The Influence of Climate Change and Climatic Variability on the Hydrologie
Regime and Water Resources (Proceedings of the Vancouver Symposium,
August 1987). IAHSPubl. no. 168 1987.
The impact of increasing atmospheric carbon
dioxide concentrations upon reservoir water
quality
B. Henderson-Sellers
Department of Mathematics and Computer Science
University of Salford, Salford M5 4WT, U.K.
ABSTRACT
The potential changes in a water body resulting
from increasing atmospheric carbon dioxide can be related
to enhanced radiative and non-radiative fluxes (especially
evaporation) at the air-water interface. The initial
perturbation due to enhanced CO2 levels can be parameterised as an increase in the downwelling longwave radiative
flux expressed as a function of time and CO2 concentration
(the so-called "transient CO2 experiment"). The only
experiments presented to date relating to the impact C0 2
levels have on water resources have concentrated on the
oceanic environment using simple box-diffusion modelsmodels which cannot take into account seasonal and daily
changes in water temperature and the consequent non-linear
effects of heat storage that occur. To investigate these
non-linear effects, an eddy diffusion thermal stratification model, which includes specification of all the
météorologie variables on timescales of 1-24 hours, has
been used to investigate further the impact of tising*
atmospheric carbon-dioxide concentrations on the aquatic
environment. For a simulation period of 50 years, the net
decrease in reservoir levels is found to be 2.05m. If,
simultaneously, stream inflow rates increase, this is
likely
to result in enhanced nutrient loading
and
accelerated eutrophication.
Impact d'un accroisement des concentrations de
dioxide de carbone sur la qualité de l'eau dans
les réservoirs.
RESUME
On peut relier les changements potentiels d'une
masse d'eau résultant d'un accroissement de gaz carbonique
dans l'atmosphère a l'augmentation des flux radiatifs et
non-radiatifs (spécialement 1'evaporation) à la surface de
séparation eau-atmosphère. Il est possible de paramétrer
la pertubation initiale due à l'augmentation de gaz
carbonique par un accroissement du flux radiatif dans les
ondes longues* Ceci est fonction du temps et de la
concentration
en gaz carbonique ("expérience du CO2
transitoire"). Les seules expériences, qui ont été faites
jusqu à présent sur l'impact des concentrations de CO2
sur les ressources en eau, ont été concentrées sur
1'environment
océanique.
Dans ces expériences, on a
571
572 B. Henderson-Sellers
utilisé des modèles de diffusion simple qui ne peuvent
tenir compte des changements saisoniers et journaliers de
la température de"l'eau et par consequent des effets nonlinéaires causés par ce reservoir de chaleur. Pour étudier
ces effets non-linéaires, un modèle de diffusion par
tourbillons pour la stratification thermique a été utilisé.
Ce modèle inclue toutes les variables météorologiques sur
une échelle de temps de 1 a 24 heures. Ce modèle permet
d'étudié l'impart dun accroissement des concentrations en
CO2 sur 1'environment aquatique. Pour une simulation de 50
années, on trouve que la diminution nette du niveau de
reservoir est de 2.05 metres. Si, en même temps, il y a
une augmentation du débit des eaux affluentes, il est
possible d'observer un accroissement d'accumulation des
nutriments ainsi qu'une accélération de 1 "eutrophication"
du lac.
Notation
A
Cm
cp
Kj^
q
t
T
z
AF
AQ
AT
X
X0
p
cross-sectional area of lake.
heat capacity of mixed layer (ML) in boxdiffusion model
specific heat at constant pressure
eddy diffusion coefficient for heat
penetrative short wave energy flux
time
temperature
depth
diffusive energy flux into deep ocean
perturbed flux due to increasing atmospheric
carbon dioxide
water surface temperature increase due to the
impact of increasing atmospheric carbon dioxide
feedback parameter (W nT^R-1)
value of X for "no-feedback" case
water density
Introduction
The climatic and hydroclimatic effects of increasing atmospheric
carbon dioxide are of great concern to mankind. Since the details of
the feedbacks operating within the Earth-atmosphere system are not
well understood, it is difficult to implement a numerical model of
the total system and research has been centred on specific areas
considered likely to be of greatest influence.
In this paper a
quantitative analysis is presented of the potential impact of
increasing atmospheric carbon dioxide on water levels and water
quality in inland lakes and reservoirs, use of which is frequently
made for public water supply (cf. discussion of Coûtant, 1981).
Effects of C0 2 on reservoir water quality
573
Box-diffussion models
To date, the impact on water bodies of increasing atmospheric carbon
dioxide has been largely confined to studies of the global ocean
using box models. For example, two recent studies (Wigley and
Schlesinger, 1985; Hansen et al. 1985) both utilise the simple boxdiffusion model of Oeschger et al. (1975). This model is represented,
essentially, by the equation
C m dAT/dt = AQ - XAT - AF
(D
where the perturbed surface energy balance difference term,AQ , is
related to the ambient carbon dioxide concentration and its temporal
evolution. For example, Wigley and Schlesinger (1985) suppose that
over the period 1850-1980 this term is given as
AQ = 34.6534xl0-4t exp(8.686xl0~3t)
(2)
In any perturbation experiment, this box-diffusion model approach
contains an implicit timescale of one year. It assumes a fixed mixed
layer (ML) depth throughout each year, identical for all latitudes.
However in water bodies there exists a strong seasonal (and indeed
diel) cycle. The mixed layer (ML) depth deepens a little during the
summer but is totally absent during the winter season. At this time
strong convection can occur in the water body, thus mixing heat
downwards. These non-linearities are also evident in the seasonal
hysteresis cycle (Gill and Turner, 1976), which is observed but is
not simulated by box-diffusion models.
In addition to retaining a fixed ML depth, box models also possess
a "fixed lid"; such that there is no change in water level, nor any
possibility of calculating changes in evaporation rates. That such
changes may occur as a result of increasing atmospheric carbon
dioxide, is self evident, yet has not been simulated quantitatively.
Some indications of the potential importance on global, regional and
local water resources of changes in evaporation rates is given by
Wigley and Jones(1985). Although they make no attempt to quantify
such effects directly, in their analysis of the sensitivity of runoff
to changes in precipitation and evaporation, they highlight the
potential changes for a number of perturbation scenarios. Conversely
it can be deduced from this study that a CO2 perturbation is likely
to affect runoff and hence inflows to lakes and reservoirs. There are
thus two impacts on reservoir levels: i) enhanced evaporation and ii)
probable increased runoff (and hence inflow).
In this paper a study is undertaken of the changes in lake level
and evaporation rate due directly to increased downwelling infrared
radiation using a high temporal and spatial resolution eddy diffusion
model.
Thermal stratification model
The lake/reservoir thermal stratification model U.S.E.D. (HendersonSellers, 1985) includes a specification of météorologie variables on
timescales of 1-24 hours and calculates the thermal structure on a
574 B. Henderson-Sellers
daily timescale. The heat transfer equation
A(z) aT/at = f- A(z) {a+ KH(z,t)} ^
H
8z
8z
+
a(Ao)/3z
pep
(3)
is solved together with a full evaluation of the surface energy
budget (Henderson-Sellers, 1986). Indeed the inclusion of the surface
energy budget is vital to the successful incorporation of both
diffusive and convective mixing necessary for successful longterm
simulations. The model U.S.E.D. utilised here is found to be stable,
with cyclic climatic forcing, over periods of centuries and hence is
considered suitable for climatic simulations, as described below.
Increased atmospheric C0 2
Since the model U.S.E.D. contains a full météorologie description of
surface energy exchanges, it is relatively easy to include a AQ-type
perturbation. Equation 2 was proposed as being representative of the
impact of atmospheric carbon dioxide over the last 130 years.
Extrapolating this for a simulation period of 50 years into the
future gives :
AQ = 34.6534xl0~A(t+130) exp[8.68xl0~3(t+130)] - 1.3934
(4)
where t runs from 0 to 50 in the experiment described below. This
formulation results in perturbation flux which begins as zero and
rises to 2.98 W m-2after a 50 year period. In the U.S.E.D. simulations, the air is allowed to change by such an amount that the perturbed
downwelling (atmospheric) infrared radiation is equal to the AQ value
given by Equation (4). Feedbacks then occur naturally since this
perturbed air temperature is itself directly responsible for a change
in the vapour pressure gradient across the air-water interface. Thus
evaporation (and consequently sensible) heat fluxes change, in
addition to enhanced longwave radiation resulting from increased
water surface temperatures. It is found that this parametrisation
produces a response which is approximately two to three times greater
than the "no-feedback" simulation - in good agreement with the value
of a feedback ratio of 2.4 proposed by Hansen et al. (1985).
Lake level simulation
The lake simulation pertains to a hypothetical lake situated at 54°N,
with a maximum depth of 40 m and realistic bathymetry. Values of lake
level changes from the control are stored and then compared to values
calculated in the perturbation simulation. At the end of the 50 year
simulation, the lake level depression is 2.05 m. In terms of water
quantity, even such an apparently small decrease in lake and reservoir
level can be a relatively large (and therefore costly) loss to the
reservoir manager in terms of volumetric loss. Over the simulation
period of 50 years, a level decrease of 2.05m can be evaluated in
terms
of volumetric loss for different sized water bodies. For
Effects of CO2 on reservoir water quality
575
example, for a water surface area of 20 km^jthe total quantity Ipst,
over the 50 year period, is 4.1xl0'm3 (i.e. an average of 0.8x10 m
yr - l). For a typical depth of 40 m, then this is a volumetric percentage loss over 50 years of 5.1%. Many water supply reservoirs are
smaller. For a 10 m deep lake of say 2 sq.km in area, the percentage
loss is 21%.
However these calculations concern only evaporative losses and do
not consider the probability that another impact of increased
atmospheric carbon dioxide is a change in the hydrologie cycle.
Wigley and Jones (1985) evaluated possible-changes in runoff for an
atmosphere in which the CO2 content has been doubled. Such changes
are directly reflected in inflow rates. For a typical residence time
of 50 years, the inflow to a lake is approximately 2 % of its total
volume. In this case, for an x% change in inflow rates, the lake
volume will change by 0.02x% per year. Over this 50 year period, it
might be reasonable to anticipate, from Wigley and Jones (1985)
figures for doubled CO2, that inflow rates could increase by an
average of approximately 10%. In that case, inflow changes might
increase lake volumes by 10% - potentially more important than the
evaporative loss. However outflow rates could also increase and an
overall rise may well be limited by the lake bathymetry and the
surrounding topography. Furthermore increased inflow rates are likely
to alter the annual thermal stratification cycle. This could lead to
a further change in evaporation resulting from changed surface
temperatures due to higher inflow rates.
Secondly, changes in nutrient concentrations in the lake will be related to changes in inflow rates and lake volume. If runoff is increased,
the leached nutrients (phosphorus and nitrogen especially) are likely
to increase roughly at a parallel rate. This would cause ambient lake
concentrations to increase, further compounded if water levels were
to fall. (For example, Wigley and Jones (1985) allow for the fact
that runoff may decrease due to increased atmospheric CO2 viz x<0 in
the above calculation). For example, for a -25-50% increase in
nutrient inflow the trophic state could change dramatically. The
increase in nutrient level between oligotrophic and eutrophic states is
typically an increase of 100% in ambient phosphorus concentrations.
Calculations must, at this stage, remain as an order of magnitude
analysis, since without better streamflow forecasts, which are likely
to be site specific (Wigley and Jones, 1985), nutrient loading is
difficult to assess with any accuracy. Suffice it to observe that the
potential impact of increasing atmospheric carbon dioxide on the
trophic status of freshwater bodies is unlikely to be negligible.
Conclusions
Initial quantitative assessment of evaporative loss from freshwater
lakes and reservoirs, made using a highly resolved thermal stratification model, suggests a significant decrease in storage, although
this deficit could be ameliorated by increased streamflow (but may
possibly be exacerbated in areas where streamflow decreases) as a
result of the impact of increasing atmospheric C0 2 . Furthermore, if
inflow rates increase, so will the nutrient loading and this is
highly likely to lead to accelerated eutrophication. Further studies,
576 B. Henderson-Sellers
using such well-resolved models,are needed, as is further information
on the likely perturbations to the hydrologie cycle, especially (in
this context) runoff.
References
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changes: freshwater concerns, Env. Conservation, 8, 285-297.
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