CHARACTERIZING AND ESTIMATING SPATI AL AND TEMPORAL VARIABILITY OF TIMES BETWEEN STORMS
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An exploratory investigation was conducted on methods to characterize times between storms and estimate a
parameter used to identify storms in a precipitation record for the purpose of simulating the occurrence of storms. The
parameter is called the “critical duration” (CD) and is fundamental to the operation of a storm–generator model. It is defined
as the minimum dry period between storms that separates bursts of rainfall into statistically independent storms. Data used
covered an area of about 225,000 km 2 in the plains area of Colorado, Nebraska, Kansas, and Wyoming for the months from
May through September. The exponential method that yields an exponential frequency distribution of times between storms
(TBS) appears to give reasonable fits to the data. However, some precipitation gauge data yield CD estimates that appear
to be outliers for individual months.
CD varies noticeably with month and location. An equation developed earlier for estimating CD over a wide range of
climates is inadequate for estimating monthly CD values. Computing CD by collapsing TBS and precipitation data into 2–,
3–, 4–, and 5–month periods did not improve the estimating capability of the equation. Separate regressions for monthly CD
data against average monthly precipitation were statistically insignificant for all months, and led to computing averages of
the data for each month. A multiplicative power equation that was successful in another study to estimate monthly CD did
not work as well in the present study. Mapping CD over the study area appears to be the best method to estimate CD on
ungauged areas for parameterization and storm modeling. The study results are useful for guiding storm–generation model
development and parameterization, and drought studies.