Salt Dynamics in prairie wetlands under changing climate

Transcription

1 Salt Dynamics in prairie wetlands under changing climate Uri Nachshon Andrew Ireson Garth van der Kamp Howard Wheater Ryan Davies GIWS, University of Saskatchewan GIWS, University of Saskatchewan Environment Canada University of Saskatchewan Imperial College London

2 Outline: Problem: Question: Hypotheses: Prairie agriculture is vulnerable to salinization What are the climatic drivers of surface salinization in the prairies? Wet conditions associated with extreme snow do not lead to signficant salinization Wet conditions associated with summer rainfall lead to severe salinization I will present various observed data to support the

3 2011: Prepare for white stuff this spring: saline soil Wetter-than-normal soil over much of the Prairies may worsen soil salinity problems this year. Soil salinization occurs when the water table is close enough to the soil s surface to allow a wicking action to carry the salty ground water to the surface dirt.

4 2013: Wet weather worsens salinity issues recent years of wetter weather have given rise to salinity problems in places where they hadn t previously been seen. As well, farmers may have even more to deal with because of snow conditions across much of the Prairies that indicate high runoff and a potentially high water table.

5 St Denis, Saskatchewan

6 Past prairie climate variability Pond water levels at St Denis, SK

7 Past prairie climate variability Pond water levels at St Denis, SK saw an extremely bad drought

8 Past prairie climate variability Pond water levels at St Denis, SK 2007 saw the highest water levels up to that point

9 Past prairie climate variability Pond water levels at St Denis, SK This level was exceeded in 2011

10 Past prairie climate variability Pond water levels at St Denis, SK And the 2011 level was exceeded again in 2012

11 Past prairie climate variability

12 Wet conditions Pond water levels at St Denis, SK Average year Extreme snow Extreme summer rain Extreme antecedent wetness

13 Change over summer Salinity (EC and salt mass) Pond water volume [m 3 ] A normal year Normal Wet (snow) Wet (snow) Wet (rain) Wet (antecedent) Wet (rain) WATER: Spring melt fills pond Pond empties due to ET and infiltration SALTS: Salt concentration (EC) rises due to ET Salt mass (M salt ) reduces due to infiltration CHANGES OVER SUMMER: Reduction in pond volume and salt mass Increase in salt concentration

14 Change over summer Salinity (EC and salt mass) Pond water volume [m 3 ] High snowpack years Normal Wet (snow) Wet (snow) Wet (rain) Wet (antecedent) Wet (rain) WATER: Same pattern as normal years, but with a higher snowmelt peak SALTS: Same pattern as normal years Only a negligible increase in salt mass Basically nothing happens! CHANGES OVER SUMMER: Directions are same as a normal years Magnitude of water loss is larger

15 Change over summer Salinity (EC and salt mass) Pond water volume [m 3 ] High summer rainfall years Normal Wet (snow) Wet (snow) WATER: Levels continue to rise throughout the summer months, post melt Wet (rain) Wet (rain) SALTS: Salt concentration and salt mass increase orders of magnitude above anything previously observed CHANGES OVER SUMMER: Everything increases!

16 Change over summer Salinity (EC and salt mass) Pond water volume [m 3 ] High antecedent wetness Normal Wet (snow) Wet (snow) WATER: Same pattern as normal years snowmelt dominated hydrograph Wet (rain) Wet (antecedent) Wet (rain) SALTS: Salt mass and concentration rise CHANGES OVER SUMMER: Water level drops Salt mass and concentration increases Evidence of groundwater exfiltration

17 Change over summer Salinity (EC and salt mass) Pond water volume [m 3 ] Observations of pond salinity Normal Wet (snow) Wet (snow) Wet (rain) Wet (antecedent) Wet (rain)

18 Different pond responses

19 The prairie salt cycle Basics: Salts of geological origin dissolve into the water and move with flow, accumulating only in evaporation front locations. Intimately linked to, and diagnostic of, the hydrology. Most prairie basins are closed, comprising fill-and-spill sequences of ponds, ending in a terminal pond, so salts are internally recycled minimal export of salts from the basin most years.

20 The prairie water cycle Weathered oxidized zone (high K due to fractures) Unoxidized zone (low K)

21 The prairie water cycle Snowpack builds over winter

22 The prairie water cycle Snowmelt fills depressions

23 The prairie water cycle Post-thaw depression focused recharge Surficial isolated aquifer

24 The prairie water cycle PE > Rainfall (normally) Minimal net infiltration Surficial isolated aquifer

25 Prairie salt distributions

26 Prairie salt distributions Shallow salt belt High solubility salts GENERAL PATTERN Low solubility salts Saline ring

27 Prairie salt distributions Magnitude of salinity depends on direction of exchanges with deep GW Low salinity recharge pond

28 Prairie salt distributions Magnitude of salinity depends on direction of exchanges with deep GW High salinity discharge pond

29 The prairie salt cycle Winter through to melt: Summer: Snowmelt runoff Diffusion and bioturbation (?) Evapotranspiration/ capillary flow

30 Prairie wet extremes Extreme snow pack Extreme summer rainfall Snowmelt runoff Subsurface flows reversed Diffusion and bioturbation (?) Evapotranspiration/ capillary flow

31 Pond-groundwater exchanges

32 20 m A

33 Transect response to rainfall

34 Conclusions The past decade has seen extreme variations in climate in the prairies, including a drought, extreme high snow, and extreme high rainfall Snowmelt and rain tend to activate different hydrological processes Extreme high snowmelt over frozen soils has almost no impact on salts Extreme high summer rainfall flushes salts from the subsurface back into the ponds, from where it