Dr. Richard Taylor - Groundwater-climate interactions: current challenges & new insight.
Groundwater-climate interactions:current challenges & new insightRichard TaylorUniversity College London (UK)IAH Commission of Groundwater & Climate ChangeUNESCO-IHP GRAPHIC programme headwater of River Limpopo, NE Botswana
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
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
groundwater is the primary source of drinkingwater for nearly half of the world’s population (Coughanowr, 1994 Kundzewicz & Döll, 2009) (C h 1994; K d i Döll urban spring Kampala (Uganda)
• 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)
for some…groundwater is their only source of water sand river, headwater of the Great Ruaha River, Tanzania
“there has been very little research on the thereimpact 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
global hydrological changehydrological system is central to the Earth’s climate system source: NASA
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)
groundwater & LSMsHow 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 dischargesand what level of model complexity is required?
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
basin-scale groundwater discharges• ET in dambos across eastern and southern Africa is sustained over dry seasons y by groundwaterAroca Dambo, northern Uganda
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
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
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
GRACE vs. observationsggood correlations observed between GRACE and observationsat regional scale (e.g. Bengal Basin) Shamsudduha et al. (in prep.)
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
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
• 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)
groundwater abstraction – ignored in LSMs Shiklomanov, 2000. Water International.
groundwater abstraction & hydrology• hydrological consequences of abstraction not considered in LSMs (reduced baseflow, enhanced recharge) Shamsudduha et al. (in press) Hydrogeol. J.
recharge induced by abstraction• challenges notion of “safe yield” based on static recharge
Groundwater & climate in East Africa• focus on observational datasets: combined groundwater-rainfall monitoring stations in: Uganda (humid) Tanzania (semi-arid)
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.
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?
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)
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
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
55-year groundwater-level record in preparation • episodic recharge (1960, 1962-3, 1968, 1989-90, 1997-98, 2006-7) linked to extreme (ENSO) events
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 BotswanaMutarara District, Mozambique, 22 February 2007 • role of basin storage - both natural and constructed?
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 decreasedNote: arrows signify lands rechargecause/effect linkages
groundwater dataThe major constraint toour understanding ofthe relationshipsbetween groundwaterand 1) climate and )2) developmentand to the development d t th d l tof LSMs effectivelyrepresentinggroundwater is…DATA.
IGRAC• no coordinated global effort to collate groundwater data!
some concluding thoughts1. groundwater plays an important role not only in the provision of freshwater but also the global climate system2.2 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
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 Africa4. inter-disciplinary collaborations in hydrological sciences required to develop more effective representation of groundwater processes in LSMs f S