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