Published in Volume 100, Number 7, Page 9 of
The Cheyenne Star, February 2001
The Rain on the Plains
Feast or Famine
J.
Peter Thurmond and Allison L. Thurmond
Here for your entertainment and edification are a few
more graphs showing some interesting patterns in local rainfall. The 15 year
running average graph at right shows the long-term trend in average annual
rainfall since 1893, the beginning of the instrumental record in western
Oklahoma (the data on which this
graph is based can be viewed at:
http://www.geocities.com/dempseydiv/table_3.htm).
Despite the ups and downs, you can see the overall trend is an increase in average annual rainfall over time, notably punctuated by the Dust Bowl of the 1930s. Average annual rainfall has increased overall by about 25% over the past century, and the rate of increase is accelerating in recent decades. This is a common pattern across Oklahoma. Average annual rainfall has increased 10-25% throughout the state during the 20th century (Karl et al. 1996).
So
it's raining more in a year's time than it used to. That has to be good, right?
Well, yes and no. What worries us is how we're getting all that rain.
Take a look at the graph below of monthly rainfall since 1990. The data come
from the USDA-NRCS office in Cheyenne. There's a pretty striking difference
between the records before and after 1995. We've traditionally gotten around
70% of our rain during the growing season months of April to September, with
May and June usually being the wettest months. But the moisture was scattered
decently through the year, so that we got something most every month.
Look again at that monthly rainfall graph. What's happening is that we're getting most of our rain in torrential rainfall events, punctuated by periods of one to three months with little or no rain. Kind of a "feast or famine" pattern, which is not good. It's hard for the soil to hang onto anything much over an inch falling all at once, and we've had some real doozies since 1996. This is the rainfall pattern that has accounted for the seeming contradiction of the county's forage production declining enough for us to qualify for USDA Livestock Assistance Program disaster relief 1997-2000, when the rain gauge shows we got 27-30".
The
second author pulled together the numbers on flooding rainfall events in Roger
Mills County several years ago for a state science fair project, and we've kept
up with it since (see graph at right). Rainfall events where we get more
than 2" in 24 hours are twice as common since
1995 as they were in the period 1950-1995. The average 24-hour rainfall for
such an event before 1996 was about 2.5", and a 72- hour storm total
rarely exceeded 3". Since 1996, 24-hour totals of
3-5" are common, and we've been seeing 72-hour storm totals of 4-10".
The record 72-hour storm total before 1996 was 5.13" in the 1970s, and
72-hour totals (barely) exceeded 4" on only two other occasions in the
1950-1995 record. A 4" 72-hour total is now the average
for a big warm season storm system. A four inch rain certainly fills the
ponds, but…
So we're in a pattern the last few years of overall rainfall lower than normal, punctuated by a few torrential rainfall events that raise the year-end total to a deceptively high number. Throw in the two hottest summers on record for Oklahoma in 1998 and 2000, and it's not surprising that the grass hasn't done too well.
There's another culprit as well, as the graphs of five year
running averages of winter and summer precipitation on the next page show. There
are remarkably regular cycles in both winter (here defined as
October-March) and summer (April-September) rainfall of about 21-22 years,
probably linked to the sunspot cycle. Study of proxy climate records that go
back tens to hundreds of 
thousands
of years from around the world show climate cycles on scales of decades,
centuries and millennia, and fluctuation in the energy output of the sun is
linked to many of these.
The decade-scale cycles in local winter and summer precipitation went out of
ase after 1960, as the period of the winter cycle shortened to about 15 years. If you compare the far right sides of the graphs, you can see that we are simultaneously at the bottom of a summer cycle and
the top of a winter cycle. It's pretty likely that we're in for several more hot, dry summers and wet winters. The NOAA long-term forecast indeed calls for next winter to be much like this one on the Southern Plains, with above-average moisture. You can see the NOAA forecast on the Internet at:
http://www.cpc.ncep.noaa.gov/products/predictions/multi_season/13_seasonal_outlooks/color/page4.gif
Feast or famine, on many different time scales. It's a tough place. Let's hope the flood/drought pattern is a temporary blip and doesn't continue, but it's one of the things climatologists have been predicting would happen here as one result of greenhouse gas forcing on climate ('global warming'). If you want to see a summary of climate changes predicted for Oklahoma over the next century, check out this website:
http://www.epa.gov/globalwarming/impacts/stateimp/Oklahoma/index.html