assessing the impact of a soil surface crust on simulated overland flow at the field scale. n....
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AAssessing the impact of a soil surface ssessing the impact of a soil surface crust on simulated overland flow at the crust on simulated overland flow at the
field scale.field scale.
N. Chahinian(1, 2), M. Voltz (2), R. Moussa (2), G. Trotoux (2).
(1) Cemagref Antony, Hydrology group(2) UMR LISAH, French National Agricultural Research Institute, Montpellier
The problemThe problem
• Surface crusts have widely been reported to reduce the infiltration rate of soils.
• Most infiltration models try to represent a crusted soil as a double layer system (Chu et al., 1985; Philip, 1998, Smith et al., 1999).
• However
- the determination of the hydraulic properties of the crust is difficult.
- the integration of a double layer in the overland flow module of a distributed hydrological model can be cumbersome.
The objectivesThe objectives
Can the impact of a crust be seen on overland flow simulations at the field scale?
Does the use of a double layer model improve overland flow simulations?
The MethodologyThe Methodology
Use cumulated infiltration experiments to determine crust and sub-soil hydraulic properties by inverse modelling at the local scale (177 cm2).
Simulate overland flow at the field scale (1200 m²) using previously determined parameters in a single layer and double layer model.
Compare the results obtained by double layer and single layer representations.
Overland flow
1200 m²
Infiltration
experiments
14 cm
1. The study zone1. The study zone
• Area: 1200 m²
• Soil texture: Silty loam
• Presence of a sedimentary crust
• Vegetation: Vine
• Infiltration capacity : 7.9 mmh-1
• High runoff coefficients (23-80 %)
RoujanNice
Paris
2.1 Infiltration experiments at the local scale2.1 Infiltration experiments at the local scale
Two cumulated infiltration experiments with a 14cm cylinder
Measure i before experiment and add known volumes of water until steady state.
NB: Measurement is representative of superficial
soil layer (0-10 cm)
(De Condappa & Soria-Ugalde, 2002)
Without crust
14 cm
Sub soil
With crust
1 cm Crust
Sub soil
2.2 Inverse modelling2.2 Inverse modelling
Determine soil hydraulic properties using HYDRUS-2D (Simunek et al., 1999) in inverse modelling in axi-symmetric mode.
No flow
Constant head
Free drainage
Subsoil
Ks, s, n, subsoil
Crust
Subsoil
Ks, s, n, crust
Step 1 Step 2 Step 3
Ks, s, n, crust+subsoil
(effective parameters)
same
exp.
data
Crust
+
subsoil
2.2 Inverse modelling2.2 Inverse modelling
0.425 m
z
r0
1.0 m
0.5 m
0.01 m
0.01 m
0.01 m 722 triangular elements
200 nodes
2.3 Results of inverse modelling:2.3 Results of inverse modelling:Soil hydraulic properties Soil hydraulic properties ((Van Genuchten, 1980)
Ks subsoil = 10* Ks crust
Ks crust+subsoil harmonic mean (Kscrust + Kssubsoil)
8.563.004.41 10-2
0.37Crust+subsoil as
homogeneous layer
23.983.002.79 10-2
0.37Subsoil
2.371.931.56 10-2
0.39Crust
Ks
(mm.h-1)
(mm-1)
s
(cm3.cm-3)
n
3.3.1 Overland flow simulations at field 1 Overland flow simulations at field scale: rainfall/ runoff datascale: rainfall/ runoff data
Experimental data available since 1993 – 12 events (i known) and note prone to measurement errors– Duration: 30 min-23 hours– Runoff coefficient: 27-79%
0
20
40
60
80
0 50 100 150
Total Rainfall (mm)T
ota
l Ru
no
ff d
ep
th
(mm
)
3.2 The overland flow model3.2 The overland flow model
Simulate overland flow using:
- 1 D Richards’ equation for infiltration;
- Diffusive wave model for transfer.
Two sets of simulations done with previously determined soil hydraulic properties:– Using a double layer representation of soil; – Using the effective parameters.
3.2 Comparison on double vs. single layer 3.2 Comparison on double vs. single layer representation on runoff datarepresentation on runoff data
Effective parameters tend to underestimate runoff depth
Double layer model tends to overestimate runoff depth
Runoff depth Peak discharge
0
25
50
0 25 50
Measured peak discharge (Ls-1)
Sim
ula
ted
pea
k d
isch
arg
e (L
s-1
)0
50
100
150
0 50 100 150
Measured runoff depth (mm)
Sim
ula
ted
ru
no
ff d
ep
th (
mm
)
3.2 Comparison on double vs. single layer 3.2 Comparison on double vs. single layer representation on runoff datarepresentation on runoff data
-3
-2
-1
0
1
2
3
-3 -2 -1 0 1 2 3
Error on Volume using single layer model (%)
Err
or
on
vo
lum
e u
sin
g
a d
ou
ble
lay
er
mo
de
l (%
)
-3
-2
-1
0
1
2
3
-3 -2 -1 0 1 2 3
Error on Qm ax using single layer model (% )
Err
or o
n Q
max
usi
ng a
do
uble
laye
r mod
el (%
)Error double layer model > Error effective parameters
3.2 Comparison on double vs. single layer 3.2 Comparison on double vs. single layer representation on runoff datarepresentation on runoff data
-100
-50
0
50
100
-100 -50 0 50 100
Nash and Sutcliffe efficiency using single layer model (%)
Na
sh
an
d S
utc
liffe
e
ffic
ien
cy
us
ing
a
do
ub
le la
ye
r m
od
el (
%)
Nash Effective parameters > Nash double layer
-100
-50
0
50
100
0 5 10 15 20 25 30 35 40 45 50
Event Duration (hours)
Nas
h a
nd
Su
tcli
ffe
effi
cien
cy (
%)
3.3 Nash & Sutcliffe efficiency3.3 Nash & Sutcliffe efficiency vs. event type using effective parameters vs. event type using effective parameters
Model gives good results for short to medium duration events (duration < 10 hours).
Infiltration front too deep
h and unknown
-100
-50
0
50
100
0 50 100 150
Infilitration front depth (cm)
Na
sh
an
d S
utc
liff
e
eff
icie
nc
y (
%)
3.3 Nash & Sutcliffe efficiency3.3 Nash & Sutcliffe efficiency vs. event type using effective parameters vs. event type using effective parameters
Model gives good results for moderate infiltration depths (depth < 70 cm).
Infiltration front too deep
h and unknown
4. Concluding remarks & perspectives4. Concluding remarks & perspectives
The cumulated infiltration test did show that the crust hydraulic properties differ from that of the subsoil.
The soil hydraulic properties determined at the local scale can be used to simulate overland flow at the field scale provided infiltration front is < measurement domain.
A refinement at the local scale is not always rewarded by a greater precision at the field scale.
As overland flow is dependent on rainfall, a misrepresentation of the soil properties may be masked by the rainfall effect.
3.3 Nash & Sutcliffe efficiency3.3 Nash & Sutcliffe efficiency vs. event type using effective parameters vs. event type using effective parameters
High intensity rainfall events are well simulated
-100
-50
0
50
100
0 50 100 150 200
Maximum rainfall intensity over 1 minute (mm.h -1)
Na
sh
an
d S
utc
liff
e
eff
icie
nc
y (
%)