**Syntax**

PrevTSValue

or PrevTSValue(Branch:VariableName)

or PrevTSValue(Branch:VariableName[Scale Unit])

or PrevTSValue(Branch:VariableName[Scale Unit, Dimension = Item])

or PrevTSValue(Branch:VariableName[Scale Unit, Dimension = Item], TimeStepsPrevious)

or PrevTSValue(Branch:VariableName[Scale Unit, Dimension = Item], TimeStepsPrevious,EndOfPreviousTSInterval)

or PrevTSValue(Branch:VariableName[Scale Unit, Dimension = Item], TimeStepsPrevious,EndOfPreviousTSInterval,
FunctionToCompute)

or PrevTSValue(Branch:VariableName[Scale Unit, Dimension = Item], TimeStepsPrevious,EndOfPreviousTSInterval,
FunctionToCompute, PercentileValue)

or PrevTSValue(MODFLOWResultVariableName[Layer, Row, Column], TimeStepsPrevious,...)

**Description**

The PrevTSValue function can be used in two ways. One is to return results
calculated in previous timesteps. The other is to return past or future
values from *Data* variables. The
parameters are:

**Branch:VariableName[Scale Unit, Dimension
= Item]:** Name of branch and variable from which to read previous
results. If omitted, then the result will be the value from the previous
timestep of the current Branch:Variable. Scale
and Unit are optional, and can be used to convert to any unit of the same
unit class (e.g., volume, area, flow). A
few result variables have extra "dimensions," e.g., source,
water quality constituent, crop or cost/benefit type. For
these, you can specify the dimension inside the square brackets, after
the unit. If
omitted, WEAP will sum up the values across all items, e.g., for costs,
it will add Capital Costs, Operating Costs and Benefits.

**TimeStepsPrevious**:
Number of previous time steps to look back. For example, a value
of 1 returns the result from the previous time step, 2 returns the value
from 2 time steps previous to the current time step, etc.. If omitted,
the function will default to 1 time step previous.

**EndOfPreviousTSInterval**:
The other end of the interval of time steps previous to use. For
example, if TimeStepsPrevious is 1 and EndOfPreviousTSInterval is 4, then
results are returned from the previous 4 time steps. If TimeStepsPrevious
is 2 and EndOfPreviousTSInterval is 5, then results are returned from
the interval from 2 time steps previous to 5 time steps previous (a total
of 4 time steps). If omitted, the function will default to just the one
time step specified by TimeStepsPrevious.

**FunctionToCompute**: Operation
to execute on results: Sum, Average, Min, Max, Median, CV, Percentile,
AverageAnnualSum. The function can be specified either by its name or
by its numerical code: 0 for Sum, 1 for Average, 2 for Minimum, 3 for
Maximum, 9 for Median, 8 for CV (coefficient of variation), 7 for percentile,
and 10 for AverageAnnualSum. If omitted, the operation will be Sum. If
any of the previous timestep values are the MissingValue (-9999), then
the operation (sum, average, ...) will also equal the MissingValue.

**PercentileValue**:
Only used if FunctionToCompute is Percentile. Values range from 0 to 100.
If omitted,
0 will be used.

**MODFLOWVariableName[layer,
row, column]**:
To access MODFLOW results for individual
cells, replace Branch:VariableName with MODFLOWVariableName[layer, row,
column].

**Examples**

If( PrevTSValue > 15, PrevTSValue / 2, PrevTSValue + 1)

This example looks at the previous value of the current branch:variable.
If it is greater than 15, then reduce by half. Otherwise, increase by
1.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume)

This example calculates Central Reservoir's storage from the previous month.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume[Million m^3, 3)

This example calculates Central Reservoir's storage from three months ago,
in million cubic meters.

PrevTSValue(Demand Sites\South City:Pollution Generation[kg, WQ Constituent=BOD])

This example calculates the amount of BOD generated by South City.

PrevTSValue(Supply and Resources\River\Weaping River\Returns\Return
Flow from South City:Pollution Loads[kg, Source=South City, WQ Constituent=BOD])

This example calculates the amount of BOD that flows into the Weaping River
return flow node from South City. (There
is also inflow from South City WWTP, but it won't be included.)

PrevTSValue(Demand Sites\Agriculture West:Supply Delivered[m^3])

This example calculates the amount of supply delivered to Agriculture West
from all sources.

PrevTSValue(Demand Sites\South City:Net Benefit[$, Cost/Benefit Type=Operating
Cost])

This example calculates the amount of operating cost at South City.

PrevTSValue(Demand Sites and Catchments:Production[kg, Crop=Corn])

This example calculates the total annual production of the PGM crop named
"Corn" for all catchments.

PrevTSValue(Demand Sites and Catchments\Big Farm:Crop Yield[kg])

This example calculates the total annual yield of all crops and branches
for the catchment named "Big Farm."

PrevTSValue(Demand Sites\South City:Unmet Demand, 1, 4)

This example calculates the sum of all unmet demands at South City for
the previous 4 months.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume, 1, 4, Average)

This example calculates the storage in Central Reservoir, averaged over
the previous four months (a moving average).

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume[Million Cubic Meter], 1, 4, Minimum)

This example calculates the minimum storage in Central Reservoir over the
previous four months in million cubic meterst.

PrevTSValue(Supply and Resources\River\Weaping River\Reservoirs\Central
Reservoir:Storage Volume, 1, 6, Maximum)

This example calculates the maximum storage in Central Reservoir over the
previous six months.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume, 1, 12, Median)

This example calculates the median (50% percentile) storage in Central
Reservoir over the previous twelve months.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\Central Reservoir:Storage
Volume, 1, 12, Percentile, 5)

This example calculates the 5% percentile storage value in Central Reservoir
over the previous twelve months.

PrevTSValue(Cell Head[1,57,30], 1, 12, Minimum)

This example calculates the minimum groundwater head elevation of
cell at layer 1, row 57, column 30 over
the previous twelve months, as calculated by a linked
MODFLOW model.

Other examples of MODFLOW results:

PrevTSValue(Recharge Volume[1,7,7], 1, 3,
Sum)

PrevTSValue(Constant Head Flow Volume[1,17,3])

PrevTSValue(Flow Right Face Volume[2,10,14])

PrevTSValue(Evapotranspiration Volume[1,16,3])

PrevTSValue(Leakage Depth to River[1,14,14])

This function can also be used to return previous values from data variables
found in the *Data* view, such
as the "Top of Conservation" for reservoirs.

PrevTSValue(Supply
and Resources\River\Weaping River\Reservoirs\North Reservoir:Top of Conservation,
1, 4, Average).

This example returns the average of the Top of Conservation variable for
the North Reservoir over the four previous months.

The function can also be used to "look ahead" at a data variable. In this case, specify the time steps to look ahead as negative numbers for TimeStepsPrevious and EndOfPreviousTSInterval (because a negative number of timesteps previous means a positive number of timesteps ahead).

PrevTSValue(Supply and Resources\River\Weaping River:Headflow, -1, -4,
Average)

This example returns the average headflow over the future four months.

**Tip**

To find out which result variables are available to use with PrevTSValue, go to the Branches tab of the Expression Builder, and drag a branch, such as a reservoir or groundwater node, to the expression area at the bottom. A menu will appear that lists all the available variables -- data variables first, followed by the result variables. The result variables will include the notation "calculated result from previous timestep."