Transcript H33B-05 Esteban G Water subsidies from mountains to deserts: groundwater-fed oases in a sandy landscape Marcelo D Pablo E http:// gea.unsl.edu.ar CONICET – GEA & Universidad Nacional.

H33B-05
Esteban G
Water subsidies from mountains to deserts:
groundwater-fed oases in a sandy landscape
Marcelo D
Pablo E
http:// gea.unsl.edu.ar
CONICET – GEA & Universidad Nacional de San Luis, ARGENTINA
CONICET – Instituto de Nivologia y Glaciologia, ARGENTINA
Dept Biology & Nicholas School of the Env. – Duke University, USA
*
Jackson
he Central Monte desert (~120,000 km2) hosts vast sandy aeolian plains dominated by short shrublands. In some areas low landscape positions are occupied by unusually high and dense
woodlands of Prosopis flexuosa. Next to the Andean Cordillera, the Central Monte receives large inflows of snowmelt from the San Juan, Mendoza, and Tunuyan rivers, which sustain ~7000
km2 of vineyards and olive orchards. Transverse dunes and discontinuous valleys create elevation gradients of 8 to 20 m. Precipitation is 156 mm/yr (>80% summer-fall). Mean annual
temperature is 18.5ºC. Local settlers live in “puestos” around hand-dug wells that supply both people and livestock. The area is grazed by goats and cattle. Large Prosopis trees provide
goats with forage, and people with timber and food (flour from seeds, syrup from seedpod carpels)
the system
What are the contributions of LOCAL rainfall vs. REMOTE mountain
snowmelt to groundwater RECHARGE in the sand dune systems of the
MONTE DESERT?
A• •
shrublands
in dune crests
rainfall: 150 mm/yr
Altos Limpios
B
Water-Soil-Plant sampling
* Groundwater sampled at existing handdug wells and ad-hoc bore holes
* Ten rainfall events collected in study area +
GNIP database for Mendoza
* Soil/sediment profiles (5-10 m deep ) done
with hand augers
* Plant xylem sap-water collected in two
dates (woody stem sections saved in vials).
•
interdune woodlands
post with well
1980 toyota
truck
APPROACHES
large & productive
Prosopis flexuosa
trees
-15
-10
-5
0
10
-25
-20
-15
-10
-5
1
0
depth (m)
-75
groundwater
(bare interdunes)
-100
-125
groundwater
(vegetated interdunes)
vegetated intedune
vegetated crest
bare interdune
δ 18O‰
-15
δ 2H‰
500
24
0
10
20
30
40
50
site C
site B
5
6
site C
site A
1
2
3
4
5
6
12
24
0
0.2
0.4
0.6
2
site A,
veg. upland
site A, bare interdune
6
site A,
veg. upland
site A, bare interdune
7
vertical profiles of moisture (gravimetric) and chloride (pore water concentration). Note
interrupted Y moisture axis expanding range towards saturation in the capillary fringe.
Water table position shown with horizontal dashed lines.
non-capillary water - RAINFALL
wet season
dry season
-10
-15
Larrea
divaricata
Prosopis
alpataco
Capparis
atamisquea
-5
0
-5
947
site A, vegetated interdune
948
949
705
950
706
951
707
site C, vegetated interdune
Intradiurnal level fluctuations (night
raise, day drop) are associated with
groundwater consumption by plants.
Based on their depth and duration
throughout the year water uptake can
be estimated. In our case suggesting a
value of 296 mm/yr. Fluctuations
disappear in bare dunes or after rainfall
events that provide a more accessible
water source to plants.
non-capillary water - RAINFALL
WEST
-15
-20
capillary water - GROUNDWATER
EAST
crops
lowland
Xylem sap water isotopic composition at site C. Horizontal colored bands represent 5% to 95%
percentiles of soil moisture δ18O‰ values within the unsaturated zone (non-capillary water)
and the capillary fringe (capillary water). Markers indicate sap water values of individual plants
(2-3 stems per individual) grouped by species and by sampling date. The first sampling date
(wet) corresponds to the end of the growing season of 2005-2005 and the second (dry) to the
end of the dry season of 2006, before the onset of the next rainfall period.
upland
bare dune
Free aquifer
irrigated oasis
sand dunes
Desaguadero river
Infiltration of Andean water brought by the Mendoza River was the dominant mechanism of groundwater recharge.
Local recharge was negligible, except in rare bare dune areas.
Woodlands in interdune positions displayed significant consumption of groundwater located 6 to 10 m underground.
Plants in the rest of the landscape (90% of the area) relied on rainfall.
Regional scaling suggests that groundwater evapotranspiration reaches 18 to 42 mm/yr across the whole landscape,
accounting for 7 to 17% of the Mendoza river flow regionally.
precipitation
evapotranspiration
surface flow
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
water fluxes and trade-offs
in the mountain –desert
continuum
Atlantic moisture
(rain in summer)
wet season
708
time of the day (hh:mm)
Monte Desert
dry season
-10
583
585
Andes
Pacific moisture
(snow in winter)
51 mm at site A
584
Hourly dynamics of water tables
captured by pressure transducers. The
simultaneous water table depth
variations at the vegetated interdunes
of site C and the neighbouring
vegetated and bare interdune of site A
are shown. The Y axis indicates relative
depth changes and is not intended for
absolute level comparisons across sites.
Points indicate actual half-hourly
measurements and lines three-hour
centered moving averages. The
occurrence of a large rainfall event is
depicted by a horizontal bar at the top.
3
site C,
4 veg. upland
site A, bare interdune
952
site C,
veg. upland
1
582
CREST plants
grasses
Larrea
divaricata
Bulnesia
retama
Prosopis
flexuosa
12
0
Soil water samples grouped according to
landscape situations and differentiated
by their vertical position in the profile
(unsaturated zone vs. capillary fringe)
and their gravimetric moisture content
(dry < 1.3%, wet > 1.3%) in the case of
the unsaturated zone.
-150
Isotopic survey. Precipitation events, sampled at the town of Mendoza and at the study are in
Telteca. Mendoza and San Juan rivers sampled at their lowest sections before entering the study
area. Two temporary ponds were sampled at site. Detailed isotopic composition of river and
groundwater samples indicating values for each sampling date.
δ18O‰
6
4
5
INTERDUNE plants
5
site B
0
unsaturated, dry
unsaturated, wet
capillary fringe
-125
hand dug wells
4
vegetated crests & bare interdune
Tricomaria
usillo
Site C
bare interdune
grasses
Site B
soil water
vegetated
interdunes
3
10
depth (m)
ground waters
Site A
2
9
-100
Site A
1
site A
rivers
-20
Irrigated oases
8
-175
Mendoza
San Juan
100 km
7
-150
-175
GW discharge area
River sampling points
3
-50
> 60 mm
Water table monitoring
* Levels monitored in ad-hoc boreholes with automated pressure
transducers with built-in dataloggers (HOBO-Onset water level
logger)
* Water use estimates based on level fluctuations assumed specific
yield of 0.3 mm/mm
pore water chloride (g/L)
2
-25
30-60 mm
-150
-25
moisture (% mass)
0
ephemeral
ponds
precipitation events
< 30 mm
-125
0
10
0
-100
Xylem sap isotopes revealed the
combined use of rainfall and
groundwater in most species in
INTERDUNE woodlands. Rainfall
was the only source for plants in
the DUNE CREST
5
25
-75
Water level fluctuations suggest
frequent groundwater
consumption in vegetated
interdunes (~300 mm/yr)
0
25
2H‰ -50
In vegetated dunes LOCAL rainfall
supplied only upper soil layers
with virtually no deep drainage.
Bare dunes, instead, experienced
deep drainage & local recharge.
epilogue
5
50
Sample prep. & analysis
* Soil & plant water extracted with azeotropic distillation
* Isotopic concentrations analyzed with Finnigan MAT Delta Plus XL
(precision +/- 1.5 and 0.1 ‰ at one SD for 2H and 18O, respectively)
* Chloride analyzed with ion selective electrode
vegetated interdunes
δ 18O‰
50
-25
Shallow phreatic groundwater
was recharged by REMOTE
snowmelt waters brought from
the ANDES by the Mendoza river.
0
woodlands = 10% landscape
C •
Altitude a.s.l. (m)
> 4500
Permanent rivers
Temporary rivers
ARGENTINA
50
0
δ
-20
study sites
•dunes
with
hand-dug wells
woodlands
0
1
2 km
CHILE
shallow groundwater
(6-12 m in interdunes)
δ 18O‰
-25
Dominant Prosopis used 50% and
90% groundwater in the wet and
dry season, respectively
groundwater and Prosopis trees
are a critical water and food source
for people and livestock
large trees in
interdune
groundwater depth from surface (cm)
Region: stable isotopic survey of water from rivers, rainfall,
groundwater and soils (3 years, Mendoza river watershed)
Landscape: deep soil profiles (moisture/isotopes/chloride) across
topographic gradients (3 crest-interdune-crest transects)
Stand: water table level monitoring (2-3 years, 3 sites) and water
stable isotopes in plants (2 dates, 1 site)
VERY large isotopic contrasts
facilitated our exploration of
sources
Study
region
City of
Mendoza
MATERIALS & METHODS
bare dune area
x Headquarters of
Telteca Reserve
City of San Juan
What is the contribution of GROUNDWATER to sustain VEGETATION
in sand dune landscapes (when, where, how much)?
findings
[email protected]
STUDY REGION
In arid regions throughout the world, shallow phreatic aquifers feed
natural oases of much higher productivity than would be expected
solely from local rainfall. While local groundwater recharge is feasible
in arid regions under very sandy soils, transfer from adjacent areas
with more positive water balance is another possible recharge source
Bulnesia
retama
Prosopis
flexuosa
quick tour
*
Villagra
Robert B
QUESTIONS
quick tour
*
Nosetto
intradiurnal depth
deviation (cm)
intro
*
Jobbágy
In central Argentina, like in many mountain –
desert contact zones of the World, water is
increasingly diverted for irrigated agriculture.
While the trade-offs between production and
SURFACE WATER-fed natural ecosystems is usually
obvious, it is less so in the case of GROUNDWATERfed ones, like those studied here. We find signs of a
long-term decline of water table levels (dead shell
beds, narrower tree rings, historic documents) at
our sites, yet their connection with irrigation
upstream is still uncertain.
EGJ-MDN-PEV-RBJ
ground flow
The West-East transect encompasses the Andean Cordillera and the Monte Desert with the irrigated oasis of Mendoza and the sand dune landscapes occupied by natural vegetation. The main findings of
this work, based on isotopic surveys, deep soil profiles, and water table records, suggest the prevalence of the hydrological fluxes indicated with the arrows. The free aquifer found under the sand dune area
is recharged by the Mendoza river (Pacific isotopic signature), likely after it leaves the irrigated area. Upland vegetation is sustained by local precipitation inputs (Atlantic isotopic signature), while lowland
vegetation consumes both local precipitation and remotely recharged groundwater. The free aquifer receives no local recharge in the sand dunes with the exception of bare dune zones, as suggested by
deep soil profiles and isotopic data. The sand dune region exports a fraction of its groundwater inputs to the Desaguadero river, likely facilitating salt evacuation from the groundwater fed oases. The onset
of irrigation could have potentially compromised lowland oases by cutting the contributions of the Mendoza river to the free aquifer, causing declines in water table level and supply to plants. However, the
timing of these potential effects is unknown a needs to be resolved to project the impacts of irrigation on natural oases. Evaporative (curved lines) and liquid (straight lines) water fluxes are differentiated.