Changes of biotope areas are covered as an annual lump sum. There is no accumulation of past (net-) losses. This approach is according to the calculation method of the NWI components 17 (agriculture) and 19 (CO2 Emissions). Regarding biodiversity, such an approach for an industrialized country like Germany, with massive declines in earlier decades, might appear unsatisfactory. However, not only for the sake of internal consistency, this way needs to be chosen: Firstly, the difficulty of selecting a reference point is avoided, and secondly, the focus is on the present development. For the calculation of costs and benefits through biotope area change the average present value per unit of area of the respective biotope is multiplied with the change in units of area. If there is no total loss through land sealing or if a biotope is created from a (in biodiversity terms) “useless” state this is considered by balancing the initial and final biotope. The valuation is only possible in all cases where for both initial and final state a estimation of the value exists.
Since there is only area data for the years 1990, 2000 and 2006, the remaining values for annual losses or gains in between those years are estimated by linear interpolation. For the years between 2007 and 2014, the value of 2006 is taken as constant. More information and details on surface measurement and monetization can be found in the final report on the updated NWI 2.0 version.