Conserving soil as a capital: main scientific challenges

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Conserving soil as a capital: main scientific challenges Global changes and Impacts of soil erosion in the Mediterranean context Y. Le Bissonnais & Al.

1 Conserving soil as a capital: main scientific challenges Global changes and Impacts of soil erosion in the Mediterranean context Y. Le Bissonnais & Al.* Institut National de la Recherche Agronomique, UMR Lisah, Montpellier, France, *MESOEROS team : INRA, IRD, BRGM, Partners in Tunisia and Marocco Laboratoire des Interactions Sol-Agrosystème-Hydrosystème Montpellier

2 Context for soil conservation Despite soil represents only a very little part of the earth, it plays a fundamental role for vegetation, life and the environment. It takes thousands of years to form, whereas it can disappear in a few minutes when exposed to heavy rain without vegetation protection. Soil erosion will be influenced during the next century both by climate and land use and management changes (although these changes are not well known for Mediterranee : increase of temperatures, rain intensities, drought periods) Negative impacts of soil erosion may thus be amplified, and potentially new and different problems could arise if some thresholds are exceeded. EFIMED Marrakech

3 What is the issue for soil erosion research? Erosion impacts are multiples: in-site impacts to soils such as soil loss, OM loss, Available water capacity decrease, gully off-site impacts to environment & water (muddy floods, turbidity, dam siltation, pollution's...) We often focus on measuring or modelling soil erosion rate at a given scale however, assessing the impacts is a step beyond : The erosion rate has to be converted into an indicator related to the impacts and cost EFIMED Marrakech

4 Runoff and erosion processes : field scale Sheet erosion Rills and gullies EFIMED Marrakech

5 Erosion and sediment transfer : catchment scale Permanent gullies Défrichement et surpâturage pertes de sols agricoles sédimentation des barrages Dam siltation EFIMED Marrakech

6 4 examples of erosion impact studies 1. Erosion in wheat fields in north of Tunisia with complete soil removal 2. Erosion in vineyard in Languedoc with impact on water quality 3. Erosion in Atlas Mountain producing large amount of sediments 4. Siltation of small dams in North African catchments EFIMED Marrakech

7 Ex. 1: Erosion in a Tunisian cultivated catchment Processes at field scale Kamech 250 ha Processes at hill slope scale Processes at catchment scale Processes at dam scale EFIMED Marrakech

8 A multi-scale erosion experiment : Kamech catchment in Tunisia gully field Small catchment Dam 4 hydrological stations soils on marl with monoclinal sandstone layers EFIMED Marrakech

9 Runoff and erosion results for a 84 mm event Runoff rate = 63% of rain Field scale erosion rate = 39 t/ha EFIMED Marrakech

10 Runoff and erosion measurement Runoff rate = 82% of rain Single gully scale Erosion rate = 20 t/ha EFIMED Marrakech

11 Runoff and erosion measurement Runoff rate = 92% of rain Small catchment scale Erosion rate = 36 t/ha EFIMED Marrakech

12 Qualitative aspect : Field scale erosion EFIMED Marrakech

13 Field scale erosion EFIMED Marrakech

14 Apparition of the sandstone layer EFIMED Marrakech

15 Large eroded areas with apparent sandstone layer EFIMED Marrakech

16 Conclusions for erosion in Tunisian wheat fields Erosion rates are very high for heavy rain event The on-site impact, depends, in this context, on the location of the field within the catchment : the distance from sandstone layer determine the remaining soil depth In some cases, the whole arable soil layer may be removed during a single rain event Such highly vulnerable fields should probably not continue to be cultivated Conservation tillage methods may be adopted in other fields Anti-erosive methods such as gully vegetalisation could be very efficient and probably cost effective Off-site impact (example 4) EFIMED Marrakech

Example 2: Roujan experimental catchment Runoff,

3000 m² and 1000 m² : vineyard with either mechanical

17 Example 2: Roujan experimental catchment Runoff, erosion and herbicide losses measurement at two scales: A catchment of 1 km² with 70% vineyard. Two fields of ca m² and 1000 m² : vineyard with either mechanical (tillage) or chemical weeding (no-tillage) (slope 8 to 12%). ditches outlet Tillage No tillage EFIMED Marrakech

18 Results: mean event erosion rate 10 years data with an average of 20 events/year 3,00 2, T/ha Mean event erosion (T/ha) 2,00 1,50 1,00 0,50 0, T/ha T/ha Herbicide no-tilled field Tillage tilled field Catchment catchment Tillage may reduce erosion rate (five times in average) compared to chemical weeding Erosion rate for an event may be 20 to 50 times higher at field scale than at catchment scale EFIMED Marrakech

An experiment for agricultural practices

surface Ruissellement et érosion 2000 m²

19 An experiment for agricultural practices assessment in mediterranean vineyard chemical weeding Delayed herbicide mecanical weeding Grass coverage États de surface Ruissellement et érosion 2000 m² Plots 5% slope Propositions EFIMED Marrakech

20 Effect of agricultural practices on runoff mean annual runoff rate (% of rain) mean annual runoff rate ( ) 10 0 chemical weeding delayed herbicide mecanical weeding grass coverage Very high loss of water by runoff (up to 40%) Tillage and grass reduce runoff rate (by half in average) compared to chemical weeding EFIMED Marrakech

21 Effect of agricultural practices on erosion 10 mean annual erosion rate ( ) mean annual erosion rate (t/ha/y) chemical weeding delayed herbicide mecanical weeding grass coverage Mean annual erosion rate ranges from 1 to more than 8 t/ha/y Grass may reduce erosion rate (ten times in average) compared to chemical weeding EFIMED Marrakech

22 Effect of agricultural practices on herbicide losses mean annual herbicide loss (g/ha/y) mean annual glyphosate loss (g/ha/y) (corresponding to up to 1000 μg /l in runoff) 0 chemical weeding delayed herbicide mecanical weeding grass coverage Mean annual herbicide loss is reduced 50 times in average for grass compared to chemical weeding, because of the reduced amount used and the reduced runoff and erosion rates EFIMED Marrakech

23 Conclusions for erosion in vineyard Grass and tillage are efficient for runoff and erosion control, with some limitations : Tillage operation should be repeated and a high risk of erosion exist in case of big storm Grass is not immediately efficient and concurrence for water and nutriments may occur sometimes The main impact, in this case, is soil and water pollution by herbicide, and the overall efficiency of these practices, is even much more important than for erosion People don t care about erosion itself: they install new vineyard in «rocky» areas with heavy machine global catchment management is also important : with grass strip, ditches, hedges EFIMED Marrakech

24 Example 3: Mountain erosion in the Atlas Rhéraya catchment : Area : 228 Km² Elevation : 925m to 4165m Situation du sous bassin de la Rhéraya dans le bassin du Tensift EFIMED Marrakech

25 Atlas landscape EFIMED Marrakech

26 Geological context Very erodible material in the north : permotriasic red clays and marls Less erodible material in high mountains : andesite, granites Rhéraya catchment EFIMED Marrakech

27 Vegetation cover Overgrazing Thuya aux basses altitudes especially at high altitudes Chaméphytes aux hautes altitudes EFIMED Marrakech

28 Permotriasic clays very sensitive to gully erosion when limestone colluviums coating are removed EFIMED Marrakech

29 Annual rainfall : 300 to 700 mm Tahanaout Variability of annual rainfall Evènement du 05/11/2004 Spatial variability of rain Taourirt Tagadirt aït ali Tamatert Tachedirt Armed Matate Imsker el bour Sidi chamharouche Imzouren Pluie (mm) Pluviomètres 1970/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / / /2006 Années Rainfall intensity : 10 most intense rains in 4 years : 24 à 118 mm.h -1 in 5 min 16 à 42 mm.h -1 in 30 min EFIMED Marrakech P (mm)

30 Arg_rav 6 long term erosion plot measurements Arg_parc Arg_pin clay ran_parc Volc_parc clay Volc_pin clay sand sand sand EFIMED Marrakech

31 Runoff measurements Evènements mean runoff rate ruisselants for runoff events Coefficient mean annual moyen runoff rate de Pourcentage (%) Arg_ravi Arg_parc Arg_pin Gran_parc Volc_pin Volc_parc EFIMED Marrakech

32 Erosion measurements Suspended sediment concentration : from 1 g/l to 300 g/l 1000 MES t.ha-1 TDF t.ha-1 Turbidité g.l ,1 0,01 Arg_ravi Arg_parc Arg_pin Volc_parc Gran_parc Sites EFIMED Marrakech

33 Comparison of catchment scale erosion vs plot scale erosion extrapolation 10 8 Exportation du Tahanaout Spatialisation des parcelles d'érosion 6 (t.ha -1 ) / / /2006 Années EFIMED Marrakech

34 Conclusions for mountain erosion Erosion rates are very heterogeneous in space depending on geologic and anthropogenic parameters with three order of magnitude of variation The off-site impact in this case, depends principally on the erosion rate from degraded permo-triasic clay soils Anti-erosion land management should focus on these specific areas representing only a few % of the total catchment area These results could also be used to assess erosion risk in other similar catchment by quantifying the % area of degraded permo-triasic clay soils EFIMED Marrakech

Example 4: Small dams siltation in Tunisia Built during last decades

Kamech lac, 1994-2004 0 Hauteur (cm) 1400 1200 1000 800 600 400 Digue

08/96 06/97 04/98 02/99 11/99 09/00 07/01 05/02 03/03 240 One problem :

35 Example 4: Small dams siltation in Tunisia Built during last decades for water storage and protection of barrages against siltation 1600 Kamech lac, Hauteur (cm) Digue Seuil Pluie (mm) Vase /94 12/94 10/95 08/96 06/97 04/98 02/99 11/99 09/00 07/01 05/02 03/ One problem : rapid siltation with annual loss of 5% of storage capacity EFIMED Marrakech

Measurement of sediment volum with bathymetry 600 550 Bathymétrie de la retenue Decrease of reservoirs capacities: 24 hill reservoirs in Tunisia (built between 1990 to 1993) were monitored initially

36 Measurement of sediment volum with bathymetry Bathymétrie de la retenue Decrease of reservoirs capacities: 24 hill reservoirs in Tunisia (built between 1990 to 1993) were monitored initially storage 2.6 millions m 3. They lost, in 9 years, 22%, or an yearly lost of 4,5 % Distance (m) Corresponding siltation relative to the area catchment ranges from 1.8 t/ha/year to 50 t/ha/year. EFIMED Marrakech

37 Spatial and temporal variability of erosion Erosion (deposit + spillway), Kamech - Tunisia Interannual Variability of erosion Erosion t/ha/year Erosion t/ha/year Rainfall mm Rainfall mm Annual erosion varies for a given catchment (2 to 45 t/ha/y) according to rainfall characteristics: Intensity Temporal distribution m3/ha/year sandstones, conservatio n Volume of silts deposited from 1994 to 2000 Spatial Variability of erosion Basalt, cultures, cordons Siltation m3/ha/year Marl, cultures, slopes Mean erosion varies between catchments depending on soil, land use and management (2 to 30 t/ha/y) El Gouazine (Tunisia) Syndianeh (Syria) M'Richet (Tunisia) Es Senega (Tunisia) Fidh Ali (Tunisia) Kamech (Tunisia) Saboun (Morocco) EFIMED Marrakech

38 Conclusions for siltation in Tunisian dams The off-site impact in this case, depends on the specific erosion rate at the catchment scale, whatever it is from interrill or gully or wadi banks erosion Erosion rates at catchment scale depends on several natural and anthropogenic parameters In some cases, the reservoir was completely filed within less than ten years The cost and efficiency of these dams should be compared to other traditional erosion management techniques such as gully vegetalisation EFIMED Marrakech

39 Conclusions We are still missing of erosion rate measurements! (for regional erosion risk assessment, model validation ) However, the consequences of erosion not only depend on its rate, but also on the nature, location and scale of the main impacts associated Therefore, assessment of the impacts of erosion requires additional informations in order to elaborate various indicators of vulnerability (uncertainty), as well as cost estimation Our research on erosion may be more interesting for end-users, if we were able to produce erosion impact and cost maps or models Erosion costs could be then compared to the cost of anti-erosive actions (vegetation cover) EFIMED Marrakech

40 Thank you for your attention! EFIMED Marrakech

Advancing hydrological research applied to watershed management: achievements and prospects in South and East Mediterranean Countries

Advancing hydrological research applied to watershed management: achievements and prospects in South and East Mediterranean Countries Jean Albergel Hydrologist at IRD Kenya EFIMED, Marrakech 29 and 3 April