US Precipitation Trends

By Paul Homewood

 

http://gis.ncdc.noaa.gov/map/cag/#app=cdo

 

I had a quick look at US drought trends the other day, but let’s look at the topic in more depth.

The above map shows that in the period October 2012 – September 2013, most of the country has had around or above average precipitation. Only Texas, California and Nevada are below normal, even then not significantly so. For instance, Texas is ranked 19th driest since 1895, hardly a rare event.

Am I cherry picking by looking at Oct – Sep numbers? Well, no actually! According to Water UK

 

The ‘Hydrological New Year’ occurs on 1st October (in the Northern Hemisphere) and it’s the point when the hydrological cycle is in balance. After 1st October rainfall starts to fill up the water reserves in the ground, until 1st April (middle point of the Hydrological Year), when evaporation starts to deplete this stored water. This carries on until 1st October, when it starts to replenish, and the cycle begins again.

 

Put another way, the rainfall that is relevant for the growing season of Spring and Summer is what has fallen in the previous Autumn and Winter, and not the seasons that follow.

 

Now let’s take a look at US precipitation trends since 1895. (Don’t blame for the colour scheme, that’s NOAA’s fault!)

 

http://www.ncdc.noaa.gov/cag/

 

A number of things become apparent:

• Precipitation this year is above average.
• Last year’s figure, while below normal, was well above many other years on record.
• Long term trends indicate an increase in precipitation.

What about the regional picture? Obviously there are regional variations, so let’s take a look at the various agricultural belts.

 

 

 

 

 

 

There is obviously a certain amount of overlap between the various belts, but they all show droughts in the 1930’s and 50’s that were much worse than anything seen in recent years. And the long term precipitation trends are either up or flat.

The bottom line is that there is no evidence at all that “droughts are getting worse” in the US.

 

 

 

References

All data from NOAA

http://www.ncdc.noaa.gov/cag/