Predicting Drought
Eric Peterson - University Extension Agent, Sublette County
"Droughts Happen!" Clearly it's been happening since at least the Pleistocene. It's probably
been happening since North America drifted into the zone of prevailing westerlies and the
Laramide Orogeny created the Rocky Mountains rain shadow 50,000,000 years ago...as long as
there have been cold ocean currents off the west coast. Unfortunately, our inability to make
reliable predictions of drought generally hamper our ability to be proactive toward management
for drought.
In this part of the world, drought affects range livestock agriculture in two important ways.
First is the effect that winter drought has upon snowpack and resultant streamflows. Many
streams in the State are very susceptible to water shortages as a result of low snowpack. There
isn’t much challenge in predicting the streamflow supplies, as the United States Geological
Survey, Natural Resources Conservation Service have excellent data gathering and analysis
programs and the data is readily available.
When the NRCS Snotel surveys alert us to critical shortfalls in snowpack, we know that
irrigation water supplies will be critically short. Without water, haylands are virtually non-
productive and our ranching industry is extremely reliant on the ability to produce winter feed
stores from those haylands. We can see this problem coming and hopefully make proactive
decisions to help get through this drought challenge.
The second way that drought can affect the ranch industry is in rangeland grass production.
Without adequate soil moisture during the growing season, range grasses just don’t produce and
the forage base for the spring, summer and fall is compromised. Predicting this element of
drought, and it’s degree of severity is much more difficult.
There are a number of factors which contribute to summer range drought and it’s severity.
Among them are the amount of soil moisture we had going into the fall, the amount of wind,
temperatures, summer rains, and the timing and amount of spring rainfall. Of all of these, the
timing and amount of spring rains is probably most important.
Predicting an annual forage yield early in the forage production and grazing season is
essential for effective livestock management planning. A fourteen-year study in the in the
Saratoga area of Wyoming shows us that a high correlation exists between forage production
and moisture timing.
Regressions were prepared for Winter (October-March), Winter plus April, April, May and April
plus May precipitations. The results are: Winter- No correlation, Winter plus April - 9%, April -
43%, May - 20%, April plus May - 34% correlation.
As you can see, in Saratoga, April provided a good predictor of available forage in a short
amount of time. It also comes early enough in the season for profitable management decision to
be made. The point to be learned, is that range plants have critical growth periods in which
adequate moisture is extremely important to the amount of production the range ultimately
produces. It is wise to factor spring rains, or lack thereof, into your management decisions.
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