From 2001 to 2005, the Arizona Department of Water Resources ("ADWR")
evaluated the Upper San Pedro River Basin ("the Basin") to see if it
should become an Active Management Area ("AMA") -- an area in which the
ADWR controls mandatory water conservation.
The entire report is online through
This digest provides the parts of the report most relevant to
current politics in Cochise County. I have added some comments to point
up interesting differences between current politics and the facts
presented by ADWR, and also between ADWR's information and information
provided from other sources. I've tried to keep each section short
enough to be comprehensible and interesting.
Under Arizona law, a watershed basin should become an AMA if either
1) active management practices are necessary to preserve the supply of
groundwater for future needs, or 2) land subsidence or fissuring is
endangering property or potential groundwater storage capacity, or 3)
the use of groundwater is causing or threatening a degradation in water
quality. In other words: if the water is disappearing or turning bad,
or the land surface is collapsing.
The ADWR's report, issued in March 2005, finds that the Basin should NOT become an AMA.
However, the report adds that the ADWR "will continue to work with
the Upper San Pedro Partnership as a Partnership member on local water
management and planning efforts." In other words: there is no water
crisis in the Basin, but the ADWR is going to support the Upper San
Pedro Partnership, whose constant message is that there IS a water
crisis. This seems a little odd.
2. Basic information and terms
The San Pedro River ("the River") drains about 700 square miles in
Mexico, and 1800 square miles in the United States. The Babocomari
River is the River's only significant tributary.
Inside the US, the River flows north from Palominas to "The
Narrows" north of Benson, where the Basin ends. The Basin includes the
Sierra Vista and the Allen Flat sub-basins, but the Allen Flat
sub-basin is negligible, so this report discusses only the Sierra Vista
For practical purposes, the Sierra Vista sub-basin is divided into
the "Sierra Vista sub-area," from the Mexican border to Fairbank, and
the "Benson sub-area," north from Fairbank.
The Sierra Vista sub-basin has two primary aquifers: a deep, regional aquifer, and a shallow, floodplain aquifer.
"Groundwater" is water that is either taken from a well, or located
in an underground aquifer; in other words, groundwater is not flowing
water such as a river.
Sierra Vista sub-area water comes almost exclusively from
groundwater, for lack of another source. However, the Benson sub-area
uses a large amount of River water for irrigation.
"Recharge" is water returning from the surface to an aquifer.
"Net use groundwater" is groundwater which does not return to the
aquifer through recharge -- water that is really, really used up and
A "cone of depression" is a drop in the water level of an aquifer, centered on the wells that are taking the water.
Some ADWR figures are EXTREMELY approximate, especially in
calculating a "water budget" of inflow and outflow for the entire
Basin. There is a good deal of guesswork, and some reasoning that seems
unnecessarily obscure. However, in apology for the ADWR, some data are
not available, and some data cannot even be theoretically calculated.
3. The Basin's 2000-year supply of groundwater
The Basin has about 20-26 million acre-feet of water.
In 2002, about 27,720 acre-feet were used, mostly for municipal and
military uses near Sierra Vista/Fort Huachuca, and farming near
Benson/St. David. About 9500 acre-feet was net use groundwater --
really used up and lost. That is less than 1/20 of 1% of the Basin's
groundwater. In other words, the Basin has a 2000-year supply of
For 2030, the ADWR predicts about 26,900 acre-feet of net use
groundwater. That is about 1/7 of 1% of the Basin's groundwater. In
other words, in 2030, the Basin will have a 700-year supply of
As county residents know, there is a current push, led by Judy
Gignac and her circle of politicians such as Pat Call, to convince
Basin residents that there is a water crisis. The push is not based on
reality. A federal law requires water conservation, but the law cannot
create an actual crisis. Whatever the law says, the Basin has a
2000-year supply of groundwater.
4. Cones of depression
Between 1940 and 1961, a cone of depression developed near Fort
Huachuca/Sierra Vista, dropping the groundwater up to 50'. By 1978, the
cone was falling up to 4'/year. In other areas, wells were steady or
dropping 9"/year or less. However, the cone is now definitely deepening
slower than it used to.
Two other minor cones of depression are forming, one associated
with pumping in the Benson area, the other associated with the Bisbee
5. Overall, the Basin's water level is dropping only slowly, and in some areas is even rising.
Basically, the Basin has two aquifers: a shallow floodplain aquifer
under the River, and a deep artesian aquifer. The two aquifers have
different patterns of rising and falling.
The floodplain aquifer has shown no long-term drops in water level;
in fact, water levels near Pomerene have risen. The deep artesian
aquifer has had a modest drop.
Climate change, especially summer drought, may be contributing to
groundwater level declines in some areas. At Tombstone, annual summer
rain fell by about 1" from 1897 to 1997.
Here are some details about water-level changes in different areas of the Basin:
-- Mexican border/Hereford: changes from +7' to -4.9', with most between +3' and -3'.
-- Naco/Bisbee: the highest concentration of major declines, from
-9.8' to -32.1'; but, toward the River to the west, changes from +1' to
-- Tombstone: drops, from less than -1' to -23.1'.
-- Sierra Vista/Huachuca City/Nicksville: Changes from -1.2' to
-14.8' in a public well; in the cone of depression, most wells dropped,
but less than 1'/year. Between Sierra Vista and Huachuca City, declines
slower since 1990-2001, but from -5' to -7'. North of Huachuca City,
from -1' to -5'; one public well was -13.4'. South of Sierra Vista,
from -35.4' to +16.6'.
-- St. David: changes from +12.9' to -11.1'.
-- Pomerene/Benson: in the shallow aquifer, +6" to -10.2', mostly
-1' to -5'. In deep wells, +4" to -18.9', with most declines from 4' to
9'. Near Benson, -5' to -11.8'.
-- "The Narrows"/North of Pomerene: south of "The Narrows,"
changes from +1" to -5.3', with an average of -1.5'. However, wells
along the River rose an average of +4.7', up to +11.1'.
6. Different kinds of water use
Water use is classified as municipal, agricultural, livestock, industrial, or riparian.
In 2002, municipal demand was the largest use, at 18,800 acre-feet; 13,700 were net use.
-- Sierra Vista sub-area municipal use was about 15,100 acre-feet,
about 80% of the 18,800 total. Of the 15,100 acre-feet, 14,500 came
from groundwater. 3500 acre-feet were recharged.
-- Benson sub-area municipal use was about 3700 acre-feet, with
3300 acre-feet, about 85%, from groundwater. About 600 acre-feet were
Agricultural use was not measured, but calculated: acres irrigated,
times crops' theoretical use of water for growth and transpiration.
Agricultural use dropped about 40% from 1985 to 2002, and is expected
to keep dropping. However, ADWR predictions through 2030 use the same
acreage as for 2002.
-- Sierra Vista sub-area agricultural use was about 2500 acre-feet
in 2002. Acres being irrigated dropped from 2000 acres in 1985, to 1400
in 1990, to 800 in 2002.
-- Benson sub-area agricultural use was about 7300 acre-feet in
2002, about 61% of the total Benson sub-area demand. Agriculture is
predicted to remain the largest user in 2030. Acres being irrigated
dropped from 3200 acres in 1985, to 2600 in 1990, to 2200 in 2002.
Livestock use was calculated at 300 acre-feet per year, based on 12
gallons per head per day (which implies about 22,500 head of cattle in
Industrial use means an industrial-type use that is served by its
own well. Sierra Vista sub-area industrial use includes five sand &
gravel facilities, and two golf courses. Benson sub-area industrial use
includes a golf course, a dairy, and the Apache Nitrogen ammonium
nitrate plant. In 2002, total use was 2100 acre-feet, all from
groundwater; 2000 were net use.
Riparian use means vegetation along the River and the Babocomari;
ADWR estimates this at 17,350 acre-feet/year. This figure will increase
as conservation efforts increase.
In 2002, 79% of the water demand was municipal in the Sierra Vista
sub-area, but only 30% in the Benson sub-area. In both sub-areas,
municipal use provides the fastest-growing demand.
7. Water management, i.e. conservation
Since 1988, there has been an increase in conservation.
Notably, Fort Huachuca reduced its use by almost 45% from 1993 to
2002 (though some people ask if troops have been moved to off-post
housing to reduce on-post water use). Conservation has been implemented
in the Sierra Vista area, but water use has increased with population.
Basin-wide, municipal demand increased by over 5000 acre-feet from 1985
to 2002, and per capita use has not changed appreciably.
The ADWR will NOT assume a reduction in per capita use, because the
estimated savings from conservation programs are affected by many
variables, including the ability to measure program effectiveness, the
impact of weather, the difficulty of modifying behavior, and the need
for ongoing efforts to ensure that initial savings are maintained.
8. Effluent, i.e. treated sewage
In 2002, effluent production was about 5300 acre-feet.
Effluent from the Sierra Vista and Benson wastewater treatment
plants is used for golf course irrigation and groundwater recharge;
about 800 acre-feet went for golf courses. Fort Huachuca has built
stormwater recharge projects.
Some plans are underway to use more effluent. For 2030, effluent
production is predicted at 7700 acre-feet, with about 7300 acre-feet
recharged or used directly. With these efforts, the Basin's net use of
groundwater MAY NOT INCREASE by 2010.
Clearly, effluent may become significant in water conservation.
9. Golf courses
Four golf courses were noted as using a large volume of water:
Course --------------- Water Provider ----- Demand
Chaffee Parade Field - Fort Huachuca ----- 53
Mountain View ------- Fort Huachuca ------ 371
San Pedro ------------ City of Benson ------ 500 (est.)
Veterans Park ---------City of Sierra Vista -- 179
All those courses except Veterans Park use effluent. Their total
use was 1103 acre-feet/year -- about 10% of the total water deficit for
the Basin. It is not unreasonable to guess that the ten biggest courses
in the Basin cause 20% of the net water deficit. Should golf courses be
10. Water usage from water systems
In the Basin, 30 water systems serve municipal users. Eight of
those systems provide 250 acre-feet or more per year, and account for
96% of total demand, but only 57% of municipal demand. The eight
Location - - - System - - - - - - - - Actual/Estimated Water
inside Sierra Vista city limits
- - - - - - - - - Bella Vista Water Company - - - - - 3640
- - - - - - - - - Pueblo del Sol Water Co. - - - - - - -1355
- - - - - - - - - Arizona W.C. – Sierra Vista - - - - - 1299
inside Sierra Vista sub-area
- - - - - - - - - Fort Huachuca - - - - - - - - - - - - - 1947
- - - - - - - - - Arizona W.C. – Bisbee - - - - - - - - 1222
- - - - - - - - - East Slope Water Company - - - - - - 306
- - - - - - - - - Huachuca City - - - - - - - - - - - - - - 250
outside Sierra Vista sub-area
- - - - - - - - - Benson, City of - - - - - - - - - - - - - - 813
ADWR estimates that a person on these systems uses between 124 and 196 gallons/day.
HOWEVER, if you actually run the ADWR's water usage numbers for a 2002
Basin population of 82,300, the 10,832 acre-feet amounts to only 117
gallons/person/day, less than the "124 to 196" range that ADWR says
the numbers show.
117 gallons/person/day is, in fact, not much above what is shown by the data from Cochise County water
companies that serve only households. A few months ago, I found that average usage to be
about 90 gallons/day/person, very close to the 87.5 gallons/day/person
figure found independently in 2003 by the International Arid Lands
If 117 gallons/person/day is an accurate average for the Basin, but
90 gallons/person/day is an accurate average for the entire
county, then households in the Basin are using 27 gallons/person/day
over the average, and to make the average work out, households
outside the Basin must be using something less than the average.
The population of the Basin is roughly 2/3 of the entire county;
if 2/3 of the people in the county are using 117
gallons/person/day, and the average use for the whole county is 90
galllons/person/day, then to make the numbers work out, the 1/3 of the
people outside the Basin must be using 36 gallons/person/day.
This is somewhat below the national average for rural well users,
of about 50 gallons/person/day (see section 11 immediately below).
However, the 36 gallons/person/day figure may be slightly lowered
because many people outside the Basin have their water hauled in by
truck. In any case, the numbers in this analysis are probably not
far from reality.
outside the Basin -- in, say, the Sulphur Springs Valley -- use
noticeably less water than households in the Basin, then, ALL OTHER
THINGS BEING EQUAL, non-Basin households should not
be held to the same water conservation standards that are imposed in
However, it is a matter for a separate report about the Sulphur
Springs Valley that ALL OTHER THINGS ARE NOT EQUAL.
11. Water usage from wells
Especially in unincorporated areas, many households use wells, that
are either private or shared with neighbors. In 2002, about 3200 active
domestic wells served about 14,400 people, concentrated southeast of
Sierra Vista north of Hereford Road, south of Sierra Vista, and around
St. David/Benson. Most domestic wells can only pump less than 35
gallons per minute, so are "exempt," meaning exempt from regulation.
The ADWR had to estimate the water taken from exempt wells, based
on usage patterns in the Tucson AMA and the San Pedro HSR. A higher
percentage of small lots is irrigated in the Benson sub-area, so ADWR
estimated 312.5 gallons/person/day in the Sierra Vista sub-area, and
491 gallons/person/day in the Benson sub-area.
HOWEVER, ADWR's estimates are seriously at variance with other figures.
International Arid Lands Consortium said in 2003 that "a typical
household that gets its water from a private well or cistern uses about
200 gallons for a family of four," or 50 gallons/person/day. The
ADWR guess of 312.5 to 491 gallons/person/day is about 6 to 10 times
the national average, so is statistically highly unlikely.
To help figure out this discrepancy, here is the entire paragraph in
which the ADWR guess is made: "Other information on municipal
water demand came from direct communication with water providers and
from estimates of water demand for domestic wells. The demand
associated with exempt wells is not metered or reported to any
entity. This demand was estimated based on large lot parcel use
in the Tucson AMA for which a long history of metered water use is
available, with an additional demand associated with irrigated lands of
less than two acres in size based on information in the San Pedro
HSR. Because proportionately more small, irrigated lands exist in
the Benson sub-area, the acreage per person demand estimate differs
between sub-areas. The estimated demand in the Sierra Vista
sub-area is therefore .35 acre-feet/person and .55 acre-feet/person in
the Benson sub-area. The pumpage for Fort Huachuca comes from the
Fort Huachuca Biological Opinion Annual Report for 2002.
Information on municipal water demand assumptions is found in Appendix
In short, all the numbers about exempt wells are based on estimates,
and the ADWR report gives no basis for its figures of .35 acre-feet
around Sierra Vista and .55 acre-feet around Benson. .35
acre-feet translates to 312.5 gallons/person/day, and .55 acre-feet
translates to 491 gallons/person/day. The .35 and .55 acre-foot
numbers come from the 1991 "Final San Pedro River Watershed
Hydrographic Survey Report." The derivation of the .35 and .55
numbers was recently tracked down, and is set out below.
The calculation is in a document not readily available. (Its relevant part is
now online at
The calculation is valid only for the Sierra Vista and Benson sub-areas
of the Upper San Pedro Watershed, with pastures and gardens of up to 2 acres, watered by domestic
wells. Other parts of the county, such as the Sulphur Springs
Valley have very
different conditions; and and the calculation does not apply to them.
The calculation begins with water use inside homes. For a certain area
near Tucson, the average residential water use
during 1992-2002 was about 107 gallons/person/day. The calculation takes
this as the basic indoor use, plus a few fruit trees and livestock, by rural areas using domestic wells in Cochise
Of course, many households also use water more intensively outside the home. This use
was calculated, not measured, for parcels containing pastures or gardens
of up to 2 acres, irrigated from domestic wells, in the Sierra Vista and
Such pastures and gardens comprised 572 acres around Sierra Vista, and
307 acres around Benson. Their total water use (according to a
complicated and imprecise calculation) was 1490 acre-feet/year around
Sierra Vista, and 906 acre-feet/year around Benson. Dividing by the
number of people using domestic wells in 1990, the average was 205
gallons/person/day (.35 acre-feet/person/year) around Sierra Vista, and 393 gallons/person/day
(.55 acre-feet/person/year) around Benson. To repeat, these figures for
around Sierra Vista or Benson cannot be applied to dissimilar areas of the county, such as the Sulphur Springs Valley.
To obtain total water use, the pasture or garden figures were added
to the 107 gallons/person/day for inside use.
Around Sierra Vista, the total was 312 gallons/person/day (.35
acre-feet/person/year).. Around Benson, the total is 500
gallons/person/day (.55 acre-feet/person/year).
However, for properties not using a private well to water pastures or
gardens intensively, the use is less, and a property with no outdoor use except a few fruit trees and livestock would use
an average of 107 gallons/person/day.
12. The Basin's population
In 2000, the population of the Basin (plus part of Bisbee outside
the Basin but using wells inside the Basin) was 79,944 people,
distributed as follows:
-- 28,113 people in unincorporated areas
-- 51,831 people in incorporated areas
-- -- Sierra Vista complex, 39,526, consisting of:
-- -- -- -- Sierra Vista 29,362
-- -- -- -- Fort Huachuca 8,413
-- -- -- -- Huachuca City 1,751
-- -- Bisbee 6,090
-- -- Benson 4,711
-- -- Tombstone 1,504
The Sierra Vista sub-area had about 56,600 people in 1990 and
70,100 in 2002, and will have about 76,500 in 2010. The Benson sub-area
had about 8700 people in 1990 and 12,200 in 2002, and will have about
15,300 in 2010.
By 2030, the Sierra Vista sub-area may add about 22,000 people, and the Benson sub-area about 6000.