The Great Lakes water levels are like a puzzle with many pieces.

I've been writing about the weather part of the water level puzzle.

The amount of water going into the Great Lakes due to weather factors is a fairly straightforward formula.

The formula is rainfall on the lake and over the drainage basin minus the evaporation of the lake water.

The drainage basin is the area of land surrounding a Great Lake where any rainfall drains into a river or stream, and then runs into the Great Lake.

The Lake Michigan and Lake Huron drainage basin is all of Michigan, and small parts of Illinois, Indiana, Wisconsin, and Ontario.

'Net basin supply' holds the key

'Net basin supply' is the total amount of water Lake Michigan and Lake Huron are getting through weather.

'Net basin supply' is rainfall over the lake plus runoff into the lakes from rivers and streams, minus evaporation.

In an earlier blog, I showed how evaporation is thought to be increasing over Lake Michigan and Lake Huron.

I use the word "thought" because we don't actually have instruments that will measure evaporation. The Great Lakes Environmental Research Laboratory has to model the evaporation.

So the numbers are theoretical. However, since the process is done the same for every year, the evaporation numbers can certainly show a trend.

I also showed in an earlier blog that precipitation amounts are on the upswing, and snowfall is on an upward trend.

Yes, that is hard to believe given the dry spells we've had the last few years.

The graphic shows 'net basin supply'

The graphic shows a five year running average of the 'net basin supply.' That means I take five years of supply and average them. Next, I kick off the oldest year, and add on the next year. I then average those.

Making a running average helps to smooth out the strange years and helps you visually pick up on a trend.

The trend for 'net basin supply' is definitely on the downswing from a high in 1986.

It should come as no surprise that Lake Michigan and Lake Huron water levels were at record highs in 1986.

It also should come as no surprise that significantly less 'net basin supply' since 1998 is one of the big factors in our recent record low water levels.

Let's use 40 inches of water supply as an average in the 1970s and 1980s, and use 32 inches of water supply on average for the last decade. That results in a reduction of 80 inches of water supply over the last ten years.

It is amazing how much water the lakes receive

The one take away I got from my research is Lake Michigan and Lake Huron are getting a lot of water each year.

From surplus precipitation, the amount of water is somewhere between 28 and 42 inches a year.

That also means if the water levels are going down, outflows and water usage are more than the water running into the lakes.

I'll have a blog for you next week that covers the outflows from Lake Michigan and Lake Huron. That's another piece of the Great Lakes water level puzzle.

If you have any questions on the information above, ask below.

Mark Torregrossa has been the Chief Meteorologist for three television news stations in Michigan. A resident of the state for 20 years, he is also a master gardener and avid hunter. Email him at mark@farmerweather.com and find him on Facebook at facebook.com/mark.torregrossa and Twitter @weathermanmark