Abstract:
This article evaluates drought scenarios of the Upper Colorado River basin (UCRB) considering
multiple drought variables for the past 500 years and positions the current drought in terms of the magnitude
and frequency. Drought characteristics were developed considering water-year data of UCRB’s streamflow, and
basin-wide averages of the Palmer Hydrological Drought Index (PHDI) and the Palmer Z Index. Streamflow and
drought indices were reconstructed for the last 500 years using a principal component regression model based
on tree-ring data. The reconstructed streamflow showed higher variability as compared with reconstructed
PHDI and reconstructed Palmer Z Index. The magnitude and severity of all droughts were obtained for the last
500 years for historical and reconstructed drought variables and ranked accordingly. The frequency of the current
drought was obtained by considering two different drought frequency statistical approaches and three different
methods of determining the beginning and end of the drought period (annual, 5-year moving, and ten
year moving average). It was concluded that the current drought is the worst in the observed record period
(1923-2004), but 6th to 14th largest in terms of magnitude and 1st to 12th considering severity in the past
500 years. Similarly, the current drought has a return period ranging from 37 to 103 years based on how the
drought period was determined. It was concluded that if the 10-year moving average is used for defining the
drought period, the current drought appears less severe in terms of magnitude and severity in the last 500 years
compared with the results using 1- and 5-year averages.
