Geotechnical News • March 2019
35
Improving Mine Site Water Balances using the
Eddy Covariance Method
Sebastián Fernández
A word on water balances
In simple words, a water balance is
equivalent to a mass balance over a
period of time applied to a certain sys-
tem. It can be simplified to the point
that the change in storage over a cer-
tain period of time is equivalent to the
inflows minus the outflows. In mining
operations, these systems could repre-
sent the comminution plant, the Tail-
ings Storage Facility (TSF), or perhaps
the Waste Rock Dump (WRD). While
the water balance in the comminution
plant could be obtained with the aid of
flowmeters and densimeters installed
in the different pipelines / flumes
that enter and exit the system, this
approach cannot be applied to spaces
exposed to the nature where TSF and
WRD are located, since variables as
precipitation, runoff, evaporation and
infiltration are also involved.
The water balance for a TSF and
WRD are based on the general equa-
tion used for a vegetated soil surface:
P – Int – Roff = ET + Perc + ΔS
Where:
P= Precipitation; Int= Interception by
vegetation; Roff= Runoff; ET= Evapo-
transpiration; Perc= Percolation; ΔS=
Change in storage
For bare soils, the interception is
zero, and the term ET is replaced by
evaporation. For an operational TSF,
the terms that account for incoming
water in tailings, recovered water
and entrainment water losses must be
considered in addition to the variables
expressed in the equation above.
Out of the presented variables in equa-
tion 1, and considering the large areas
involved in TSFs and WRDs, only
precipitation can be measured accu-
rately through the use of rain gauges.
All the other variables are estimated,
evaporation being the most difficult to
estimate (Cui and Zornberg, 2005).
Importance of estimating evapo-
ration accurately
For a TSF located in an arid region,
the evaporation rates are related to the
water losses that have to be replaced
from an alternative source, such as
fresh or desalinated water, in order to
continue processing minerals at the
same rate. An accurate evaporation
rate estimation and forecast could
lead to improvement of the tailings
discharge efficiency via increasing the
number of discharge points or even
by increasing the tailing discharge
concentration if thickeners could be
added in the process. In severe cases
of water scarcity, it could even lead to
the construction of desalination plants.
In the case of tailings located in areas
were the precipitation is higher than
the evaporation, improving the drying
cycles of the discharged tailings lifts
may improve the overall efficiency of
dewatering.
In the case of waste rock dumps,
evaporation plays a critical role for
the closure design (Carey, 2005). The
use of an engineered cover that limits
the water percolation and the oxygen
diffusion to minimize the transport
and reaction of the contained material
is based on a proper account of the
evaporation rate.
Why use the Eddy Covariance
Method?
The most widely used method to esti-
mate the actual evaporation is based
on pan evaporimeters. Daily readings
are made to estimate the raw evapora-
tion, and these values are adjusted by a
coefficient that should take into consid-
eration among others the water content
in the soil, the edge effect, and the heat
capacity and albedo coefficient differ-
ence between the pan and soil/pond in
study. As a consequence, the obtained
evaporation rates are a mere approxi-
mation, hence the closure of equation 1
is rarely satisfied unless adjustments in
the pan coefficients are made.
The Eddy Covariance method (ECV)
offers a unique advantage difficult to
match: it can measure actual evapora-
tion rates in a direct manner.
How does ECV work?
The Eddy Covariance method is a
micrometeorological technique that
measures water vapor fluxes fast
enough to account for the air turbu-
lences that occur in the surface.
The air flow has a net horizontal
component and is composed by many
rotating eddies in 3D. Each eddy
transports compounds at different
concentrations and rotates at different
velocities. The eddies closer to the
ground tend to rotate fast, while higher
eddies tend to rotate slow (Burba,
2013).
The Eddy Covariance method can be
understood as the covariance between
a concentration of interest and wind
speed. If we know the amount of
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