Safe Yield Wizard

The Safe Yield Wizard can help optimize and evaluate tradeoffs among human and environmental water supply needs by finding the maximum safe yield for a given WEAP model.  For example, you could explore different policies for reservoir release, instream flow requirements and demand management to see what impact they had on the maximum safe yield from the reservoir and on instream flow.

The Wizard will attempt to maximize the "goal variable" in each selected scenario, such that all selected demand sites and flow requirements have 100% reliability (no unmet demand) for all years in the study period, and all selected reservoirs refill completely at least once after their lowest point.

The first step in the Safe Yield Wizard is to select the Goal Variable that you want to maximize, along with its bounds (minimum and maximum values, and an initial value to try).  This variable should be linked to water demand, such that increasing the value of the variable will increase demand.  For example, the variable could represent the population (or any other activity level, such as units of production or land area under cultivation), or it could be an absolute volume of water demand.  When WEAP varies this value to find the maximum safe yield, it will have a single value for all years and timesteps.  If it is population, for example, each iteration of the Safe Yield calculation will set the population to some value for all years of each scenario.  Rather than model a rising population over time, the idea here is to find the maximum population that can be supported by the water system modeled.

Next, select the Demand Sites and Flow Requirements that you want to ensure have 100% reliability.  You must select at least one demand site or flow requirement.  

Next, select one or more Reservoirs that must refill at least once after their lowest point.  The lowest point may correspond to the drought of record.  The requirement about refilling after the lowest point ensures that there is not an unsustainable trend in reservoir use that would eventually drain the reservoir.  If instead you want to track the reservoir storage but not require that they refill, uncheck the "Require reservoirs to refill at least once after their lowest point" checkbox.

Finally, choose one or more Scenarios to evaluate.  For each scenario selected, the Wizard will find the maximum value of the goal variable that has 100% reliability and reservoirs refilling.

The Iterations setting dictates how many calculation iterations will be made for each scenario in searching for the maximum safe yield.  The Safe Yield Wizard uses a "binary search" algorithm to find the maximum safe yield within the bounds given for the goal variable.  For example, if the bounds were 0 and 16, with an initial value of 8, and the maximum safe yield corresponded to a value of 10.23, here is the sequence of guesses that would be tested: 8, 16, 12, 10, 11, 10.5, 10.25, 10.125, 10.1875, 10.21875,10.234375, ....  Each guess divides by half the distance to the maximum safe yield.  If a guess results in reliability less than 100%, or a reservoir that does not refill, the next guess will be lower.  Conversely, if reliability = 100% and the reservoirs refill, then the next guess will be higher.  Therefore, the greater the number of guesses, the more precise the answer, but also the longer the time to calculate.  Keep in mind that the WEAP model of your system has many different uncertainties and inaccuracies, so a precision for the goal variable of 1% or 2% is probably sufficient.  This can be achieved in 6 iterations.

If the Include Current Accounts box is checked, then the value for the goal variable will be set in the Current Accounts as well as the scenarios, and WEAP will check that the yield is safe in the Current Accounts as well as in the scenarios.

Click the Calculate button at the bottom to start the calculations.  The status bar at the bottom of the WEAP window will show the current status, in terms of the scenario and iteration being calculated, the value of the goal variable being tested and the maximum safe goal variable value found so far.  Once the wizard has iterated the chosen number of times for each scenario to find the maximum safe yield for each, it will create a view in the Scenario Explorer showing the value of the goal variable in each scenario (in the upper Data Section), and the corresponding yield (Supply Requirements and Instream Flow Requirements) and the reservoir storage for each scenario (in the lower Results Section).

Menu option: Advanced, Safe Yield Wizard