative priority.

The Release Decision Process

The ResSim decision logic for determining the reservoir releases in each timestep is called the Release Decision Process. It involves determining an allowable release range, calculating the desired guide curve release, and comparing the two to arrive at a release decision.

The allowable release range is defined by a minimum allowable release and a maximum allowable release. The reservoir's physical and operational constraints are used to determine the allowable release range.

The desired guide curve release is, by definition, the release the reservoir should make to get to or stay at guide curve in this timestep. This calculation derives from the basic Conservation of Mass Equation:

Inflow minus Outflow equals Change in Storage.

There are few limitations on how the desired guide curve release is computed since guide curve operation is the primary objective of the reservoir operation. However, the computations do include logic to limit or prevent oscillating releases as the reservoir pool elevation approaches the guide curve.

The steps that make up the release decision process are:

  • Estimate Physical Limits—The decision logic begins by estimating the physical capacity of the reservoir to release water, given the reservoir's state at the start of the timestep and an estimate of the pool elevation at the end of the timestep. Unless the dam leaks, the physical minimum release limit is usually zero. The physical maximum release limit is a function of the estimated average elevation for the timestep. The physical release limits establish the initial values for the minimum and maximum limits of the allowable release range.
  • Identify the Current Zone — To determine the current zone, the decision logic computes the elevation value at the end of the current timestep from each top-of-zone curve or relationship. Then, working from the bottom up (based on the zone-sort order), it compares the reservoir pool elevation computed at the end of the previous timestep to each "current" top-of-zone elevation until it finds the zone elevation value that is greater than or equal to the current pool elevation.
  • Identify the Active Rules — If necessary, the decision logic evaluates all relevant State Variables, IF-Blocks, and Rule Modifiers to identify the active rules in the current zone and assemble them in an ordered list based on priority.
  • Apply the Rules—The decision logic then works its way through the list of active rules from the highest priority rule to the lowest. As each rule is evaluated, its current release limit is applied to the allowable range. The maximum allowable release is reduced if a rule calls for a lower maximum, and the minimum allowable release is increased if a rule calls for a higher minimum. However, if a rule has a desired maximum limit that is greater than the current maximum allowable release or has a desired minimum limit lower than the current minimum allowable release, the allowable range will remain unchanged. Since rules are evaluated from highest to lowest priority, the allowable range can never be widened, because the rule attempting to do so would violate the higher priority rule or physical constraint that set the limit.
  • Calculate the Guide Curve Release—After evaluating all the rules, the decision logic then determines the desired guide curve release. This is the release needed to bring the reservoir pool elevation to the guide curve in the current timestep (computation interval) based on the starting pool elevation, the prior release, and the current inflow.
  • Determine the Release — In this final step, the decision logic compares the guide curve release to the limits of the allowable range. If the guide curve release is within the allowable range, the release decision will be the guide curve release. However, if the guide curve release is outside the allowable range, the release decision will be the limit of the allowable range nearest the guide curve release.
    For example: if the desired guide curve release for this timestep (guide curve release) is 35,987 cfs but the final maximum limit of the allowable release range is 10,000 cfs, then the release decision will be 10,000 cfs.

Why are rules "prioritized"?
Sometimes, the rules that describe the desired operation of the reservoir conflict with one another and a means for resolving the conflict is needed. A conflict is when two rules call for a desired release or a release limit that cannot both be satisfied.

For example, a reservoir might have a minimum release rule of 500 cfs and a maximum downstream control rule of 12,000 cfs. At a given timestep during high flow conditions, the downstream control rule evaluates to a maximum release of 100 cfs from the reservoir. If the reservoir releases 500 cfs, the downstream rule is violated. If the reservoir only releases 100 cfs, the minimum flow rule is violated.

ResSim uses rule prioritization as the method for resolving rule conflicts — the highest priority rule wins. And, it is up to you, the modeler, to order your rules appropriately for all circumstances.

The fundamental rule for applying rules to the allowable release range is:

As each rule is applied, from highest priority to lowest, the allowable range may narrow, by reducing the maximum limit or increasing the minimum limit, but it may not widen.

  • Therefore, a minimum limit rule may raise but not lower the minimum limit of the allowable range.
    For example, if the current minimum allowable release is 100 cfs, a minimum limit rule of 500 cfs may raise the minimum allowable release to 500. This is allowed because a release of 100 cfs is still met by a 500 cfs release so the higher priority rule (min 100) is not violated by the lower priority rule (min 500).
    But, if the next rule calls for a minimum release of 10 cfs, the allowable range would not change. The minimum would stay at 500 cfs. This is not considered a violation of the 10 cfs minimum rule, since a minimum release of 500 cfs still meets the minimum 10cfs objective
  • And, a maximum limit rule may lower the maximum limit of the allowable range but not raise it.
    For example, if the current maximum allowable release is 9000 cfs, a maximum limit rule of 5000 cfs may lower maximum allowable release to 5000. Remember a maximum limit of 9000 allows for the release to be any value less than 9000. By reducing the maximum limit to 5000, a value less than the previous limit of 9000, the 9000 cfs limit is not violated.

These examples can be considered illustrations of the following two Rules of Thumb that you should remember when prioritizing rules:

  • When two maximum-limit rules are beside one another in the rule stack, their relative priority doesn't matter — the smaller max wins (sets the maximum limit of the allowable range).
  • When two minimum rules are beside one another in the rule stack, their relative priority doesn't matter — the larger min wins (sets the minimum limit of the allowable range).