Water, forests and footprints – finding the right scale for sustainabilityProf. Kevin BishopUppsala University& Swedish Univ.of Agricultural Sci.

Less than 4% forest in Blue NileDeforestation widely blamed for water problemsWill more trees help? Or will they “steal” water?

65 %

35 %M.Falkenmark March 09

Green-Blue Water sees forests as consumers of water

Plynlimon – Hydrologist HeavenDocumented how forests took water – at the the 2 km2 catchment scale

My own background Digging in small catchments –

Miss a bigger picture?

©Bill Waterson

Hydrology and Forests in the Blue Nile:

What can be learned from half a century of observations and community perception for water management?

Solomon Gebreyohannis

Addis Ababa UniversityandSwedish University of Agricultural Sciences

Popular Perception vs Science

Popular hopes for reforestation:– Increase dry-season flow– Decrease erosion

But science suggests…

While Peak flows and erosion will decreaseTotal flow will probably decreaseDry season flows will probably decrease, (but site-specific information needed)

Interdisciplinary PhD Thesis: Half a century of quantitative and qualitative observations

• A dozen catchments have over 40 years of daily flow data.

• Remote imagery exists for land use/land cover change analysis

• Community perception will be used help in the analysis.

*= Flow gauge

Some large declines in forest area

1957 Landcover16% Forest = Green

2000 2% Forest

Statistical and Modeling Analyses:No obvious deforestation impact

y = 0.0008x + 0.3844

y = 0.0321x - 17.170

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1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Pea

kflo

w (m

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00.511.522.533.544.5

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eflo

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r)

Peakflow (m 3̂/sec) Baseflow (m 3̂/yr)

Community Perception: A complex understanding of the forest-water relationship

Deforestation influence on streamflow:Revisting what has science “proven”

Deforestation

Baseflow sometimes increases sometimes decreases

Region-specific data needed to know response

Total flow usually increases

Peakflow always increases

04/10/2023 13

Watershed Mass Balance

Precipitation (P)

Evaporation+

Transpiration (ET)

Runoff (Q)

Are Forests Good for Water Resources?

The Hydrologic Cycle: Trees remove water at one point, return it to the region

The Forest-Water Debate: Supply vs. Demand-Side

Demand-side:Increasing forest cover reduces runoffEcosystems subtract from the water budgetStrong observational basis (<2 km2)

Supply-side:Increasing forest cover positively impacts precipitation

and runoffSometimes labeled the “romantic” or even

mythological viewLimited empirical evidence Regional Modeling is the best evidence

Source data generously provided by Michael Bosilovich, NASA

Balatic Recycled ET Ocean

Annual 40% 60%

Summer 60% 40%

DJF MAM JJA SONBaltics

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Water Recycling Modelled at Regional Scales

Source data generously provided by Michael

Bosilovich, NASA. Highlighting indicates Terr_shr

> Oc_shr.

Note: local share included in terrestrial share.

DJF MAM JJA SONMississippi

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DJF MAM JJA SONMacKenzie

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DJF MAM JJA SONSiberian

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DJF MAM JJA SONBaltics

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DJF MAM JJA SONHUB

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DJF MAM JJA SONTibet

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DJF MAM JJA SONGTR

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DJF MAM JJA SONCAT

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DJF MAM JJA SONAmazon

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Average Estimated Precipitation in Major River Basins by Source (1948-1997)

Should Egypt be more concerned about deforestation of the Congo that dams on thd Blue Nile?

Much Precipitation is Recycled Evaporation:East African over 50% of rain from trees

The Forest Water Yield Debate:A Question of Scale

• All Agree: Trees Increase Evapotranspiration– Demand-side View: Reduces runoff – Supply-side View: Recycles Water, Increases Rain

• View point depends on Scale– Demand at Small Scale– Supply at Large Scale

What is Forest ET’s impact on precipitation??

Forest and wetland cover are the two most efficient methods for promoting evapotranspiration (ET)

Only oceans can evaporative as effectively- 90% of ocean evaporation falls back on the ocean

Cropland exhibits comparatively low evaporative efficiency

Land conversions to agriculture will reduce precipitation

Between 2000-2005, global forest cover has decline by 3% (Hansen et al 2010)

Suggests a 4.7-5.3% decline in global precipitation

Conclusions • Forest cover important role for the global hydrologic cycle.

• Increasing forest cover increases regional precipitation and runoff .

• Global impact of forest cover does not rule out local demand.

• Trans-boundary impacts of local decision-making about forests.

• Forest provide ecosystem services beyond biomass

M.Falkenmark March 09

Remember Recycling!

Are forests good for water and food security in Ethiopia?

Swedish Ministry of Foreign Affairs as Project on global food security

Two MSc Students to the Koga Irrigation Dam

Two PhD Students fromCentre for Natural Disaster Science

New Interdisciplinary research initiativeby the Swedish Government

Climate Adaptation = Disaster Mitigation