Plenty of rain – Neat statistics

(30-day rain, SE River Forecast Center)

Since I love weather and I love the river, it’s natural that I like to look at hydrology sometimes. Recently, I’ve been compiling some statistics from USGS and the NWS River Forecast Center, and some calculations using the output is interesting to me, and it might be to you, also.

The main theme…we’ve had a lot of rain across most of Alabama so far in 2009! We shouldn’t have to worry, at least for a good while, about drought conditions and heat waves, especially if the weather pattern stays active, bringing us rain about once every 3-5 days. The rivers and lakes are pretty full, the ground water table has improved significantly since 2007, and the amount of water that has gone down the major rivers of Alabama and into the Gulf of Mexico so far this year is simply amazing.

The chart above shows estimated rainfall over the past 30 days, and this excludes the major flood event in January. Most of central Alabama has received 5-10″, while south Alabama has received 10-15″! Some places from Baldwin County over into NW Florida have received over 20″.

Where has that water, and all the water that fell in earlier rain events this year, gone so far? A large amount of it has gone into the rivers. Most of the water in central and southern Alabama, from about Cullman south, flows into the Coosa, Tallapoosa, Cahaba, Alabama, Warrior, Tombigbee, and Conecuh Rivers. The first 3 all eventually drain into the Alabama River, and the Warrior flows into the Tombigbee at Demopolis. So, looking at the combined streamflow past a site south of Demopolis (Tombigbee), and another near Monroeville (Alabama), should give a good estimate of the water flowing into the Gulf from Alabama rain.

Look at the amounts here…they are in billion gallons per day! A peak of 181 bg/day occurred on Jan 11, after the floods along the Warrior River, another of 175 on March 4 after a heavy rain event, and then the largest occurred on March 30, when 206 went into the Gulf due to heavy rains, especially across south Alabama.

If you add up the flow over the whole year, about 6 trillion gallons of water have flowed into the Gulf of Mexico out of Alabama’s major rivers so far in 2009. Nature is awesome! Normally by this time of year, it’s only 5 trillion. So, it’s a lot of water either way, and we’ve had more than normal this year.

These two river systems drain about 37,000 square miles of land (Alabama’s total is 52,423 square miles). Over that area, an average of 20″ of rain has fallen so far this year. If you do the math, that’s 12.9 trillion gallons of rain so far this year. So, only half of it has run off into the Gulf. Some of it still in the now mostly full rivers and lakes, some of it is being used in homes, some has evaporated into the atmosphere, and some has gone into the soil and water table.

To look at the ground water table, here is data from a well in Cullman County. Notice how, in late 2007 after the drought, the water was about 51 feet below the surface. More recently, water levels have recovered to 30 to 35 feet below the surface. So, there is a lot more water under the ground than there was two years ago.

Looking back at daily streamflow at Demopolis and Monroeville since 2000, you can see the drought in 2007 were only 10-20% of normal, and how flows have returned to more normal levels this year.

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