In his presentation Kevin Bishop tried to unfold how forests impact water partitioning at different scales. At a local catchment scale, a removal of forests usually increases the total flow, always increases the peak flows, and can increase as well as decrease the base flow. Although there exists some ambiguity regarding the role of forests, all scientific studies confirm that forests have larger evapotranspiration (ET) than most other land uses. (Evapotranspiration is the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere) However, there are studies that consider impacts of forests on water availability for annual mass balances at the local watershed level misleading. More at www.siani.se

1. Water, forests and
footprints –
finding the right
scale for
sustainability
Prof. Kevin Bishop
Uppsala University
& Swedish Univ.
of Agricultural Sci.

2. Deforestation widely blamed for water problems
Less than 4% forest in Blue Nile
Will more trees help?
Or will they “steal” water?

3. Green-Blue Water sees
forests as consumers of water
65 %
35 %
M.Falkenmark March 09

4. Plynlimon – Hydrologist Heaven
Documented how forests took water
– at the the 2 km2 catchment scale

5. My own background
Digging in small
catchments –
Miss a
bigger
picture?
©Bill Waterson

6. 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 University
and
Swedish University of
Agricultural Sciences

7. Popular Perception vs Science
Popular hopes for reforestation:
– Increase dry-season flow
– Decrease erosion
But science suggests…
While Peak flows and erosion will decrease
Total flow will probably decrease
Dry season flows will probably decrease,
(but site-specific information needed)

8. 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

9. Some large declines in forest area
1957 Landcover
16% Forest = Green
2000
2% Forest

10. 120
100
y = 0.0008x + 0.3844
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
3
Peakflow (m 3/s)
140
Baseflow (m /year)
Statistical and Modeling Analyses:
No obvious deforestation impact
80
60
40
20
y = 0.0321x - 17.17
0
1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Peakflow (m^3/sec)
Baseflow (m^3/yr)

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

12. Deforestation influence on streamflow:
Revisting what has science “proven”
Deforestation
Total flow
usually increases
Peakflow
always increases
Baseflow
sometimes increases
sometimes decreases
Region-specific data
needed to know response

13. Watershed Mass Balance
Evaporation
+
Transpiration (ET)
Precipitation (P)
Runoff (Q)
11/14/2013
13

14. Are Forests Good for Water Resources?
14

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

16. The Forest-Water Debate: Supply vs. Demand-Side
Demand-side:
Increasing forest cover reduces runoff
Ecosystems subtract from the water budget
Strong observational basis (<2 km2)
Supply-side:
Increasing forest cover positively impacts precipitation
and runoff
Sometimes labeled the “romantic” or even
mythological view
Limited empirical evidence
Regional Modeling is the best evidence

17. Recycled ET
Ocean
Annual
40%
60%
Summer
60%
40%
Source data generously
provided by Michael Bosilovich, NASA
Baltics
SON
Balatic
JJA
DJF
Water Recycling
Modelled at
Regional Scales
MAM
160
140
120
100
80
60
40
20
0

18. Average Estimated Precipitation in Major River Basins
by Source (1948-1997)
120
100
80
60
40
20
0
200
150
150
100
100
50
50
0
DJF MAM
JJA
200
0
DJF MAM
JJA
SON
150
300
250
200
150
100
50
0
100
50
0
350
300
250
200
150
100
50
0
DJF MAM
JJA
SON
DJF MAM
JJA
SON
HUB
Mississippi
500
400
300
200
100
0
300
100
200
50
100
0
GTR
400
150
SON
SON
Tibet
200
DJF MAM
JJA
SON
Baltics
Siberian
MacKenzie
DJF MAM
JJA
DJF MAM
JJA
SON
0
DJF MAM
JJA
CAT
SON
Source data generously
provided by Michael
Bosilovich, NASA.
Highlighting indicates
Terr_shr > Oc_shr.
DJF MAM
JJA
Amazon
SON
Note: local share included in
terrestrial share.

19. Much Precipitation is Recycled Evaporation:
East African over 50% of rain from trees
Should Egypt be more concerned about deforestation
of the Congo that dams on thd Blue Nile?

20. 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

21. 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

22. 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

23. Remember
Recycling!
M.Falkenmark March 09

24. Are forests good for water and food
security in Ethiopia?
Swedish Ministry of Foreign Affairs as
Project on global food security
24

25. Two MSc Students to the
Koga Irrigation Dam

26. Two PhD Students from
Centre for Natural Disaster Science
New Interdisciplinary research initiative
by the Swedish Government
Climate Adaptation = Disaster Mitigation