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AwaterbudgetstudyofPugetSoundanditssubregions

ArticleinLimnologyandOceanography·January1972

DOI:10.4319/lo.1972.17.2.0237

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A WATER BUDGET STUDY OF PUGET SOUND AND ITS SUBREGIONS’

Mark A. Friebwtshauser and Alyn C. Duxbwy Department of Oceanography, University of Washington, Sc,attlc 98195

ABSTRACT

A water budget study for Puget Sound and its principal subregions, based on the fresh- water, seawater, and total water budgets, determines the mean fluxes of water into and out of these regioas, the freshwater content, and the change in freshwater content by month and by year. Data used to calculate the budgets include monthly mean values of precipita- tion, evaporation, runoff, changes in sea level, mean salt content, and the s’alinities of inflowing and outflowing waters for each subregion. The fluxes of water calculated in the budget analysis yield an approximation of flushing efficiency and replacement time for Puget Sound and its subregions. The monthly freshwater content, along with the fluxes of water and their salt load, indicates that river runoff does not by itself control the fresh- water content of a particular subregion; considerable freshwater is added by the inflow of water from an adjacent embayment.

INTRODUCTION

Estuaries, the partially isolated arms of the sea along the coast whcrc freshwater dilutes seawater, have a net circulation that is driven by the addition of freshwater and modified by tidal and wind mixing. This net circulation controls the flushing of estuaries and dctermincs many of the prop- crties of the water within them.

The net circulation, or inflow and out- flow of water, at the mouth of an estuary can be dctermincd by direct measurements of flow with depth across the mouth. Direct measurements must bc made over a considcrablc period to permit dctcrmination of both a temporally and a spatially inte- grated net flow. The expcnsc and effort required to obtain a measured net flow distribution with depth on a seasonal basis lead most investigators to arrive at esti- mates of the net circulation in an estuary by an indirect approach using a budget analysis.

The budget approach in its simplest form

1 Contribution No. 618 from the Department of Oceanography. This research was supported by National Science Foundation Grant GN-66 under the Washington Sea Grant Program and done under the auspices of the Department of Ocean- ography, University of Washington. The Wash- ington Sea Grant Program is part of the National Sea Grant Program, which is now maintained by the National Oceanic and Atmospheric Adminis- tration of the U.S. Department of Commerce. LIMNOLOGY AND OCEANOGRAPHY 237 MARCH 1972, V. 17(2)

assumes that the volume of water and total salt content in an estuary, as avcragcd over a given period, are constant. Therefore water from all sources flowing into an estuary must bc compcnsatcd for by an out- flow of equal magnitude, and the salt carried in by one flow must equal the salt carried out by another. Thcsc assumptions arc crude and tend not to hold in real cstuarics, especially if periods over which budgets are calculated arc less than a year.

Rivers adding freshwater to estuaries suffer periodic changes in their discharge rates, and thus the amount of freshwater contained in an estuary must vary and the total salt content as well. Changes of den- sity within an estuary, and seasonal fluctu- ations in atmospheric pressure, cause a variation that is measurable as a sea level change. The change in freshwater content and in the volume of the estuary affects the exchange of water with the sea at the estuary mouth to complicate further the assessment of the actual budget. Processes external to the estuary also play a role in the budgets; coastal upwelling or downwell- ing alters the density of the water delivered to the estuary and may significantly change the inflow rate if this water can bodily displace the decpcr water lying behind the entrance sill of the estuary.

Puget Sound has many more factors con- trolling the influx and efflux of water than

238 MARK A. FRIEBERTSIIAUSER AND ALYN C. DUXBURY

123O 45’ 122” 30’ 15’ 122O

FIG. 1. Subregions of the Puget Sound system.

WATER BUDGET STUDY OF PUGET SOUND 239

arc assumed in the simplest budget ap- proach; the equations that define the water and salt budgets cannot be the simple ex- prcssions of “water in equals water out” and “salt in equals salt out,” Marc variables must be considered. This paper presents an effort to determine physically mcaning- ful fluxes and budgets of Puget Sound as a total system, using a more applicable set of budget equations. Since Puget Sound may also bc divided into subregions that lend themselves to the same analysts, cal- culations have been made accordingly. This allows comparison of the net circulation of one subsection with another and with the total Puget Sound region and leads to further understanding of the factors causing circulation to vary within the sound.

WATER BUDGET EQUATIONS

Analysis of a time variable and complex estuary involves determining three budg- ets : total water; freshwater; seawater or salt. These may be used to calculate fluxes of water and salt over varying time bases for the entire estuary or its subregions.

Puget Sound includes all waterways in- side a lint between Middle Point at the at the north end of Quimper Peninsula and Point Partridge on Whidbey Island. The body of the sound in this analysis has been subdivided into four subregions: Whid- bey basin, Puget Sound basin, southern Puget Sound, and Hood Canal (Fig. 1). Statistical data required to determine arcas and volumes, precipitation, evaporation, and runoff contribution from ungaged por- tions of the watcrshcd wcrc compiled but arc not rcportcd here.

This study presents the budget analysts over monthly and annual periods for all of the sound and its subregions, using the method described by Waldichuk ( 1957). The analysis also allows calculation of flushing times, which give insight into the ability of the sound to exchange water in response to the factors controlling the water budgets.

According to Waldichuk ( 1957)) the equations governing total water budget, freshwater budget, and seawater budget rc- spcctivcly are:

AF=R+(v)T,-(y)T,;

AP=AW-AF

whcrc Ti = average rate of inflow of water from sea to basin, To = avcragc rate of outflow of water from basin to sea, R = total frcshwatcr input, gaged and ungaged river runoff + direct precipitation - direct evaporation, AW = change in water volume of basin as indicated by change in sea level, AF = change in freshwater content of the basin, AP = change in seawater content of the basin, AW - AF ( all in vol/time); Sb = mean salinity of seawater available to fill basin if no freshwater were present (a rcfcrencc salinity); Si = mean salinity of inflowing water ( Ti water type ) ; So = mean salinity of outflowing water (TO water type). Thcsc equations can be combined to give

T 4=&lR-aW) +&(hW-AF), .- % 0 .- z 0

and

T,=T,i-R-AW.

The flushing time, assuming that the out- flowing water ( To) dots not mix with the inflowing water (T,) at the entrance but flows continually seaward, is given by Vo/To, whcrc VO is the volume of water within the basin and T,, is the average rate at which the water is removed. This flush- ing process also assumes that within the basin Ti water is mixed completely.

The problem now becomes one of de- termining the appropriate values of each of the parameters in the budget equations for each month or year.

RUNOFF CALCULATIONS

The freshwater contribution to the sys- tcm dcsignatcd here as runoff, R, is com- posed of river discharge and direct precipi- tation on the water surface of Puget Sound minus the direct removal of water by evap- oration from the surface of the sound.

240 MARK A. FRIEBERTSIIAUSER AND ALYN C. DUXBURY

TABLE 1. Gaged river contribution* by month, X 100 m’, in Puget Soundi-

WB SPS IIC EPS

Jan 2,891.5 274.1 345.2 4,203.O Feh 2,549-g 219.2 284.0 3,677.l Mar 1,921.2 186.6 254.0 2,760.4 Apr 1,958.g 143.6 159.2 2,661.7 May 2,569.2 93.2 133.7 3,223.l Jun 3,278.6 87.8 158.0 3,970.5 Jul 2,144.4 60.3 91.2 2,576.B Aug 1,132.B 63.2 49.6 1,458.2 Sep 988.9 64.9 68.3 1,321.4 Ott 1,775.l 109.0 194.6 2,325.6 Nov 2,093s 139.3 255.7 2,790.l Dee 3,017.5 213.0 374.4 4J64.2

Total 26,321.l 1,654.2 2,367.B 35,132-l

* Determined from data in Wate,r Resour. Data for Wash. (1965, 1966, 1967, 1968).

t WB = Whidbey basin; SPS = southern Puget Sound; IV.3 = Hood Canal; EPS = entire Puget Sound area.

The monthly mean values of gaged river discharge arc given in Table 1. However, freshwater can also cntcr the rivers below the lowest gaging station as ungaged river flow, and the sum of the gaged and ungagcd portions is the total river discharge rc- quired here. Gaged river flow was cvalu- atcd for years of low stream flow (Water Rcsour. Data for Wash. 1952, 1958, 1966) and for years of high stream flow (Water Resour. Data for Wash. 1954, 1956, 1959) to dcterminc if there was any significant variability; the annual mean gaged dis- charge into the sound could vary from 0.8- 1.2 times the annual mean discharge for the period 1965-1968 used hcrc.

The ungagcd portion of the river con- tribution was estimated as equal to prc- cipitation minus evaporation times the un- gaged land area. WC assumed that all of the excess precipitation over evaporation falling on the portions of the drainage basins below the gaging station returns to the sound as surface runoff and determined the monthly mean precipitation and evapo- ration values over the ungagcd land area for each subregion (values were obtained from Climatol. Data for Wash. 1965, 1966, 1967, 1968). Data from three weather sta- tions in the ungagcd drainage basin area of each subregion were used to estimate the monthly mean precipitation by subregion.

Evaporation values were not available for Puget Sound on a monthly basis, but on an annual basis the evaporation rate for the entire region is about 64 cm yr-l (P. E. Church, personal communication; Puget Sound and Adjacent Waters Study 1970). Monthly values were estimated by taking those of Waldichuk ( 1957) for the Strait oE Georgia arca and increasing them all by a factor of 1.05 to make their annual value equivalent to 64 cm. WC assumed that the evaporation pattern in Puget Somld is similar to that of the Strait of Georgia so that the adjusted values would reasonably estimate the evaporation pattern.

The land area comprising the ungagcd portion for each subregion (Puget Sound Lit. Sure. 1954, v. 1, See. 3) was used to dctcrminc the ungaged river contributions. During summer months when evaporation cxcccded precipitation, this flow is assumed to bc zero. The annual average of the un- gaged river contribution was 9.3% of the gaged flow and thus is a minor component of the freshwater added to Puget Sound. A second small source of freshwater is that supplied by direct precipitation on the sur- fact of the estuary. This contribution was dctcrmincd from direct precipitation times subregion water surface arca. The monthly mean precipitation rate is assumed to bc the same as that used in the previous cal- culation of ungagcd river flow. The weather stations used to determine the monthly mean precipitation over the ungagcd land areas wcrc located in the Puget Sound low- lands, and it was assumed that the precipi- tation over water was similar to that over thcsc areas.

The monthly loss of frcshwatcr from the water surface of each subregion was cal- culated from the evaporation rates used in the calculation of ungaged river flow. This again assumes that the monthly evaporation rates for the sound arc similar to those for the Strait of Georgia and vary with time but not location. The monthIy loss of water from each subregion due to evaporation is comparable in magnitude to direct prc- cipitation. The gain of freshwater by prc- cipitation cxcccds its loss by evaporation

WATER BUDGET STUDY OF PUGET SOUND 241

TABLE 2. Total fedawater input, R, by month, X l@mms

WB SPS IIC EPS

Jan 2,934 648 549 5,010 Fcb 2,605 436 394 4,169 MiU* 1,940 357 425 3,299 Apr 1,960 193 187 2,767 May 2,560 85 127 3,169 Jun 3,271 JUl 2,135 ;7

150 3,956 82 2,523

A% 1,127 45 1,436 SeP 977 5": act 1,849 357 3::

1,269 2,897

Nov 2,195 453 433 3,597 Dee 3,220 640 725 5,593

Total 26,773 3,429 3,502 39,685

Mean 2,231 286 292 3,307

over the annual cycle, but evaporation cx- cccds precipitation during summer.

The total frcshwatcr input (R) for each subregion as calculated from the com- poncnts, river flow (gaged and ungaged), direct precipitation, and direct evaporation, is listed in Table 2. The total dctcrmincd by this method closely approximates the freshwater influx of 41,780 m3 x 10G given in the Puget Sound and Adjacent Waters Study ( 1970).

Two distinct periods of high runoff arc apparent in the annual cycle: Dcccmber and June. The Dcccmbcr peak coincides with the time of maximum precipitation. The second peak occurs in June and is caused by the spring melt of snow at higher elevations of the drainage basin. If the sub- region’s drainage basin has littlc arca in the snow-covcrcd clcvations, as do southern Puget Sound and Hood Canal, then the June peak is almost insignificant. IIowcver, for Whidbey basin the June peak caused by snowmclt cntcring the Skagit River is nearly as great as the Deccmbcr peak (this is the largest river cntcring Puget Sound, contributing 40-50% of the total river input ) .

A calculation of potential water supply was made to cheek on the annual average runoff and to estimate if a large volume of water rcachcs the sound by way of sub- surface or groundwater flow. This potential was evaluated by establishing a curve of

annual precipitation vs. clcvation and using the amount of drainage basin land area by elevation from the Puget Sound Lit. Surv. ( 1954, v. 1, see. 3). Assuming an cvapora- .tion rate of 64 cm yr-l for the cntirc drain- age basin arca, the amount of frcshwatcr available to the rivers flowing into Puget Sound equals the total rainfall for elevation interval times arca of elevation interval minus evaporation times drainage basin arca. A rough calculation, using the aver- age precipitation between 0 and 305 m times the watcrshcd arca for this incrcmcnt plus the average precipitation bctwcen 305 and 1,982 m times the area of this increment minus the assumed annual average evap- oration rate of 64 cm yr-l times the total arca, gave a potential freshwater supply of 38,325 x 10” 1213 yr-l, close to the value shown in Table 2. This suggests that the supply of frcshwatcr to the sound by groundwater is minor compared to surface runoff. The annual volume of frcshwatcr ( R) contributed to all of the sound would have a height of 15.9 m and an area equal to Puget Sound.

CALCULATION OF TIIE CIIANGES IN WATER

VOLUME OF THE BASINS, Arv

The monthly change in basin water volume ( AW) due to change in sea lcvcl height (Ah) has been calculated using the monthly mean change for the 1933-1963 period for Scattlc. Thcsc unpublished data, obtained from the National Oceanographic Survey Environmental Data Service, are a record of the vertical displaccmcnt bctwcen a benchmark and the mean level of the water surface in Puget Sound by month in Scattlc. The volume change is assumed to bc Ah times the surface arca of Puget Sound. It is assumed that the change in sea lcvcl at Scattlc is representative for all of the sound and that the surface area does not change with Ah. Values of Ah and AW

arc shown in Table 3.

CALCULATION OF THE FRESIIWATER

CONTENT AND ITS CHANGE, AF

The monthly frcshwatcr storage of the sound and its subregions can bc evaluated

242 MARK A. FRIEBERTSIIAUSER AND ALYN C. DUXBURY

Tmm 3. Change in water volume (AW) due to change in sea level height, by month, x 10fi ms

Ah WB SPS HC EPS (cm)

Jan +0.58 +0.42 +0.37 +2.52 +O.l Feb -37.50 -27.46 -24.22 -163.57 -6.6 Mar -30.38 -22.25 -19.63 -132.52 -5.3 APT -7.22 -5.29 -4.66 -31.50 -1.3 May -2.88 -2.11 -1.86 -12.57 -0.5 Jun -1.45 -1.06 -0.93 -6.31 -0.3 Jul -2.88 -2.11 -1.86 -12.57 -0.5 As $5.78 +4.23 +3.74 +25.22 +l.O SCP +14.46 +10.59 +9.34 +63.08 +2.5 Ott +27.43 +20.09 +17.72 +119.65 +4.8 Nov +39.06 $28.61 +25.23 +170.39 +6.9 Dee -5.78 -4.23 -3.74 -25.22 -1.0

from the expression ( S1, - S,) Sb-1 x vol of water in a basin = vol of freshwater present, where St, is the salinity of the source sea- water undiluted by freshwater, and S, is the average salinity of the water volume in question. If there were no freshwater present to dilute the estuary, it would have the salinity of the source water; if fresh- water is present, the estuary will have a salinity lower than Sa. The average salinity can be determined for the diluted estuary and one can calculate from the above equation the volume of freshwater required to reduce the salinity from Si, to S,.

The value of Sb is assumed to be 33.8%0, the value used by Waldichuk ( 1957) in his study of the Strait of Georgia, which also has as its source water the Strait of Juan de Fuca. The value of 33.8%0 approxi- mates the salinity found at midchannel near the bottom of Juan de Fuca Strait; it may approach 34.0%0 in late summer or drop to 33.5%0 in winter (Puget Sound Lit, Surv. 1954, v. 3, Sec. 12). The freshwater content calculated in this manner is not rigorously quantitative since the value of 33.8%0 assigned to Sb is somewhat arbitrary, but the volume evaluated on this basis can be rcgardcd as relative ( Waldichuk 1957).

It was necessary to examinc the monthly salinity distribution for each subregion to determine a reasonable value for S,. The volume of water in lo-fathom (18.3 m) depth incrcmcnts was available from Mc- Lellan (1954) and was used to obtain the

volumes for the depth increments O-10 m, lo-50 m, 50-100 m, and 100 m-bottom for each subregion.

An S, value for each subregion was ob- taincd by determining the total salt con- tent by month for each depth interval of each subregion. Salinity data required for this process were chosen from Collias (1970). Four stations were selected for each subregion, Two salinity profiles with depth from a common month were drawn for each station, and from these profiles the average monthly salinities and total salt content determined for each depth interval. The sum of the salt content by depth layer was then determined for a subregion. This total monthly salt content divided by the total volume of the subregion, assuming that the density of seawater equals 1.00, yielded the average salinity of that subregion.

The monthly freshwater content obtained by using the S, and S, values derived in this manner was then used to determine the monthly change in freshwater content (AF). Although the arbitrary assignment of a value to Sb makes the evaluation of AF arbitrary, the values found for AF in the subregions have significance in that the amounts of freshwater possessed by each subregion and their change in time arc rcla- tive to each other. More serious is the problem of evaluating S, for each basin. Salinity data, taken over a great many tidal cycles each month and uniformly dis- tributed over each basin, are needed to assure an accurately integrated value of total salt content, but thcsc arc not avail- able. WC hope that the values of S, used here are reasonable approximations of those that would be found if more data were available, The total salt content of each depth increment is expected to have a greater error than the total salt content of the entire volume formed from their sum.

The relationship between the monthly variation of runoff and total freshwater con- tent of the basins for each subregion is dc- pitted in Figs. 2-6. In Whidbey basin the large amount of freshwater that enters from the Skagit River results in a delay of less than a month between the runoff peak and the peak of the frcshwatcr content curve. In

WATER BUDGET STUDY OF PUGET SOUND 243

TABLE 4. Average salinities (go) of outflowing (S,-left) and inflowing ($-right) water, by month

WB SPS HC EPS WB SPS 1x2 EPS

26.5 28.5 27.8 28.8 J ‘an 30.0 29.5 29.9 30.8 25.5 27.8 27.8 28.9 Feb 29.2 29.1 30.0 31.3 26.9 29.2 28.5 28.9 Mar 29.8 29.7 29.9 32.4 26.6 27.9 28.9 28.4 Apr 28.8 28.5 30.2 33.0 26.4 28.0 28.4 28.4 May 29.8 29.1 30.3 32.5 26.4 28.3 27.9 30.1 Jun 29.2 29.1 30.8 33.0 26.8 28.4 28.5 29.0 JuI 29.8 29.3 30.8 33.0 26.7 28.7 28.9 30.9 Aug 29.6 29.5 30.8 33.1 29.1 29.8 29.4 29.8 Sw 30.5 30.4 31.1 33.2 26.0 28.6 28.9 30.2 act 30.3 30.1 31.1 32.7 26.0 29.8 29.5 29.6 Nov 30.5 30.4 30.9 32.5 28.0 28.7 26.8 27.8 Dee 30.1 29.9 29.8 31.8

26.7 28.6 28.4 29.2 Mean 29.8 29.6 30.5 32.4

Puget Sound basin, southern Puget Sound, and Hood Canal, the delay is from 2-3 months, probably because of the lower in- put by local rivers and the grcatcr contribu- tion of freshwater from adjacent basins by the entering T$-type water. In all of the graphs, there is a noticeable decrease in the freshwater content for March and a recovery in April, caused by high S, values determined from survey data for March and low ones in April (which cause oscilla- tions in the calculated frcshwatcr content). These S, values and their effect may not bc typical.

DETERMINATION OF THE AVERAGE SALINITY

OF THE OUTFLOWING AND IN-FLOWING

SEAWATER, So AND Si

WC determined the average monthly sa- linity of the outflowing water (So) by drawing distributions of salinity with depth for stations on the inside of the sill for each subregion. The haloclinc almost always oc- curred at 10 m. It was assumed that out- flowing water, 2’0, was primarily of the type found at depths above that, and the average salinity of the top 10 m was used as the monthly value of So for each subregion,

The average monthly salinity of the in- flowing water (S,) was determined from station data at the entrance side of each subregion slightIy seaward of the sill. Again monthly salinity profiles with depth were drawn, and the average salinity below sill depth (below 50 m for Whidbcy basin)

was used as the value for SL. Values for So and Si are shown in Table 4.

The accuracy of So and Si values, like that of S, values, is in doubt. Again samples taken over many tidal cycles would bc needed to obtain reprcscntative values of the salinity at the mouths of the subregions. It would also be desirable to have vertical current profiles integrated with time over each month to determine the depth distri- bution of Ti and TO flows, which could then be used to establish the depths over which Si and So should be determined and to serve as a check on the values of Ti and To calculated from the water budget anal- ysis.

Despite the errors in Ti and To that may bc gcncrated by &, So, and S, values, a budget analysis of this type giving values of exchange rates to even a -t- 30% accuracy is better than none at all. The overall accuracy of the cstimatcs of To and Ti in this analysis cannot be asscsscd, and the * 30% is stated only for the sake of perspcctivc.

The values of the component parts of the equation used in calculating Ti are in Tables 2, 3, and 4; tabulations of Ti and To are in Table 5. The monthly replacement or flush- ing time as calculated from the basin volume and To is given in Table 6.

DISCUSSION

The budget calculations, despite their shortcomings, yield some insight into the behavior of Puget Sound and its subregions.

244 MARK A. FRIEBERTSIIAUSER AND ALYN C. DUXBURY

TABLE 5. Average flux of inflowing (T$--left) ancl outflowing (T ,--right) water, by month (X lo” m”)

WB SPS IIC EPS WB SPS 1x2 EPS

15,403 13,602 2,209 39,889 Jan 18,337 14,251 2,753 44,896 23,851 10,014 8,422 76,560 Feb 26,493 10,478 8,842 80,890 13,826 11,059 3,271 5,251 Mar 15,797 11,438 3,716 8,683 26,519 16,712 7,516 27,728 Apr 28,487 16,910 7,709 30,526 16,570 4,067 983 24,338 May 19,133 4,155 1,113 27,518 30,954 8,757 2,121 47,068 Jun 34,225 8,842 2,280 51,030 26,389 5,871 4,843 38,221 JUl 28,527 5,922 4,928 40,755 16,879 6,686 3,871 37,014 Rug 18,000 6,746 3,914 38,425

8,221 5,902 4,990 11,235 SCP 9,184 5,950 5,044 12,441 13,140 8,986 3,177 39,351 Ott 14,961 9,323 3,483 42,129 9,079 2,300 -816 7,088 Nov 11,235 2,724 -408 10,5 15

44,267 12,888 6,700 29,056 Dee 47,495 13,531 7,428 34,672

The monthly mean addition of frcshwatcr by surface runoff and the freshwater content of the sound ( Fig. 2) show that the latter always cxcceds the former. Further, it ap- pears that the maximum or minimum frcsh- water content occurs 1.5-2 months after the maximum or minimum influx of freshwater. Several factors crcatc this time lag.

Within a basin, mixing of the freshwater with seawater by tidal turbulence causes a change in the cfficicncy of the seaward transport of surface water, To, in removing freshwater from the basin. If equal parts of freshwater and basin water form the sur- fact water flowing seaward, then the sca- ward flow rate must bc about twice the river contribution and contain about half in- flowing-type water. However, tidal mixing outside the basin between To and Ti waters tends to add freshwater to the Ti-type water. Thus, the Ti flow must incrcasc to

TABLE 6. Replacement time” (in days)

WB SPS IIC

Jan 46 33 272 Feb 32 45 85 Mar 54 41 202 Apr 30 28 97 May 44 114 672 Jun 25 54 328 Jut 30 Aug 47 ;:

152 191

Sep 93 Ott 57 i:

149 215

Nov 76 174 -183 Dee 18 35 101

Mean 40 56 177

* Replacement time = basin volume/T,.

EPS

113

5:; 166 184 99

124 132 407 120 480 146

152

bring in salt to suppress the local frcshen- ing of the basin water and T,-, flow must incrcasc to maintain continuity. Hence, changes in the supply of freshwater trigger a change in the TO ancl Ti flows. Since response of these flows is not immcdiatc, some time lag is crcatcd.

Other factors affecting the ability of the freshwater content to keep pact with the local contribution by rivers are outside in- fluences that produce changes in the frcsh- water content of Ti water in addition to mixing between inflowing and outflowing water at the basin entrance. In spring, con- siderable freshwater is added to the Strait of Juan dc Fuca system from Canadian sources, where tidal mixing tends to add freshwater to the water forming the T,i flow. In late summer, coastal upwelling tends to increase the salinity of the water forming the Ti-type available to the sound. The net

FIG. 2. Total freshwater contribution and Freshwater content to Puget Sound, monthly mean values 1965-1968.

WATER BUDGET STUDY OF PUGET SOUND 245

I I ’ -iAkiN ;:,,:,,:,-;-AT JeiH FEB MAR r\pR

FIG. 3. Total freshwater contribution and frcshwatcr content to Whidbey basin, monthly mean values 1965-1968.

result of these factors is the relationship bc- twccn freshwater content and river runoff dcpictcd in Fig. 2.

Although the two peaks of river runoff, in winter from heavy precipitation and in spring from snowmclt, arc of obvious origin, the maxima and minima in the frcshwatcr content in February, March, and April have no clear explanation. It appears that the avcragc salt content dctermincd during March might have been too high (or for April too low ) . Howcvcr, a cheek of the calibration data and duplicate sample anal- ysis pcrformcd at the time the samples wcrc run indicates that the salinities were correct and acceptable. Since no fault could bc found in the salinity values and the data wcrc trcatcd the same for all months, the irregularity in the freshwater content curve must stand. Presumably if more data were available to determine the avcragc salt con- tent for a given basin for each month, S, irregularities in the frcshwatcr content curve would bc minimal,

The relationships bctwccn freshwater content and river runoff in the subregions (Figs. 3-6) show the diffcrcnccs bctwccn thcsc subbasins and their individual con- tribution to the total pattern. Whidbcy basin has winter and spring peaks in runoff of comparable magnitude ( Fig, 3)) with the freshwater content closely coupled to river discharge. This indicates that the pri- mary sources of frcshwatcr in Whidbcy

FIG. 4. Total freshwater contribution and freshwater content to Puget Sound basin, monthly mean values 1965-1968.

basin are local rivers, with the importance of frcshwatcr added by Ti-type flow ac- cordingly rcduccd. Whidbcy basin has one feature not common to the others : It has no sill to restrict inflow and outflow of water at its cntrancc, and has less tidal vertical mixing at its mouth for this reason. Lessened vertical mixing allows the T,, and Ti flows to occur readily and not affect each other appreciably at the basin mouth. Fresh- water content lags behind freshwater input less for Wbidbcy basin than for any of the others. The runoff in Whidbcy basin is by far the largest source of frcshwatcr to Puget Sound ( Table 2) and can thus cntcr into the other subbasins as a freshwater com- poncnt in the Ti-type flow.

The central basin is the common region into which the subbasins cast their To flow and from which they receive their Ti-type water. A meaningful Ti, To transport cannot hc calculated for this central basin by the methods applied to the cntirc sound or its major appcndagcs. Howcvcr, its frcshwatcr runoff and frcshwatcr content can bc calcu- lated and arc shown in Fig. 4. Thcsc two curves show that the peak freshwater con- tent lags behind the peak wintertime input of freshwater in a manner similar to that for the total system. As frcshwatcr input dccrcascs, the time lag bctwcen the mini- mum frcshwatcr content and freshwater supply dccrcascs bccausc the summer influx of high salinity water produced by coastal upwelling and large values of Ti (Table 5)

246 MARK A. FRIEBEliTSIIAUSER AND ALYN C. DUXBURY

Lb&--+ ;;,--I t I i-- I I I I- JPlN LlPiR MB “UN JUL AU0 SEP OCT NO” DE’:

FIG. 5. Total freshwater contribution and freshwater content to southern Puget Sound, monthly mean values 1965-1968.

rapidly lower the apparent freshwater con- tent of the central basin,

Southern Puget Sound and Hood Canal behave similarly in their freshwater cycles (Figs. 5 and 6), although the time lag bc- twecn the peak of the local freshwater run- off and the peak of the freshwater content is greater for the latter than for the former. The delay during late winter-early spring is because the freshwater in the inflows of these basins has its primary source in Whid- bcy basin and has worked its way through the central basin to the entrances of Hood Canal and southern Puget Sound. This in- dicates that Hood Canal has, in gcncral, a poorer circulation than southern Pugc t Sound. The recovery of freshwater content for Hood Canal in fall, however, appears to be more rapid than in southern Puget Sound, due probably to the closer proximity of the entrance of IIood Canal to the source of freshwater from Whidbey basin, which increases at the same time as local river run- off to Hood Canal.

The replacement time ( in days) gives an indication of the ability of a basin to flush itself ( Table 6) ancl may bc looked upon also as a flushing efficiency. If a month is 30 days, then 30 days divided by the tabulated monthly flushing times and multiplied by 100 gives a percent efficiency of flushing for each basin and each month. A month showing a flushing time of 60 days for a given basin indicates that half

FIG. 6. Total freshwater contribution and freshwater content to Hood Canal, monthly mean values 1965-1968.

the water in the basin can be exchanged that month, a 50% efficiency.

The flushing times or efficiencies so dc- tcrmined do not necessarily indicate what actually happens in nature, because the in- flowing water does not completely mix with the water in a basin. During late summer, the Td water entering the central basin from the Strait of Juan de Fuca is at its dcnscst and may spill over the sills to dis- place the less dense, deeper water of the various basins. Thus a 100% displacement of dccpcr water in a basin may occur over a rclativcly short period, with a less efficient cxchangc occurring above. Once this deep water replacement has occurred, then the further introduction of water of lesser den- sity may increase the exchange with the shallower water, but have little effect on the decpcr water.

The irregularities in the freshwater con- tent already mcntioncd produce changes in the sign and magnitude of the freshwater content that may not be real but that pro- duct irrcgularitics in Ti and To affecting the calculation of replacement times. Ncga- tivc values of replacement times have no significance. The -183 days rcplaccmcnt time calculated for the period bctwecn No- vember and December in Hood Canal prc- sumably reprcscnts a period when flushing was active and replacement time was short.

The irregularities in calculated freshwater content arise primarily from the determina- tion of the mean salt content of a basin for

WATER BUDGET STUDY OF PUGET SOUND 247

a given month from inadequate data. Even be used to aid in verifying numerical model though the area has been extensively moni- schemes presently under study and furnish tored for its oceanographic conditions, sa- a guide for developing smooth functions linity measurements taken at four or five describing annual changes by month of midchannel locations once a month in a freshwater input. subregion are not sufficient for accurate A numerical model using this information integration of the total salt content of the to make predictions of the changes of chem- entire basin over a l-month period. ical constituents and transports can in fact

The salinities used for SO and Si for each produce refined estimates of SO, Si, and S, month also are based on less than adequate that can upgrade the functions required in data since measurements of salt content the model. Until representative numerical are not available for averaging over tidal cycles. Salinities measured near the en- trance to the major subbasins yield a dif-

models arc available to tell us more about the mass budgets of Puget Sound, and until WC have more measurements in both time

fercnt pattern on the rising than on the and space, refinement of the water budget falling tide. This variation must bc aver- will be difficult. aged out several times during the month to obtain values rcprcsen tativefor the whole month.

However, despite the overall problem in evaluation of the budgets owing to the lack of available basic data, the results are rea- sonable and consistent with what we know about Puget Sound from a descriptive view- point. Thus it stems reasonable to accept the results of this analysis as a first approxi- mation of the month-by-month water budg- ets of the sound and its subregions.

The water fluxes found here represent the base data necessary to carry on many studies in Puget Sound. The R, TO, and Ti transports can be used along with additional chemical data to dctcrmine the efflux and influx of nutrients. This coupled with ob- served changes of nutrients within the sound and its subregions can be used to obtain a first order approximation of organic pro- duction. The data presented here also can

REFERENCES

CLIMATOLOGKAL DATA FOR WASHINGTON. 1965, 1966, 1967, 1968. Annual summaries. U.S. Nat. Oceanic Atmos. Admin., Dep. Commerce.

COLLIAS, E. E. 1970. Index to physical and chemical oceanographic data of Puget Sound and its approaches, 1932-1966. Univ. Wash., Dep. Oceanogr. Spec. Rep. 43.

MCLELLAN, P. M. 1954. An area and volume study of Puget Sound, Washington. Univ. Wash., Dep. Oceanogr. Tech. Rep. 21. 39 p.

PUGET SOUND AND ADJACENT WATERS STUDY. 1970. Puget Sound Task Force of the Pacific Nosrthwest River Basins Commission, Append. 3, 6. Pac. NW River Basins &mm., Van- couver, Wash.

PUGET SOUND AND APPROACHES, A LITERATURE SUWEY. 1954. Univ. Wash., Dep. Oceanogr., v. l-3.

WALDICHUK, M. 1957. Physical oceanography of the Strait of Georgia, British Columbia. J. Fish. Res. Bd. Can. 14: 321486.

WATER RESOUHCES DATA FOR WASHINGTON. 1952, 1954, 1956, 1958, 1959, 1965, 1966, 1967, 1968. U.S. Geol. Surv., Dep. Interior.

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