Yield Analysis

Firm yield estimates the amount of water that can always be supplied.  The ability to store water increases the firm yield by allowing water to be saved and used when streamflow is less than demand.  Thus, the process of identifying firm yield also identifies the critical period as the driest in the record that identifies the firm yield for a given storage volume.  Greater storage volume produces greater firm yield by providing the ability to supply more water to meet demand during a drier period or a longer dry period.  A different available storage volume might have a different critical period for a given period of streamflow.

A modeler can manually perform a yield analysis in ResSim by iteratively computing a period-of-record simulation until the yield has been identified.  With each iteration, the modeler makes a small change to the demand.  This process continues until a run is made that just barely empties the water supply storage in the reservoir.  That level of demand is the yield for the period-of-record conditions.

The Yield Analysis alternative type, added in ResSim 3.3, automates the iteration process and makes calculation of yield a more easily reproducible.  There are two types of yield analysis currently implemented in ResSim:  Reservoir Yield and Storage Account Yield.  Reservoir Yield uses the entire conservation pool of the reservoir to meet the demand and can be calculated for an individual reservoir or for a system of reservoirs.  Storage Account Yield calculates the yield of a single water storage account among multiple accounts at a reservoir. The ResSim Water Accounts feature allows you to divide the total conservation storage into multiple storage accounts based on either volume or percent of pool.  ResSim can then track water use for each account and account holder based on the rules attached to it. 

The ResSim yield feature can be used to calculate firm yield for water supply or hydropower, or it can be used to calculate a specific Exceedance Drought Yield by using a particular time series of inflows.  This chapter covers the use of the ResSim Yield Analysis feature for the purpose of calculating water supply firm yield.

ResSim Yield Analysis Feature

The ResSim Yield Analysis feature automates the iterative work necessary to determine firm yield. 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. The bisection search method is one method used to determine the next demand to try, and there is also an option to use a heuristic search based on mass balance to attempt to converge in fewer iterations. The bisection method is more reliable for finding the firm yield, but typically requires more 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".

 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".

The Yield Analysis tab has two 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 for the total yield from a reservoir pool or
• Water Account Yield for yield based on a specified water storage account within a reservoir.

The edit panel for each Yield Analysis Type is described in the next two sections, and the results analysis is described afterwards.

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