4 Outline of part 4 - philippe-choler.com · Advanced readings • Cappelaere, B., et al. 2009. The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger - Investigating

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


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


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


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


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.


44.3

4.3. Determinants of woody cover in Africahttp://news.nationalgeographic.com/news/bigphotos/30639457.html


Global distribution of forests


Woody cover along mean annual rainfall


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


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


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


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


4.5. Desertification and land degradation


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


High correlation between RUE and rainfall -> RUE not a good proxy of land degradation assessment

Residulal trend analysis


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)


Introduced irrigated agriculture in Syria

Positive trend of residuals related to increased carrying capacity

Residulal trend analysis (example 2)


Negative trend of residuals related to rangeland degradation in South-Africa

Regional analysis (west-Africa)


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 ?


Ongoing issues


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


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


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.


Documents

4 Outline of part 4 - philippe-choler.com · Advanced readings • Cappelaere, B., et al. 2009. The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger - Investigating