In the water resources field, the concept of yield can be understood a few different ways. USACE defines yield as follows:
“Yield is the amount of water that can be supplied from the reservoir to a specified location and in a specified time pattern” (EM 1110-2-1420) across a period of streamflow.
Yield includes all water supplied, possibly at differing rates. Reservoir or system firm yield is an additional important calculation used for water supply studies. USACE defines firm yield as follows:
“Firm yield is the largest consistent flow rate (demand) that can be provided [without fail] throughout a period of historic stream-flow” (EM 1110-2-1420).
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 3.6, automates the iteration process and makes calculation of yield a more easily reproducible. There are Five types of yield analysis implemented in ResSim: Reservoir Yield, Water Account Yield, Reverse Reservoir Yield, Reverse Water Account Yield, and Reservoir Storage vs. Yield Analysis.
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. Water 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.
HEC-ResSim has to iteratively solve for the required amount of storage necessary to satisfy a specified flow demand without fail. These reverse cases will allow convenient analysis of the other typical study questions over how much storage needs to be reallocated between storage zones in the reservoir or how much storage a water user needs to purchase to sustain their newly requested flow demand.
With the Reservoir Storage vs. Yield Analysis HEC-ResSim computes multiple Reservoir Yield Analyses to produce a Storage versus Yield curve for the Reservoir. The new feature will allow input of minimum and maximum range limits for the Guide Curve level as well as the number of intervals within that range, over which discrete Storage Yield Analyses can be computed and produce discrete storage versus yield values.
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.