9568

• UCREFRP Catalog Number: 9568
• Author: Resource Consultants Inc.
• Title: Sediment Transport Studies of the Little Snake, Yampa, and Green River Systems.
• USFW Year: 1991.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

4.5 Analysis of Historical Climate Records of the Upper
Green River / Yampa River Bal\iol\

4.5.1 Introduction

The purpose of this analysis is to determine whether significant climate change has occurred from
1895 through 1989 in the Upper Green and Yampa River basins of the western United States (Balling and
Miller, 1990). The results of this analysis will support refmement of the geomorphic analysis of the
Yampa/Little Snake/Green River system. Three monthly climatic variables -- mean temperature, total
precipitation, and the Palmer Hydrological Drought Index (PHDI) -- were examined for six climate divisions
in Utah, Colorado, and Wyoming (F'JgU1"e 4.7). A variety of statistical techniques were used to ascertain
whether (a) there have been any significant trends over the 95-year record, and (b) whether there have been
any significant climatic changes since the closing of Flaming Gorge Dam in 1962.

4.5.2 Study Area and Climatic Data Bases

The study area (Figure 4.7) comprises the drainage basins of the Upper Green River, the Little
Snake River, and the Yampa River. Six climatic divisions lie within the boundaries of the study area.
These divisions are defined by the National Climatic Data Center (NCDC), and are used to aggregate
climatic data to larger areal units. Three divisions are in Utah (North Mountains, Uinta Basin, Southeast),
one in Colorado (Colorado Drainage), and two are in Wyoming (Green and Bear Drainage, Upper Platte).
Divisional mean temperature and total precipitation are calculated for each month by NCDC. From these
values, several drought indices are computed -- among these the Palmer Hydrological Drought Index.

The PHDI is based on the balance between the natural water supply and the natural water
demand. Many semi-empirical equations are used to convert routinely-measured temperatures into
estimates of potential evapotranspiration. These estimates are compared to precipitation levels, and the
results are integrated into a set of soil parameters that may vary through space; a soil moisture
"bookkeeping" scheme is essentially developed using the procedure. Palmer "normalized" the index to allow
drought or wetness severities to be directly compared from area to area. Values near 0 indicate near
normal conditions, values below -4 indicate "extremely dry" conditions, while values of +4 or above are
indicative of "extremely wet" conditions.

Most researchers use the Palmer Drought Severity Index (PDSI) to represent meteorological
drought or wetness conditions. In this investigation, the Palmer Hydrological Drought Index (PHDI) is
chosen to better represent the impact of climate on a hydrological system. Karl and Knight, in their Atlas of
Monthly Palmer Hydrological Drought Indices (1895-1930) for the Contiguous United States (NCDC
Historical Climatology Series 3-6,1985, page ix), stated:

"The PDSI is a meteorological drought index and it attempts to classify spells of
weather. This means that once the weather begins to return to a new regime,
regardless of soil moisture conditions, streamflow, or lake levels, etc., the index will
rapidly respond and return to near normal values. The PHDls should more closely
reflect water availability (i.e. soil moisture, streamflow, and lake levels) when a
drought or wet spell is ending than the traditional PDSI . . ."

The computational details, the sensitivities, and the limitations of the PDSI and the PHDI are available in
a number of sources; the PHDI appears to be the superior choice in this study given the focus on changes
in a hydrological system.

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