Monday, April 20, 2009

Rain & Gallon Buckets; How Do They Relate?

Pretend we had a method to collect every single drop of moisture that fell from the sky before it evaporated, got absorbed into the ground, or ran off...and we filled empty gallon buckets.

How many would we have needed in the Denver metro area this past weekend?

To put this storm a little more into perspective...let me give you a conversion to gallons of water instead of inches.

Of course this is an estimate only -- and meant to illustrate a point.

Denver International Airport

The Denver International Airport sits on 53 square miles of land, the largest US airport in terms of size and the 2nd largest in the world.

During our 3-day storm, the airport recorded 2.45 inches of water. (some melted snow)

Assuming that the moisture fell uniformly over the entire airport, meaning each square mile of the 53 square miles saw 2.45 inches of water, we can make the following calculation of converting the moisture that fell into gallons.

Before we convert into gallons, let's establish some standards for when 1 inch of rain falls over 1 acre of land.

  • 1 inch of rain over 1 acre of land is equal to 27,154 gallons of water

  • 1 inch of rain over 1 square mile is equal to 17.38 million gallons of water. (Note: There are 640 acres in 1 square mile)

    OK...let's calculate how many gallons of water fell over Denver International Airport.

    First take 53 square miles and multiply that by 17.38 million gallons. This tells us that 921,140,000 gallons of water falls with 1 inch of rain over the airfield.

    So now take the 1-inch standard we calculated above and multiply that by the total amount of moisture collected.

    921,140,000 gallons of water (X) 2.45 inches of precipitation = 2,256,793,000 gallons!!

    So over 2 billion gallons of water fell over the 53 square-mile airport property.

    City of Denver

    So when looking at a larger footprint, such as the city of Denver, which is about 155 square miles, that calculation would look something like this...

    17.38 million gallons of water over 1 square mile (X) 155 square miles (X) 2.45 inches of precipitation...

    = 6,600,055,000 gallons of water over the city of Denver

    7-County Denver Metro Area

    And to really blow your mind, let's say the average precipitation over the entire Denver metro area was 2.45 inches...

    The 7-county Denver metro area is about 4,531 square miles, which is roughly the size of the state of Connecticut.

    17.38 million gallons of water over 1 square mile (X) 4,531 square miles (X) 2.45 inches of precipitation...

    = 192,934,511,000 gallons of water over the 7-county Denver Metro area -- yes that is over 192 billion gallons of water

    Water, The Lifeblood

    So now let's turn the tables and say the area is in a drought, and finally a little tiny storm system moves through and drops a quarter-inch of rain over the 7-county Denver metro.

    That is still over 19 billion gallons of water. (19,687,195,000 gallons of water to be exact)

    We have a saying in CoCoRaHS and that is "EVERY DROP COUNTS" -- and now hopefully you see why that is so.

    What About Runoff?

    Now does all this water get absorbed into the ground?

    Of course not. There are a number of factors involved with what happens to the water once it falls from the sky, including evaporation and runoff.

    Here are some other factors that determines how the water gets absorbed and where it might end up.

  • Rate of rainfall -- luckily this event we just had in Colorado was either snow or a light but steady rain, so the ground will absorb a lot more than if that all fell within a few hours during intense thunderstorms

  • Topography -- this obviously impacts the runoff as water falling over unlevel land will run downhill until it becomes part of a river or stream, finds a place to accumulate like in a lake, or gets absorbed into the ground

  • Soil conditions -- clay soils don't absorb water as well as sandy soils for instance

  • Density of vegetation -- the more plant cover, the less runoff and erosion

  • Amount of urbanization -- the amount of impervious surfaces (i.e. concrete or asphalt) impact runoff and force the water into creeks, rivers and streams where nature didn't necessarily intend for the water to go -- which is why despite the engineering put into to flood mitigation, sometimes with huge precipitation events, we still see flooding in our cities

    Today's Weather

    Well get ready for a warm one across portions of the country, especially the west.

    90-degree heat is likely all the way into northern California and the greater Phoenix area will be knocking on the door of their first 100-degree temperature of the year.

    This is early for that -- typically the first 100 doesn't get recorded until May.

    The cause of the early season hot streak is a large ridge of high pressure. The picture below is compliments of the NWS office in Phoenix.

    In fact, this high pressure is bringing 90-degree temperatures to the San Francisco Bay area today where a heat advisory in in effect.

    But don't worry, it won't last long -- by Friday you will be back to highs in the upper 50s with a chance for rain.

    And although it is cool and damp in the northeast, things will dry out and warm up later in the week.

    Severe storms are possible along the southeast coast of the US today.

    Your Comments

    There were a few comments left on yesterday's blog that I want to answer real quick.

    OSNW3 -- yes, Denver is one of the "sunniest" cities in the states. I really think ranking the sunniest locations depends on the criteria set forth, such as what days count (i.e. days with 50% of the possible sun, 60%, 90%, etc.) and how many years of data are available.

    Here is a web site that ranks the top major cities in the US for sunshine. Click here. You will often see Denver advertise itself as having over 300 sunny days a year.

    Due to our dry climate, most all days start out sunny. We very rarely have a morning with all clouds, and if we do, it is more often during the cold season.

    The National Climate Data Center ranked cities based off data available through 2004. Here is that link.

    I find it interesting to see my home town of North Little Rock, Arkansas ranked higher than Denver, Colorado. I would have never guessed that being as humid as the climate is in Arkansas.

    And this question came from an observer in Texas.

    A couple of weeks ago on our NWS site one of the weather historical facts was a snow in TX that totaled 25" but never exceded a depth of 4" because it was melting as fast as it fell. Question: as a CoCoRahs observer how does one get a measurement of such a snow?

    I am going to send it to Nolan and I will post his answer when he gets back to me.

    1. Fantastic equations! Gallons upon gallons of water. The WGN weather blog (Channel 9 out of Chicago) just put together a Great Lakes tidbit. Sorta relates to the this massive water talk... Great Lakes tidbit

      As far as the sunshine goes, I like it. It's great for our serotonin levels. :) Amazing that most days during the year, you Denver folk get to see a sun rise. Cool.

      I keep track of "Days with Sun" here at OSNW3. It's crude... but I like it. Days with Sun

    2. Great post Chris! I am amazed when I see just how much water really falls from the sky. Thanks for crunching the numbers for us!

      I feel like I need to take a day off of work to dig into all the great info.

      Thanks for your hard work on the blog.