Salinity Management in the Rio Grande
Bosque
Jan M.H. Hendrickx
J. Bruce J. Harrison
Jelle Beekma
Graciela Rodriguez-Marin
Abstract—This paper discusses management options for salinity
control in the Rio Grande Bosque. First, salt sources are identified
and quantified. Capillary rise of ground water is the most important
cause for soil salinization in the bosque. Next, a riparian salt
balance is presented to explain the different mechanisms for soil
salinization. Finally, the advantages and disadvantages of three
salinity control options are discussed: drainage systems, adaption
to natural conditions, and managed flooding.
The Rio Grande has two important functions. One is to
deliver water to aquatic and terrestrial habitats as well as to
urban and rural communities linked by the river; the other
is to transport salts and other wastes generated by those
habitats and communities to the Gulf of Mexico.
Until the beginning of this century an apparent salt
balance existed in the Rio Grande Valley. The accumulation
of salts in the soils adjacent to the river was counteracted by
frequent inundations that provided surplus water for salt
leaching. Infrequent major floods resulted in a reshaping of
the river landscape and reallocation of salt sinks. Construction of flood protection and irrigation works since the early
1900’s have resulted in a more regular river flow without
major flooding events. In addition, much river water has
been allocated for agriculture, wetlands, and domestic and
industrial water supplies. The combined effects of decreasing river water supplies and channelization of the river to a
permanent streambed have resulted in environmental conditions where soil salinity is no longer controlled by the
river (for example Crawford and others 1993; Conover 1954).
Since soil salinity determines to a large extent the success
of revegetation projects in the Rio Grande Bosque management measures are needed to control soil salinity levels. The
objective of this study is to discuss management options for
salinity control in the Rio Grande Bosque.
Salt Sources ___________________
A good understanding of salt sources in the Rio Grande
Bosque is needed for the development of effective salinity
In: Finch, Deborah M.; Whitney, Jeffrey C.; Kelly, Jeffrey F.; Loftin,
Samuel R. 1999. Rio Grande ecosystems: linking land, water, and people.
Toward a sustainable future for the Middle Rio Grande Basin. 1998 June 2-5;
Albuquerque, NM. Proc. RMRS-P-7. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.
Jan M.H. Hendrickx, J. Bruce J. Harrison, Jelle Beekma, and Graciela
Rodriguez-Marin are with the Department of Earth and Environmental
Science New Mexico Tech, Socorro, NM 87801.
68
control options. Five salt sources contribute to the salinity of
riparian areas under natural vegetation: atmospheric deposition, mineral weathering, capillary rise, seepage from
higher areas West and East of the river, and flooding.
Atmospheric deposition in the interior of most continents
is about 40 kg/ha (Bresler and others 1984). Mineral weathering is an important process that affects the chemistry of
waters in rivers, soils, and shallow ground water. It involves
the dissolution, alteration, and precipitation of minerals and
for the most part takes place in the zone of aeration of soils.
Since the mineral weathering rate of a given soil is proportional to the amount of water that flows through the soil
(Langmuir 1997), it will increase with the leaching fraction.
Rhoades and others (1973, 1974) and Wierenga and others
(1972) studied in lysimeters the compositions of drainage
waters resulting from irrigation with eight synthesized
river waters of the Western USA under conditions of alfalfa
production on calcareous and noncalcareous sandy loams.
For Rio Grande water, leaching fractions above 25 percent
resulted in a net increase of salts in the drain water by
approximately 2-10 percent of the salts in the irrigation
water. When the leaching fraction is less than 25 percent,
salt precipitation will occur and will result in a decrease of
salt content in the drainage water. These percentages indicate that the contribution of mineral weathering to soil
salinity in the bosque is of the same order of magnitude as
atmospheric deposition.
The most important salt source at many locations is
capillary rise which results from upward water flow originating at the ground water table. The amount of capillary
rise depends on the depth of the ground water table and the
physical characteristics of the soil profile. Table 1 presents
capillary fluxes in a number of homogeneous soil profiles.
For example, a ground water table depth at 2 m will result
in capillary fluxes of 0.2 mm/day in a sand soil and 8 mm/day
in a loamy sand. Even if the ground water quality is good,
such rates can result in considerable salt depositions near
the soil surface or in the root zone. For example, ground
water with a total amount of dissolved solids of 500 mg/l will
accumulate 5 kg of salts per hectare for each millimeter of
capillary rise. This results in an annual salt load of, respectively, 365 and 14,600 kg/ha in the previously mentioned
sand and loamy sand soils.
Salt accumulation by capillary rise will rapidly decrease if
the ground water table is lowered. However, in many cases
the ground water table remains relatively shallow as a
result of ground water seepage. Such seepage may originate
from higher areas West and East of the river or from manmade sources. An example of man-made seepage is found at
USDA Forest Service Proceedings RMRS-P-7. 1999
Table 1—Maximum capillary rise (cm) for five fluxes (cm/day) in
homogeneous soil profiles (Hendrickx and others 1990).
Texture
Sand
Loamy Sand
Sandy Loam
Loam
Silty Loam
Clay Loam
Silty Clay Loam
2.0
69
151
29
73
59
126
7
Capillary Flux (cm/day)
1.0
.6
.1
82
185
45
99
90
165
12
92
213
61
123
121
197
18
135
334
158
247
321
342
65
.01
213
572
484
577
1025
639
219
Santa Ana Pueblo where surplus irrigation water from the
golf course on the mesa is seeping into the lower lying
riparian area. Another more important example are the
wetlands created to provide food and shelter for migratory
birds. The flooding of wetlands and the irrigation of adjacent
agricultural fields result in shallow water table depths
around the ponds and an increase in salinization as a result
of capillary rise. This can be observed in the Bosque del
Apache, La Joya, and San Bernardo reserves.
Another salt source is flooding since the water in the Rio
Grande contains approximately 300 to 500 mg/l total dissolved solids (Crawford and others 1993). This means that a
flood of 0.5 m water would deposit 150 to 250 kg salt per
hectare.
The most important salt source in riparian areas is capillary rise since its potential contribution exceeds by far the
salt amounts originating from atmospheric deposition, mineral weathering, and flooding. Areas with relatively shallow
ground water tables (1 to 3 m) that discharge ground water
seepage have the highest risk for salinization.
Riparian Salt Balance ____________
Salinization will occur when the amount of salt added to
the soil by rain water and flood water is greater than the
amount of salt removed by drainage waters. The salt balance
of the riparian root zone can be expressed as
R.Cr + F .C f + G.Cg = P.Cg + ΔS
[1]
where R is rainfall entering the root zone (mm/year), Cr is
the salt concentration of the rain water (mg/l), F is the
amount of flood water entering the root zone (mm/year), Cf
is the salt concentration of the flood water (mg/l), G is the
amount of capillary rise into the root zone (mm/year), Cg is
the salt concentration of the ground water, P is the amount
of deep percolation or drainage water leaving the root zone
and entering the ground water, and DS is the amount of salt
stored in the profile. It is assumed that the salt concentration of the drainage water and capillary rise water is equal
to the salt concentration of the ground water which is quite
reasonable for annual periods but less so for shorter periods.
Equation [1] reveals several important aspects of riparian salinity balances. Firstly, soil salinization becomes
inevitable if no drainage takes place to evacuate the salts
from the root zone, i.e. if P=0 then ΔS will increase.
Without some form of drainage soil salinization cannot be
prevented even if the irrigation or flood water contains only
USDA Forest Service Proceedings RMRS-P-7. 1999
small amounts of salt. Of course, under the latter conditions
salinization would take a long time.
We have found evidence in the Bosque del Apache that
older soils contain more salts than younger ones. The age of
soils can be approximately determined from aerial photographs by observation of channel continuity and the degree
of organic matter and aeolian dust accumulation which are
reflected in soil color. The oldest river deposits show complicated patterns of channel breaches and different degrees of
organic matter and aeolian dust accumulation. The younger
deposits show more continuous channels and less diversity
in soil color.
Using these criteria we selected two locations with comparable point bar deposits and water table depths, but different estimated ages of isolation from the active river. At both
sites soil salinity was measured at several hundreds of
points using electromagnetic induction (for example
Hendrickx and others 1992; Sheets and others 1994). Average topsoil salinity in the younger deposits was 24 against
280 mS/m in the older deposits. Average subsoil salinity was
31 mS/m in the younger deposits and 225 mS/m in the older.
This shows a ten fold increase in soil salinity for comparable
deposits in terms of soil texture and depth to the groundwater as a result of age, i.e. the duration during which capillary
rise has contributed to salt accumulation in the soil profile.
Secondly, flooding only will lead to a decrease of soil
salinity if the flood water leaches out salts, i.e. if P>0 so that
the salts percolate into the ground water and can be transported towards a drain. Otherwise, the salts in the flood
waters actually will contribute to an increase in soil salinity.
Whether flooding will result in drainage depends on the
water retention and hydraulic conductivity of the soil profile. Before drainage occurs the root zone needs to be sufficiently wetted to allow downward flow. In most riparian
areas with shallow ground water tables this requirement
will not pose a problem. In addition, the hydraulic conductivity of all soil layers must be large enough to allow the
drainage water to pass through. This requirement poses
serious limitations on the use of flooding in soils that contain
clay layers with a low permeability. In such soils the flood
water will become perched on the clay layer. A large part of
the perched water will evaporate or transpire while leaving
the salts behind in the root zone and, thus, increase soil
salinity. This scenario is quite common in riparian areas.
Another important factor that determines the effect of flooding is the leaching efficiency. For an effective leaching it is
not sufficient to replace once all soil water by flood water. On
the contrary, the soil should be flushed several times to
make it salt free (Hoffman 1980).
For example, in the Bosque del Apache on soil unit 29
natural regeneration of cottonwood is being stimulated by
applying an annual flood of approximately 0.3 m. The flood
water carries cottonwood seeds that settle at the high
water lines. After several weeks seeds emerge and young
trees start growing. In unit 29, the soil profile consists of a
layer of 50 cm to 100 cm of sandy loam to loamy sand
overlying clay loam to clay. Before flooding the soil water
content in the top 40 cm and from 40 to 80 cm is, respectively,
3 and 9 volume percent. The soil water content in the
underlying clay loam is close to saturation. During flooding the soil water content increases to saturation which
is approximately 35 volume percent. This means that
69
(0.35-0.03)*40 + (0.35-0.09 )*40 = 23.2 cm water will be
stored in the top 80 cm of the soil profile during flooding.
Most of this water will evaporate during the gradual infiltration of the flood water into the soil and contribute little
to salt leaching. Instead it will add to the salt load. The
total amount of water stored in the soil profile at unit 29 is
about 30 cm. This means that an efficient leaching will
require at least 60 to 90 cm of water.
The relatively low permeability of the subsoil of unit 29
and the small amount of flood water applied have resulted
in an increased salinity after flooding. Salinity measurements before flooding in April 1997 and after flooding in
August 1997 revealed an increase of soil salinity from 150 to
262 mS/m in the subsoil and from 112 to 204 mS/m in the
topsoil. Standard deviations of the measurements were
high, indicating considerable variability from place to place.
A third aspect of equation [1] is the importance of capillary
rise. Since the salt concentration of the ground water is at
least one order of magnitude larger than that of rain water,
even a relatively small capillary flux, G, can cause a large
increase of soil salinity. Under the arid conditions of the Rio
Grande Valley it is a constant challenge in riparian nature
reserves to maintain a water balance in which the deep
percolation rate exceeds the capillary upward flux. Without
such condition no favorable salt balance can be maintained.
Salt Management Options ________
The physical principles for salt management in soils are
well understood (Bresler and others 1984, Smedema &
Rycroft 1983; Van Schilfgaarde 1974) and lead to three basic
strategies for salinity management in the Rio Grande bosque:
(1) Installation of drainage systems; (2) Adaptation to natural conditions; (3) Managed flooding.
Installation of drainage systems is the classical response
to salt problems in irrigated agriculture. The functions of a
drainage system are to lower the ground water table and to
discharge saline drainage waters percolating from the root
zone. The ground water table is lowered to a depth at which
the upward capillary flow becomes too small for any significant upward salt movement. This depth depends on the soil
type and can vary from 50 cm in a coarse sand to more than
400 cm in loamy soils (see also table 1). A drainage system
can consist of surface drains or of underground pipes
(Smedema and Rycroft 1983; Van Schilfgaarde 1974). The
outlet is the terminal point of a drainage system where it
discharges into the river.
In mountainous terrain with large elevation differences
drainage waters can be discharged by gravity but in many
arid river valleys the slope of the land surface is not sufficient
to drain by gravity. For example, the gravity system installed
near Las Nutrias (New Mexico) performs only marginally
since sufficient head is not available to discharge the drainage waters. Therefore, it is common practice to have the
drain waters accumulate in a sump and to discharge by
pumping. Such a system has been in use until a few years ago
in the Bosque del Apache. Although the system performed
well during a period of about fifteen years, it has been shut
down and will be not be replaced. The main reasons for this
decision are: (1) The initial installation costs are high (approximately $200,000 for 120 acres); (2) The life of the
system was shorter than expected due to clogging of the
70
Table 2—Relationship between cottonwood tree vigor class and soil
salinity (Hendrickx and others 1997).
Salinity (mS/m)
Tree vigor class
N
Meana
Std Dev
Healthy Tree
Marginal Growth
Dying Tree
30
23
28
60a
111b
210c
25
64
112
a
Minimum Maximum
25
24
85
140
225
534
Means with the same letter are not significantly different.
tile drains; (3) The system was difficult to maintain
(John Taylor, personal communication 1998). These complaints are typical even in large irrigation projects with
trained personnel.
DralZage systems work well for moist soil management
and agricultural fields covering large areas. However, due to
high costs and problematic maintenance their use for restoration of native vegetation appears limited.
Adaption to natural conditions involves identifying optimal locations for revegetation of native species such as
cottonwoods. Hendrickx and others (1997) conducted a systematic study near El Paso and statistically confirmed the
dependence of cottonwood survival on soil salinity level
(table 2). They also observed a close relationship between
geomorphology and soil salinity. The lowest levels often
were found on small levees which naturally are the preferred
areas for cottonwoods. The highest levels occur in depressions such as old river channels with a fine soil texture that
enhances capillary rise.
At the Rio Bosque Park near El Paso it was found that 50
percent of the variability in salinity could be explained by
soil type and ground water table depth. This indicates that
in areas with soil and ground water table maps the location
of suitable restoration areas is relatively straightforward.
Otherwise an inexpensive salinity survey using electromagnetic induction can quickly indicate the areas with revegetation potential (Sheets and others 1994).
Managed flooding is often propagated as an efficient
and natural way to restore cottonwood and other native
vegetation. Flooding can only be successful for restoration if
it contributes to an effective salinity control. This requires
(1) a soil profile of sufficient permeability for salt leaching
and (2) sufficient quantities of flood water. If either of these
factors is not fulfilled, flooding will not lead to sustainable
restoration.
Managed flooding is a relatively inexpensive method to
control soil salinity and to promote bosque restoration.
However, the design of a flood management plan requires
detailed knowledge of the interaction between flood water,
ground water, soil, and vegetation.
References _____________________
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