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:
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.
revised 1 Aug 2006