Interaction of Climate Change and Acid Deposition - Climate Change Impacts on Freshwater Ecosystems - page 164

1983-88
1989-93
1994-98
1999-2005

40
20

Temporal change in modelled ANC
at different percentile flow values

0

-20

-40
-60
-80

1.359

1

10 25 50 75 90 98

0.502

Modelling
ANC from
flow

0.269

0.1

0.130
0.067

Linear regressions of ANC
against log (flow) for the four
subperiods

0.043

Flow duration curve based on
15 min frequency continuous
monitoring (1989-2002)

0.01

0.1 0.5 1 2 5 10 20 30405060

% time flow not exceeded

70 80 90 95 98 99 99.5 99.9

Figure 7.10 Afon Hafren, Wales. Relationships between ANC and discharge (log flow)
for four different periods. As the streamwater has recovered (increasing ANC over time),
the ANC at peak discharges has also increased. (From M. Hutchins, unpublished data.)

between ANC and discharge, and this relationship has changed over time due to
chemical recovery (Fig. 7.10). Since the mid-1980s, there has been a relative shift
from SO 4 -dominated events to those dominated by sea salts. Principal component
analysis of the major ionic species in Hafren streamwater suggests that about
70% of the discharge-related chemical variation is explained by sea salts. Episodic
deposition of sea salts causes acid pulses due to cation exchange in the soil of Na
with H + and Al n + ; the resulting streamwater has a molar Na: Cl ratio appreciably
lower than that in standard sea water.
Wright (2008) used the same procedure to examine episodes in the Birkenes
catchment, southernmost Norway. Here, the 30-year record of weekly (or more
frequent) samples of streamwater shows gradual improvement in water quality
from mean annual ANC less than −70 m eq l −1 in the mid-1980s to greater than
−30 m eq l −1 in 2000-4 due to decline in S and N deposition. The severity of acid
episodes has also declined. Whereas, prior to 1993, ANC during episodes decreased
to less than −100 m eq l −1 , since 2000 the ANC has not dropped to lower than
-75 m eq l −1 (Fig. 7.11). At Birkenes, about a third of the acid episodes were driven
by sea-salt inputs. As the frequency and severity of storms are projected to increase
with future climate change, the expectation is that sea salt-driven episodes will
become more frequent. But since the general level of acidification at Birkenes is
improving, the severity of the acid pulses may decline in the future.