assessment of soil erosion and sedimentation rates in “my bouchta
TRANSCRIPT
Assessment of Soil Erosion and Sedimentation
Rates in “My Bouchta” Watershed in North
Morocco using Fallout Radionuclides and Stable
Isotopes
M.Benmansour1, N.Amenzou1, A.Zouaghi1, M.Sabir2, A.Nouira1, H.Marah1, H.Benjelloun2, A.Benkdad1, F.Taous1
1: Centre National de l’Energie des Sciences et des Techniques
Nucléaires (CNESTEN), Rabat Morocco 2: Ecole Nationale Forestière des Ingénieurs (ENFI), Salé, Morocco
FAO/IAEA International Symposium on Managing Soils for Food Security
and Climate Change Adaptation and Mitigation
Vienna, 23 – 27 July 2012
Content
General background
Study site
Material and Methods
Results
Conclusions
General background
Agriculture 8.7 millions ha ( 12.%)
13 -20 % to GDP (PNB)
Half of the active population
Water Resources Semi-arid - Arid Climate
Potential : 21 Milliards m3/yr
80 % used for agriculture
Forest 9.037. 500 ha ( 12.7 %)
Bio-ecologic diversity
General background
Great pressure on arable soil and water resources Increase of Population
Inadequate agricultural practices
Deforestation
Overgrazing
Use of fertilizers and pesticides
Impacts Soil degradation and desertification
Pollution and Degradation of water quality
Reduction of water reservoir capacities
General background
Soil erosion impact in Morocco
15 Millions hectares of land under serious threat
Soil loss : 100 Millions t yr-1
Annual erosion rates : 5 - 50 t ha-1 yr-1
Sedimentation in reservoirs: 75 Millions m3 yr-1 ( ~ 0.5 % of the total reservoir capacities)
Sustainable management of soil and water resources
Management of Morocco watersheds: 22 W
Strategies of Agriculture Ministry and associated Departments (eg. Green Morocco Plan)
Research Project
Title Use of fallout radionuclides , stable isotopes and conventional
techniques to assess soil redistribution rates and to identify primary sediment source areas at the watershed scale in Morocco
FAO/IAEA Coordinated research project. D1.20.11 Integrated Isotopic Approaches for an Area-wide Precision
Conservation to Control the Impacts of Agricultural Practices on Land Degradation and Soil Erosion
Institutions Centre National de l’Energie, des Sciences et des Techniques
Nucléaires, Rabat, Morocco
Ecole Nationale Forestière des Ingénieurs, Salé, Morocco
Study Site
Issoumatène
Rifiine
Taghbaloute
Barrage T
ala
mb
ote
Boubiyine
Jbel Agoumir
Vers Tétouan
Vers Chefchaouen
BattaraIfkiren
O. M
y. Bouch
ta
2000 0 2000 Meters
N
La légende
Projection: Lambert conformal conic
Spheroid : Clarke 1880
Sources:
Carte topographique 1/50 000
Photographie aériennes 1/20 000 (1986)
Agriculture
Forêt naturelle
Matorral
Arboriculture
Pin d'Alep
Pin maritime
Pin radiata
Eau
Réseau hydrique
Route Tétouan--Chefchaouen
Pistes
Carte de situation des perimètres de reboisement
496000
496000
500000
500000
504000
504000
508000
508000
512000
512000
5120
00
512
000
5160
00
516
000
520000
520000
524
000
5240
00
52
8000
52800
0
« My Bouchta » Watershed
North of Morocco: Occidental RIF
Study site
Characteristics
Total Area: 76.6 km2 , Perimeter : 41052 m
Climate: Humid (Chaouen) – Sub humid (Tetouan)
Precipitation: 604 – 1700 mm
Altitude 1821 m max. and 321 m min
Topography: Multitude of classes dominated by high slopes
Land Use
Agriculture (60%): Wheat, Maize, Barley, Beans, Olives, Almonds…
Forest, Shrub ( 40%): Pine, Oak
Soil Vertisols, Alfisols, Aridisols, Entisols
Study site
Agricultural areas Forest areas
Study site My Bouchta River Water Reservoir «Talembout »
Erosion forms: sheet, rill, gully…
Material and Methods
Assessment of Soil erosion and sedimentation rates Fallout Radionuclides (FRNs) : 137Cs, 210Pbex ,
7Be
Conversion models: MBM2, DMM, PD
Dating Models: CFCS, CRS
g: HPGe
a: SSBD
Material and Methods
Identification of the primary
sediment source areas
Stable Isotopic signatures: 13C,
15N
Bulk signatures
Compound Specific Isotope Analysis
( CSIA) after separation of fatty acids
with collaboration of University of
Hohenheim, Stuttgart, Germany
Mass Spectrometer
coupled with
elemental analyser
Sampling Strategy
ARCGIS : 48 Units but only 22 Units
covering more than 90% of the total area
are concerned
Land Use (3)
Agriculture, Forest,
Shrub areas
Soil Type (4) :
4 classes:
Vertisols, Alfisols,
Aridisols, Entisols
Slope (4)
0-5, 5-15, 15-30, >
30 %
Sampling equipments: FRNs
Preliminary results
137Cs and 210Pbex profiles
Reference site
MB/RF2
0 10 20 30 40 50 60
2
6
10
14
18Depth
(cm
)
137Cs (Bq.kg-1)
Agricultural site
MB/AG5
0 1 2 3 4 5 6 7 8
2
6
10
14
18
De
pth
(cm
)
137Cs (Bq.kg
-1)
Agricultural site
MB/AG5
0 2 4 6 8 10 12 14
2
6
10
14
18De
pth
(cm
)210
Pb (Bq.kg-1
)
Reference site
MB/REF2
0 50 100 150 200
2
6
10
14
18De
pth
(cm
)
210Pb (Bq.kg
-1)
137Cs inventories: Reference sites
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 2 4 6 8 10 12 14 16
Reference point
137C
s (
Bq
.m2)
2704 Bq m-2 (CV: 21%) 3549 Bq m-2 (CV: 12%)
653 mm 1777 mm
y = 1.7199x + 1029.9
R2 = 0.712
0
1000
2000
3000
4000
5000
0 500 1000 1500 2000
Precipitation (mm)
13
7C
s in
ven
tory
(B
q m
2)
Agricultural fields (AG): 300 – 2398 Bq m-2, Forest Fields
(FR), 1350 – 6830 Bq m-2, Shrub fields ( SR) 1340 – 3036
Bq m-2
Medium-term soil erosion rates by 137Cs
period: ~ 50 years
Net Soil
Erosion Rate:
21.1 t ha-1 yr-1
Site Iso-sector
Land use,
Soil type
Slope
Area (ha) % of total
area
Net Erosion
rate
(t ha-1 yr-1)
Net
Erosion
rate
( t yr-1)
% of
total
erosion
AG1/AG2 AG, VE, 5-15 % 654 8,8 27.4 17887 15.4
AG3 AG, AR, 15-30 % 1723 23.1 18.5 31876 27.4
AG4 AG, VE 15-30 % 382 5.1 22.5 8595 7.4
AG5 AG, AR, > 30 % 1564 21.0 36.1 56460 48.5
AG6 AG,AR,5-15% 578.9 8.4 18.8 10883.6 8.3
AG7
AG, VE 0-5% 288.2 4.2 - -
FR1 FR, AL, > 30 % 121 1.6 1,6 194 0.2
FR2 FR, AL, 15-30 % 146 2.0 0.8 121 0.1
SR1 SR, AR, >30 % 472 6,3 2.4 1133 1.0
SR2 SR, AL, 15-30 % 23 0,3 4.5 104 0.1
SR3 SR, AR 15-30% 119 1.7 6.9 3254.5 2.5
Total 6421 88.1 21.1 130487 100
High rates of
soil erosion in
AG fields
compared to
FR and SR
fields
Long-term soil erosion rates by 210Pbex
3 fields (AG1, FR1, SR1)
Activities
Reference sites ( FR1, FR2): 3220 Bq m-2, 4800 Bq m-2
Agricultural field ( AG1): 2685 Bq m-2
Forest field ( FR1): 3088 Bq m-2
Shrub field (SR1): 2950 Bq m-2
Soil erosion rates corresponding to AG1, FR1 and SR1 fields over 100 years
AG1: 12 t ha-1 yr-1,
FR: 0.61 t ha-1 yr-1
SR1: 1.3 t ha-1 yr-1
Sedimentation in the water reservoir
0
5
10
15
20
25
30
35
40
0 1 2 3 4
137Cs concentration ( Bq kg-1)
Dep
th (
cm
)
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35
210Pbex concentration ( Bq kg-1)
Dep
th (
cm
)
1963
Assuming a Constant flux of 210Pbex and Constant sedimentation
rate: CFCS
R = 0.34 cm yr-1 50 t ha-1yr-1
This rate is higher than the net erosion obtained from the upland
fields of the watershed
13C and 15N signatures
Upland areas: 3 fields – 6 points/field
Forest f ield
0
5
10
15
20
25
-29,00 -27,00 -25,00
Delta 13C %
Dep
ht (
cm)
Shrub field
0
5
10
15
20
25
-29,00 -28,00 -27,00 -26,00 -25,00
Delta 13C %
Dep
ht (
cm)
Agriculture field
0
5
10
15
20
25
-29,00 -27,00 -25,00
Delta 13C %
Dep
ht (
cm)
Forest fie ld
0
5
10
15
20
25
0 5
De lta 15N
de
pth
cm
S h ru b fie ld
0
5
10
15
20
25
0 5
De lta 15N
de
pth
cm
Agricu ltu re fie ld
0
5
10
15
20
25
0 5
De lta 15N
de
pth
cm
13C and 15N signatures
Upland areas (3 fields)
0
5
10
15
20
25
-29 -28 -27 -26 -25
13C(
0/ 00)
dep
th (
cm
) Forest
Shrub
Agriculture
Mean of 6 points for each field
0
5
10
15
20
25
0.0 1.0 2.0 3.0 4.0
15
N(0/00)
De
pth
(c
m) forest
Shrub
Agriculture
C3 plants – Stable isotopes increase with depth especially
for FR and SR fields
Total C and N
0
5
10
15
20
25
0 5 10 15 20 25
C (g/kg)
de
pth
(c
m)
Forest
Shrub
Agriculture
0.00
5.00
10.00
15.00
20.00
25.00
0.00 0.50 1.00 1.50 2.00
N (g/kg)
de
pth
(c
m)
Forest
Shrub
Agriculture
C decrease with depth for FR, SR and AG fields
N decrease with depth for FR and SR fields and almost
uniform for AG fields
Mean values
Mean 13
C(0/00) Mean C (g/kg) Mean
15N(0/00) Mean N (g/kg) Mean C/N
FOREST -27.49 ± 0.18 12.99 ± 1.39 1.76 ± 0.75 1.15 ± 0.33 11.29
SHRUB -27.24 ± 0.23 13.05 ± 2.49 2.12 ± 1.00 1.10 ± 0.10 11.86
AGRCULTURE -27.33 ± 0.40 9.77 ± 3.96 1.59 ± 0.52 1.28 ± 0.04 7.63
Correlations
Agriculture field
y = -3,453x - 84,609
R2 = 0,2367
0,00
5,00
10,00
15,00
20,00
25,00
-29,00 -28,50 -28,00 -27,50 -27,00 -26,50 -26,00 -25,50
Delta 13C %
% C
Shrub field y = -5,9432x - 148,74
R2 = 0,7483
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
-29,00 -28,50 -28,00 -27,50 -27,00 -26,50 -26,00 -25,50
Delta 13C %
% C
Forest field
y = -3,8232x - 92,782
R2 = 0,6048
0,00
5,00
10,00
15,00
20,00
-29,00 -28,50 -28,00 -27,50 -27,00 -26,50 -26,00 -25,50
Delta 13C‰
C%
Correlation of
C with 13C
for FR and SR
fields
Decomposition
of organic
matter for AG
fields under
high erosion
13C and 15N signatures
Water reservoir – Sedimentation area
0
5
10
15
20
25
30
-25.00 -20.00 -15.00 -10.00 -5.00
13C(
0/0 0)
de
pth
(c
m)
0
5
10
15
20
25
30
-2 -1 0 1 2 3 4
15N(
0/ 00)
dep
th (
cm
)
C3 plants C4 plants 15N decrease from 5 cm to 0
cm depth
Increase of the fertilizers
during the 15 last years ?
Total C and Total N
Water reservoir – Sedimentation area
0
5
10
15
20
25
30
3 8 13 18C (g/kg)
dep
th (
cm
)
0
5
10
15
20
25
30
0.5 0.7 0.9 1.1 1.3 1.5N ( g/kg)
dep
th (
cm
)
Maximum of C and N were observed at 7 cm depth
Conclusions
The net soil erosion rate, over a period of 50 years using 137Cs, estimated for “My Bouchta” watershed was about 21 t ha-1 yr-1 with an important contribution of Agricultural fields.
Results obtained by 210Pbex for 3 different fields indicate that soil erosion has increased significantly during the last 50 years
High sedimentation rate about 0.50 g cm-2 yr-1 (eq. to 50 t ha-1 yr-1) was obtained for “Talembout” water reservoir suggesting an additional contribution of gully erosion and mass movement
Conclusions
Similar behaviours of stable isotopes (13C , 15N ) with depth were obtained for forest and shrub fields and good correlation between 13C and total Carbon was obtained for forest and shrub fields
Using 13C profile, the sediment deposits in the water reservoir, seemed to be originated mainly from fields under maize culture (C4 plant)
The sediment profile of 15N indicate an increase of fertilizers during the 15 last years
Work will be continued by using 7Be for soil conservation methods assessment and CSIA for more precise information about the sediment sources
Thank You
for Your Attention