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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7 are the historical PDSI plots for the drought periods described in paragraph
3-01. These plots can be used to compare current index trends with historically
similar PDSI trends. For storage based water supplies the duration of the
drought and its intensity are indicators of severity as it relates to the use
of conservation storage.

A drought is considered severe when there is insufficient water to meet
the needs of users based on forecasted inflow, existing conservation storage and
projected water usage. Some of the factors to consider in determining what
initial action to take as a drought progresses are: snowmelt runoff forecasts;
stream flow compared to normal flow for the same time of year; rainfall as
compared t.o normal; the PDSI; remaining conservation storage; >>end proj ected water
usage.

The PDSI indicates how severe a drought is in a particular division of a
state. It is an indicator on how severely a drought might affect agriculture
and small water storage reservoirs, more so than identifying critical conditions
in Corps multipurpose reservoirs. For example, a reservoir with full
conservation storage could possibly be located in a division with a PDSI
indicating a severe drought.

(2) Precipitation. The ultimate cause of a hydrologic drought is
the shortage of rainfall. This shortage can be used to indicate the severity
of the drought. The NWS publishes precipitation totals monthly and provides an
annual summary. Also provided are monthly and annual departures from normal for
individual stations and for the NWS divisions mentioned above. These records
show how much the experienced rainfall has departed from the average. A high
negative departure from the normal rainfall would indicate an increased severity
of drought, if it persists for a duration sufficient to cause a problem. A
slight departure from the normal rainfall, for a long period, will have little
effect on the reservoirs in the Basin. Likewise, a large departure from the
normal rainfall for a short period will have little effect on the reservoirs in
the Basin. Drought severity will be indicated by a large departure from normal
rainfall over an extended period of time. Normal monthly precipitation as
compiled by the NWS for the Arkansas River Basin in Colorado is presented in
Table 3-4. The monthly precipitation figures in Table 3-2 are weighted values
for the Arkansas River Drainage (climatic division # 1). The average annual
precipitation for the Arkansas River Basin is 14.68 inches.

Table 3-4

Arkansas River Drainage
Normal Arkansas River Basin Precipitation Amounts
(inches)

Jan Feb
.47 .55

Mar Apr May Jun Ju1 Aug Sep
.99 1,44 2.09 1.57 2.24 2.13 1.16

Total
14.68

Oct
.89

Nov
.64

Dec
.51

(3) Snowmelt Runoff. Another factor to consider in comparing trends
during a drought is snowmelt runoff in the Basin. The average annual snowmelt
runoff for the Purgatoire River at Trinidad is 41,000 acre-feet (Apr-Sept). The
average annual snowmelt runoff on the Arkansas River above Pueblo, Colorado is

3-5