By Becky Bolinger
Originally published at http://livestockwx.com/drought/top-states-and-counties-in-drought-over-the-last-two-decades/
Note: Figures 2-3 and 6-7 have interactive features. Clicking on the maps will take the user off-site where you can access the interactivity.
Some Regions are More Drought-Prone than Others
Drought is a pretty complicated climate extreme to define. Most commonly, it is defined as a precipitation deficit - much less precipitation falls than what is typically expected for a given time period.
But drought can be observed in lots of ways: lack of snowpack, low flows in rivers and streams, dry soils, higher evaporative losses, low lake/reservoir levels, etc. Because the climate varies all the time, it’s a safe assumption to say that all areas experience drought similarly because all areas will see times when precipitation is (or the other water variables) less than average.
But drought is not equally distributed across the U.S. Just as there are climatologically wetter areas and drier areas across the country, the same could be said about the occurrence, frequency, and severity of drought.
So, some might be asking "where are the most 'drought-prone' areas?"
First, let's look at the states that experience drought most often, according to data from the U.S. Drought Monitor.
The Drought Monitor has been tracking drought conditions across the country since 2000.
Each week, the map is updated with the latest conditions ranging from none, to D0 (which is an “abnormally dry” category), to D1-D4 (which represents increasing severities of drought categories).
How the map is made is a big process that includes analysis of data that encompasses local/regional variability in all the water variables and adds a human component, like impacts.
The map categories are also based on the assumption that the more severe a drought is, the rarer it is. For example, a D1, or Moderate Drought, is expected to occur approximately 2 or 3 times out of every 10 years, and a D4, or Exceptional Drought, is expected to occur 1 or 2 times out of every 100 years (think of the commonly referred to 1-in-100 year event).
Since the Drought Monitor has been monitoring drought for nearly 20 years now, we may think that most locations would have experienced drought conditions about 30% of the time. To test this idea, I counted the number of weeks that at least half of each state experienced at least a Moderate Drought (D1) as defined by the Drought Monitor.
The above map shows the percent of time in the last 20 years that each state has been "in drought"based on these criteria. Right away we see that parts of the Great Lakes region, and most of the northeast U.S., have experienced drought less than 10% of the time (or not at all).
We begin to see an increasing frequency of drought as we move west of the Mississippi River. Drought may typically be expected to occur about 30% of the time, but states that are shaded dark orange, red, and maroon experience drought more than that. Interestingly, Nevada and Arizona (encompassing some of the driest regions of the country) have experienced drought more than half the time the Drought Monitor has been in operation.
Pasture, Rangeland, and Forage Insurance 2007-2018
Another way to look at recent drought trends, and the costs associated with them, is through insurance products like the Pasture, Rangeland, and Forage (PRF). PRF insurance offers livestock producers and hay growers protection against drought. Payments, based on acres insured, are triggered when precipitation for a specific area falls below historic average during a specific time period.
Looking at PRF data from 2007-2018 for each state, the total indemnity payments and the average loss ratio (indemnity compared to the total premiums paid) roughly tracks the Drought Monitor. See your state using the below interactive map. The average loss ratio is labeled for each state while you can use your mouse to click on your state of interest to see the total indemnity payment. Total indemnity payments, however, should be viewed with caution since some states have been in the program longer and the amount of indemnity payments are based on the value of the acres being insured.
USDA's Livestock Forage Disaster Program (LFP)
Disaster programs through USDA are yet another way to see drought. An important component of the Drought Monitor outside of tracking drought, is that it is used to trigger payments under USDA's Livestock Forage Disaster Program (LFP). The LFP provides payments to livestock producers that have experienced grazing losses due to a qualifying drought (this must occur during the normal grazing period for each county that is established by USDA's Farm Service Agency).
I looked at LFP data between 2011-2018 and plotted the total amount of payments and the number of payments for each state. The map is interactive. If you use your mouse you can see the payment information if you're having trouble reading the map.
Texas and Oklahoma far exceed the other states in payments since 2011. This is not surprising given livestock numbers in those states and given the intense droughts that have occurred in the Southern Plains since 2011.
Drought Frequency by County
Statistics for counties is very important in the world of drought. Many state drought response plans, and even several federal bills, rely on statistics for when the Drought Monitor depicts certain drought severity categories across county lines like the LFP. So, let's look at some of these county statistics.
Figure 4 shows how many weeks each county has had any amount of Extreme (D3) or Exceptional Drought (D4) within its boundaries. Again, you'll notice the Great Lakes region pop up because much of the area has yet to experience any Extreme Drought (D3) conditions since the start of the Drought Monitor. A region in Alabama/Georgia/South Carolina pops up as having more weeks with Extreme Drought. Counties throughout the high plains, west through the Rockies, and to the Pacific, have experienced many weeks of extreme drought in the last 20 years. Counties in red and dark red have had at least 200 weeks in Extreme/Exceptional conditions. That's around 4 years!
Figure 5 shows the number of weeks each county has experienced the highest drought category, Exceptional Drought (D4), and theoretically the most rare in occurrence. There are many counties across the country that have never seen Exceptional Drought. More than 100 weeks of Exceptional Drought have occurred in a handful of counties. Wilbarger County in Texas has seen Exceptional Drought in more than 200 weeks. The area around Wilbarger County, which also includes Wichita County and Tillman County in Oklahoma has really been ground-zero for intense drought since 2000. Most of the Exceptional Drought was observed between 2011 and 2015 and to a lesser extent in the 2006 drought. Figure 6 shows the Top 25 Counties observing Exceptional Drought over the last 20 years.
Counties Seeing the Most Intense Droughts: Exceptional Drought (D4)
Why are Some Areas More Vulnerable to Drought than Others?
Overall, these maps really highlight the areas of the country most prone to drought and specifically to the most intense droughts. But why do these areas experience drought and intense droughts more frequently? The answer is not simple, and it's not the same answer for every location every time. But here are a few reasons we could see these patterns:
- In statistics, we talk about average and standard deviation. With climate, we regularly talk about the average precipitation at a location, but don't often talk about standard deviation, which is what describes the variability of a location. Locations with a larger standard deviation in precipitation experience more variability, therefore can experience droughts more frequently.
- Water limited areas can more easily go into drought conditions throughout the water cycle. A region that gets over 60 inches of precipitation in a year may have a drier than average year that doesn't really impact streamflows or soil moisture. An area that gets less than 20 inches a year on average and is drier will more likely show up in other water variables, exacerbating those dry conditions.
- Impacts are an important piece of the puzzle when monitoring drought. One common saying in the drought community is: "if a drought occurs and nobody is there to experience it, did it really happen?" Basically, without impacts, how can there be a drought? Water limited areas, croplands and grazing lands, regions with more vulnerable populations (people and animals) will experience more impacts, hence can be more prone to drought.
Check out your county. Using your mouse, click on the map below to see the percentage of time or the number of weeks your county as been in these intense drought categories (Extreme Drought or greater).
Are you surprised by how many times it has experienced extreme drought? Is it what you expected?
About the author: Dr. Becky Bolinger is the Colorado Assistant State Climatologist. She is based at Colorado State University and is a frequent contributor to Livestock Wx, the U.S. Drought Monitor and tracks all things climate around Colorado.