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Arid Grasslands
And Water Resources
Charles W. Stockton, David Meko, and Mary F. Glueck1
ABSTRACT
The grasslands of the Sonoita-Elgin area are among the best
preserved in the state. These grasslands have been occupied for
years, but the rate of occupation has increased rapidly in the past
ten years. Whether or not there is enough water to maintain the
grasslands and their riparian areas is the subject of this paper that
examines water supplies and projected uses.
GRASSLANDS ARE ATTRACTIVE FOR DEVELOPMENT
The old saying .. lush green valley.. has a pleasant ring to most people, and grasslands have
traditionally been attractive to human settlement. In the past, when grassland valleys were relatively
remote, the pressure of development was slight. Grassland valleys near Sonoita and Elgin were
homesteaded relatively late, in the period 1910-1920. Much of the land was already occupied by
large ranches such as the Empire, Babocomari, and San Rafael and only a few 160 acre parcels
were available. For some reason, perhaps because the grassland area appeared similar to the
Great Plains region, many of the homesteaders thought the land could be tilled and began farming
operations in the area. Most commonly, the crops were rain-fed, and consisted of cereals, such as
wheat and barley. The soil in the grassland area is relatively fertile, and because of an anomalous
wet period extending from roughly 1905-1925, many small farming operations flourished. From
the end of this wet period to the present, however, the primary use for water has been for livestock.
Because of the abundance of wind in the Sonoita-Elgin region, a common early source of water
was ground water from either drilled or hand-dug wells pumped by windmills. These water sources
were of course subject to the vagaries of the wind, and an additional source of stock water was
needed. The rancher was quick to realize that grassland produces a high percentage of runoff, and
many stock ponds -- most with less than 5 acre-feet capacity -- were built during the period
1920-1950. Contracting companies developed the expertise to construct stock ponds on draws
and washes capable of providing water to livestock on an annual basis and yet able to withstand
large runoff events. These stock ponds, built with minimal engineering design, have been
remarkably durable and many have survived and are in use today. The total amount of water
retained by stock ponds and produced from wells pumped by windmills has resulted in only minimal
use of the water resources and little stress on local water supplies. But this scenario seems to be
changing, and the point of this paper is to address some of the possible ramifications of the
changing patterns of water use on the valley grasslands.
1
All at the Laboratory of Tree-Ring Research, University of Arizona, Tucson AZ.
177
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BASIN AND RANGE PROVINCE: VALLEYS ARE PRESUMED TO BE
LARGE SOURCES OF GROUND WATER
As cities in southeastern Arizona have grown, they have tended to look at nearby valleys as
potential sources for expansion of municipal supplies of water. This is an understandable approach,
because in the Basin and Range Province it is not uncommon for the valleys to be large gravel filled
grabens with abundant ground water. Examples are the Tucson Valley and Avra Valley west of
Tucson. Both valleys are classic in that they are occupied by pervious valley fill thousands of feet in
thickness with large quantities of stored water. Not all valleys in the Basin and Range Province,
however, are of this type. Two that are not are the San Rafael and the Sonoita-Elgin Valleys, both
classic grassland valleys in which geologic structural events have produced valleys with fill
conducive to neither storage nor transmission of large quantities of water. In the San Rafael Valley,
there appears to be only a relatively thin layer of surface alluvium capable of storing and
transmitting water to wells. This layer is underlain by older volcanic and well indurated sedimentary
rocks. The tectonic map of the Lochiel quadrangle (Simons, 1974), which includes the San Rafael
Valley, confirms that, with isolated exceptions, faults and folds have not had an important impact
on the occurrence of ground water in this valley. In general, the thinness of the alluvium and the
character of the underlying volcanic and sedimentary rocks make large quantities of ground water
rare in this Valley and wells yield only moderate quantities of water.
In the Sonoita - Elgin Valley, the geologic framework is totally different, and structural features
along with rock types play an important role in the occurrence of ground water. Two faults, the Lone
Mountain-Sawmill Canyon fault complex along the western edge of the valley and the Babocomari
fault running approximately along the channel of the Babocomari River along the eastern edge of
the Valley have tremendous influence on the availability of ground water (Figure 1). There is
disagreement among geologists as to the types and ages of these two faults (see for example
Drewes, 1976) but their influence on the occurrence of ground water in this Valley is proved by the
numbers of dry holes. Nearby and immediately to the east of Elgin, the Pantano Formation, a
siltstone capable of storing and transmitting only small quantities of water, is exposed at the
surface and only small quantities of water are available to wells. This is the up thrown block of the
Babocomari fault, according to Drewes (1976). Towards the center of the valley, the alluvium is
thicker and moderate quantities of water are available to wells at moderate depths. This portion of
the valley is the down-thrown block, if in fact the faults are high angle faults -- segments of the
Lone Mountain-Sawmill Canyon and the Babocomari faults. Apparently, the resulting trough is not
deep and the valley fill is not very thick. Consequently, the water is available to wells in only small
to moderate quantities. These two faults merge, according to Drewes, 1976, near the head waters
of Gardner Canyon. Toward the northern end of the valley, wells have been drilled with production
ranges of 1oo•s to 1000•s of gallons per minute, but ground water appears to be available only in
moderate amounts (100 spin or less) over most of the valley.
CLIMATIC VARIABILITY AND MISCONCEPTIONS
ABOUT WATER AVAILABILITY
Water resources management is often based on a period of record rainfall, runoff, or ground
water recharge -- in general, a period of supply -- presumed to be average or "normal. .. This
average of past values is considered to be the best estimate of the future supply. A problem arises
in identifying the most representative period of record to be used to compute the average. This is
because climatic variations are not random in time and the average computed from climate records
179
can vary considerably depending on length of record and the time period involved. Examples of
water resource development in_the western United States can be cited in which the period of record
used to estimate the future supply has been anything but average. Is the present state of the water
resources in the Sonoita-Elgin and San Rafael Valleys the result of average rainfall?
To examine this issue, we have analyzed some of the longest term precipitation records
available in the Sonoita-Elgin and San Rafael Valleys along with a 296 year tree ring record
collected in the Huachuca Mountains in October of 1995. The precipitation series include records
from Tombstone, Ft. Huachuca, Elgin and Canelo. The Tombstone record is the longest and most
complete, 1899-1996.
The Canelo record is also complete but covers the shorter period 1918-1996. The Fort
Huachuca record is very sparse and missing values have been estimated based on records from
nearby stations. The Sierra Vista record was used to extend this record from 1982 to the present.
The Elgin record contains a period of 5 years, 1986-90 with no data. These values were estimated
from the monthly regional averages.
We have chosen to group the monthly distribution of rainfall into annual values composed of
the sum of monthly total rainfall for November through October. This monthly grouping facilitates
splitting the annual total into a cool-season and warm-season component. We define the cool
Tombstone
Canelo
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1900 1920 1940 1960 1980
1900 1920 1940 1960 1980
Elgin
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50
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1900 1920 1940 1960 1980
1900 1920 1940 1960 1980
Year
Year
Figure 2. Annual precipitation time series expressed as a percentage of the long term
mean for the period of record for Tombstone, Canelo, Elgin, and Ft. Huachuca. Annual
period defined as November-October. (See text). A seven weight binomial filter was used
to obatin the low frequency overlay curves.
180
season as November through April, a period corresponding to predominantly winter-type
atmospheric circulation patterns, and convenient for comparison with winter runoff and soil
moisture recharge for use by cool season grasses. We define the warm season as May through
October, a period encompassing summer rains and occasional tropical storm rainfall.
Plots of the annual time series expressed as percentages of the long term mean for the period
of record for all four stations are shown in Figure 2. Similar plots of the cool season values are
shown in Rgure 3, and of the warm season, in Rgure 4. On each time series plot of the individual
station values, we show an overlay of the series filtered by a 7 weight binomial filter. This allows
comparison of the low frequency component of the series. Examination of the annual low frequency
series reveals an important aspect of climate in southwestern United States. During the early
19oo•s, from about 1905 though 1930, the precipitation tended to be above normal; in some
years as much as 170% of normal, but on average about 115% of normal. This is especially
evident in the Tombstone record but is, in general, corroborated by the other records. This wet
period is followed by an extended period of generally below normal precipitation extending from
about 1931 through 1982. This dry period again shows in all four records with individual-year
severity varying from one record to the other. Rainfall during this period averaged about 90% of the
Tombstone
Canelo
Elgin
Ft. Huachuca
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1900 1920 1940 1960 1980
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Figure 3. Cold Season (November-April) Precipitation time series as a percentage of the
long term mean for the period of record for Tombstone, Canelo, Elgin, and Ft. Huachuca.
A seven weight binomial filter was used to obatin the low frequency overlay curves.
181
- ·,·,
long term mean. This extended dry period has been followed by another wet period extending from
1983 to the present. Again the mean for this period may be as much as 110-115% of the long
term mean. The large swings in annual precipitation point out an important characteristic of
precipitation in this area: precipitation occurrence is non-random in time and above /below mean
occurrences predominate for extended periods of time.
The time series variations in precipitation for the cool season are broadly similar to those for the
annual series, though the peaks and troughs differ slightly in location (Figure 3). The wet period of
the early 1900's appears to extend only about to 1920, the extended dry period ends in the late
1970's and in at least the Canelo record, the wet period beginning in 1978 has a magnitude nearly
equal to that of the early 1900's.
Station-to-station consistency in precipitation variations deteriorates for the warm-season totals
(Figure 4). Each station, although showing some synchrony from year to year and over short periods
with the other stations, does not exhibit the same regional synchrony in low frequency variance as
the annual and cool season records. All four records, however, show the last few years as being
predominantly below normal during the warm season.
In Arizona, the seasonal origin of storms changes from the warm season, monsoon and tropical
Canelo
Tombstone
Ft. Huachuca
Elgin
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1900 1920 1940 1960 1980
1900 1920 1940 1960 1980
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Figure 4. Warm season (May-October) precipitation time series expressed as a
percentage of the long term mean for the period of record for Tombstone, Canelo, Elgin,
and Ft. Huachuca. A seven weight binomial filter was used to obatin the low frequency
overlay curves.
182
storm, to the cool season, frontal type storms from the west and northwest. The atmospheric
circulation changes associated with the seasonal storm type changes are roughly the same as
warm and cool season distinctions. These distinctions are also important from the hydrologic
viewpoint because cool season precipitation is associated with runoff from higher elevations
resulting in reservoir storage replenishment and recharge to the ground -water basins.
Comparison of the plots for the warm season shows some high frequency agreement, especially
in dry years but little low frequency agreement. This is most likely because most warm season
storms are extremely local whereas the dry periods (droughts) are caused by more regional
atmospheric phenomena. The cool season plots, however, show a greater uniformity in low
frequency variance. In the Tombstone record, the longest and most complete of the four, the
1905-25 period shows rainfall as high as 200-300% of the long term mean (i.e. the mean for the
entire period of record), for the period 1979-95, the wet years are 200-250% of the long term
mean. The other three records are incomplete but show evidence of similar wet periods in the early
1900's, Ft. Huachuca and Elgin, or the period 1979-95, Cane lo. The two wet periods are
separated by an extended period of below average rainfall, reaching a downward trending low
around 1972 around 25% of the record mean. These same trends are seen in the variance in the
annual records even though the cool season amount is only about 30% of the annual total at these
4 stations.
It is apparent that the variance of the cool season precipitation is greater than that of the warm
season. These extended periods of wetness in the cool season precipitation are associated with the
extended, southward displacement of mid latitude storm tracks. The cause of this displacement is
currently unknown.
A natural follow-up question is "How rare are these anomalous wet \dry periods in the annual
and cool season precipitation when viewed in the extremely long term context"? To answer this
question, we use a Douglas-fir tree-ring chronology collected in September 1995 in Scheelite
Canyon, on the north side of the Huachuca Mountains. This series appears to give a good
representation of cool season precipitation at least back to the year A.D. 1700. Correlation
coefficients with the four precipitation stations plotted in Figures 24 are about 0.40 for the annual
series, 0.63 for the cool season and near zero for the warm season.
The Scheelite Canyon tree ring record has also been filtered with the 7 -term binomial filter used
on the precipitation records. The original annual tree-ring series and its filtered version are plotted in
Figure 5. These data show that the long term variance so evident in the station precipitation data
are not unusual. In fact, there appear to have been at least 4 extended, anomalously wet periods
during the past 296 years The first centered around 1770 was rather short when compared to the
others and showed moisture in the range of 170% of normal, the second, centered around 1835,
was extended and shows moisture around the 180% of normal, the third centered around 1914,
also was extended and also was around the 180% of normal, the last apparently centered around
1988 and whose duration is continuing, also shows moisture in the 180% of normal. Apparently,
the extended wet periods are not too uncommon, occurring about every 7 4 years, but the dry spell,
extending from 1925 - 1972 has no analog in the long-term record. In the worst year, 1954,
moisture is only about 25% of normal. The cool season of 1995-96, not shown here, could easily
have been as severe as that for 1954.
183
RAMIFICATIONS FOR GRASSLANDS OF
SONOITA-ELGIN AND SAN RAFAEL VALLEYS
The climatic record clearly shows large swings in precipitation over periods of decades. The
health and appearance of the grasslands would be expected to vary with these climatic swings. With
recent variations (except for the exceptionally dry 1995-96), it is mostly the cool season grasses
that have benefitted. Such grasses as plains love grass, alkali sacaton, and vine mesquite are most
directly affected by the increase in cool season precipitation, though warm season grasses may
also benefit from the high soil-moisture reserves in the dormant season. It is important that the
recent anomalously wet period not lead to a misconception that the land is less arid than it really is,
and to ill-conceived changes in land use, such as increased farming operations similar to those of
the early 19oo•s. The limited ground water reserves in the region are unlikely to be able to support
extended increased ground water development for such ventures. The immediate impacts would be
lowered ground water tables, with increased pumping costs, and possibly dry wells. A possible
related detrimental effect of lowered ground water tables would be harm to the vegetation in the
cienegas of the Sonoita-Elgin region.
Regardless of the known limitation on supplies of water, these arid grassland valleys are likely to
remain targets for future expansion. Besides stressing ground water supplies, additional
development could be detrimental in altering patterns of runoff. The problem of changing land use
Scheelite Canyon Tree-Ring Index
2
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1700
1750
1800
1850
1900
1950
Year
Figure 5. Douglas Fir tree-ring chronology collected in September 1995 from Scheelite
Canyon on the north side of the Huachuca Mountains. As in the precipitation analyses a
seven weight binomial filter was used to obatin the low frequency overlay curves.
184
in grassland areas and increasing percentages of runoff per unit of rainfall, especially in upland
valleys like the Sonoita-Elgin area, upper drainage's for the Pantano Wash and the San Pedro River
and the San Rafael Valley, upper drainage area for the Santa Cruz River can have deleterious
effects on downstream communities. It has been noted that the magnitude of peak flows on the
Santa Cruz at Tucson (and probably the Pantano Wash at Tucson) have increased dramatically in
the past 30+ years (Saarinen et al. 1984). These increased flow magnitudes are attributed to large
flows resulting from tropical storms and increased runoff associqted with upstream changes in land
use. When storms of the magnitude and intensity of tropical storms dump
SUMMARY
Grassland valleys in southeastern Arizona present unique challenges to water development. A
trend toward wetter cool season conditions in recent years might lead to misconceptions about the
available water. Considerations in water development include limited aquifer storage, health of
cienegas, and possible problems with increased erosion, and runoff.
ACKNOWLEDGMENTS
This research was supported by National Biological Service Agreement A950-AI-0012.
Precipitation data for 1970-present at Elgin was supplied by Ray Thibault.
REFERENCES
Drewes, H., 1976, Laramide tectonics from Paradise to Hells Gate, southeastern Arizona:
Arizona Geological Society Digest, v. X, no. March 1976, p. 151-167.
Gray, W. M., 1983, Atlantic seasonal hurricane frequency: Colorado State University
Atmospheric Science Paper No. 370, Fort Collins, 48 pp.
Saarinen, T. F., Baker, V. R., Durrenberger, R., and Maddock, T., Jr., 1984, The Tucson,
Arizona, Flood of October 1983: Report No. CETS-CND-029: National Academy Press.
Simons, F. S., 1974, Geologic map and sections of the Nogales and Lochiel Quadrangles,
Santa Cruz County, Arizona: Misc. Investigation Series, Map 1-762: U.S. Geological
Survey.
185