939 - PARTITIONING AND SOURCING OF DRY SEASON EVAPOTRANSPIRATION WITHIN EDDY COVARIANCE TOWER FOOTPRINT

Enrico BaluganiITC, Uniiversity Twente, Enschede, Netherlands

Maciek Waclaw LubczynskiITC, Uniiversity Twente, Enschede, Netherlands

Leonardo Reyes-AcostaITC, Uniiversity Twente, Enschede, Netherlands

Christiaan Van Der TolITC, Uniiversity Twente, Enschede, Netherlands

Klaas MetselaarWageningen University, Wageningen University, Wageningen, Netherlands

Evapotranspiration (ET) is a lump term that represents two different processes, evaporation (E) and transpiration (T). Both processes are characterized by different principles (physical and biological) and different spatial and temporal patterns. The two fluxes, E and T, have also different subsurface sources, i.e. originate either from groundwater, Eg and Tg, or from unsaturated zone, Eu and Tu.

In this study carried out in the granitic, shallow water table Sardon Catchment in Spain, we attempted to quantify all the four subsurface ET components and to compare their sum with ET estimated by eddy covariance tower. The experiment was carried out in two dry seasons (2009 and 2010), i.e. when grass was dormant, within sparsely vegetated by oak trees (~7% crown cover) footprint of the eddy covariance tower. The T of the oak trees was estimated by sap flow measurements whereas its sourcing into Tg and Tu components, by stable isotopes. The bare soil E was estimated by coupled liquid water, water vapor and heat flow, HYDRUS modeling, carried out in soil moisture profiles, while its sourcing into Eg and Tg, by HYDRUS/SOURCE modeling.

Due to low canopy cover, T accounted only for ~ 6 % of ET (~ 0.5 mm d-1) while Tg was approximately equal to Tu, did not depend on water table depth and did not change significantly between dry seasons with different moisture status. The E was the main ET flux while Eg was ~1/3 of Eu. The comparison of the sum of all estimated ET components with ET of the eddy covariance tower, for dry season 2009 indicated very good match with only 1% error while for dry season 2010, the error was ~32%, mainly because of large number of low quality, rejected eddy data.

The study emphasized the importance of typically neglected Eg. It also pointed at importance of Tg, which in this study was low, but only because of sparse oak tree cover.

In dry environments, the Eg and Tg groundwater fluxes represent important components of groundwater balances so must not be disregarded in groundwater modeling and groundwater resources estimates. Therefore, more research in that direction is needed.

Session S5.8 - Aqua 2015 - 42nd

IAH Congress

1

RESULTS:

PARTITIONING AND SOURCING OF DRY SEASON EVAPOTRANSPIRATION WITHIN EDDY

COVARIANCE TOWER E. Balugani1, M. W. Lubczynski1*, J. Leonardo Reyes-Acosta1, C. van der Tol1

1 - Water resources, ITC-Twente University, Hengelosestraat 99, 7514 AE, Enschede, The Netherlands; * - poster presentar

MATERIALS AND METHODS:

Evaporation (ET): • Eddy covariance tower placed

in a semi-arid, water-limited

climate, with almost no rain

events during summer

• Eddy tower footprint

calculated using the approach

of Hsieh et al. (2000).

• The footprint calculation

shows that all the footprint

measurements come from a

distance < 2 km from the

station.

OBJECTIVES: 1. To partition evapotranspiration

(ET) measured by eddy

covariance (EC) tower into

evaporation (E) and transpiration

(T) during dry season in a semi-

arid environment.

2. To source E and T into their

groundwater (Eg, Tg) and

unsaturated zone (Eu, Tu)

components.

RESEARCH NICHE: • The sap flow measurements are

usually taken with the most economic

Granier method, which if not properly

handled, typically results in serious

overestimation of T.

• Calculation of bare-soil

evaporation is usually over-simplified

by not accounting for vapor flow,

which can result in substantial

underestimation of E.

• To our knowledge, nobody ever

tried to source and partition ET into its

components Eg, Eu, Tg and Tu..

Transpiration (T): • Two main tree species in the study area: Quercus pyrenaica and

Quercus ilex.

• Sap-flow measured and optimized using combination of thermal

dissipation probe (TDP) and heat field deformation (HFD) sensors

• Sap-flow upscaling procedure:

1. Tree classification map (canopy coverage, species type)

2. Develop an upscaling function for each species

3. Apply the upscaling function to obtain a transpiration map for

the whole investigated area

• Isotopic experiments to measure (source) Tg and Tu .

• Elaboration of half-hour maps of transpiration (Tg and Tu) for a

square area of 2 km side with the eddy tower in the centre.

Evaporation (E): • Sandy loam soils, laying on weathered and fractured granite.

• Shallow groundwater (0.5 m to 10 m).

• Water table and soil hydraulic properties measured.

• Soil profiles equipped with soil moisture, temperature and

matric potential sensors at different depth.

• Soil modeled with Hydrus1D using equations for the coupled

water vapor flow.

• E sourced into Eu and Eg with the SOURCE post-processing

package.

• Elaboration of half-hour maps of evaporation (Eu and Eg) for a

square area of 2x2 km with the eddy tower in the centre.

CONCLUSIONS: • The eddy covariance

tower did not record any

difference in ET between

shallow (0.5 m) and deep (> 5

m) water table depth locations

• In the savannah

landscape, with low canopy

cover, such as 7% in the

investigated area, E is the

most relevant flux.

• Eg can be relevant in

very dry soils, because

groundwater may flow as

vapor through the unsaturated

zone to the ground surface.

Picture of the

study area, with

Tu, Tg, Eu and

Eg.

It was assumed

that T was the

only flux within

the ground

projection of

the tree canopy

area and E

outside that

area.

? ET

Tg + Tu Eg + Eu

Eg

Eu Tu

Tg

E T

The footprint of an EC tower is a

probability density area function

that indicates which area is

responsible for the ET measured

by the tower.

This footprint changes in time

due to the wind magnitude and

direction.

Hydrus1d

SOURCE

Sap flow

estimation SUM Eddy tower

E (mm d-1) T (mm d-1) ET (mm d-1) estimated

ET (mm d-1) measured Eg Eu Tg Tu

Summer 2009

0.137 0.389 0.017 0.017 0.560 0.565

24% 29% 3% 3% 99% 100%

Summer 2010

0.170 0.483 0.016 0.016 0.685 0.518

33% 93% 3% 3% 132% 100%

Here the EC tower

was on top of very

shallow water table