WSP03745

• File Number: 8111.815
• Description: Arkansas River Compact Administration
• Basin: Arkansas
• Date: 4/1/1990
• Author: UCACOE
• Title: Drought Contingency Plan - John Martin and Trinidad Lake - Appendix D
• Water Supply Pro - Doc Type: Publication

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Trinidad Lake experienced approximately the same type of droughts which occurred
at John Martin Reservoir. When it appears that a drought will be of
significance, the PDSI can be compared with previous droughts to establish a
reference for comparison. The inflow, outflow, and PDSI plates can be used to
identify the onset of a possible drought.

3-02. Severitv.

a. Drought Indicators. The impact of drought is proportional to its
severity and its duration. Its effects vary on different segments of the
popu!ation. Some of the indicators which may be used to describe droughts are
presented in the following paragraphs. Each measures one or more effects of
drought, but none in itself presents a comprehensive picture of the drought
period. It is from indicators such as these, however, that the Corps and other
water management agencies must make decisions in response to drought conditions.

(1) Palmer Drought Severity Index. The National Weather Service
(NWS) utilizes a numerical index, referred to as the Palmer Drought Severity
Index (PDSI), to quantify the status of climatically induced water balance
between rainfall and soil moisture. This index (PDSI) is routinely calculated
on a statewide basis for Colorado and separately for each NWS climatological
division within the state. Plate 3-1 shows the five climatological divisions
in Colorado. These indexes are made available on a weekly basis. The PDS1 is
regionally specific and reflects the cumulative excess deficiency in moisture
relative to seasonal norms. It typically ranges from +7.5 to -6.5 but may exceed
these values in the case of long duration or abnormal rainfall. The PDSI scale
along with the NWS characterization of the corresponding moisture status is shown
in Table 3-3.

Table 3-3

Palmer Index
Classes for Wet and Dry Soils

PDSI
> 4.0
3.0 to 3.9
2.0 to 2.9
1.0 to 1.9
O. 5 to O. 9
0.4 to -0.4
-0.5 to -0.9
-1. 0 to - 1. 9
-2.0 to -2.9
-3.0 to -3.9
< -4.0

Class
Extremely Moist Spell
Very Moist Spell
Unusual Moist Spell
Moist Spell
Incipient Moist Spell
Near Normal
Incipient Drought
Mild Drought
Moderate Drought
Severe Drought
Extreme Drought

The PDSI is a standardized indicator and is insensitive to man-made
drought. It provides a drought severity indicator that can be used to compare
diverse climatological regions. It is published by the NWS and is readily
available. Plate 3-7 shows the PDSI values for the Arkansas River Basin for the
period 1895 through 1987. Also, presented previously in Plates 3-2 through 3-

3-4