The rule dialog works like a Hewlett-Packard calculator, using reverse Polish notation. The way to use it is to figure out the calculation sequence that would be needed to solve a problem and then save that sequence in the table on the right side of the calculator.

The tree on the left side of the calculator contains all of the variables both in the simulator and in the accounting system. The simulator variables should be self explanatory. There are four accounting variables, having to do with gain and loss transactions. These transactions are generated by the simulator and represent virgin flow and system losses at each node in the water network. Since these transactions will be passed in turn to each bin in the node, the calculator may deal with either the original amount or with whatever remains of the transaction by the time it gets to the bin having this calculator.

In the sequence shown in the image, the first thing on the stack (which is the last item to be used in the equation) is the total amount of the gain transaction. That amount is to be multiplied by 15% if the storage in Roosevelt is above 1.5 million acre feet. Otherwise, the amount is zero.

Finally, this rule is valid during all time steps. If we wished, we could have specified a within year period when this rule is to be used.