1. Fig 1: Location and geology of the Boussouab catchment
Applying radiometric techniques for assessing soil erosion in the NE Rif of Morocco
Sadiki, A1. Faleh, A.2, Navas, A.3, Bouhlassa, S. 4
1 Department of Geology, Facuty of Sciences, Fès (Morocco)
2 Department of Geography, Faculty of Saiss, Fès (Morocco)
3 Department of Soil Science, Estación Experimental de Aula Dei. CSIC Zaragoza (Spain)
4 Laboratory of Radiochemistry, Faculty of Sciences, Rabat (Morocco)
Methods
Soil samples were collected along six transects representative of materials, slopes and land uses for the study area (Fig. 1). Profiles were 30 cm depth and sample treatment is as follows (Fig. 2):
Objectives
Estimate erosion rates by applying 137Cs technique
Analyse the erosion factors and the soil degradation status
Meso-Rif
(Bouhaddoud nappe) 
Post-nappes deposits  
Sillon South-Rif 
Terni - Mezgout 
Rif
Pre-Rif external
Foreland
Pre-Rif internal x
Transect
Fig 1: Location and geology of the Boussouab catchment
50 km
Prérif N
1km
Anguied
Ain Zora
Jbel Bouhajjer
J. Binet
J. Tanout
34°35’
34°30’
3°35’
3°40’
Mezguitem 1 2 6 4 5 T 3 A
300 km
Maroc
Fig. 2: Soil sampling and treatment
30 cm
5 cm
- Dry out
- weight
137Cs gamma analyses
622 KeV
137Cs activity (Bq/kg)
137Cs inventory (Bq/m2)
Study area
The Boussouab catchment (eastern Rif) has an abrupt landscape developed on the marls and limestones nappes (Fig. 1).
The climate is of semiarid – arid type with average annual rainfall ranging between 150 and 350 mm.
The natural vegetation in the area (Alep pine, juniper and thuya species) and some Eucalyptus plantations show important degradation features. On the smooth slopes, there is steppe vegetation (alfa and artemisa) and on the flat valley floor, soils are cultivated for cereals that have very low yields.
Erosion rates by 137Cs are estimated by comparing with values of reference inventories for the study area (Ritchie et al., 1974; Navas & Walling, 1992).
Estimation of erosion rates are calculated after calibration of 137Cs data. The models used are the proportional model for the cultivated land and the mass balance model (Walling & He 1997) for the uncultivated land
RESULTS
- slope = 10°
- cereals
marls
Glacis-terrace
For the same condition of slope and land use, soil losses are double on the marls (40 t/ha/year) that on the glacis-terrace materials (20 t/ha/year).
0,00
5,00
10,00
15,00
20,00
25,00
30,00
35,00
40,00
45,00
C13
C14
C33
C32
C31
C30
C29
C28
t/ha/year
Lithology
Conclusions
The 137Cs technique allows to quantify the soil losses since 1963 and to distinguish the key factors of soil erosion in the study area.
A large range of variation was found and soil losses ranged between 5 and 49 t/ha/year.
It was found that the anthropogenic impact was the main factor responsible for the high erosion rates. For the areas that had not suffered from human intervention, the slope is a main factor triggering soil erosion. The high contrast in lithology with highly erodible materials such as marls also results in strong differences in soil erosion rates that double the values found in glacis-terrace materials.
- slope= 10°
- lithology = glacis terrace
C27
C26
C25
C24
C23
C22
Matorral
cereals
The erosion rate under matorral is 10 t/ha/year, but it reaches 20 t/ha/year for deforested and cultivated soils.
Therefore, in just 40 years soil losses in deforested land double that under matorral.
Land use
References
Navas, A., Walling, D. (1992). Using caesium-137 to assess sediment movement on slopes in a semiarid upland environment in Spain. In: Erosion, Debris Flows and Environment in Mountain Regions, IAHS Publ. nº 209,
Ritchie J. C., Spraberry J. A. & Mchenry J. R. (1974) - Estimating soil erosion from the redistribution of fallout Cs-137. Soil Sciennce Society of America Proceeding, 38, pp. 137 – 139.
Walling D. E. & He Q. (1997) - Models for converting 137Cs measurements to estimates of soil redistribution rates on cultivated and uncultivated soils, Report to the IAEA as a contribution to IAEA Coordinated Research Programmes on Soil Erosion (D ) and Sedimentation (F31001) Exeter : Department of Geography, University of Exeter.
0,0
5,0
10,0
15,0
20,0
25,0
30,0
35,0
40,0
C21
C20
C19
C18
C17
C16
C15
15°
20°
22°
25°
- Lithology : marls
- Land use : alfa
Soil losses increase when slope increases:
21 t/ha/year (15 º) to 36 t /ha /year (25°).
A variation of 10 º means a 75 % increase in soil losses.
Slope
t/ ha/ year
Fig. 3 : 137Cs reference inventory (T) R 2
= 0,9741 5 10 15 20 25 30
depth (cm)
137
Cs Bq kg -1
Reference inventory
- The 137Cs profile decays exponentially with depth.
- Total inventory is 4258 Bq/m2
- 70 % of 137Cs is concentrated within the first 15 cm.
t/ ha/year
137Cs inventories and erosion rates according to the lithology, land use and slope.
Comparing with the reference inventory for the area (4258 Bq/m2), inventories range between 245 and Bq/m2 corresponding to 137Cs losses of 94 and 13 % respectively. After calibration of 137Cs data, the erosion rates estimated ranged between 5 and 49 t/ha/year.
Transect 5
(cereals)
C 28
400
800
C 29
C 30
200
C 31
C 32
Erosion rates
t/ha/year :
12,9
14,7
19,9
18,8
23,6
26,6
137Cs Bq/m2 5 10 15 20 25 30
Depth (cm)
C 33
Soils on glacis terrace
Soils on marls
Transect 2, (cereals)
38.9
39.2
39.5
41.7
41.5
38.2
C 9
100
C 10
C 11
C 12
C 13
C 8
300
37.9
Transect 6
(badlands)
43,4 47
C 35
Bq/m2
C 36
C 34 )
t/ha /year :
48,8
Transect 4 (Matorral) 14
10,7
12,5
10,8
10,1
C 22
500
1000
C 26
C23
C 25
C 27
C 24
5,1
Transect 3
(alfa)
C 16
C 17
C 18
C 19
C 20
C 15
32,8
33,6
36,2
33,5
35,5
28,9
C 21
C 14
20,6