Dr. Richard Taylor - Groundwater-climate interactions: current challenges & new insight.
1. Groundwater-climate interactions:
current challenges & new insight
Richard Taylor
University College London (UK)
IAH Commission of Groundwater & Climate Change
UNESCO-IHP GRAPHIC programme
headwater of River Limpopo, NE Botswana
2. outline
• groundwater - a fundamental resource and key
component of the Earth’s hydrological system
p y g y
• representation of the groundwater system in Land
Surface Models (GCMs)
• satellite monitoring of groundwater (GRACE)
• groundwater, hydrological change & adaptation
3. groundwater – a global resource
- world’s largest accessible store of freshwater
- estimate of 23 400 000 k 3 (K
ti t f km (Korzun, 1974) i hi hl
is highly
uncertain
4. groundwater is the primary source of drinking
water for nearly half of the world’s population
(Coughanowr, 1994 Kundzewicz & Döll, 2009)
(C h 1994; K d i Döll
urban spring
Kampala (Uganda)
5. • 30% of the world’s irrigated land is supplied by
groundwater
d t (Foster & Chilton, 2003)
• primarily responsible for dramatic increases in
food p oduct o in Asia ( d a, Bangladesh, C a)
ood production s a (India, a g ades , China)
groundwater-fed irrigation of Boro rice (Bangladesh)
6. for some…
groundwater is their only source of water
sand river, headwater of the Great Ruaha River, Tanzania
7. “there has been very little research on the
there
impact of climate change on groundwater…”
p
p. 185, Chapter 3 (Freshwater), IPCC AR4 (Kundzewicz et al., 2007)
, p ( ), ( , )
Makgadikgadi Salt Pan – an evaporated lake in Botswana
9. global hydrological change & GCMs
• climate system represented by General Circulation Models
(GCMs); terrestrial hydrology is simulated by an embedded
Land-Surface Model (LSM)
• in LSMs, groundwater is either excluded
OR represented very crudely (e.g. Niu et al., 2007)
- no lateral flow
- simplistic K & water table estimations
• LSMs essentially ignore groundwater
storage and flows operating at larger
spatial and longer temporal scales
Schaller & Fan (2009)
10. groundwater & LSMs
How important are groundwater-controlled processes:
• capillary flow from the water table sustains ET
• shallow groundwater exchanges with surface water
• regional groundwater discharges to surface waters
• submarine discharges
and what level of model complexity is required?
11. groundwater & LSMs
• groundwater influences soil
moisture and, hence, the
, ,
magnitude and spatio-
temporal variability of ET
(land-atmosphere feedbacks)
Kollet and Maxwell (2008)
• improved river flow simulations by considering
shallow groundwater exchanges
12. basin-scale groundwater discharges
• ET in dambos across eastern
and southern Africa is
sustained over dry seasons
y
by groundwater
Aroca Dambo, northern Uganda
13. LSM calibration & data availability?
• absence of global groundwater dataset is a major
impediment to improve representation of
groundwater processes is LSMs
• as th resolution of LSM improves, the omission
the l ti f LSMs i th i i
of groundwater processes is expected to become
more problematic
• LSM are, h
LSMs however, already overparameterised so
l d t i d
more sophisticated representations of
groundwater require careful consideration
14. GRACE
• gravity variations
represent fluid mass
changes
• fluid mass changes
represent total
(terrestrial) water
storage changes after
removing atmospheric
and oceanic mass
d i
changes
15. GRACE: total water storage changes
∆TWS = ∆GW + ∆IS + ∆SM + ∆SW
• attribution of GRACE signal to specific
components of the hydrological system (above)
requires independent measures or simulation
16. GRACE vs. observations
g
good correlations observed between GRACE and observations
at regional scale (e.g. Bengal Basin)
Shamsudduha et al. (in prep.)
17. GRACE
• low resolution
of GRACE data
f d t
(160 000 km2)
constrains utility
0.25° x 0.25° grid (reference)
of data for sub-
regional water
g
management
GRACE resolution
18. GRACE & storage co-efficients
• direct comparison between GRACE data (water
depth) and groundwater-level observations
requires a storage co-efficient
q g
• few reliable assessments of
groundwater storage and Gelhar (1986)
subject to scale dependency
observed in other
hydrogeological parameters
(dispersivity)
• unconfined aquifer conditions
typically assumed
19. • uncertainty in storage co-efficients is non-trivial
-0.75km3/year (spatially distributed Sy)
-1.36km3/year (assuming Sy = 0.10)
groundwater-fed irrigation of Boro rice (Bangladesh)
21. groundwater abstraction & hydrology
• hydrological consequences of abstraction not considered
in LSMs (reduced baseflow, enhanced recharge)
Shamsudduha et al. (in press) Hydrogeol. J.
22. recharge induced by abstraction
• challenges notion of “safe yield” based on static recharge
23. Groundwater & climate in East Africa
• focus on observational
datasets: combined
groundwater-rainfall
monitoring stations in:
Uganda (humid)
Tanzania (semi-arid)
24. Projected changes in the intensity of P
• fewer low and medium intensity precipitation events
fewer,
• more, very heavy precipitation events (i.e., “extreme events”)
Allen & Ingram, 2002. Nature 419, 224-232.
Trenberth et al., 2003. BAMS 84, 1205–1217.
Allan & Soden, 2008. Science 321, 1481-1484.
surface runoff – Kampala
Mileham et al., 2009. HSJ, Vol. 54(4), 727-738.
25. test this hypothesis in East Africa?
• historical (~1940s to ~1980s), sub-daily (tipping-bucket)
rainfall records are available (in hardcopy) for many
stations in East Africa
• transcribe and analyse historical dataset and compare with
more recent records from revamped, strategic stations?
26. Impact of changing P intensities on groundwater resources
p g g g
• over 200 protected springs in Kampala
• 60% of the low-income population with access to springs use
them for all or part of their domestic water needs
protected spring in Bwaise (Kampala)
27. Impact of heavy rainfall events on water quality
• high-frequency monitoring of spring discharges shows a rapid
high frequency
deterioration in bacteriological quality following heavy rainfall
Taylor et al., 2009. Groundwater & Climate in Africa IAHS Vol. 334
28. Impact on groundwater recharge?
• groundwater recharge
d t h
correlates better to the sum
of heavy rainfall events than
y
the sum of all rain events
Owor, Taylor et al., 2009. ERL Vol. 4, 035009.
• shift to more frequent, very
heavy precipitation events
favours recharge in tropics –
contrary to suggestions
y gg
reported in IPCC AR4 for
SW Africa and Brazil
30. 55-year groundwater-level record in preparation
• episodic recharge (1960, 1962-3, 1968, 1989-90,
1997-98, 2006-7) linked to extreme (ENSO) events
31. variability in African water resources
• most variable river discharge in the world
McMahon et al., 2007. J. Hydrol. 54, 727-738.
projected to increase - more frequent and intense floods &
droughts
headwater of River Limpopo, NE Botswana
Mutarara District, Mozambique, 22 February 2007
• role of basin storage - both natural and constructed?
32. role of land-cover change?
• non-intuitive basin responses
Population
growth (‘Sahelian Paradox’)
Descroix et al. (2009) J. Hydrol., Vol. 375, 90-102.
Expanded
cropping
i
area
Land clearance
Reduced
and deforestation
fallow
Soil
Nutrient erosion
mining
i i
Alterations
to water
balance
Extreme
Stagnant or
degradation
falling yields
of 95m ha
Increased use Increased or
of marginal decreased
Note: arrows signify lands recharge
cause/effect linkages
33. groundwater data
The major constraint to
our understanding of
the relationships
between groundwater
and 1) climate and
)
2) development
and to the development
d t th d l t
of LSMs effectively
representing
groundwater is…
DATA.
35. some concluding thoughts
1. groundwater plays an important role not only in
the provision of freshwater but also the global
climate system
2.
2 despite new advances (e g GRACE) there
(e.g. GRACE),
remains an urgent need for a global system of
archiving hydrogeological data - analogous to
g y g g g
WMO, GRDC, WGMS
36. 3. critical role of groundwater as a natural basin
store in adaptation to climate variability and
change i sub-Saharan Af i
h in b S h Africa
4. inter-disciplinary collaborations in hydrological
sciences required to develop more effective
representation of groundwater processes in LSMs
f S
