HYDROCLIMATIC VARIABILITY IN THE ROCKY MOUNTAINS
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WATER RESOURCES BULLETINVOL. 27, NO.5 AMERICAN WATERRESOURCES ASSOCIATION OCTOBER 1991
HYDROCLIMATIC VARIABILITY IN THE ROCKY MOUNTAINS'
David Changnon, Thomas B. McKee, and Nolan J. Doesken2
ABSTRACT: The spatial and temporal variability of hydroclimaticelements were investigated in the central and northern RockyMountains (Colorado, Idaho, Montana, Utah, and Wyoming) duringthe 1951-1985 period. The three hydroclimatic elements studiedwere total water-year (October 1-September 30) streamllow (ST),winter (October 1-March 31) accumulated precipitation (PR), andApril 1 snowpack (SN). An analysis of 14 virgin watersheds showedwide spatial djfferences in the temporal variability of SN, PR, andST, and these were found to be caused largely by basin exposure tomoist air flows. The more stable (low variability) basins were thoseexposed to prevailing northerly to westerly flow, while unstable(high variability) basins were exposed to occasional southwesterlyto southeasterly moist flow. Snowpack was the better indicator ofST in 11 of the 14 watersheds, explaining 37 to 87 percent of the STvariance.
Analysis of the spatial variability, based on all SN and PR datafrom across the study area, revealed 11 discrete climatic regions.Both SN and PR exhibited coherent regions of stable and unstabletemporal variability. The average variability between stable andunstable regions differed by a factor of two, and the differenceswere best explained by the exposure of the mountain barrier tomoist air flows.(KEY TERMS: hydroclimatic variability; snowpack; streamflow;watershed; aspect; winter precipitation; snowmelt runoff.)
Each year the available water stored in the wintersnowpack of the Rocky Mountains provides much ofthe usable surface water in the western UnitedStates. Much of the economic and environmental wel-fare of the West is affected by the availability of thiswater (Weiss, 1982) stored in the winter snowpackthat is derived from winter precipitation. Thesesurface-water resources are controlled directly by cli-matic factors. Because winter precipitation, wintersnowpack, and annual streamfiow are interrelated
elements and part of the hydrologic cycle and climatesystem, they are referred to as hydroclimatic ele-ments.
Recent periods of drought and excessive runoff tiedto wide fluctuations or variability in winter precipita-tion and snowpack have created major problems forthose who design facilities for and manage the distri-bution of the available surface water. Understandingthe hydroclimatic variability of the Rocky Mountainregion is particularly important because winter pre-cipitation stored in the winter snowpack represents85 percent of the area's total streamfiow (Grant andKahan, 1974). Few studies have examined the rela-tionships of winter snowpack variability to stream-flow variability in this region (Meko and Stockton,1984; Peterson et al., 1987) or the existing connec-tions between physical variables and hydroclimaticvariability in the Rocky Mountains (Barry, 1981).
The major objective of this research was to defineand explain the magnitude of the variability in keyhydroclimatic elements in a number of watershedsand across a large region of the Rockies where muchof the western surface water is produced.
DEVELOPMENT OF DATABASE FORHYDROCLIMATIC RESEARCH IN THE ROCKIES
The five-state study region (Colorado, Idaho, Mon-tana, Utah, and Wyoming) located in the central andnorthern Rockies, was chosen because it representsthe water source for six large watersheds in theUnited States. The streamfiow from this region issupplied predominantly from higher elevations
Paper No. 91093 of the Water Resources Bulletin. Discussions are open until June 1, 1992.2Respectively, Regional Research Climatologist, Southeast Regional Climate Center, South Carolina Water Resources Commission, 1201
Main St., Suite 1100, Columbia, South Carolina 29201; Professor of Atmospheric Sciences, Colorado State Climatologist, Dept. of AtmosphericSciences, Colorado State University, Ft. Collins, Colorado 80523; and Assistant State Climatologist for Colorado, Dept. of AtmosphericSciences, Colorado State University, Ft. Collins, Colorado 80523.
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through winter precipitation and accumulated snow-pack followed by melting and runoff in the spring andearly summer. The six large watersheds include theMissouri, the Arkansas, the Rio Grande, theColorado, the Great Basin, and the Columbia. Thethree primary data elements for study of hydroclimat-ic variability in this five-state region are precipita-tion, snowpack, and streamfiow. Each of theseelements is related to climate. However, these ele-ments are observed and archived by various federalagencies and used for a variety of purposes. Thedevelopment of a combined database requires thatdata sources be identified, data acquired and checkedfor quality and completeness, individual sites chosen,and limitations of the data recognized.
Each of the three historical observations chosen forstudy were acquired from three different federalagencies. Monthly precipitation records were acquiredfrom the National Climatic Data Center, monthlysnow course measurements were acquired from theSoil Conservation Service, and monthly streamfiowrecords were acquired from the United StatesGeological Survey. The study period of 195 1-1985 waschosen for analysis because data studies revealed itwas the period with the largest number of continuallymonitored snowpack sites and precipitation stations.
April 1 measurements of snow water equivalentwere greater than other measurements taken throughthe winter; thus, the April 1 snowpack (SN) was cho-sen to represent the snowpack each year throughoutthe study. The winter accumulation period was arbi-trarily chosen to begin October 1, the start of thehydrologic year. The October 1-March 31 precipitation(PR) represented the accumulating period up toApril 1, the time of the maximum average snowpackmeasurement, to allow for consistent comparison withSN. The output of the hydrologic cycle in this studywas the streamfiow. Total water-year (October 1-September 30) streamfiow (ST) was chosen to com-pare with PR and SN.
Evaluation of Data
The data for all potential SN sites and PR stationsin the five-state region, with a period of record from1951 to 1985, were examined. Each record was exam-ined thoroughly to identify station relocation orchanges in observation technique.
Figures 1 and 2 show the distribution of PR sta-tions and SN sites that had complete and consistent
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records for the 35-year period. A total of 266 PR sta-tions and 275 SN sites are included. A completedescription of all sites used in this study is describedin Changnon et al. (1990). An examination of Fig-ures 1 and 2 reveals that the two types of observa-tions were not generally co-located. PR stations havebeen placed to monitor climate and are spatially dis-tributed and located where volunteers can make dailyobservations. Although there is a fairly uniform spa-tial distribution, there is a distinct bias toward mea-surements in valley locations and lower elevations.SN sites have been specifically designed to monitorsnow for the projection of seasonal streamfiow andthey are usually located at higher elevations wherethe snow is heaviest. Snow course sites are usuallymeasured once a month.
The number of ST gauges selected for studyinghydroclimatic variability was limited by the availabil-ity of within basin data throughout the RockyMountains. Watersheds were chosen using a basinselection criteria to examine the variability of eachelement, PR, SN, and ST, as well as to compare thevalues within each watershed.
Figure 1. Distribution of 266 PR Stations Used in Study.
Hydroclimatic Variability in the Rocky Mountains
Figure 2. Distribution of 275 SN Sites Used in Study.
The physical variables influencing hydroclimaticvariability in the five-state region are remarkablydiverse. The study stratified the SN and PR data intodescriptive categories, including latitude, elevation,and aspect. These descriptors were identified for eachstation and watershed. Latitude is related primarilyto the large-scale atmospheric circulation and stormtracks that vary with time. Elevation is importantbecause wetter locations are generally highe. Aspectdefines the direction of the inflowing air that causes
vertical motion. Aspect is important at all latitudesbecause the vertical motion caused by the terrain isoften a greater influence on precipitation productionthan the lifting associated with synoptic-scale storms.
Table 1 shows the distribution of 266 PR stationsand 275 SN sites listed by approximate latitude(states from north to south) and elevation categories.One observation is that PR stations and SN sites arelocated at higher elevations in Colorado than inMontana or Idaho. This is related to the general ele-vation trend in the Rocky Mountains such thatColorado has the highest average elevation.
Comparison of elevations of the PR stations andSN sites reveals that only 16 percent of the 266 PRsites are above 2000m elevation, while 78 percent ofthe 275 SN sites are above this level. The concentra-tion of PR stations at elevations lower than SN sitesconfirms that PR and SN generally do not sample thesame portions of the total water supply. However,they are not independent because they monitor simi-lar storm systems. A summary of the advantages anddisadvantages related to each type of data is given inTable 2. Based on characteristics of the three hydro-climatic elements described in this section, all threeelements were identified as useful for analyzing vari-ous water resource issues in the West.
HYDROCLIMATIC VARIABILITYCHARACTERISTICS OF ROCKY
One way to understand the relationships betweenclimatic and hydrologic elements in the western U.S.is by studying individual watersheds (Gleick, 1987;Cayan and Peterson, 1990). In this study, a group ofwatersheds was identified and analyzed to describethe spatial and temporal hydroclimatic variability
TABLE 1. Number of PR Stations and SN Sites by Elevation Rangefor the 1951-1985 Period in Each State.
0500 2 (0) 0 (0) 0 (0) 0 (0) 0 (0) 2 (0)5011000 11 (1) 46 (0) 0 (0) 0 (0) 0 (0) 57 (1)
10011500 11 (5) 36 (5) 10 (0) 9 (0) 18 (0) 84 (10)15012000 12 (25) 11 (23) 14 (1) 21 (1) 22 (0) 80 (50)20012500 0 (23) 2 (31) 7 (26) 3 (10) 20 (0) 32 (90)25013000 0 (5) 0 (2) 0 (21) 0 (13) 9 (40) 9 (81)30013500 0 (0) 0 (0) 0 (5) 0 (3) 2 (35) 2 (43)TOTAL 36 (59) 95 (61) 31 (53) 33 (27) 71 (75) 266 (275)
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characteristics of individual watersheds, and throughcomparisons, determine the factors that contributedto the variability among the watersheds.
TABLE 2. Advantages and Disadvantages ofPR, SN, and ST Observations.
Element Advantages DisadvantagesPR + Greater areal coverage
at low elevations, Variable point
+ Includes data throughyear.
+ Captures wet winterswith large area,
Few sites in highelevation.
Doesn't account forother atmosphericelements duringwinter.
SN + Located in maximumsnow zone.
+ Integrates severalatmospheric elementsthrough winter, such asevaporation, wind,temperature, andprecipitation,
Few sites at lowelevations to monitorwet years.
April 1 date isvariable relative tomelt.
ST + Actual water to exitbasin,
Few basins havenatural flow.
+ Integrates all hydrologicand geologic processes.
Variability not alldue to climate.
Basincharacteristicsvary in all basins.
Watershed Selection Criteria
The selection of a set of watersheds adequate tostudy the climate variability across a wide region wasan initial priority. The procedures used includedselecting a number of candidate watersheds,- develop-ing a basin selection criteria, and identifying the finalset of watersheds that qualified for detailed study.
Over 600 basins of various sizes, monitored by ST,are located within the boundaries of the five-stateregion. Several physical characteristics of the regionand of the watersheds within the region made thor-ough study of all watersheds an unproductive task.Based on discussions with state USGS offices, a groupof watersheds with high quality long-term data wereselected. Thirty-one of these watersheds had less than
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25 percent of their flow diverted, did not receivetransbasin diversions, and were initially selected ascandidate watersheds.
The watershed selection criteria were developed toyield a group of basins with similar physical charac-teristics. However, it was important that the criterianot be too restrictive; rather, it should be flexiblebecause no one basin is exactly like another in theRocky Mountain region (Pilgrim, 1983). Four majorcriteria, described below, were developed to judge the31 watersheds.
The amount of water diverted from the naturalrunoff was the first limiting factor examined in water-shed selection. Meko and Stockton's study (1984)selected watersheds if less than 7 percent of the totalflow was used for irrigation and less than 7 percentwas used for reservoir capacity. In this study, water-sheds were selected if less than 10 percent of the totalflow was diverted for any reason such as transmoun-tam diversion, reservoir storage, or irrigation.
Initially, pairs of watersheds, one located up-wind(air ascending) and another down-wind (air descend-ing) of a mountain barrier, and along the mountainchain in the study region, were chosen to study theimpact of large winter synoptic patterns on surface-water supplies east and west of the ContinentalDivide (C.D.). However, because the mountain chainvaries in width and topography, not all watershedscould be selected in pairs. Watersheds that were geo-graphically situated in locations where the up-windflow was favorable for precipitation and was notimpeded by other local mountain ranges passed thissecond selection criterion.
The third selection criterion was elevation.Watersheds selected were located at or above an ele-vation where snowmelt runoff was the most impor-tant contributor to the annual streamfiow (Cayan andPeterson, 1990). The watershed elevation criterionvaries with latitude and climate regime; consequently,no fixed elevation minimum was applied throughoutthe region.
Once the first three watershed criteria were met,questions regarding the data availability and qualitywere addressed. Because of the sparse networks ofNWS cooperative stations, USGS streamflow gauges,and SCS snow courses, only a limited number ofwatersheds had all three data sets available. Mostdata sets for watershed analyses included records forthe study period 1951 through 1980. A longer periodof study was not chosen because the availability ofhydroclimatic data before 1951 or after 1980 dimin-ished in many watersheds and...