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SWCv
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SWCv_tree
SWCv_between rows
Transpiration
The effects of rainfall partitioning and evapotranspiration on the temporal and
spatial variation of soil water content in a Mediterranean agroforestry system 1Biel, C., 1Molina, A.J., 1Aranda, X., 2Llorens, P. and 1Savé, R. 1IRTA - Torre Marimon, 08140 Caldes de Montbui, Spain. 2IDAEA-CSIC, Department of Geoscience, Barcelona, Spain
*Corresponding author: e-mail: [email protected] Telephone: +34 934 674 040, Fax: +34 938 650 954
Introduction and objectives Tree plantation for wood production has been proposed to mitigate CO2-related
climate change. Although these agroforestry systems can contribute to maintain
the agriculture in some areas placed between crops and secondary forests,
water scarcity in Mediterranean climate could restrict its growth, and their
presence will affect the water balance. Tree plantations management (species,
plant density, irrigation, etc), hence, can be used to affect the water balance,
resulting in water availability improvement and buffering of the water cycle.
The present work studies how the temporal and spatial variation of soil water
content is affected by transpiration and interception loss fluxes in a
Mediterranean rainfed plantation of cherry tree (Prunus avium).
Materials and methods
Results and discussion
Annual rainfall and ET0 during 2011 were 771 and 1088 mm
respectively. During the growing season (from March 30th till November
1st) rainfall and ET0 were 537 and 942 mm (Fig. 1), and climatic deficit
(ET0-Rainfall) was 405 mm. Until the end of June, tree transpiration
was maintained to the highest rate, due to SWC availability and optimal
climatic conditions. However, a drastic reduction of tree transpiration
occurred during summer, paralleled to a decrease of the SWC under
tree and the tree coverage (Fig.1).
SWC dynamics “under tree” and “between tree rows” were similar till
April 2011, despite the water consumption estimated as SWCv was
higher in “under tree”. From that moment, SWCv was always higher in
“between rows” in days with and without rainfall (Fig. 2 and 3). In rainy
days, this SWCv difference can not be explained by throughfall alone
(mean of 83 % of rainfall, Fig.4), due to the clear disagreement with the
SWCv estimated “under tree”. On the other hand, in rain-free days, tree
transpiration explained 64 % of the SWCv (Fig.5). Similarly, the higher
evapotranspiration in the “between rows” should reflect a higher
transpiration in this area, including effect of the herbaceous cover and
the tree.
Acknowledgements This work has been partially funded through Spanish Ministry of Science and Innovation at the project entitled
“Ecohydrological relationships in agroforestry interfaces. What is their role in the water and carbon regulations?” (AGL2010-21012) and
a grant to A.J. Molina (BES2011-043748). Authors want to acknowledge the participation of C. Morales, M. Ferrer, E. Serra and B.
Grau in the field work and data processing.
Figure 1. Rainfall and ET0, soil water content, transpiration and tree
relative green cover during the growing season. Values of soil water
content , transpiration and tree relative green coverage are the average of
3 trees.
Figure 3. Soil water content variation (SWCv) under tree cover and
between rows, tree transpiration and ET0 in rain-free days. Values are
the average of 7 days.
Conclusion
Our preliminary results show that there is a different SWC pattern
depending on the measurement point. In this sense, apart from
the hydrological processes of transpiration and interception that
are believed to be the main driving forces, other factors not
controlled in the experimental design (e.g. crust formation, soil
water content redistribution, soil heterogeneity), affected the
results.
A five-years-old cherry tree plantation located in northeast of Spain at 4x4 m
spacing was studied without any soil tillage. Tree height and trunk diameter
were 4.46 m±0.28 and 5.42 ±0.29 cm at the beginning of the year.
From April till December 2011 were continuously monitored:
Meteorological variables: rainfall, air temperature, solar radiation, wind speed
and relative humidity.
Throughfall and stemflow in 6 trees, with 7 automatic tipping-bucket per tree.
Transpiration in 3 trees by means of heat pulse sap flow sensors (HRM30,
ICT Int. Australia). Daily sap flow (L day-1tree-1) was converted to daily tree
transpiration (mm day-1) according to the tree density of 625 tree ha-1.
Soil water content (SWC) at 25 and 50 cm depths under selected trees (at a
distance of 60 cm from the trunk) and between tree rows exposed to direct sun
with capacitive sensors (10HS, Decagon Devices, USA).
Tree relative green coverage through zenith digital pictures.
Soil water content variation (SWCv) was compared under tree and between
tree rows at different time scales in order to know the effect of:
- Tree transpiration: as the accumulated depletion in the first 50 cm soil depth in
successive days without rain.
- Rainfall interception: as the soil water content recharge at the first 25 cm soil
depth as the difference between the SWC before and after rainfall events from
1 to 30 mm.
Figure 5. Relationship between tree transpiration and
Soil water content variation (SWCv) under tree cover
in dry periods. Values are the average of 7 days.
Figure 2. Relationship between rainfall and soil water content variation
(SWCv) under tree cover and between rows. Values are the average of
3 points for rain event.
Figure 4. Relationship between rainfall and
throughfall.
0
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8
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ET0
(mm
)
Rain
fall
(mm
)
20
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60
70
80
SWC
(mm
)
Tree 25 cm
Tree 50 cm
Between rows 25 cm
Between rows 50 cm
RAINFALL AND ETO
0.0
0.1
0.2
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0.4
Tree
tran
spir
atio
n (m
m)
SOIL WATER CONTENT
TRANSPIRATION
0
10
20
30
40
50
60
31-Mar 30-Apr 30-May 29-Jun 29-Jul 28-Aug 27-Sep 27-Oct 26-Nov 26-DecRe
lati
ve g
reen
cov
er (%
)
Days
TREE RELATIVE GREEN COVER
y = 2.50x - 0.01R² = 0.64**
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0.0 0.1 0.2 0.3 0.4
SWCv
(mm
.d-1
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T (mm.d-1)
y = 0.13x - 0.17R² = 0.15
y = 0.90x - 2.47R² = 0.79
0
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SWCv
(mm
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Rainfall (mm)
SWCv tree
SWCv between rows
y = 0.83x + 0.95R² = 0.94
0
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0 10 20 30 40 50
Thro
ugh
fall
(mm
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Rainfall (mm)
Picture 1. Photography of the plot with block
distribution (August 16 2011 (institut cartogràfic de
Catalunya).
Picture 2. Tree with sap flow sensor, and tree with
raingauges.
Block 1
Block 2 Block 3
