Establishing the influence of climate, water extraction and tectonics on the water level of the Prespa Lakes (N Greece)

Transcription

1 Establishing the influence of climate, water extraction and tectonics on the water level of the Prespa Lakes (N Greece) Tim van der Schriek & Christos Giannakopoulos National Observatory of Athens Institute for Environmental Research and Sustainable Development In collaboration with The presented work is part of the project CLIM HYDROLAKE (Improving future projections of climate change induced hydrological responses by looking into the past: the Lake Prespa / Aliakmonas River case study in Greece). This project is supported by the European Community under a Marie Curie Career Integration Grant (Framework Program 7, Grant 7)

2 Why is the water level of Lake Prespa falling? 84,00 8,00 80,00 848,00 84,00 844,00 Ιαν- Ιαν-0 Ιαν-0 Ιαν- Ιαν- Ιαν-8 Ιαν-8 Ιαν-7 Ιαν-7 Ιαν- Ιαν- Ιαν- 84,00 Ιαν- Stage height (m.a.s.l. FYROM) Monthly Level of Lake Megali Prespa Years Caused by water extraction, groundwater outflow changes and/or climate? Threat to: global biodiversity & regional water resources (Lakes Prespa and Ohrid; Drim River and Lake Skardar) Urgent need to understand Lake Behaviour future extraction, climate change impacts?

3 Methods & Data Aim: to explain lake level fluctuations with available data Documentary analyses, basic statistics, regression analyses, spreadsheet based analyses of extraction impact Most comprehensive analyses so far, covering the entire catchment (SPP data base) for 004 Rainfall (7 stations; at lake level) Water extraction data (compilation) Evaporation (open pan; extended by Penman formula) Lake level Megali Prespa (FYROM data) Fluvial Discharge (Brajcinska River only) Estimates for groundwater flow Earthquake measurements Snow records

4 Lake Water Balance & Geology Direct precipitation Fluvial discharge Evaporation Extraction Groundwater Karst drainage Main elements of the Lake Water Balance In: Direct Precipitation, Fluvial Discharge, Groundwater Out: Evaporation, Extraction and Karst Drainage Geology / Tectonics: Internally draining basin, surrounded by mountains (400m) All groundwater / fluvial discharge is locally generated Intrusive rocks (N,E,basement) aquicludes Limestone overlying in W (aquifer), downfaulted SW (lake) Subsiding Basin, alluvium (N,E) with small aquifers Underground outflow only in SW area of lake (Ohrid) Mikri Prespa is separated by an alluvial isthmus Earthquakes ( 4 Richter) frequent

5 Rainfall, snowfall and evaporation 0 Average monthly rainfall at Lake Level Mediterranean climate with continental influences There is a positive rainfall evaporation balance from Oct Apr (c. 80m near Lake Level) wet season These months also receive significant snowfall (at Vrondero; 000m) Monthly evaporation at Lake Level 0 Min 00 Avg Max 0 Rainfall / snowfall must be much higher in the surrounding mountains (up to 400m) Monthly rainfall evaporation balance at Lake Level Min 00 Avg Max Monthly snowfall (Vrondero) Min 0 Avg 40 Max

6 Hydrological characteristics 004: annual rainfall evaporation balance at lake level is mostly negative Annual Rainfall minus Evaporation Balance Difference (mm) Suggests significant fluvial input (and minor input from direct snow fall?) Apr May are the months of max. discharge fed by snow melt. Hydrological Years (Oct Sept 004) Max. monthly rainfall: Nov Dec Max. monthly snowfall: Dec Jan 0, Monthly Runoff Data [m³/s], Brajcinska River, FYR of Macedonia, 004 0, 0,0 4 Min Avg ,0 Max 0, 0, , Average monthly water level fluctuations (cm): Lake Megali Prespa ( 0)

7 Volumetric conversion Stage height (m.a.s.l. FYROM) Monthly Level of Lake Megali Prespa 84,00 8,00 80,00 848,00 84,00 844,00 Ιαν- Ιαν-0 Ιαν-0 Ιαν- Ιαν- Ιαν-8 Ιαν-8 Ιαν-7 Ιαν-7 Ιαν- Ιαν- Ιαν- Ιαν- 84,00 Years Water level fluctuations are a function of in /outflow factors AND bathymetry Lake level change (m) needs to be converted to volumetric change (m) to compare different years thus correcting for the difference in bathymetry at different stage heights. This conversion is based on the DEM of the basin; the DEM for the upper part of the lake is most reliable and thus annual volumetric differences have been calculated

8 Volumetric changes Annual volumetric change (0 m) of Lake Megali Prespa ( 0) 000,00 800,00 00,00 400,00 00,00 0,00 00,00 400, ,00 Lake volumetric change per hydrological year (October year to October year ) as is common for Mediterranean river/lake systems and ones dominated by snow melt Highly variable, perhaps increase in duration/intensity of large negative volumetric changes

9 Annual Extraction 000,00 800,00 00,00 400,00 Lake volumetric change without extraction (observed annual volumetric change + extracted volume) 00,00 0, ,00 400,00 Observed lake volumetric change 00,00 Extraction data (incl. groundwater abstraction) have been collected from various sources. A time series of abstraction was reconstructed for 004 (best estimate). Extraction prior to 7: 0 0 0m, after 7: 0 4 0m Extraction after 7: max ~0.00% of total lake volume

10 Extraction: Impact on Lake Level lake level 004 (Oct) Theoretical calculations (LakeAnnual Megali Prespa) support this analysis. 84 Stage Height (m.a.s.l.) Average8lake level (no extraction): ~80.m from to 87. This corresponds to a lake surface area of 77.8 km. Annual evaporation over this surface area: 0.8m 80 *77.8 km 848 =. 0 m. Subtracting 4 0 m (yearly lake water extraction) from m gives 7. 0m for the required lake surface evaporation to stabilise. 084 lake level. Dividing 7. 0m by 0.8m gives a lake surface area of 0. km at 844 a lake stage height of ~84.4m Average lake level (no extraction): ~84.m after. This corresponds to a lake surface 88 area of. km; annual evaporation over this surface area is 0.8m m. If we subtract 4 0m (yearly lake water extraction) *.Reconstructed km =.4 (no 0 extraction and extra evaporation; top) vs observed (bottom) hydro yearly stage height change 0 (Oct)m of Lake Prespa ( 004). from.4 wemegali obtain a value of m for evaporation to stabilise lake by 0.8m gives a surface area of 4. km at a lake level. Dividing lake 07.4level 0m Reconstructed change (no extraction scenario). Annual extraction was added to annual volumetric change (both in m ); the resulting value was converted to Lake level change in m. Subsequently the stagelake height of ~844.m. lake surface area difference was calculated between actual/computed lake level; annual evaporation over the extra surface area was calculated and subtracted from the computed lake level. Extraction has alevel significant cumulative lake level (not visible when looking at Note that lake falls are dependentimpact on the on bathymetry! annual volumetric changes) Balance is reached after multiple decades (lake surface evaporation decrease due to lake surface area shrinkage equivalent to extracted volume)

11 Groundwater & Tectonics 84,00 8,00 80,00 848,00 84,00 844,00 Ιαν- Ιαν-0 Ιαν-0 Ιαν- Ιαν- Ιαν-8 Ιαν-8 Ιαν-7 Ιαν-7 Ιαν- Ιαν- Ιαν- 84,00 Ιαν- Stage height (m.a.s.l. FYROM) Monthly Level of Lake Megali Prespa Years Groundwater flows poorly known: inflow is estimated at 0 0 0m per year. The most reliable outflow estimates are 8 0m per year. Recent modelling suggests steady outflow rates, except for : Oct 78 Sep 8 (8 years: 0 0m total reduction in outflow, or extra inflow), and Oct 8 Sep (7 years: 0 0m total reduction in outflow). Extra inflow following fall? The timing of significant lake level changes does not coincide with recorded earthquakes. There are no R>4 earthquakes; R 4 earthquakes occur regularly throughout the observation period (not concentrated during periods of significant lake level change).

12 Relationship Lake Level Climate Lake Volumetric Change vs R E ( 004) Annual lake level change (Oct Oct) is strongly related to the cumulative Oct Apr rainfall evaporation balance 00,0 y = 0,708x +,84 R² = 0,870 Oct Apr R E (0m) 000,0 800,0 00,0 Correlations improve when corrections for extraction and extra inflow are incorporated 400,0 00,0 0,0 00,0 400,0 00,0 0,0 00,0 400,0 00,0 800,0 000,0 Hydro yearly Lake Volumetric Change without Extraction (0m) Points representing lake level falls are close to the regression line (thus not supporting a tectonic cause) Lake Volumetric Change vs R E ( 004) 00,0 y = 0,708x + 4, R² = 0,0 Oct Apr R E (0m) 000,0 800,0 Regression analysis between annual lake volumetric change and Brajcinska River Discharge has an R of 0.74 (catchment: 0% of Prespa). Snow melt fed discharge exerts a strong control on lake level. 00,0 400,0 00,0 0,0 00,0 If more months are included correlations become poorer 400,0 00,0 0,0 00,0 400,0 00,0 800,0 Hydro yearly Volume Change without Extraction & with Inflow (0m) 000,0

13 Discussion The extreme lake level rise matches a wet event that is recognised throughout Greece; significant falls (74 78 & 87 ) correspond with major Mediterranean wide droughts. Levels of e.g. Lakes Skardar, Ohrid & Dorjan also fall. This regional synchronisation, suggests that local tectonics do not control the major Prespa lake level fluctuations [] the lack of correlation between earthquakes lake level change and [] the good correlation of lake volume change with precipitation support this conclusion. Even minor water extraction has a progressive and serious impact on lake level, with a lag time of multiple decades (dependent on the bathymetry). Extraction is responsible for 0% of the overall fall since 87. It is likely that water extraction will increase under future climate change scenarios, thus amplifying climate change impacts on lake level. The SE Balkans experience decreasing snowfall and an increase in extreme (drought) events; future climate change scenarios suggest this trend to continue. Lake level variations are therefore expected to increase while lake levels will follow a downward trajectory (that is strongly influenced by the bathymetry.). Falling lake levels lead to: i) the reduction of lake volume thus increasing pollutant concentrations (strongly accelerating the current eutrophication), and ii) reduced underground outflow to lake Ohrid. The entire Prespa Ohrid Drim catchment may be affected with significant consequences for water resources and biodiversity.

14 Conclusions This work proves for the first time that annual fluctuations in Prespa Lake Level are strongly related to the cumulative precipitation during the first 7 months of the hydrological year (Oct Apr). The long term falling lake level trend is amplified by minor water extraction (which is not visible from regression analyses of annual data) Abrupt, large falls in lake level may induce one off emptying of groundwater stores (through the adjustment of the groundwater table in the adjacent aquifers) There is no link between earthquake occurrence over the observation period ( 004) and major lake level fluctuations. The presented linear regression and extraction impact models will help steer adaptation and mitigation strategies by informing on lake response under different climate change and extraction scenarios There are two key issues to should be the focus of any strategies dealing with climate change impacts on lakes in the S Balkans: address the causes of pollution / eutrophication (to protect water quality and biodiversity) and decrease water extraction through better irrigation techniques (to ensure future water availability and biodiversity)

15 THANK YOU Any Questions?