## Carbon Stock DifferenceMethod

The "Stock-Difference" method includes all processes that bring about changes in a given pool. The carbon stocks are estimated for each pool at two points in time, namely t1 and t2. The duration between the two points could be 1 year or several years, say 5, 7 or 10 years. As discussed in Chapter 4, the frequency of measurement of most of the carbon pools is once in several years - 5 years, for example, for soil carbon. Thus, the estimated stocks at t2 need to be deducted from the estimated stock at t1 and the difference divided by the number of years between the two periods (t2 - t) The "Stock-Difference" must be estimated separately for each carbon pool.

Carbon stock change in a given pool as an annual average difference between estimates at two points in time (Stock-Difference method) is given by:

where AC is the annual carbon stock change in the pool, C& the carbon stock in the pool at time t1, and the carbon stock in the same pool at time t2.

Changes in carbon stock using this method are estimated for a given land-use category or project area as follows:

• Estimate the stock of a pool at time t1 and repeat the measurement to estimate the stock at time t2.

• Estimate the change in the stock of selected carbon pool by deducting the stock at time t1 from that at t2.

• To obtain the annual change in stock, divide the difference in stocks by the duration

• If the estimates are made for sample plots, extrapolate to per hectare basis.

• To obtain the total for the project area, extrapolate the per hectare estimate to the total project or land-use category area.

The periodicity or frequency of measurement varies from pool to pool. Therefore, add the annual changes in each pool to obtain the total change in carbon stocks for the total project area over the selected period. It is important to ensure that the area under project activities between the two periods is identical; if it has changed, it is important to account for the change by calculating the changed area and multiplying it by the per hectare values.