Current Studies of the 

Hydrology of Prairie Potholes

GEOLOGICAL SURVEY 

CIRCULAR 472

PREPARED AS PART OF A PROGRAM OF THE DEPARTMENT OF THE INTERIOR FOR DEVELOPMENT OF THE MISSOURI RIVER BASIN
Current Studies of the Hydrology 

of Prairie Potholes

By J. B, Shjeflo and others

Prepared as part of a program of the

Department of the Interior for 

Development of the Missouri River basin

GEOLOGICAL SURVEY CIRCULAR 472

Washington 

1962
United States Department of the Interior

STEWART L. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U.S. Geological Survey, Washington 25, 0. C.
CONTENTS

Page Page

Abstract ___............ _______............ _______ 1 Ecology, by R.S. Aro and F. A.

Introduction, by H.M. Erskine _________ 1 Branson___________________________ 5

Description of the study areas_________ 2 Chemical quality of water and sediLocations--------_-_---_---______ 2 mentation, by D.M. Culbertson ______ 7

Climate _______ ___ ___________ 3 Instrumentation ________ __ ________ 8

Topography and drainage _ ________ 4 Seepage and evaporation computations,

Project plan _______________________ 4 by G.Earl Harbeck, Jr. ____ _____ 9

Geology and ground-water conditions of Future project plans __________________ 11

the project areas, by H. M.Jensen____ 5 References __________________________ 11

ILLUSTRATIONS

Page 

Figure 1. Map of prairie pothole region in the United States showing location of study

areas ___________________________________________________________________ 2

2. Pothole with dense stand of emergent aquatic vegetation and vegetation gage______ 3

3. Pothole clear of emergent aquatic vegetation with water-stage recorder,

anemometer, and rain gage________________________________________________ 3

4. Hypothetical net seepage loss as determined from relation between fall in pond

stage and product u(eo-^o) --__--___-__----------_________________________ 10

III
Current Studies of the Hydrology of Prairie Potholes

By J. B. Shjeflo and others

ABSTRACT

The prairie potholes in the North-Central States and in 

Canada are of glacial origin. Because many of them contain 

ponds or marshes, they are important in the production of 

livestock and waterfowl. The objective of the present investigation is to determine the amount of water that accumulates in and is used in the potholes in their natural environment.

Two study areas were selected in North Dakota, and 

construction of gaging facilities was completed in 1960. 

Each study area contains four potholes. Three of the potholes for each area contain dense growths of aquatic vegetation, and one is clear of aquatic vegetation. The instruments 

to provide the basic data for computing water losses include: 

water-stage recorder, water-temperature recorder, rain gage, 

anemometer, and hygrothermograph. Observations will also 

be made of ground-water levels, rate of growth and type of 

vegetation, and quality of water.

INTRODUCTION

Throughout great sections of the prairies 

of the northern States and of Canada are more 

than a million shallow depressions formed by 

the melting of glacial ice. They range in size 

from less than an acre to several hundred 

acres and are known in the region as potholes. Each pothole receives the drainage, if 

any, from the small closed basin of which it 

occupies the lowest part. Thus most potholes 

contain marshes, shallow ponds, or lakes, depending on the amount of drainage. Dense 

growths of aquatic vegetation emerge from 

most of these water bodies an important 

factor in the water economy of potholes and 

their usefulness to wildlife.

The prairie pothole region is one of the 

greatest waterfowl production habitats on the 

continent; hence its preservation and possible 

improvement is of concern to wildlife interests. Already there has been considerable 

shallow flooding of lowlands by impoundments 

and dikes in some parts of the region, particularly in the Souris River basin, in connection 

with migratory waterfowl production and refuge areas. The demand for water for other

needs is sometimes in competition with this 

usage. In areas where livestock grazing is 

practiced, potholes often provide the principal source of water. In some areas that are 

suitable for cultivation, agricultural interests 

encourage the draining of such potholes thus 

bringing new land into cultivation.

Irrigation projects that could eventually 

place more than 1.5 million acres under irrigation are being planned in the Dakotas. A 

large part of this area is in the prairie pothole region and thus many potholes would be 

eliminated by land leveling and the construction of drainage systems. Many of the lost 

potholes would be replaced by new marsh 

areas created by flooding certain lowlands 

through diversions and impoundments, and by 

construction of dikes and similar works.

The amount of water that would be required 

for these proposed undertakings is not known. 

Demands on the water available are made by 

seepage, evaporation, and transpiration from 

aquatic vegetation. Very little information is 

available for evaluating these demands so the 

present investigation was undertaken to 

measure the water losses at representative 

natural potholes. Such information should 

substantially aid in determining the feasibility of many of the proposed projects. The 

project is a part of a program of the Department of the Interior for development of the 

Missouri River basin.

Other questions raised by the drainage of 

potholes are: (a) What effect does it have on 

ground-water levels in the vicinity, (b) to 

what extent is the flood hazard to downstream 

areas increased, (c) how much runoff accrues 

to the stream receiving the drainage, and (d) 

what is the chemical quality of the drainage 

water. These questions however, are beyond 

the scope of the present investigation.
CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES

DESCRIPTION OF STUDY AREAS

LOCATIONS

The many types of observations needed in 

the investigations impose several requirements on the study areas to be selected. 

Each area should include several potholes. 

One should be clear of emergent aquatic vegetation, while the others should support a 

considerable amount of it. The ground under 

the potholes should be highly impervious in 

order to reduce seepage losses. The potholes 

should be unaffected by the activities of man, 

and the area should be accessible.

Two study areas were selected in North 

Dakota (fig. 1). One is in south-central Ward 

County about 80 miles south of the Canadian 

border, and the other is in the southwestern 

part of Dickey County, 2 miles north of the 

South Dakota State line. These areas are 

about 200 miles apart.

Preliminary selection of the areas was 

made from an airplane in regions preselected 

on geologic maps and aerial photographs.

Final selection was made on the ground. 

Each study area comprises four potholes. 

Three of the potholes support various 

amounts and types of aquatic vegetation (fig. 

2), while the fourth one is clear and almost 

entirely void of emergent vegetation (fig. 3). 

The potholes in the Ward County group were 

assigned numbers 1 4; pothole 3 is clear of 

emergent aquatic vegetation. Those in the 

Dickey County group were assigned numbers 

5-8; pothole 5 is clear. The four potholes in 

each group are contained within a circle 8 

miles in diameter, where soils and weather 

are expected to be very similar. The potholes generally are elliptical and range in 

size from 5 to 40 acres and in depth from 1 

to 4 feet. All of these particular potholes 

hold water the year around except during 

extreme drought.

Many potholes do not hold water the year 

around. Some hold water for only a few days 

after the spring runoff or heavy rains. Others 

have never been known to go dry. It has been 

estimated that there are about a million potholes in North Dakota alone. The volume of 

water in each depends upon the amount of 

precipitation on its drainage basin.

I05C 101°

49°

45°

Figure 1. Map of prairie pothole region (shown by ruling) in the United States showing location of study areas.
DESCRIPTION OF STUDY AREAS

Figure 2. Pothole with dense stand of emergent aquatic 

vegetation and vegetation gage.

The drought that has prevailed in much of 

the prairie pothole region between 1953 and 

1961 probably aided in the selection of the 

study areas, for thousands of potholes were 

dry at the time of the reconnaissance for the 

selected study areas.

CLIMATE

Because of its midcontinental position and 

its location near the paths of many cyclonic 

storms, the general area has a markedly continental climate with extreme summer heat, 

extreme winter cold, and rapid fluctuation of 

temperature. The average July temperature 

is about 68°F for the Ward County group and 

70°F for the Dickey County group, but daily 

maximum temperatures of 100°F or more are 

common.

The average annual precipitation in the vicinity of the Ward County group is about 16 

inches, and at the Dickey County group it is 

about 19 inches. General rains can be expected in the spring until about the end of 

June. Summer rains are usually the result 

of thunderstorms, some of which are quite 

severe. Occasionally, several inches of rain 

may fall in a relatively short time on an area 

of only a few square miles. This is about the 

only type of rainstorm that will produce runoff in the prairie pothole region during the 

summer and fall.

Snowfall is probably the most significant 

factor in replenishing the water supply of 

potholes. Potholes, especially those with 

dense emergent vegetation, usually trap a 

great deal of snow. Also, deep snow drifts 

often develop on the periphery of some potholes while the surrounding hills may be

bare. Normally about one-fourth of the precipitation in the study areas occurs as snow 

between October and March. Much of it 

reaches the potholes when melting occurs in 

the spring while the ground is still frozen.

The average wind velocity for the area is 

about 10 miles per hour with the highest velocities during April and May. The prevailing winds are northwest,but during the summer southeast winds are almost as common.

The area is generally thought of as being 

extremely dry, since it lies in a dry climatic 

region; but the relative humidity is not as 

low as some might expect. The long-term 

averages at noon during July show the relative humidity to be between 45and 50percent 

in the study areas.

According to the U.S. Weather Bureau, the 

greatest seasonal evaporation (April through 

September), as measured at an evaporation 

pan for the period 1907 1945 at theDickinson 

Experiment Station (about 150 miles west of 

the study areas), was 51.43 inches in 1936. 

The rainfall was 3.49 inches during the same 

period. The lowest seasonal evaporation was 

26.15 inches in 1942, when the total rainfall 

was 17.27 inches.

The potholes usually freeze over during 

the early part of November and remain frozen until the first or second week in April. 

The shallower ones (less than 3 feet deep) 

freeze to the bottom as will about half of 

those under study. No records will be collected during the period of ice cover.

Figure 3. Pothole clear of emergent aquatic vegetation with 

water-stage recorder, anemometer, and rain gage.
CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES

TOPOGRAPHY AND DRAINAGE

Both study groups lie along the eastern 

edge of the high country known as the Coteau 

du Missouri; an area covered by glaciers in 

Pleistocene time. (Flint, 1955.) This is a 

region of many small lakes without outlets 

and of rolling prairies marked by ranges of 

rounded hills, some of them high.

The potholes in the Ward County group 

are in deep bowllike depressions where water 

would have to rise as much as 15 feet before 

it could flow into another pothole. Those in 

the Dickey County group are in an area where 

the water courses are better defined. The 

potholes in this area are in chains of saucerlike depressions, and in places, a rise of only 

2 feet would cause water to flow from one 

pothole to another. However, since little or 

no runoff is expected except during early 

spring, the shallowness of these basins is not 

expected to detract from the study.

PROJECT PLAN

The plan is to determine the water budget 

for each of the potholes, and the seasonal 

variation of each of the items in the water 

budget. These include runoff, precipitation, 

seepage, and evapotranspiration. Changes in 

ground-water levels, chemical quality of water, and aquatic vegetation also will be observed.

The construction phase of the project was 

completed during the summer of 1960. For 

each pothole this included a stilling well and 

shelter for the water-stage recorder, supports for the anemometer and rain gage, 

water-temperature gage, outside staff gage, 

and a ground-water observation well. Vegetation gages were installed at each of the potholes having emergent aquatic vegetation to 

aid in determining the growth (fig. 2).

A shelter was erected near the center of 

each area to house a hygrothermograph used 

to serve the group of four potholes.

A detailed topographic survey was made of 

each pothole covering the expected range in 

stage, and stage-area curves, and stagecontents curves are being computed. The 

potholes will be resurveyed at the end of the 

field investigations to determine the rate of

sediment accumulation. Transit-stadia traverses have been run to determine the drainage area of each pothole to determine unit 

runoff.

The operational plan is as follows:

1. Operate the water-stage recorder, anemometer, rain gage, water-temperature recorder, and hydrothermograph continuously 

throughout the open-water season. No attempt will be made to obtain records during 

the winter when the ponds are frozen, except 

to determine the elevation of the ice surface 

before the spring thaw.

2. Observe the ground-water level weekly 

at each observation well in the pothole group.

3. Determine the extent, density, and kinds 

of vegetation at each pothole at least three 

times each growing season (preferably in the 

late spring or early summer, midsummer, 

and shortly before the first frost) by measuring from the vegetation gages and by photographs. Aerial photographs will also be 

obtained to aid in determining the density and 

extent of emergent aquatic vegetation.

4. Obtain water samples at each pothole 

about once a month during the summer and 

once during the winter for chemical analysis.

5. The seepage-stage relation for each 

pothole will be determined. This will consist 

of measuring the net seepage at about four 

different reservoir stages.

The observations obtained will be used to 

compute the items in the water budget. Runoff can be obtained from stage records and 

area and capacity curves of the pothole. 

Evapotranspiration can be computed from 

the records of stage,precipitation, and computed seepage. The evaporation records from 

the clear pothole may be used to separate 

evaporation from the total evapotranspiration 

at the pothole shaving emergent aquatic vegetation. The records of ground-water levels 

will be used to determine what relation, if 

any, exists between ground-water fluctuation 

and water stages of potholes. The chemical 

analyses of water samples will be used to 

investigate trends in water quality and its 

possible effect on ground water and aquatic 

vegetation.
ECOLOGY

GEOLOGY AND GROUND-WATER CONDITIONS OF 

THE PROJECT AREAS

By H. M. JENSEN

The term "pothole" has great descriptive 

value and is a colloquial synonym, for the 

geologic term "kettle." The pothole region 

is primarily in areas of morainal complexes 

glacial drift deposited by ice advances in the 

Wisconsin stage of the Pleistocene epoch. 

The region is characterized by what geologists refer to as knob and kettle topography 

small closely spaced hills and ridges with no 

integrated drainage pattern and local relief 

of from 20 to 100 feet.

The groups of potholes in this investigation 

are in the Missouri Plateau glaciated section 

of the Great Plains physiographic province, 

the northeastern strip of which is known as 

the Coteau du Missouri. (Simpson, 1929, p. 

11.) (See fig. 1.) The border of the Coteau is 

a northeast-facing escarpment that rises 300 

to 500 feet above the Central Lowland to the 

east and owes its prominence in part to topographically nigh bedrock. The high bedrock not only acted as a buttress to advancing 

ice sheets but it also influenced the distribution and landforms of the glacial drift depos - 

ited on the Coteau. Drift deposits of variable 

thickness were left on the Coteau, and, as the 

ice front retreated, isolated blocks of ice 

wholly or partly buried in the drift mass 

melted, leaving shallow saucer-shaped depressions or kettles. The morainic complex 

associated with the Coteau has been traced 

from northwestern North Dakota to southeastern South Dakota and has been called the 

Altamont end-moraine complex (Flint, 1947, 

p. 274).

The glacial drift in the areas of investiga - 

tion is composed predominantly of till. Till 

is that part of the drift deposited by and underneath the ice mass, with little or no transportation by water. Till is a heterogeneous 

mixture of the rock fragments picked up by 

the advancing ice sheets, namely: clay, silt, 

sand, gravel, and boulders. In the study 

areas, a preliminary field reconnaissance 

showed that the till was composed predominantly of clay and could be expected to be 

impermeable. This fact was important in determining the suitability of the pothole areas 

to be studied.

Test wells were drilled in the immediate 

vicinity of the selected potholes as part of 

the investigation of ground water. Because 

no continuous sand and (or) gravel lenses 

were penetrated in drilling, and because the 

clayey till surrounding the potholes has relatively low permeability, it is inferred that 

there is little or no recharge or discharge of 

ground water in the immediate vicinity of the 

areas under investigation. To obtain additional ground-water information, one test 

hole at each pothole was converted to an ob - 

servation well. The observation wells, consisting of l|--inch steel pipes, range in depth 

from 17 to 22 feet, and the bottom 6 to 10 feet 

of the pipe columns is slotted. When the 

pipes were installed, a gravel pack was 

placed outside the slotted part of the pipe. 

The remainder of the hole was packed with 

clay till to prevent the entrance of surface 

water.

The configuration of the water table in the 

vicinity of the potholes is uneven, reflecting 

to some extent the morainic topography. 

Water-level measurements in observation 

wells will be influenced by the topographic 

location of the observation well, the distance 

of the well from the pothole, and the type and 

amount of vegetative cover near the well. 

The period of record now (1961) is too short 

to form definite conclusions as to the effec - 

tiveness of measurements of the groundwater levels.

ECOLOGY

By R. S. ARO AND F. A. BRANSON

The ecologic phase of this investigation 

will involve the measurement and description 

of the vegetation associated with the evapotranspiration rate obtained for each pothole 

and an appraisal of the importance of the 

physical and biological factors at work in the 

existing plant communities. Water depth and 

quality, soil chemistry, and weight of emergent vegetation will be measured for each 

pothole, while less important factors, such as 

waterfowl and submersed plantlife, may be 

described in qualitative terms. Until the 

quantitative data are assembled, only a 

sketchy ecological picture of the eight potholes can be given. The following descriptions were prepared as the result of a brief 

visit to each pothole in October 1960.
CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES

The prairie pothole region is an extensive 

glaciated area stretching from the hardwood 

forests of Minnesota across the tallgrass and 

mixed prairie associations of North and South 

Dakota into the aspen parkland of Canada. All 

potholes being studied in the current project 

are located in the mixed prairie association, 

and are mostly surrounded by native grass 

hayfields or pastures, dominated by such 

species as needle-and-thread (Stipa comata), 

blue grama (Bouteloua a/acilis), western wheat 

grass (Agropyron smithii), and little bluestem 

(Andropogon scoparius).

Cultivated fields that border parts of potholes 2, 3, 6, and 8 may constitute a source 

of salt and sediment in the ponds; but these 

possible small additions should be negligible. 

Mechanical harvesting of vegetation within 

the margins of potholes 1 and 8, and heavy 

grazing of plants in pothole 7 probably reduced 1960 gross transpiration below natural 

losses from these sites. Such land uses, 

which remove live pothole vegetation, must 

be recognized and evaluated along with estimates of subducted plant material when 

evapotranspiration rates are compared with 

the qualities and quantities of vegetation in 

affected ponds.

Typical of aquatic habitats, the pothole 

vegetation discussed here involves only a few 

dominant species and notably simple community structure. The following table indicates the presence and relative abundance of 

the major emergent plant species found, 

roughly in decreasing order of their tolerance to deep water.

Deep-water species dominate the appearance of all the containing potholes covered 

with emergent vegetation and the margin of 

one of the clear ponds. An exception, prairie 

bulrush (Scirpus paludosus), occurs twice as a 

codominant in relatively shallow water and 

saline soil. Species present on intermittently 

flooded or continually moist shorelines occur 

in zones coincident or parallel to pothole water levels and indicate, in part, fluctuations 

and gradations in soil moisture and salinity.

The emergent vegetation of pothole 1 

consists mainly of hollowstem (Scolochloa 

festucacea)* a tal1 rhizomatous grass occurring in the deeper water as round colonies, 

which merge into a nearly continuous belt 

around the margin of the pond. Just outside 

the hollowstem is a narrow band of broadleaved cattail (Typha latifolia), with a scattering

Plant species and their relative abundance, observed in three ecologic niches at eight prairie potholes in North Dakota, 

listed approximately in decreasing order of their tolerances to deep water

[Relative abundance indicate by: R, rare; O, occasional; F, frequent; A, abundant; V, very abundant]

Ecological niches and their associated species

Deep water: 

Common bulrush Scirpus validus _________________________

Hollowstem Scolochloa festucacea ____________________

Broad-leaved cattail Typha latifolia _________________________

Shallow water: 

Prairie bulrush Scirpus paludosus _______________-__----_

Intermittently flooded or moist shorelines: 

Willow Salix sp._ ____________________________

Northern reedgrass Calamagrostis inexpansa __________________

Sedge Caret sp. ____________________________

Prairie cordgrass Spartina pectinate _______________________

Desert saltgrass Distichlis stricta _______________________

Rayless aster Aster brachyactis _______________________

Foxtail barley Hordeum jubatum ________--__-_--------_

Alkali sacaton Sporogolus airoides. _ ____________________

Cottonwood Populus deItoides-__--__-------_--------

Aspen P, tremuloides _________________________

Species abundance in given 

pothole number

1

A 

A

F 

O

0

2

V 

A

R

3 4

V 

A

F 

F 

F 

F

5

O

R

6

A

A

F

0

7

V 

A

F 

F

8

A 

V 

0

R 

R
CHEMICAL QUALITY OF WATER AND SEDIMENTATION

of northern reedgrass (Calamogrostis inexpansa) 

and sedge (Carex sp.) along its outside edge. 

On the south and east banks, aspen (Populus 

tremuloides) groves form a discontinuous community.

The dominant vegetation of pothole 2 is 

common bulrush (Scirpus validus), which occupies about two-fifths of the pond in one pure, 

dense stand at its southeast end. The remaining part is mostly open water containing water milfoil (Myriophyllum exalbesceus) and 

pondweed (Potamogeton sp.). In the shallower 

water, a fringe of broad-leaved cattail completely encircles the pothole, and on drier 

ground at the north edge prairie cordgrass 

(Spartina pectinate) is present.

Pothole 3 is free of all but submersed 

plants, such as water milfoil and pondweeds.

Common bulrush is the most important 

plant species throughout pothole 4, occurring 

in dense, scattered colonies; yet at least 25 

percent of the pond is open water. All along 

the shallow saline margin prairie bulrush 

forms a distinct community; and above it, on 

the moist saline shoreline, a zone containing 

alkali sacaton (SporogoJus airoides), foxtail barley (Hordeum jubatum), desert saltgrass (Distichlis 

stricta),and rayless aster (Aster brachyactis)separates the aquatic vegetation from the mixed 

prairie.

Pothole 5, in the Dickey County group, is 

virtually free of vegetation, except at the 

water's edge, where common bulrush and a 

few patches of cattail form a continuous, narrow belt around the pond.

The plant communities of pothole 6, from 

the center to the shore, are a contrast between irregular patches of hollowstem, covering about half the deep-water area, and the 

narrow, ordered ranks of prairie bulrush. 

Mixed cottonwood and willow saplings mark 

their respective zones of water-level tolerance around the perimeter. Above these rows 

of young growth are scattered a few large 

willow trees.

Pothole 7 exhibits typical aquatic community zonationfrom its dense stand of common 

bulrush, which dominates the central 80 percent of the pond, to the prairie grasses at its 

rim. Hollowstem occurs mixed with the outer 

fringe of bulrushes, then as almost a pure 

belt about 30 feet wide. Next outside is a

20 -foot band of foxtail barley and desert salt - 

grass bounded by a similarly mixed zone of 

foxtail and midgrasses, and finally by a belt 

of little bluestem.

Common bulrush and hollowstem express 

simple communities in pothole 8. The deepest one-third of the marsh contains a dense 

growth of bulrush and occasional clumps of 

cattail. Hollowstem forms a complete cover 

over the remainder of the pothole, extending 

outward to the surrounding native grassland.

Detailed investigations during the 1961 

growing season will help to define and relate 

all of the principal components operating in 

these potholes, but even now several causative factors are apparent. For example, water depth seems to be controlling not only the 

ratio of vegetation-covered to open-water 

areas, but also species zonation and community structure. The presence of aspen and 

willow trees along several pothole margins 

suggests stable water levels in such ponds; 

and relatively high salinity and shallow water 

probably account for the two perimeter bands 

of prairie bulrush noted above. Observation 

of these study areas and comparisons of their 

ecological characteristics with other pothole 

communities over a period of years should 

reveal useful information on aquatic plant 

succession. Knowledge of these and other 

ecologic agents, processes, and changes will 

be essential to sound water and wildlife 

management of prairie potholes.

CHEMICAL QUALITY OF WATER AND 

SEDIMENTATION

By D. M. CULBERTSON

The ecology of the vegetation in the pothole 

area is affected by the chemical quality of 

the water. Although the effects are mostly 

indirect, information on chemical quality of 

water will aid in understanding the water requ^rements and the physicochemical relationship bet ween the environment and the vegetation. In closed lakes, evapotranspiration and 

re-solution of deposited salts cause considerable fluctuation in water quality with time. 

Although the dissolved-solids content of the 

water directly retards evaporation by reduc - 

ing the vapor pressure of the water, the effect on evaporation is not significant. The 

dissolved solids, however, may influence considerably the type of plants that will thrive in 

the lakes and thereby indirectly affect the
8 CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES

rate of transpiration from the vegetation in 

the lakes.

The investigation provides for determination of the chemical quality of water in each 

of the potholes during the field seasons. 

These determinations will be made approximately every month. A comparison of the 

1960 pothole survey with later surveys will 

provide the information for determining 

whether any significant deposition of sediment occurs. These data will be analyzed to 

relate changes in water quality to changes in 

depth of water, volume of water, and water 

temperature associated with the evapotranspiration rate computed for each pothole. 

Also, some correlations may be found between the change in water quality and the 

ecology in the potholes.

Prelinimary data for potholes 1 4 indicate 

that the chemical composition of the water 

differs considerably from one pothole to another. For example, in April 1960 water from 

pothole 3 had 237 parts per million (ppm) of 

dissolved solids, which was mostly calcium, 

magnesium, and bicarbonate; water from pothole 4 had 1,380 ppm of dissolved solids, 

which was mostly magnesium, sodium, and 

sulfate. Probably the inflow to each of the 

potholes contains principally calcium and bicarbonate, and the differences in chemical 

composition are partly the result of differences in the depth of water in the potholes. 

Although, as evaporation takes place, calcium 

and bicarbonate tend to precipitate, the other 

constituents tend to remain in solution until 

the potholes become nearly dry; consequently, 

the relative proportions of the more soluble 

constituents increase. In potholes that dry 

up occasionally, precipitates of even the most 

soluble constituents may have little chance 

to accumulate because they are susceptible 

to removal by wind. The data also indicate 

that the chemical composition of water in a 

pothole may differ considerably from time to 

time.

From April to October 1960, the dissolved 

solids in water from potholes 1, 2, and 4 

more than doubled; however, the dissolved 

solids in water from pothole 3, which is clear 

of vegetation, increased only about 10 percent.

The deposited sediment, represented by a 

few samples obtained in 1960 from each lake, 

ranged from mainly sand to mainly silt and 

clay. The deposited sediment from pothole 3

was nearly all sand and was very low in 

organic matter, that from pothole 4 was 

predominantly sand and was low in organic 

matter; and that from the other potholes was 

predominantly silt and clay and was high in 

organic matter.

The deposited sediment from pothole 5 

contained abundant mollusk (gastropod) shells, 

generally less than one-eighth inch in diameter. The sediment from potholes 3, 6, 7, and 

8 contained less abundant shells, and the 

sediment from potholes 1,2, and 4 contained 

very few or no shells. The relations among 

the kind and abundance of invertebrate forms, 

the chemical quality of the water, the type of 

suspended and deposited sediment, and the 

aquatic vegetation have not been established 

for these potholes.

INSTRUMENTATION

To provide the necessary data for computing water losses it is necessary to install 

instruments that will measure the following 

variables: water-surf ace elevation, wind 

movement, rainfall, water temperature just 

below the surface, air temperature, and humidity.

The change in water-surf ace elevation is 

measured by a continuous water-stage recorder in a shelter mounted on a 24-inch 

pipe stilling well near the edge of each pond 

(fig. 3). A 10:12 gage height ratio (1 foot per 

width of chart) and 4.8 inches per day time 

scale is used to produce the accuracy required. Auxiliary pen attachments were 

added near the right and left margins of the 

recorder chart to record the wind movement 

and rainfall. The chart is usually removed 

near the end of each month.

Wind movement is obtained by a three-cup 

totalizing anemometer with a dial showing 

linear movement to one-tenth mile. The anemometer is placed on an adjustable standard 

and set at about the 4-meter level above the 

water surface. An electrical connection between the anemometer and one of the auxiliary pens on the recorder permits the recording of each 10 miles of wind movement.

Rainfall is measured in a tipping-bucket 

rain gage (fig. 3). An electrical connection 

between the rain gage and the other auxiliary 

pen on the recorder permits the recording of 

each 0.10-inch of rain collected. After the
SEEPAGE AND EVAPORATION COMPUTATIONS

rain actuates the tipping bucket mechanism it 

flows into a standard rain-gage can. The 

total collected is measured by means of a 

standard tube and graduated stick at the time 

of each weekly inspection.

The water -surface temperature is recorded 

on the chart of a weekly recorder with 10- 

foot capillary tube and sensing element. The 

sensing element is adjusted weekly when the 

chart is changed, to keep it below but as close 

to the water surface as possible.

Air temperature and relative humidity are 

recorded on a single chart of a hygrothermo - 

graph. This is a weekly, vertical drum, 2- 

pen recorder with a hair element for humidity 

and a bimetal element for temperature. A 

sling-psychrometer is used in checking and 

setting this instrument. One hygrothermo - 

graph is used for each pothole group and is 

as centrally located as possible with respect 

to the four potholes.

Two vegetation gages are located in each 

pothole where emergent aquatic vegetation is 

significant, to aid in determining the rate of 

growth. They are 1-inch by 6-inch boards, 

painted alternately black and white at 1 -foot 

intervals, and set to gage datum (fig. 2).

Other measurements made on a weekly 

basis are the determinations of the water 

surface in each observation well, and photographs in the vicinity of the vegetation gages. 

Aerial photographs will also be obtained once 

or twice during the summer to aid in determining density of vegetation and percent of 

coverage.

During the first season of operation, 1960, 

only the Ward County group was fully instrumented and ready for operation. All instruments performed satisfactorily during the 

entire season except for a few days when the 

the water level dropped below the lowest intake holes in the stilling wells or the sensing 

element of the water-temperature recorder 

became exposed to the air.

Two areas of improvement in the records 

during the next season concern the humidity 

recorder and the rain gage. At times during 

the past season the humidity would be off 

several percent when checked by the slingpsychrometer. Accurate results will require 

repeated and careful checking and resetting, 

preferably during that part of the day when

there is little or no change. The amount of 

rainfall was rather uncertain at times during 

intense storms, as the jogs on the chart were 

not sufficiently distinct and their total as 

counted did not check very Closely with the 

amount in the standard rain gage. It is suspected that at times the tipping-bucket may 

have bounced and showed 0.2 or 0.3 inch of 

rain instead of 0.1 as it should have. Evaporation from the standard rain gage can further complicate the situation. Some sort of 

a maximum depth indicator placed inside the 

8-inch rain-gage can may aid in determining 

more closely the actual amount of rainfall.

During the summer of 1960 the watersurface of pothole 1 (Ward County group) declined 31 inches with 7 inches of rainfall; 

during the same period the water surface in 

pothole 2 dropped 20 inches with 11 inches of 

rainfall, and the water surface in pothole 3 

fell 22 inches with 12 inches of rainfall. The 

remainder of the gages were either under 

construction or awaiting delivery of instruments so no further results are available. 

Construction was completed, however, and 

data gathering began on all eight potholes by 

the spring of 1961.

SEEPAGE AND EVAPORATION COMPUTATIONS

By G. EARL HARBFCK JR.

The basic objective of the prairie pothole 

project is to determine the water requirements of marsh and pothole areas. The volume of water in a pothole decreases gradually during the periods of no inflow because 

of seepage, evaporation, and transpiration 

losses. One of the criteria for selecting the 

two groups of potholes was that seepage 

losses should be small. Although the geology 

of both areas, and the test drilling indicate 

that is probably true, the seepage losses 

must be measured. The clear potholes have 

little or no emergent vegetation so that transpiration losses are negligible and only 

evaporation need be measured. It would be 

desirable to measure separately the evaporation and transpiration losses from the potholes containing aquatic vegetation, but adequate techniques are not now available, and it 

is necessary to measure the sum of the two, 

or the evapotranspiration loss. The potholes 

in each group, however, do not differ greatly 

in their physical characteristics and it appears reasonable to assume that annual 

evaporation rates would be approximately
10 CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES

the same for all ponds in each group if none 

had vegetation. Differences in evapotranspiration rates between ponds are due partly to 

differences in transpiration rates and partly 

to differences in evaporation. The turbulent 

transport of water vapor away from the water 

surface is doubtless decreased by the emergent vegetation, so that evaporation from a 

pond covered with dormant or dead vegetation 

(no transpiration) presumably is less than 

evaporation from a clear pond.

The technique to be used in this study for 

measuring seepage and evapotranspiration 

losses was first suggested by Langbein, 

Hains, and Culler (1951) and has been successfully applied in other studies, among 

them being the measurement of seepage gains 

and losses from Lake Washington, Miss., 

(Harbeck and others, 1961). The basic assumption is given in the following equation:

E = Nu(e0 -efl) (1) 

in which E = evaporation 

u = wind speed

e0,= saturation vapor pressure corresponding to 

the water-surf ace 

temperature

ea. = vapor pressure of the 

air

The value of the coefficient N is of no consequence. Many mass-transfer equations have 

been proposed for the computation of evaporation, and the coefficient N is usually an extremely complicated mathematical expression. Nearly all the mass-transfer equations 

are similar in two respects: evaporation is 

directly proportional to the wind speed, u, and 

to the vertical humidity gradient, as represented by the vapor pressure difference, (e<>-«a).

Using the equation developed by Yamamoto 

(1950), it was shown byMarciano and Harbeck 

(1954) that evaporation into still air was extremely small, even with free convection resulting from appreciable air-water temperature differences. If the vapor-pressure difference, (e0 -ea) is zero, evaporation is zero 

regardless of the wind speed. In the technique described by Langbein and others 

(1951), it is assumed merely that evaporation 

is negligible when the product ufe0 -ea) is 

zero.

In the absence of surface inflow or outflow, 

the measured fall in stage in any of the potholes may be considered to have two compo - 

nents, seepage and evaporation. The seepage 

component is outseepage minus inseepage, 

and the difference may have either a positive 

or negative sign. Lake Washington, for example, has a seepage gain in spring when the 

stage of the nearby Mississippi River is high 

and a seepage loss in fall when the Mississippi River is low.

To determine the seepage loss or gain,the 

fall in stage, AH, and the average values of 

the product u(e0 -ea^ are computed for selected periods during which neither surface 

inflow (including precipitation) nor surface 

outflow occurred, and the data plotted as 

shown in figure 4. The net seepage loss is 

the intercept on the y-axis. If the net seepage loss were zero, the best-fitting line 

should go through the origin, for then the fall 

in stage, AJW, would be evaporation only, 

which is zero when the product u(eo-ea) is 

zero.

If there is a substantial seasonal change in 

stage in any pond, this type of analysis should 

be made for periods of high, low, and mediumpond stage, so that the relation between pond 

stage and seepage loss or gain may be defined. In this manner the daily seepage loss 

or gain from each pond can be determined.

During periods of no surface inflow or outflow, evaporation or evapotranspiration can

UJ

o

go

z 

o

Q.

PRODUCT u(e0-ea)

Figure 4. ~Net seepage loss as determined from relation between 

fall in pond stage and product tt (GO - &a) 
CURRENT STUDIES OF THE HYDROLOGY OF PRAIRIE POTHOLES 11

be computed from the fall in reservoir stage 

minus the net seepage loss. During both inflow and noninflow periods, evaporation can 

also be computed using equation 1, with the 

value of N being the slope of the best -fitting 

straight line of figure 4.

The technique thus provides a means of 

computing the net seepage loss (or gain) from 

each pond, including the seasonal variation 

therein if significant, and the evapotranspiration losses from each pond. By continuing 

observation over a period of years it will be 

possible to determine the effect of variations 

in water levels on the consumptive use by 

different kinds and amounts of aquatic vegetation.

FUTURE PROJECT PLANS

Observations described in the previous 

sections are scheduled to continue until 1965. 

If climatic conditions vary widely during the 

5-year period, there will be provided an opportunity to explore fully the relations between pothole water levels and rainfall, runoff, evaporation and evapotranspiration, ecol - 

°gy» ground-water levels, and water quality.

REFERENCES

Flint, R. P., 1947, Glacial geology and the 

Pleistocene epoch: New York, John Wiley 

and Sons, 535 p.

 1955, Pleistocene geology of eastern 

South Dakota: U.S. Geol. Survey Prof. Paper 262, p. 12.

Harbeck, G. E., Jr., Golden, H. G., and Harvey, 

E. J., 1961, The effect of irrigation withdrawals on the stage of Lake Washington, 

Mississippi: U.S.Geol.Survey Water-Supply Paper 1460-1.

Langbein, W. B., Hains, D. H., and Culler, R. 

C., 1951, Hydrology of stock-water reservoirs in Arizona: U.S. Geol. Survey Circ. 

110, p. 13-15.

Marciano, J. J., and Harbeck, G. E., 1954, 

Mass-transfer studies, in Water-loss investigations: Lake Hefner studies, technical report: U.S. Geol. Survey Prof. Paper 

269, p. 61.

Simpson, H. E., 1929, Geology and groundwater resources of North Dakota, U.S. 

Geol. Survey Water-Supply Paper 598, 312 p.

Yamamoto, G., 1950, Investigations of evaporation from pans: Am. Geophys. Union 

Trans., v. 31, no. 3, p. 349-356.

U. S. GOVERNMENT PRINTING OFFICE : 1963 O - 681904