Causes of Depletion of 

the Pecos River in 

New Mexico

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1619-G

Causes of Depletion of 

the Pecos River in 

New Mexico

By HAROLD E. THOMAS

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1619-G

Effects of recent ground-water development, drought, and phreatophytes

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. - Price 15 cents (paper cover)
CONTENTS

	Page 

Abstract___ _ -_---------_-_-__-_________________________-_-______ G-1

Introduction._ ____________________________________________________ 1

Evidence of depletion______________________________________________ 3

Possible causes of depletion.________________________________________ 4

Drought..__-----________________________----_-____ 6

Phreatophytes ________________________________________________ 8

Wells in Roswell area__________________________________________ 8

Wells in Carlsbad area_________________________________________ 11

Summary___________________________________________________ 13

References cited_________________________________________________ 14

ILLUSTRATIONS

FIGURE 1. Map of Pecos River basin________________________________ G-2

2. Trends of precipitation and runoff, 1900-58-__-_____--_----_ 5

3. Precipitation-runoff relation, 1905-57.___________-----_---_ 7

4. Trends in recharge, discharge, and storage, 1936-56, for Roswell

basin ________________________________________________ 10

in

CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

CAUSES OF DEPLETION OF THE PECOS RIVER 

IN NEW MEXICO

By HAROLD E. THOMAS

ABSTRACT

The contribution of water to the Pecos River in the reach between Alamogordo 

Dam and Red Bluff Reservoir has been trending downward for the past 40 years. 

This reach, the middle basin of the Pecos River, has been the scene of several 

significant changes in recent decades, each of them capable of causing a depletion of the Pecos River flow. Although several other factors may be involved 

in the streamflow depletion measured to date (1958), the principal causes are 

conceded to be the ground-water developments near Roswell and near Carlsbad, 

and the natural factors of drought and encroaching phreatophytes.

INTRODUCTION

The Pecos River Compact, entered into by the States of New Mexico 

and Texas (U.S. Congress, 1949), contains the following key provision : "* * * New Mexico shall not deplete by man's activities the 

flow of the Pecos River at the New Mexico-Texas State line below an 

amount which will give to Texas a quantity of water equivalent to 

that available under the 1947 condition." In any river basin such a 

provision would pose interesting problems in hydrology, first to 

establish the fact of depletion below the "1947 condition" and then to 

determine to what extent that depletion results respectively from 

man's activities and from natural causes. In the Pecos River basin 

the determination of the effect of man's activities is complicated by 

the close interrelation of surface water and ground water and by the 

great development and use of ground water within the basin.

The Pecos River (fig. 1) rises in the squthern Rocky Mountains, 

flows south across eastern New Mexico and southeast across west 

Texas, and enters the Rio Grande which forms the international 

boundary between Texas and Mexico. The Pecos River has a drainage basin of about 35,000 square miles, which is conveniently divided 

into an upper basin of 4,000 square miles above Alamogordo Reservoir 

in New Mexico, a middle basin of 16,000 square miles above Red Bluff

G-l
G-2 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

FIGURE 1. Map of Pecos River basin.

Reservoir also in New Mexico, and a lower basin of 15,000 square miles 

in Texas (Natl. Resources Plan. Board, 1942a). The basin has a 

semiarid continental climate, and winters that are cold enough but 

usually too dry for snow, and summers that are hot and dry but 

punctuated by frequent, local, and intense convectional storms. In 

four-fifths of the basin the average annual precipitation ranges 

from 12 to 16 inches, although small headwater areas receive more 

than 30 inches. About 75 percent of the annual total falls during
DEPLETION OF THE PECOS RIVER IN NEW MEXICO G-3

the 6-month growing season May through October. Irrigation is a 

prerequisite in all parts of the basin suitable for agriculture because of 

the low average seasonal rainfall and because of great variations from 

that average, both from place to place and from time to time.

Soluble rocks are of critical importance in the hydrology of the 

Pecos River basin, especially in the parts that lie in New Mexico. 

Strata of limestone, anhydrite and gypsum, halite, and other evaporites (including the valuable mineral sylvite) were deposited about 

200 million years ago during the Permian period. Although there 

has been great reduction of their original thickness in many places by 

solution and other forms of erosion, the Permian strata in the Pecos 

River basin presently have an aggregate thickness of 6,000 to 18,000 

feet. This includes great thicknesses of soluble rocks in practically 

all the basin in New Mexico, except for small areas in the mountainous 

headwaters. Where the soluble salts have been removed, there remain 

caverns (of which the Carlsbad Caverns are most famed), sinkholes, 

so-called bottomless pits, ponds, closed depressions, leaks in constructed surface reservoirs, areas of slumping, and changes in the 

pattern of surface drainage. The soluble salts now being removed 

contribute mineral matter to ground water, and thence also to the 

Pecos River and to irrigated soils in many places. Some of the 

limestone strata, after long exposure to solution by circulating water, 

are now very productive aquifers.

Obviously ground water has special significance in the Pecos River 

basin (Natl. Resources Plan. Board, 1942b, p. 27-102), chiefly because 

of the abundance of soluble rocks. Wells and springs furnish most 

of ,the water used for irrigation and for the relatively small municipal 

and industrial supplies. Much of the rainfall, streamflow, and water 

6ollected in surface reservoirs is diverted underground or evaporated 

in closed basins. Much of this ground water eventually reaches the 

main stream, and the Pecos River naturally has a substantial dryweather flow. However, some of the ground water contributed to 

the river is by brine springs, which increase the salinity of the river 

substantially and make the entire supply less suitable for irrigation. 

Some of the ground water maintains water-loving but useless vegetation along the river bottoms so that the water supply for beneficial 

use downstream is reduced.

EVIDENCE OF DEPLETION

The outflow of the Pecos River from the middle basin (as it enters 

Red Bluff Reservoir), and the outflow from the upper basin (above 

Alamogordo Reservoir) have been gaged for more than 50 years. 

For the first 30 years until 1934 the runoff into the lower reservoir
G-4 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

averaged about 50 percent greater than that into the upper reservoir. 

From 1935 to 1958 however, the middle basin has contributed substantially to the river in only 5 of the 23 years. In the decade 1946-55, 

the outflow from the middle basin was 23 percent less than the 

quantity entering the middle basin from the upper basin. 

. In the upper basin the irrigated acreage and irrigation practice have 

changed little in the past half century, or indeed since the days of 

Spanish colonization; the annual outflow from the upper basin has 

been within 20 percent of the median in more than half the years of 

record, reflecting the stability in pattern of use as well as the natural 

stability provided by ground-water storage. By contrast, the water 

production of the middle basin, as computed by the difference in runoff measured at the Alamogordo and Red Bluff gaging stations, has 

trended generally downward for the past 40 years (fig. 2). This 

depletion, however, was most marked in the 30 years preceding 1947. 

Although the Pecos River Compact sets 1947 as the beginning of a 

new era in the basin, any analysis of possible causes of depletion must 

include also the records for earlier years, when there was clear evidence 

of depletion of the flow of the river as it traversed the middle basin 

(Pecos River Compact Comm., 1948).

POSSIBLE CAUSES OF DEPLETION

+ s 

The middle basin .of the Pecos River has been the scene of several

significant changes in, recent' decades, each of them capable of causing 

a progressive depletion of the xPecos River flew (Thomas and others, 

written communication, 1959). Some of these changes are clearly 

products of man's activity. Use of water from the Roswell groundwate.r basin*for irrigation increased from about 160,000 acre-feet upon 

55,000 acres in 1930 to 450,000 acre-feet upon 125,000 acres in 1954. 

Pumping from wells in the Carlsbad area was negligible prior to 

1945, but has exceeded 60,000 acre-feet in some years since then.

Natural phenomena are also partly responsible for the streamflow depletion, and drought is one of these. Precipitation at representative localities has been less than the long-term mean in 18 of the 

25 years during 1932-56. In the same quarter century the bottom 

land area infested by the water-loving saltcedar (Tamarise pentandra) 

increased from about 10,000 to 40,000 acres.

Although several other factors may also be involved in the streamflow depletion measured to date (1958), the principal causes are conceded to be the ground-water developments at Roswell and Carlsbad, 

and the natural factors of drought and encroaching phreatophytes.
' PRECIPITATION: PERCENTAGE OF 1901-50 ANNUAL AVERAGE

II + +

ANNUAL RUNOFF«IN MILLIONS OF ACRE-FEET
G-6 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

DROUGHT

In the Pecos River basin, as in most of the arid southwestern United 

States, the term "drought" is not applied to a few rainless weeks or 

even to a year of deficient precipitation, because such events are commonplace and expected, and are ameliorated by water that had previously been accumulated in surface- or ground-water reservoirs. Instead, drought is commonly reserved for a succession of dry years, 

when the cumulative deficiency in precipitation is sufficient to be 

reflected in depleted storage in those reservoirs.

In the Great Plains, which lie east and north of the Pecos River 

basin, extended droughts occurred in 1909-18, 1933-40, and 1951-57 

inclusive. Graphs of cumulative departure from average precipitation of representative localities (fig. 2) show that the precipitation 

within the Pecos River basin was deficient during each of these periods 

(shaded on fig. 2), and especially so during the most recent one (1951- 

57). The uppermost graph shows the cumulative departure from 

average precipitation at two localities (Las Vegas and Santa Rosa, 

fig. 1) in the upper basin. The second graph shows the annual 

outflow from the upper basin less during droughts than during the 

intervening wetter years but no indication of progressive diminution.

The third graph shows the cumulative departure from average precipitation at three places (Roswell, Artesia, and Carlsbad, fig. 1) 

in the midclle basin. In the 40 years from 1917 to 1957, there was 

a net deficiency of precipitation equivalent to four times the annual 

average, reflected in the record of streamflow generated by the middle 

basin, which is computed as the difference between the measured 

inflow and outflow, as shown in the lowest graph.

A cumulative-mass diagram of precipitation and runoff in the 

middle basin (fig. 3) shows that the contributions from the middle 

basin to the Pecos River per unit of precipitation have decreased progressively since 1919, and have been negative (that is, the Pecos has 

lost water within the middle basin) since 1943. This decrease may 

be ascribed in part to drought, for abundant precipitation causes not 

only higher runoff but a higher proportion of runoff. But since there 

were droughts also in the early part of the record, it is not possible to 

attribute the progressive change in precipitation-runoff relation 

entirely to drought.
DEPLETION OF THE PECOS RIVER IN NEW MEXICO G-7

0.5

1910 i

>1905

 1915

11920

'1930

1945

200 300 400 500 

PRECIPITATION, IN INCHES

600

FIGURE 3. Cumulative-mass diagram of precipitation-runoff relation in the Pecos River

middle basin, 1905-57.
G-8 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

FHBEATOFHYTES

The Pecos River flood plains apparently provide an ideal environment for the saltcedar, for it has spread rapidly and displaced most 

of the native phreatophytes since it was first observed near Lake 

McMillan in 1914. Starting with about 500 acres within the middle 

basin of the Pecos River in 1915, the area covered by saltcedar increased to 15,000 acres by 1939, 25,000 by 1946, and 40,000 acres by 

1957 (Thompson, 1959). The water consumed by saltcedar in the 

middle basin during 1957 is estimated at 100,000 acre-feet.

The water consumed by saltcedar and other phreatophytes along 

the Pecos River is a direct draft upon ground water that might otherwise have contributed to riverflow (or in some areas, ground water 

that must be replaced by seepage from the river). Thus the progressive depletion of riverflow since 1920 may result in part from the 

concurrent increase in saltcedar infestation. In particular, the net 

reduction in recent years of riverflow between the gaging stations at 

the upper and lower ends of the middle basin is caused by consumption 

of water within the flood plain, and this is chiefly by phreatophytes.

WELLS IN BOSWELL AHEA

The Roswell ground-water basin is an eastward-sloping area 

bounded by a mountain crest on the west and the Pecos River on the 

east; the basin extends nearly 150 miles from north to south (fig. 1). 

The broad pattern of the hydrologic cycle in the Roswell basin includes inflow (by precipitation) which is greatest at the west boundary, eastward movement of this water both underground and in 

ephemeral streams with some losses by evapotranspiration, and eventual disposal to the Pecos. The streams flowing eastward have 

formed an alluvial basin that extends along the Pecos River for about 

70 miles and ranges in width from 10 to 25 miles. Alluvial plains .and 

terraces built by these streams are now the areas most desirable for 

agricultural use, and include practically all the area presently under 

irrigation. The alluvial sediments are less than 200 feet thick in 

most places, but they constitute an important aquifer in the Roswell 

area. >

The major source of water in the Roswell area, however, is the San 

Andres limestone of Permian age, which is more than 1,000 feet thick 

and yields water for irrigation of 75,000 acres. In the irrigated 

area the top of the San Andres ranges from 250 to 1,200 feet below 

the surface; it is covered by red beds, sandstone, shale, and gypsum of 

the Permian Chalk Bluff formation, and by the shallow alluvial 

aquifer. The San Andres also occurs east of the Pecos, where it has
DEPLETION OF THE PECOS RIVER IN NEW MEXICO Gr-9

an eastward dip and therefore occurs at increasing depth, but wells 

east of the Pecos have generally produced brine.

A quantitative study of the Roswell ground-water reservoir by 

Hantush (1955) confirms the conclusion reached by Fiedler and Nye 

(1933) that the San Andres artesian reservoir is capable of a sustained yield of about 240,000 acre-feet per year. Hantush's analysis 

includes the shallow alluvial aquifer, the San Andres, and the less 

permeable formations that bound these aquifers because they are all 

elements in a relatively continuous hydraulic system from the western 

edge of their outcrop areas to at least as far downdip as they contain 

usable water. The uppermost graph of figure 4 shows the annual 

recharge to the San Andres reservoir as computed by Hantush 

greater in each year during 1936-44 than the long-term average but 

less than that average in most subsequent years. The estimated withdrawal from artesian wells is indicated by shading below this graph, 

forming a band that is narrowest in wet 1941, when withdrawals were 

less than 100,000 acre-feet, and becomes progressively wider until 

1953, when withdrawals exceeded 300,000 acre-feet. After 1944 the 

withdrawals exceeded the calculated recharge in all but 3 years. 

Hydraulic head was sufficient to promote upward leakage from the 

San Andres until 1953, although at diminishing rates; since 1953 the 

leakage has been downward and into the San Andres. As shown by 

the graph of summation of gains and losses, recharge exceeded discharge in only 4 years (1941-43, and 1949) during 1935-55. The 

gains in these -years, however, were sufficient to offset the computed 

losses until 1953, as shown by the graph of cumulative changes in 

storage. This constructed graph has trends similar to trends in water 

level in a well near Artesia, except the well indicates that the increased storage resulting from 1941 precipitation was entirely 

dissipated by 1950. Pumping since 1950 has reduced the storage in 

the limestone aquifer to less than ever before in history.

The recharge to the shallow alluvial aquifer is by leakage from 

^the artesian aquifer, local precipitation, and seepage from irrigation. 

Pumjpage from the shallow reservoir was greater than the total recharge by leakage, precipitation, and irrigation in 1947 and 1948, 

and in every year since 1950. This pumping has replaced seepage to 

the Pecos Eiver as the principal means of discharge from the shallow 

reservoir. However, seepage to the river has not been eliminated even 

in recent years of heavy pumping. The annual discharge to the Pecos 

River, estimated from differences in runoff of the Pecos River at 

Acme and Artesia, is shown by the lowest graph of figure 4. During 

,the pumping season, however, seepage is reduced markedly, and 

much of it may never reach the river because of evaportranspiration.
CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

500

400

300

1935 1940 1945 1950 1955

FIGTJEB 4. Trends in recharge, discharge, and storage, 1936-56, for the Roswell groundwater basin.
DEPLETION OF THE PECOS RIVER IN NEW MEXICO G-ll

Partly because of this seasonal reduction, the annual gain in riverflow as it traversed the Roswell basin was much less during 1951-54 

than in any previous year of record. Nevertheless, reduction in the 

natural discharge accounts for only a small part of the excess of 

pumpage over recharge to the shallow aquifer; most of the difference 

has been made up by withdrawal from storage in the shallow aquifer. 

The pumping that has depleted storage at rates approaching 100,000 

acre-feet in some years would constitute good reservoir management 

provided the water is replaced in future years when the supply is 

greater and the demand less. As precipitation can meet a large 

proportion of crop needs in some years (as shown especially in 1941), 

a constant rate of annual withdrawal by pumping is not necessary or 

desirable for a stable irrigation economy because the total water resources can be used most effectively by heavy pumping during drought 

and light pumping in the wetter years. Such variations in draft are 

possible because of the large reserve in storage. Existing records 

indicate that reservoir storage in 1951 was not much less than in 1940, 

and that therefore the wet year 1941 provided enough water to carry 

through several subsequent years of drought. The depletion since 

1951, however, cannot be made up unless the future includes years wet 

enough to permit a drastic reduction in annual pumpage. Certainly 

a part of the increased use of water within the Roswell basin during 

the drought has been at the expense of users of the Pecos River 

downstream.

WELLS IN CARLSBAD AREA

The Carlsbad area, downstream from the Roswell basin, is a second 

major area of ground-water development in the middle basin of 

the Pecos River (fig. 1). As in the Roswell basin, the principal use 

of water in the Carlsbad area is for irrigation, but it differs from the 

Roswell in that it depended upon surface water almost entirely prior 

to 1945 and predominantly in the succeeding decade; only in 1953 and 

1954 has the water pumped from wells comprised more than 50 percent of the total water applied for irrigation.

The shallow alluvial aquifer along the Pecos River is the principal 

source of water pumped for irrigation in the Carlsbad area. Under 

natural conditions the alluvial aquifer was recharged by precipitation 

and by flood runoff in tributaries to the Pecos, and there were many 

springs and seeps along and near the stream channels; but for more 

than half a century an important source of water in the alluvium has 

been the downward seepage of water diverted from the Pecos River 

and applied for irrigation. The water table in the alluvium in the 

Carlsbad area probably remained fairly close to the land surface until 

about 1946, when the construction of irrigation wells spread rapidly.
G-12 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES

One effect of pumping from the alluvial aquifer has been to create 

a pronounced cone of depression in an area south of the city of 

Carlsbad and west of the area irrigated by surface water. The center 

of this cone was more than 60 feet deep after 8 years (1947-54) of 

pumping. In wells that tap the alluvial aquifer near the Pecos Eiver 

or those in areas that receive irrigation water from the river, there 

has been less lowering of water levels because of recharge from surface 

water, but that recharge necessarily causes some depletion in flow of 

the river downstream from the Carlsbad area.

The Capitan limestone of Permian age, recharged chiefly by precipitation upon the outcrop area, constitutes an underground tributary 

to the Pecos River, as shown by the gentle eastward gradient in head. 

Some water in this aquifer may be discharged into the alluvial aquifer 

near Carlsbad, but a major point of natural discharge is the Carlsbad 

Springs which discharge into the Pecos Eiver. The Carlsbad Springs 

also discharge Pecos River water that has leaked into the limestone 

from Lake Avalon and from canals that carry water to the Carlsbad 

irrigation project; the computed leakage from Lake Avalon reached 

a peak of 35 cfs (cubic feet per second) each winter until the drought 

year 1952. The discharge of Carlsbad Springs, which includes this 

leakage, averaged 60 cfs in 1939-40, rose above 100 cfs in 1941^i2, 

and declined to minima less than 30 cfs in 1953, 1954, and 1956 

(Bjorklund, 1958).

Pumping from wells in the limestone aquifer has not caused a 

marked decline in water levels; the principal effect of pumping has 

been to reduce the spring discharge. In some months in 1953 and 

1954 the discharge from Carlsbad Springs was less than the computed 

leakage from Lake Avalon; in those months the wells were drawing 

more than the local yield of the aquifer, and it is likely that some 

water from Lake Avalon was diverted toward the wells.

Since 1947 pumping from wells has reduced the inflow to the river 

 at Carlsbad Springs, increased the seepage from the river and from 

distribution canals, and created storage space which will doubtless be 

filled, at least in part, during future years of more abundant supply. 

All these effects tend to reduce the amount of water available downstream. To the extent that the pumped water was used on lands with 

long-established water rights in lieu of failing stream supplies during 

drought, the wells constitute a development of additional storage 

facilities along the river, facilities which in some future year may,
DEPLETION OF THE PECOS RIVER IN NEW MEXICO G-13

like a surface reservoir, capture and store floodwater that would 

otherwise continue downstream. To the extent that the pumped water 

was used on new land, the result must be a persistent reduction in the 

supplies available downstream.

SUMMARY

The Pecos River in New Mexico is an excellent example of a river 

which has had a progressive diminution of inflow from a large part 

of its drainage basin. Drought in recent years has been responsible 

for part of this reduction in inflow, but not for the progressive reduction throughout the past four decades. There has been increasing 

infestation of the river flood plains by saltcedar, and the increasing 

consumptive waste of water by this phreatophyte has certainly been 

responsible for some of the depletion of riverflow.

Part of the depletion in flow of the Pecos River is attributed to 

the development and use of water from major ground-water reservoirs 

within the basin. New Mexico was the first State to develop statutory regulation of ground-water development and use, and for this 

regulation there are reliable estimates of the ground-water supplies 

in practically all developed areas in the State, based upon studies 

that were begun more than 30 years ago. In both the Roswell and 

Carlsbad ground-water reservoirs in the Pecos Kiver basin, the state 

engineer has endeavored to limit the withdrawals from wells to a 

quantity that can be sustained perennially.

Under natural conditions these ground-water reservoirs discharged 

into the Pecos River. As might be expected, the development and use 

of ground water has been partly at the expense of this natural contribution to the river. Most of the pumped water is replenished by 

natural recharge, and the wells serve to divert the water that would 

otherwise have been discharged at springs or seeps, of which much 

would have eventually reached the river. Some of the pumped water 

has been withdrawn from underground storage with negligible effect 

upon the riverflow during the recent years of drought. However, in 

future years of more abundant precipitation and recharge this storage 

may well be replaced before there is increased ground-water flow to 

the river. Consequently, this pumping from storage can be expected 

eventually to affect the riverflow.
G-14 CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES 

REFERENCES CITED

Bjorklund, L. J., 1958, Flow analysis of Carlsbad Springs, Eddy County, New 

Mexico: U.S. Geol. Survey open-file rept., 25 p.

Fiedler, A. G. and Nye, S. S., 1933, Geology and ground-water resources of the 

Rosewell artesian basin, New Mexico: U.S. Geol. Survey Water-Supply 

Paper 639, 367 p.

Hantush, M. S., 1955, Preliminary quantitative study of the Roswell groundwater reservoir, New Mexico: New Mexico Inst. Mining and Technology, 

106 p.

Pecos River Compact Commission, 1948, Report of Engineering Advisory Committee : U.S. 81st Cong. 1st sess., S. Doc. 109, p. xxv-xxxiv, 1-72.

Thompson, C. B., 1959, Importance of phreatophytes in water supply: Intersociety Conf. Irrigation and Drainage, 1st, San Francisco, 1957, Proc., 

p. 25-32.

U.S. Congress, 1949, Public Law 91, Pecos River Compact: U.S. 81st Cong. 

1st sess., S. Doc. 109, p. iii-ix.

[U.S.] National Resources Planning Board, 1942a, Regional planning, Part X, 

Pecos River Joint Investigation Summary, analyses, and findings: Washington, U.S. Govt. Printing Office, 202 p.

 1942b, Pecos River Joint Investigation Reports of participating agencies: 

Washington, U.S. Govt. Printing Office, 407 p.

o