Modelling Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 208

with the C content estimated from typical
or averaged soil C values for the major soil
types identified in the cell (Batjes, 2009).
The uncertainty due to the structural
and input errors can be quantified by evalu-
ating the model at field scale, but using only
input drivers that are available at the larger
scale. In order to represent the uncertainty,
a range of sites across the whole area to be
simulated should be included in this field-
scale evaluation. Good model performance
is indicated statistically by simulations and
measurements that are both coincident (in-
dicating a close fit) and associated (indicat-
ing the trends in measurements are repli-
cated) (Smith and Smith, 2007). The degree
of coincidence can be used to represent the
size of the uncertainty in the simulations.
Where measurements are replicated, the
coincidence between simulated and meas-
ured values can be expressed as the 'lack of
fit' statistic, and the significance of the coin-
cidence determined using an F-test (Whit-
more, 1991). If a data set is not replicated,
the degree of coincidence can instead be de-
termined by calculating the total error as
the root mean squared error and the bias in
the error as the relative error (Loague and
Green, 1991; Smith et al ., 1996b, 1997). The
structural and input errors should be calcu-
lated separately to allow the source of errors
to be understood and reduced, but the com-
bined errors are then used to determine the
accuracy of the model simulations at large
scale.

change situations. The IPCC provide their
computational model for this (IPCC, 2006).
In addition, tools such as the Agriculture
and Land Use National GHG Inventory
Software ( http://www.nrel.colostate.edu/
projects/ALUsoftware) have been developed
to make application of the method easier;
however, the IPCC also encourage countries
to use more sophisticated models where
possible (IPCC, 2006).
With increasing interest in climate change
mitigation potential from the agriculture,
forest and land-use sector, agencies that
fund land management activities are look-
ing for ways to estimate the climate change
impact of their activities. The US govern-
ment has funded development of an online
tool, COMET-VR, which allows farmers to
estimate C sequestration and net GHG emis-
sions from soils for farms in the USA. This
allows farmers to take part in the US govern-
ment's voluntary reporting of GHGs scheme.
COMET-VR links a large set of databases con-
taining information on soils, climate and
management practices to run the CENTURY
ecosystem simulation model dynamically
to estimate soil C stocks and changes (COMET-
VR, 2012). The user supplies basic land
management information to the online sys-
tem essentially to run a highly complex eco-
system model.
There are also examples of SOC models
being used in the voluntary C market. The
Verified Carbon Standard (VCS) recently ap-
proved a protocol that uses the RothC model
in the estimation of SOC stock changes
under sustainable land management pro-
jects - SALM (VCS, 2011). It is expected
that more protocols involving models will
be approved by the voluntary market as
user-friendly interfaces are developed, mak-
ing models more accessible. To date, several
international funding agencies have devel-
oped their own tools for the reporting of C
changes (including SOC). In the main, these
are based on the IPCC method, examples
being the Carbon Benefits Project's Simple
and Detailed Assessments (Milne et al .,
2010), FAO's EX-Act and the Cool Farm
Tool. New reporting tools are likely to
emerge in the near future, with continuing
advances in graphical user interfaces that

SOC Models in Practical
Application Today

Soil organic C models are in use today in
many areas, including research of SOC dy-
namics and agricultural studies to estimate
the long-term impacts of fertilizer and ma-
nure additions. They are also used increas-
ingly in the reporting of GHG emissions. For
national GHG inventories compiled for the
United Nations Framework Convention on
Climate Change (UNFCCC), reporters are re-
quired to estimate emissions of GHGs from
soils under different land use and land-use