 So let's look at a simplified example of water supply modeling. In this case we're going to consider having a reservoir somewhere, a dammed up river or a lake or some place where water has been accumulated and is being used as a supply of water for some municipality. And in a case like this we have a very simple, it's basically a budget that we're going to work on that we're going to consider how much water is in the reservoir which is something we call storage. And that storage is going to be a volume. Usually that volume is going to be something measured. A volume might be measured in cubic meters and in this case we're going to actually talk about a million cubic meters, million meters cubed or mcm, a million cubic meters. Okay so that's the term we'll use here is a million cubic meters. So what's a large amount of water that we're dealing with? And our budget basically says that we're going to have things that are flowing in inputs to our reservoir and outputs to our reservoir. And it's simply a matter of keeping track of those inputs and outputs that we're going to have a change in our storage and that change in the storage is going to depend on the inputs minus the outputs. And then what we can think about is if we care about how much storage we have at any one time the storage later is going to be equal to the storage earlier plus that change in storage. It's the other way that we can kind of look at it that we basically change the storage by adding the inputs and outputs. In other words our storage later is equal to the storage earlier plus my inputs minus my outputs. And again it's as simple as maybe managing money or some other thing along that line. You're basically keeping track of what goes in and what goes out. What makes a problem like this difficult is knowing what those numbers are. How do you know what the numbers are? How do you know how much stuff people are going to use? How do you know what the inputs are going to be? And generally we don't know what those inputs are ahead of time but what we need to do is we can use historical information to make an estimate of what we expect to happen. We can go back and do an average of what's happened over past years five years or ten year time frame and gather that information or we can actually um and we can do the same thing with our outputs what people have used or make estimates or we could even limit that if we're talking about how much gets used we can actually sort of set a certain amount that we're actually going to use and disperse. So in a simple system like this I might consider for example that the inputs might simply be precipitation that we're able to measure how much rain is falling off and or running off input from precipitation as a result of um measurements that we've seen in the past and we might also talk about use. Notice the precipitation is an input and the use is an output and if I actually want to model my water supply to see how much water I am going to have over a period of time I simply I need to know how much water I'm starting with so let's start with a certain water amount here we'll say we have a storage of 1000 mcm okay we'll start with a certain storage now it's important in most those cases that there's going to be some level a maximum level of storage at which point we're overflowing the reservoir we can no longer hold any more water and there's also going to be some sort of minimum level of storage and this is what we call dead volume or dead storage which is a level at which the water it's either below our ability to draw the water out or it's below some sort of safe level for the the wildlife or the aquatic vegetation or whatever might be there it's some level at which we can no longer even though there is water there we can no longer go below that level and obviously we want to try to avoid reaching that dead volume so how do we go about doing this again it's simply a matter of accounting I can look for example in the month of June let's assume that it's at the end of the month of May and here's our initial storage that we have and we can go and look and compared to if we see here we have a value of 117 million cubic meters that we estimate are going to flow in based on history of what happens in June precipitation in June so we can add our 117 mcm but at the same time we're expecting that there's quite a bit of use that's occurring at this time from the nearby municipality of 142 minus 142 million cubic meters notice that net amount let me write that here minus 142 million cubic meters notice that net amount is going to result in us having less storage over our time period so let's see here that's seven minus two is going to be let's say a thousand one woman seven seven minus two is going to be five and then we get 11 minus four is seven and then I believe it's going to give us around 975 mcm and that's going to be our storage at the end of June okay and notice that is again a reduction in the amount of storage now we may not know exactly what that translates to in the height of the reservoir but we do know that volume similarly if we want to keep going with this process and this is something that is easily implemented on as something like a spreadsheet we can do the same thing in July we might expect a larger amount of rain looking at the value here I see 158 million cubic meters and then we use 138 million cubic meters perhaps there are fewer people historically people go on vacation or something along those lines or we have reasons to believe that we're going to use a little bit less and in this case you'll notice the net there is an addition of about 20 and we end up with 995 million cubic meters and that would be our storage for the months of July so as you can see it's simply a matter of adding and subtracting these pieces the difficult part of this is making the appropriate estimations for these values or gaining these values either from history well generally from history the other thing that can be difficult is that there are other pieces that balance it isn't always as simple as precipitation or just the use there might be other things that might occur for example evaporation particularly in the hot summer months you're going to have a fair amount of evaporation that evaporation is going to depend on the surface area of your reservoir which will depend in turn on how much storage you have so it can get fairly complex fairly quickly and making estimates on things like evaporation similarly things like groundwater exchange and or other uses might complicate that system a little bit however you can do some simple water supply modeling using addition and subtraction and this relationship between storage input and output