30 



BIOLOGICAL REPORT 31 



Y Precipitation 

 1957-1985 



H^ 



l 1 1 1 1 1 1 1 1 1 i r- 



Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 



Jan Feb Mar Apr May Jun Jul Aug Sep 

 1970-1971 



Fig. 3.2. A. Precipitation and mean monthly discharge 

 of the Westport River, normalized by its drainage area. 

 B. Comparison of normalized discharge from 2 years 

 of data from the Westport and Weweantic Rivers. 

 From Signell ( 1987) 



inputs and discharges results primarily from strong 

 seasonal shifts in recharge rates that are due to losses 

 via evapotranspiration and to a lesser extent the stor- 

 age of ice and snow during winter until spring melt. 

 Annual return of rainwater within the watershed to 

 the atmosphere is about 65% (45% recharge; 

 LeBlanc et al. 1 986). The integrated result of the 

 cycles of precipitation, temperature, and evapo- 

 transpiration is a distinct seasonal variation in water 

 table elevation with resulting variations in discharge. 

 Although river discharge data are limited, long- 

 term measurements were conducted on the major 

 river system, the Westport River (Fig. 3.2B), with 

 smaller data sets available for the Weweantic River 

 (cf. Signell 1 987) and Red Brook (Moog 1 987). 

 The seasonality of river discharge is clear in the 



Westport and Weweantic rivers (Tig. 3.2B). Similar 

 temporal variations caused by seasonal changes in 

 hydraulic gradient were found in groundwater 

 discharge into Buttermilk Bay (Weiskel 199 1 ). 



The similar discharge rates per unit of water- 

 shed for the Westport and Weweantic rivers over 

 the same period (Fig. 3.2B) support the use of a 

 generalized ratio of discharge/subwatershed area 

 for each of the major rivers discharging to Buzzards 

 Bay (Signell 1 987). The ratio from the long-term 

 Westport data is 0.0198 (m7s)/km : , similar to a 

 study by Bue ( 1 970) for a nearby Cape Cod River 

 of 0.0191 (mVs)/km 2 . The bay- wide total freshwa- 

 ter inflow estimated from this approach is 22 mVs with 

 the Westport and Weweantic rivers accounting for 

 about one-third of the total flow (Table 3.1). While 

 this technique does not separate the contribution of 

 runoff versus groundwater inflow to total discharge, 

 baseflow within this watershed is probably signifi- 

 cant based on the geology and Red Brook, where 

 approximately 69% of the total flow is baseflow 

 (Moog 1 987). Because of the relatively small wa- 

 tershed area versus bay area, the freshwater inflow 

 (22 m 3 /s) is nearly equivalent to the direct rain input 

 to the bay surface ( 1 8 mVs), although evaporation 

 of bay water must also be considered. Nonethe- 

 less, the importance of considering direct precipi- 

 tation is clear. Although direct precipitation leads 

 to dilution of bay salinities, it is less important than 

 streamflow in producing salinity gradients within the 

 bay waters. 



The apparent temporal variation in freshwater 

 discharge through surface and groundwater path- 

 ways and the nearly uniform monthly precipitation 

 input directly to bay waters are consistent with the 

 salinities observed in the open bay surface waters 

 near the mouth. Salinity measurements collected 

 over 14 years in Woods Hole, which receives a 

 mean mass flux of water from Buzzards Bay and 

 has almost no nearby freshwater discharges, indi- 

 cate a small annual range of less than 1 ppt. with a 

 minimum in April, maximum freshwater discharge 

 in February- April (Fig. 3.2), and a maximum of 3 1 .9 

 ppt in October at the end of the low discharge 

 period (cf. Signell 1987). 



