Example 2 - How likely is it that the drought will end in x months?
Droughts can last for months or years. If we could forecast the end
of a drought accurately, we could sometimes avoid drought impacts. Looking
ahead a month or two, for instance, we might tell farmers it was OK to
plant crops because it was going to rain and that would restore soil
moisture levels. If we had 1 or 2 year forecasts, we might continue
to make normal releases from reservoirs because we would know the
reservoir would not run dry. These decisions must be made in preparing
drought response plans and in reacting to droughts while they occur.
Estimating the severity and duration of the drought is just one part of
assessing the risks of a particular drought response.
Water managers sometimes assume a "worst case" condition, in which no precipitation is assumed to occur in the
next x months. If this worst case scenario is very unlikely to happen,
then water managers may be depriving people of water unnecessarily. Water
managers can improve their understanding of the risks of this
"conservative" strategy by estimating the chances that:
-
the region will experience "record low rainfall" in the
next x months
-
the region will experience a 50 year drought, even if that sets a new record for low
precipitation in the next x months
-
the region will recover from the
drought in the next x months.
1. To estimate the chance that the region
will experience record low rainfall in the next x months,
determine the record low precipitation for the x month period,
calculate the ratio of record low to the average precipitation for the same
period, select the Atlas precipitation cluster and spreadsheet for the
region in question, and then compare the ratio just computed to the ratios
for that cluster for each non-exceedance frequency. Interpolate or
bracket the answer.
2. To estimate the chance that the region will
experience a very severe (50 year) drought in the next x months, calculate the average precipitation for each station in the
analysis, select the Atlas precipitation cluster and spreadsheet for the
region in question, and find the ratio for the 2% non-exceedance frequency
for x months duration starting at the current month, and then
multiply the average precipitation for each station times that ratio.
3. To estimate the chance that the region will recover
from drought in the next x months, calculate the amount of
precipitation needed to recover, calculate the ratio of that amount to the
average precipitation for the same duration and starting month, and then
determine the non-exceedance frequency for that ratio by examining the table
of ratios for that cluster.
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revised 1 Aug 2006
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