The ResSim Yield Analysis feature automates the iterative work necessary to determine firm yield in Reservoir Yield and Water Account Yield analysis types and firm storage in Reverse Reservoir Yield and Reverse Water Account Yield analysis types.

In the Reservoir Yield and Water Account Yield analysis types a single minimum or specified flow rule (or downstream control rule) is used to represent the demand at the location of interest; the demand can be constant or seasonally-varying (though repeating each year). The yield is computed by repeatedly adjusting the value of the demand rule and re-simulating until the conservation pool is emptied once during the period-of-record; seasonally-varying demands are scaled proportionally during the search process. After each simulation, the demand is increased or decreased to get closer to the goal of exactly emptying the pool once, with no failure to meet the minimum rule.

In the Reverse Reservoir Yield Analysis and Reverse Water Account Yield Analysis instead of selecting a demand rule, a Storage Level is selected to iterate over and maximize. 

There are two convergence methods available in yield analysis: Bisection Search Only and Heuristics and Bisection Search. The bisection search is more reliable in yield analysis, but typically requires more iterations. The heuristic search is based on mass balance to attempt to converge in fewer iterations. The heuristic approach looks at the remaining storage and demand volumes and computes an estimate of exactly-meetable demand for the next iteration. While it requires fewer iterations than the Bisection method, the Heuristic method is currently successful only in fairly simple, single-reservoir watershed analyses.

The ResSim Yield Analysis Tool also allows the user to set flow rate and storage tolerances for defining convergence, to limit the number of iterations needed. The maximum number of iterations can also be directly limited.

A Yield Analysis is performed as part of a special ResSim alternative Type, a Yield Analysis Alternative. To create a Yield Analysis Alternative, you must set the Alternative Type to Yield Analysis on the Run Control tab of the Alternative Editor as illustrated in "Figure: Alternative Editor - Run Control Tab - Yield Analysis Alternative Type".  

Figure: Alternative Editor - Run Control Tab - Yield Analysis Alternative Type

 When the alternative type is Yield Analysis, the content of the Yield Analysis tab will be enabled as illustrated in "Figure: Alternative Editor - Yield Analysis Tab with Reservoir Yield selected".

Figure: Alternative Editor - Yield Analysis Tab with Reservoir Yield selected

The Yield Analysis tab has five views depending on the selection of the Yield Analysis Type at the top of the tab. As illustrated in "Figure: Yield Analysis Types", the Yield Analysis Type options include:

  • Reservoir Yield
  • Water Account Yield
  • Reverse Reservoir Yield
  • Reverse Water Account Yield
  • Reservoir Storage vs. Yield Analysis

Figure: Yield Analysis Types

The edit panel for each Yield Analysis Type is described in the next five sections.

Also see Guide for Running a Reservoir Pool Firm Yield Analysis with ResSim.