WATER RESOURCES BULLETIN VOL. 13, NO. 2 AMERICAN WATER RESOURCES ASSOCIATION APRIL 1977

HYDROLOGY OF TWO SMALL WETLAND BASINS IN EASTERN MASSACHUSETTS'

Arnold L. O'Brien'

ABSTRACT: A hydrologic budget was prepared for two geologically different wetland basins in eastern Massachusetts for the 1971 water-year. Water table conditions prevailed at one wetland underlain by peat while an artesian system functioned at the other wetland which was underlain by muck. Hydrologic responses were generally similar at both wetlands, although each functioned differently in detail. Both wetlands exhibited high spring discharges and depressions of low flow. Ground water accounted for an estimated 93% of the total annual discharge from both wetlands; in late summer the peat deposit recharged the regional groundwater body. Evapotranspiration in the spring was retarded in probable consequence of the extreme wetness of the wetland soils. (KEY TERMS: wetlands; hydrologic budget; groundwater drainage; streamflow; evapotrans- pira tion .)

I NT RO DUC T I 0 N

Freshwater wetlands are those lands which are periodically flooded and which have ground water a t , or near the surface for a major part of the year. In addition, they are commonly floored with organic deposits such as peat and muck and are characterized b y a distinctive suite of plants. As a class o f land, wetlands have remained relatively undeveloped. Recent expansion and development in the New England area, however, threatens to destroy or seriously change many of the existing wetlands which, according t o one study cover 6%% of the total land surface of Massachusetts (Larson, 1973). In some localities where wetlands are numerous they may cover 50% of the area. Clearly wetlands in these areas are a major component of the regional hydrologic system.

The effects of wetland alteration on the hydrologic environment are difficult t o assess as there is little data on how the unaltered wetland responds in the natural hydrologic environment. Indeed certain types of wetlands may respond differently from other types necessitating hydrologic distinctions. The purpose of this research is t o investigate two geologically different freshwater wetlands as complete hydrologic systems.

'Paper No. 761 15 of the Water Resources Bulletin. Discussions are open until December 1, 1977. *Department of Earth Sciences, University of Lowell, Lowell, Massachusetts 01 854.

325

326 O’Brien

INSTRUMENTATION, CLIMATE, AND GENERAL DESCRIPTION

Two small wetland basins were selected in Lincoln, Massachusetts, a suburban town located 1 4 miles west northwest of Boston (figure I ) . The basins were instrumented and monitored during the I97 1 water-year t o record rainfall, snow accumulation, runoff, groundwater fluctuations, and soil moisture changes. On site precipitation measurements were supplemented by data from a permanent weather station a t L. G. Hanscom Field, Bedford, Massachusetts, located within three miles o f both wetlands. Continuously operating monitors were installed on some wells and stream gages were fitted with continuous monitors t o provide complete stream hydrographs. Both basins were checked for losses due to underflow and groundwater leakage. Measurements in 7-day units were applied t o the elements in the hydrologic equation: precipitation = surface runoff t baseflow t E T t leaks t change in soil moisture +change in groundwater storage. Specific yield (used t o calculate groundwater storage) was determined from the budget by convergent approximation. Measured soil moisture trends were matched t o calculated soil moistures t o yield the quantities used in the budget. Evapotranspiration was calculated as a residual from the hydrologic budget and a sinusoid was fitted t o these values by double integration. Quantities from the determined curve were entered in the budget.

The 1971 water-year was unusually dry; precipitation was nearly 20% less than the mean annual precipitation as judged by weather data from L. G. Hanscom Field, Bedford, Massachusetts. Monthly precipitation is compared to the period 1943-1965 in table 1 (data for the “standard normal” period, 1931 -1960 is not available for the Hanscom Field station). On a seasonal basis, spring and fall fell well below the norm, while summer

TABLE 1. Weather Data - L. G . Hanscom Field. Bedford. Massachusetts.

197 1 Water-Year Period 1943 - 1965

Mean Mean Precipitation Temperature Precipitation Temperature

Inches ( O F ) Inches ( O F )

October November Decein ber January February March April May June J uly August

September

Yea1

2.39 3.36 5.28 1.54 3.76 2.1 2 1.98 4.27 3.21 3.34 I .97 1.90

54.5 44 .2 25.9 19.9 28.5 35.5 44.5 55.9 67.4 70.8 69.4 69.5

~ ~~

3.52 4.76 3.17 3.82 3.34 3.99 3.85 3.64 2.74 2.99 3.80 3 .50

52.5 41.7 28.9 25.8 27.3 35.8 46.4 57.2 66.4 71.7 69.3 61.5

35.1 2 48.8 43.71 48.7

Hydrology of Wetland Basins 327

0 Lowell

Lincoln 0

Worcester 0

0 Lowell

7

V-

0 I0 20 - miles

Figure 1 . Eastern Massachusetts.

precipitation was slightly below the mean. Temperature data indicates an unusually cold January followed by a slightly cooler than normal spring. Summer temperatures approximated the long-term mean while September (1971) was unusually warm as were the months of October and November (1970).

Route 2 Wetland

The northernmost swamp, called the Route 2 wetland, is drained by Hobbs Brook which flows easterly a distance of nearly one mile from the wetland and then turns south t o flow into Stony Brook (figure 2). The wetland covers 26.6% of the drainage basin being 0.329 square miles in extent and lying a t the mouth of a basin of 1.23 square miles. Here muck, approximately 3 feet thick, developed on glacio-fluvial deposits. Ground

328 O'Brien

0 .s 1 bow map by U.S.Geological Survey milos

Figure 2. Map of Wetland Basins.

Hydrology of Wetland Basins 3 29

water near the center of the wetland was observed to be under artesian pressure from October through July. The southern portion of the wetland is transected b y Route 2, a busy four-lane east-west highway that leads to Boston. Except for drainage ditches and the highway, there has been n o development within the wetland itself and the drainage basin is sparsely inhabited.

Cotiant Road Wetlarid

The Conant Road wetland lies two miles south of the Route 2 wetland and drains southward through an unnamed brook that empties into Stony Brook (figure 2). The wetland deposits cover an area of 0.279 square miles and lie a t the mouth of a drainage basin o f I . 12 square miles. This swamp covers 24.8% of the total drainage area, and is composed of peat averaging tcn feet thick which has accumulated in a basin bordered and bottomed by till.

Both wetlands are covered with stands of Red Maple broken occasionally with White Pine, Elm, and Tamarack. They drain southeasterly into the Charles River system which discharges into Boston Harbor. All drainage from both basins is derived from precipitation within the basin - no stream enters either basin. A history of past use, ditching, and recent reversion t o forest makes these wetlands quite typical of many of the small freshwater wetlands found in eastern Massachusetts.

GEOLOGY AND HYDROGEOLOGY

Route 2 Wetland

The Route 2 wetland developed on a tongue of glacio-fluvial deposits that formed in a north-trending valley and were graded to standing water levels of glacial lake Sudbury (Koteff, 1964). There are no excavations in this deposit t o clearly reveal its structure, but 2% miles west, a t the Concord Town Landfill, there is a similar and probably contemporaneous deposit. Here deltaic deposits of silts and poorly sorted sands, gravels, and cobbles indicate rapid deposition near stagnant ice masses. Borings north of the Route 2 wetland indicate that a t this location the glacio-fluvial deposits may be more than 70 feet thick and occupy a northward sloping valley.

Mechanical analyses of sediments derived from a borehold and from near surface samples confirm that the glacio-fluvial deposits of the Route 2 Basin are poorly sorted. In fact, many of the layers are generally similar in grain size and sorting t o the till exposed throughout the area.

The hydrologic character of the deposit was investigated a t the mouth of the wetland where test borings were made. With the exception of the topsand layer, the permeability of the deposit averaged 3 gal/da/ft2 as determined with a variable head permeameter. Although the glacio-fluvial material is generally of low permeability, significant variation exists througbout the deposit and there are doubtless many lenses and layers of highly permeable sands.

The wetland deposits are generally shallow and are composed of an upper layer of black muck approximately one foot thick and a lower layer of brown silty peat. Test borings made with a harpoon borer (see figure 3) showed that the wetland deposits are

330 O'Brien

wetland

bO""dorY-/\

\\

of section

feet

Route 2 Roadbed 7- 1 000 1,.

b 2000 I,.

a 0

1 I 1 1 1 1 0 - 0 Y

c -

.- f

-10 8

Figure 3. Route 2 Swamp Sectional Profile.

generally three feet thick north of Route 2, but increase in thickness south of Route 2 where a maximum measured thickness of 9 feet was recorded.

Tests with a variable head permeameter showed that the muck which overlies the glacio-fluvial deposits was quite impermeable in the vertical direction but much more permeable in the horizontal direction (see table 2A). Low permeabilities for muck have been reported in the literature, but the observation of high horizontal permeability was previously unknown. Presumably it results from the effects of laterally spreading roots.

Conant Road Wetland

Unlike the Route 2 wetland, the Conant Road wetland is entirely surrounded by till. The organic deposit is peat averaging ten feet in depth with a maximum observed

Hydrology of Wetland Basins 33 1

thickness of 30 feet occurring along the eastern edge of the wetland (see figure 4). The upper foot of the wetland deposit is black and tends to be muck-like. Below this lies a mostly brown, but commonly red, aromatic peat. Near the bottom of the deposit is a grey-brown, compact, and fully decomposed peat. The peat is underlain by blue-grey silt

of section

0

feet

a 0

b

\ Limit of Boringr

-0

-to

C .- -20 f

cf a

-30

t:igure 4 . Conant Road Swamp Sectional Profile.

332 O'Bnen

which overlies the till and which probably represents a lake bot tom deposit. Several hillocks of till occur within the wetland and may stand as much as 5 feet above the level of the peat.

Permeameter measurements of the peat at the Conant Road Swamp indicate that the permeability decreases very rapidly with depth. High horizontal permeabilities were measured at the surface, but at greater depth horizontal permeabilities were quite similar t o vertical ones (see table 2B). Although high horizontal permeabilities have been reported in the literature, they were not observed here (with the exception of the surface layer), nor were they reported by Boelter (1965) who conducted an extensive investigation of peat permeability in Minnesota.

TABLE 2. Permeability Determinations on Peat and Muck.

Sample Depth Orientation Kpv* General Description

A . Route -3 Swamp

0 - 5 inches vertical .94 gpd/ft2 black muck 3% - 5 inches horizontal 110 gpd/ft2 black muck 3% ~ 5 inches horizontal 216 gpd/ft2 black muck 14 - 15% inches horizontal 2.8 gpd/ft2 silty brown peat 25 inches unoriented 102 gpd/ft2 fine sand

B. Conatit Road Swamp

0 - 5 inches vertical 28.6 gpd/ft2 black mucky peat 0 - 5 inches vertical 42.6 gpd/ft2 black mucky peat % - 2 inches horizontal 433 gpd/ft2 black mucky peat 4 - 5% inches horizontal 16 gpd/ft2 black mucky peat 16 - 21 inches vertical 2.1 gpd/ft2 fibrous peat 18% - 20 inches horizontal 1.6 gpd/ft2 fibrous peat

* Permeability in gallons per day per square foot determined on oriented samples in the field with a variable-head permeameter.

Two wells were installed at well site 6 t o measure groundwater fluctuations in the peat and in the underlying material. Well 6 was screened a t the bot tom of the peat deposit while well 6A was driven through silt deposits that underlie the peat into an oxidized sandy deposit. Hydrographs for the t w o wells are shown in figure 5.

The similarity in fluctuation and overall response suggests that the peat deposit is part of the same groundwater body that extends below the silt deposits. Water levels in well 6A were maintained at approximately 0.1 feet higher than well 6 indicating discharge of ground water into the peat from late November until August, For six weeks in the fall, water levels in well 6A fell below those of 6 and the swamp deposits recharged the regional groundwater body with an estimated 7 million gallons of water. Calculations of total areal recharge t o the basin for the week when maximum head differential occurred (week ending July 30) equaled 0.13 inches. This is approximately two orders of magnitude greater than the total runoff for that week (0.002 inches) suggesting that

Hydrology of Wctland kasins 333

1,'igure 5 . Conant Road Swamp Wcll Hydrographs.

recharge from the swamp deposits to the underlying aquifer could be significant during dry periods.

HYDROLOGIC BUDGET

The hydrologic budget indicates that storage was depleated in both wetland basins a t the end of the water-year. Total groundwater deficit for the Route 2 basin was 0.46 inches or I .3% of the total precipitation. The Conant Road basin recorded a deficit of 1.2X inches or 3.6% of the total precipitation. The budget further indicates a basin wide soil moisture deficit of 4.23 inches. This is, however, a theoretical loss. Field evidence suggests the wetland soils had the same moisture content a t the end of the water year as a t the start.

Total evapotranspiration for the 1971 water-year is summarized below. PET and AET were calculated by the Thornthwaite method (Thornthwaite and Mather, 1957) as an aid in the interpretation of the data. The measurements suggest that the

measured ET Thomthwaite AET Thornthwaite PET Route 2 Basin 20.77 inches 22.81 inches 26.54 inches Conant Road Basin 23.37 inches 22.81 inches 26.54 inches

334 O’Brien

Conant Road Basin lost 12!4% more water to evapotranspiration than did the Route 2 Basin. Such a difference between two areas with nearly identical annual runoff and similar hydrologic response is difficult t o explain. In view of this, the data for total evaporative loss must be viewed skeptically.

The distribution of yearly evapotranspiration is shown in figures 6 and 7. Both wetland curves are depressed during the spring and early summer with respect to the theoretical curves, but the late summer and fall portion of the ET curves follow the theoretical curves (AET and PET) more closely. The occurrence of this phenomenon in both wetlands suggests the observation may be significant. Further, McCuinness and Bordne (1972) compared Thornthwaite’s theoretical PET to a lysimeter derived PET t o check its applicability t o the humid northeast. They found that the Thornthwaite curve was “consistently below” the lysimeter values and that the greatest divergence occurred during the spring. Its is unlikely, therefore, that the depression of the ET curves is an illusion caused by the tendency for the comparison curves t o “over-register.”

It is possible that the extreme wetness o f the wetland soils may retard evapotrans- piration in the spring. It is known that wet soils warm slowly due t o their high specific heat and the cooling effect of evaporation from the surface. These combined effects produce soils that may be 6” to 12°F cooler than dryer, well drained soils (Buckrnan and Brady, 1969). Soil temperature is a critical factor in the spring; low temperatures are known t o depress root development and delay germination (USDA, 1955). As a consequence transpiration would be retarded and the overall ET from the basin would be less than expected from theoretical estimates.

In the fall plants would not experience severe moisture deficiencies due to the high moisture capacity of the wetland soils. Evapotranspiration rates would therefore remain high during this period.

HYDROLOGY At both wetlands total runoff amounted t o approximately 48% o f the precipitation;

discharge varied considerably with the seasons (see figures 8 and 9). Both basins exhibited

Figure 6. Route 2 Swamp Measured ET (smoothed).

Hydrology of Wetland Basins 335

Figure 7. Conant Road Swamp Measured ET (smoothed).

low winter flows followed by heavy spring runoff which receded rapidly in June. The spring months (March, April, and May) accounted for 69.4% and 74.5% of the total annual discharge of the Route 2 and Conant Road basins respectively.

The relative importance of direct surface runoff versus drainage from the groundwater zone was determined from an analysis of well and stream hydrographs according to a semi-analytical technique developed by the writer. The technique matches well hydrographs with stream hydrographs and utilizes the concept of groundwater recession constants as developed by Barnes (1937) and Langbein (1940). A more complete description of the technique is given by O’Brien (1973).

Figure 8. Route 2 Basin Total Discharge.

3 36 O'Brien

Figure 9. Conant Road Basin Total Discharge.

Results o f this analysis show that for both wetlands nearly 93% of the total discharge for the 1971 water-year resulted from groundwater drainage. At both wetlands more than half of the direct surface runoff occurred during the months of April and May. The highest weekly percentage of surface runoff for the spring occurred during the week of May 7-14, 1971 when 2.16 inches o f rain fell a t the wetlands. For this week, direct surface runoff accounted for 2670 of the weekly discharge at the Route 2 Swamp and 15% at the Conant Road Swamp.

Base flows during the low flow period are greatly depressed at both of the study wetlands. Flow duration curves were constructed t o illustrate the low flow characteristics (see figure 10). For comparison, a flow duration curve for the Shawsheen River was prepared froni data collected by the United States Geological Survey at gage 1006 near Wilmington. Massachusetts (drainage area, 36.6 square miles). This basin was selected because of its proximity to the study site, lack of artificial control, and similarity of topographic and geologic characteristics.

Flow duration curves were compiled froni weekly measurements for the two wetland basins. Although daily discharge nieasurements are preferred, they could not be obtained for the entire year as the stream gages froze several tinies during the winter. Use of the longer time period reduces the effect of sudden lloods that subside after a period of a few days. Consequently, the high flow end of the curve is somewhat flatter than daily discharges would show. Little difference, however, is to be expected in the low flow end of the curves.

The steepness of the two curves representing the wetland basins illustrates the great variability of discharge and lack of perennial storage for sustaining stream flow. The rapid drop in the low flow portion of the curves is particularly evident with respect to the Shawsheen River where the weekly discharge per square mile exceeds that of the wetlands 65% of the time. Discharge data is tabulated in table 3.

Hydrology of Wetland Basins 337

Percent of Time Indicated Discharge was Equaled or Exceeded

Figure 10. Flow-Duration Curves (1971 Water-Year).

338 O'Brien

TABLE 3. Discharge Data - 1971 Water-Year.

Daily Data* weekly Data* Annual*

Drainage Area 2 4 - h ~ Max. 24-hr. Min. Median Mode Mean Total Discharge

Shawsheen River 0.1 7 0.0043 0.23 0.20 0.33 17.24 date Mar. 2 Sep. 6

date Mar. 17

date Mar. 17 Sep. -- 0.10

Route 2 Basin 0.53 0.00 0.16 0.20 0.32 16.74 _ _ _

Conant Road Basin 0.40 0.00003 0.1 1 1 .oo & 0.32 16.73

* Inches of runoff for the period indicated.

In the writer's view, the lack of low flow augmentation in the wetlands is not primarily due to low permeability in the groundwater zone or to excessively high evapotrans-, piration as some writers have suggested. It results rather from rapid groundwater discharge from a shallow upper layer of the groundwater body.

At both of the study wetlands, the streams have become graded to till barriers at the mouths of the basins and are consequently shallowly incised into the groundwater body. Following a rain, groundwater discharges rapidly to the streams from a shallow upper layer. Normally the discharge zones of aquifers are quite small, but the wetland represents an expanded discharge zone in the regional aquifer. T h s provides an opportunity for rapid goundwater discharge which is doubtless aided by the high density of drainage channels within the wetlands and by each wetlands particular hydrogeology. As groundwater discharge and evapotranspiration lower the groundwater level a few feet from the spring high, the water table drops below or coincident with the stream bottom. Flow either ceases altogether, as was the case at the Route 2 Swamp or becomes minimal due to groundwater gradients approaching zero as occurred at the Conant Road Swamp.

Thus the shallow upper portion of the wetland groundwater body fluctuates rapidly allowing relatively little perennial storage or moderation of rainfall events. This contrasts with basins where streams are more deeply incised, storage available for sustaining streamflow is greater; the deeply moving ground water is characterized by long lag times allowing for greater streamflow moderation.

FIELD OBSERVATIONS Discharge characteristics of both wetlands are generally quite similar despite their

geologic differences. Field observations suggest, however, that the wetlands function differently in detail. At the Conant Road wetland, the upper layers of peat are so permeable that there can be little direct surface runoff. It seems more likely that interflow - water moving horizontally through the unsaturated zone - plays an important role in rapid runoff. Because of the great absorptive capacity of the peat and its relatively high specific yield, changes in groundwater level are never great or abrupt. In

Hydrology of Wetland Basins 339

comparison t o the Route 2 wetland, this swamp tends t o smooth out hydrologic events and maintain a more even discharge.

At the Route 2 wetland, a piezometric head was above or near the top o f the wetland from October t o July producing an artesian system for a major part of the year. The pressure head is produced at the margin o f the swamp where groundwater levels are slightly higher than the organic deposits a t the center. During the spring, water levels in monitoring wells stood above the top of the organic deposits, yet the surface remained relatively dry. Presumably a silty peat layer beneath the muck acts as a pressure transducer and reduces the movement of water into the muck. Water which does move into the muck layer is laterally discharged due t o its anisotropic permeability. As a result artesian pressure cannot be effectively transmitted t o the surface.

Stream channels have cu t through the muck, exposing the underlying sands and a considerable amount of ground water must be discharged through the bot tom of the channels. A further mechanism of discharge is through seeps which line the sides of the stream channel. These seeps - holes and moist spots - occur without exception at the interface between silty peat and muck and are a foot t o a foot and a half above the stream bed. Measured discharges from seeps ranged from 0.50 gpm for a 2-inch diameter hole t o 0.01 gpm from a moist area. Estimates indicate that nearly the entire baseflow of the stream could be due t o discharge from the seeps. The seeps were observed t o function through most of the year and cease discharging when the piezometric head falls below their level.

Stream levels a t the Route 2 wetland fluctuate through greater ranges, exhibit sharper hydrograph peaks, and crest sooner than a t the Conant Road Swamp. I t seems likely that the rapid stream level fluctuations result from the artesian system. Pressure which develops in one part of an artesian system is transmitted rapidly t o other parts of the system. Therefore, as recharge increases groundwater levels around the perimeter of the muck deposits, the piezometric surface beneath the muck must rise a t approximately the same rate. In response t o this pressure increase, ground water is discharged to the stream shortly after a storm starts. Verification of this postulate is available from well monitor records which show groundwater levels rising beneath the swamp deposits, in concert with water levels outside the swamp area, within a few hours of the start of a storm.

CONCLUSIONS

I . Spring evapotranspiration rates are depressed relative t o non-wetland areas while fall rates are high and reflect little moisture deficiency. The spring depression is presumably due t o the extreme wetness o f the organic soils which result in a high specific heat and a consequently slow warming rate in the spring. The wet, cool organic soils may, therefore, retard moisture loss in the spring, but retain sufficient moisture to prevent moisture deficiencies from developing in the fall.

2 . Both wetlands experienced high spring flows - nearly three-quarters of the total annual discharge ran off during the months of March, April, and May.

3 . Groundwater drainage accounted for 93% of the total annual discharge from the wetlands. Moreover, groundwater entered the streams rapidly following a rain and contributed significantly to storm crests.

340 O'Brien

4. Baseflows during the low flow period are greatly depressed at both Lincoln wetlands. The writer attributes the lack of low flow augumentation t o rapid water table fluctuations in the shallow upper layer of the wetland groundwater body allowing little perennial storage or moderation of rainfall events.

5 . The wetlands are part of the regional groundwater body and receive water from it thereby serving as discharge areas, This flow was reversed at the Conant Road Swamp during six weeks in later summer - early fall, when the swamp deposits recharged the regional groundwater body. Estimates suggest that total groundwater recharge for that period may be several orders of magnitude greater than surface runoff.

6. At the Route 2 wetland, muck deposits have a low vertical permeability and cause the groundwater to be under artesian pressure for the major portion of the year. High horizontal permeability in the organic deposit allows considerable lateral discharge and prevents artesian pressure from being transmitted t o the surface so the top of the wetland remains relatively dry.

7. Storm crests a t the Route 2 wetland tended t o be greater, t o exhibit sharper hydrograph peaks, and to crest sooner than at the Conant Road Swamp. It seems that this may be primarily due to the action of an artesian system which functions for a major part of the year. Despite the somewhat flashier nature of the Route 2 wetland, the total annual discharge for both wetland basins was practically identical.

ACKNOWLEDGMENTS

The author would like to express his thanks to Dr. Dabney Caldwell, Boston University, and Dr. Ward Motts, University of Massachusetts, for their advice and suggestions.

LITERATURE CITED

B m c s , B. S., 1939. The structure of discharge-rccession curves. American Geophysics Union,

Boelter, D. H., 1965. Hydraulic conductivity of peats. Soil Science, Vol. 100, pp. 227-231. Buckman and Brady, 1969. The Nature and Properties of Soils. MacMillan Company, New York, 7th

Edition, p. 265. Currier. L. W., 1941. Disappearance of the last ice sheet in Massachusetts by stagnation Lone rctrcat.

Gcological Society of America, Vol. 52, abstract, p. 1895. Koteff. C., 1964. Surficial Geology of the Concord Quadrangle. U.S. Geological Survey, Massachu-

setts, CQ 331. Langbcin, W. B., 1940. Some channel storage and unit hydrograph studies. American Geophysics

Union, Transactions, pp. 620-627. Larson, J . S., 1973. A Guide to Important Characteristics and Values of Freshwater Wetlands in the

Northcast. Water Resourccs Research Center, University of Massachusetts, Amherst, Massachusetts, Publication No. 3 I , p. 23.

McGuinness and Bordnc, 1972. A Comparison of Lysimeter-Derived Potential Evapotranspiration with Computed Values. U.S. Department of Agiculture, Research Service, Technical Bulletin 1452, p. 18.

O'Brien, A. L., 1973. Hydrologic Investigations of Two Wctlands in Lincoln, Massachusetts. Boston University Graduate School Dissertation, pp. 147-157.

Thornthwaite, C. W., and J . R. Mather, 1957. Instructions and Tables for Computing Potential Evapotranspiration and the Water Balance. Drexel Institute of Technology, Laboratory of Climatology, Publications in Climatology. Vol. 10.

U.S. Department of Agriculture, 1955. Watcr - The Ysarbook of Agriculture. U.S. Government Printing Officc. pp. 493,606.

Transactions. pp. 721-725.