4 outline of part 4 - philippe-choler.com · advanced readings • cappelaere, b., et al. 2009. the...
TRANSCRIPT
Outline of part 4
• 4.1. Tree-grass coexistence in savannas (patch scale) • -> see computer lab session
• 4.2. The Grasslands – Shrublands dynamics (community scale)
• 4.3. Land use change and the water cycle (watershed scale)
• 4.4. Determinants of woody cover in Africa (continental scale)• -> reading discussion
• 4.5. Desertification and land degradation (global scale)• -> see Lecture 3
4
4.3. Land use change and the water cyclehttp://news.nationalgeographic.com/news/bigphotos/30639457.html
Climate vegetation feedbacks in the Sahel
• Example of a landscape scale hydrological changes as a result of land degradation explained by increases in regional runoff and streamflows in the Sahel (Leblanc et al., 2008; Favreau et al., 2009).
• Land cover dynamics and soil water balance at the catchment level
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
44.3
West Africa : Regional context
1960 2020
Aléas climatiques
Africa: Atlas of our Changing Environment (UNEP)
Demographic growth
Overgrazing
utilisation des terres
% déforestation (2000-2005)
Land use changes
Sahel and West Africa Club / OECD 200
UNEP/ISRIC and CRU/UEA 2005
Fink & al. 2010
44.3
The AMMA-Catch observatory• Objectives
• Observing the west African monsoon and its impact on ecosystem functioning
• Investigating key (eco)hydrological processes along a rainfallgradient
• Hydro-climatic modelling (local->regional)
Site d’étude du projet AMMA (Analyses Multidisciplinaires de la Mousson Africaine)
Ouémé
Niamey
Gourma
Kergoat & al. Atm. Sc. Lett. (2011)
44.3
Landscape changes
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Land clearance and increase in the density of the drainage network as documented by aerial photographs.
Hillsslopes & drainage network
Gullies & ponds
Millet fields
4
Source: Leblanc M.J., et al. (2008) Global Planet. Change, 61, 135-150.
4.3
Associated hydrological changes
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Rise in the watertable depth
Pre
cipi
tatio
n an
omal
ySevere droughts
4
Source: Leblanc M.J., et al. (2008) Global Planet. Change, 61, 135-150.
4.3
Processes
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Land clearance (agriculture, firewood supply)
Reduced soil infiltration capacity
Increased runoff (densification of gullies network – new ponds)
Groundwater recharge and long-term water table rise (in a context of severe droughts in the 70s and the 80s !)
44.3
Landscape changes in Niger
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
44.3
Advanced readings
• Cappelaere, B., et al. 2009. The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger -Investigating water cycle response to a fluctuating climate and changing environment. - Journal of Hydrology 375: 34-51.
• Boulain, N., et al. 2009. Towards an understanding of coupled physical and biological processes in the cultivated Sahel-2. Vegetation and carbon dynamics. - Journal of Hydrology 375: 190-203.
• Ramier, D., et al. 2009. Towards an understanding of coupled physical and biological processes in the cultivated Sahel-1. Energy and water. - Journal of Hydrology 375: 204-216.
• Leblanc, M. J., et al. 2008. Land clearance and hydrological change in the Sahel: SW Niger. - Global Planet. Change 61: 135-150.
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
44.3
4.3. Determinants of woody cover in Africahttp://news.nationalgeographic.com/news/bigphotos/30639457.html
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Global distribution of forests
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Woody cover along mean annual rainfall
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
4
Upper enveloppe –> potential woody cover Fire, herbivory
854 obs. sites across Africa
784mm
« Stable savannas »
« Unstable savannas »
516 mm
Source: Sankaran M., et al. (2005) Nature, 438, 846-849.
4.4
The mean wet season rainfall depth
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
4.4
1 yr
Length of the wet season (Tw)
Pw : total amount of precip. during the wet season (mm)w : mean depth of a precip. eventTw : Length of the wet season (days)nw : number of rainy days during the wet season (days)w : average frequency of precip days = nw / Tw
Pw = w nw= w w Tw
w
Seasonnal distribution of rainfall events matters
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
More frequent and less
intense rainfall events
Rare andintense rainfall
events
≈780mm
≈ 40 000 sites with TRMM estimates of seasonal precipitation MODIS estimates of woody cover
4
Source: Good S.P. & Caylor K.K. (2011) Proc. Natl. Acad. Sci. USA, 108, 4902-4907.
4.4
Seasonnal distribution of rainfall events matters
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Rare andintense rainfall
events
More frequent, less intense
events
Carrying capacity
4
Source: Good S.P. & Caylor K.K. (2011) Proc. Natl. Acad. Sci. USA, 108, 4902-4907.
4.4
generalized logistic curve
Potential woody cover and sensitivity to climate
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
4.5. Desertification and land degradation
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Plant responses to precipitation in semi-arid grasslands
Land degradation
• A process of land degradation in arid, semi-arid and dry subhumid areas that causes
• Increase in bare soil • Loss of soil resources (nutrients and water holding capacity),• Increase in soil salinity and toxicity • Changes in vegetation composition (perennial grasses -> annual
plants, grassland -> shrubland)• Reduction in the capacity of ecosystems to supply services
• Under the control of climate and land use drivers
• Remote sensing provides the unique quantitative and repeatable measure of desertification at regional and global scale
4
Seasonal dynamics o NDVI
Version du 14/01/2014206
0 50 100 150 200 250 300 350
0.0
0.2
0.4
0.6
0.8
1.0
Day
ND
VI
NPP ≈ NDVIintRUENDVI = NDVIint / Rainfallgs
Climate Forcing(rainfall)
Local drivers (carrying capacity, species composition, herbivory, fire…)
Interannual variability
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
High correlation between RUE and rainfall -> RUE not a good proxy of land degradation assessment
Residulal trend analysis
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
high inter-annual rainfall variations in drylands rainfall as a predominant driver of NPP→ Superimposed human-induced trends are difficult to detect
ND
VI in
t
Rainfall gs
residual
Res
idua
lYear
Residulal trend analysis (example 1)
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Introduced irrigated agriculture in Syria
Positive trend of residuals related to increased carrying capacity
Residulal trend analysis (example 2)
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Negative trend of residuals related to rangeland degradation in South-Africa
Regional analysis (west-Africa)
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
r value of the residual analysis (NDVI-rainfall 1996-2007)
Evidence for limited anthropogenic land degradation in the Sahel-Sudanian zone.
Ongoing debate
• The usefulness of residual analysis is still controversial (see Wessels 2012)
• If severe degradation with ΣNDVI decreases above 20%, method becomes innappilicable
• Need to have a high reponsiveness of NDVI to rainfall for the RESTREND analysis to work
• Way forward: upper enveloppe fitting in the NDVI rainfall relationship and examining residuals ? Quantile regression ?
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Ongoing issues
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
ND
VI in
t
Rainfall gs
residual
ND
VI in
t
Rainfall gs
residualSlope highly sensitive to outliers
Use of quantile regression ? -> upper enveloppe
ND
VI in
t
Rainfall gs
residual
Reference period / paddock
Compared dataset
Combating desertification
• Need to understand the main causes of desertification• Poor or questionable land management (overgrazing, firewood,
intensive use of the land beyond its carrying capacity) -> Reduce pressure on land
• Objective : Conservation of natural resources – sustainable development
• As a rule of thumb, deterioration processes are rapid, natural reclamation processes are slow
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Restoring soil fertility & productivity
• Control Soil erosion (e.g. mulching with crop residues, transplanted vegetation, afforestation in mesic areas)
• Remedy soil salinization (e.g. leaching of salts)
• Change grazing management (e.g. control livestock mobility, grazing restrictions, early warning of droughts)
• Improve irrigation management (e.g. rainwater harvesting systems, traditional water conservation practices)
• Change crop varieties
• Management of fire (e.g. shrublands)
• Stakeholder involvement
• Change land tenure policies, land reclamation policies
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)
Advanced readings
• Bai, Z. G., et al. 2008. Proxy global assessment of land degradation. - Soil Use and Management 24: 223-234.
• D'Odorico, P., et al. 2013. Global desertification: Drivers and feedbacks. - Advances in Water Resources 51: 326-344
• Fensholt, R. and Rasmussen, K. 2011. Analysis of trends in the Sahelian 'rain-use efficiency' using GIMMS NDVI, RFE and GPCP rainfall data. - Remote Sens. Environ. 115: 438-451.
• Fensholt, R., et al. 2012. Greenness in semi-arid areas across the globe 1981-2007 - an Earth Observing Satellite based analysis of trends and drivers. - Remote Sens. Environ. 121: 144-158.
• Wessels, K. J., et al. 2012. Limits to detectability of land degradation by trend analysis of vegetation index data. - Remote Sens. Environ. 125: 10-22.
Fundamentals of Ecohydrology (Philippe CHOLER – CNRS – France). Wuwei (09/2013)