Although the indicators described thus far are all derived from the dynamic balance they do not yield information on the time horizon when standards will be violated. Comparing sustainability times identifies the compartment for which the quality standard is exceeded soonest, assuming continuation of current conditions. The critical sustainability time (tc) identifies the compartment for which the quality standard (soil, crop or groundwater), if exceeded, is exceeded first and is thus defined as:
tc = MIN(te,tu,ts), where te = time at which the ecological quality standard is exceeded, tu = time at which the crop quality limit is exceeded, ts = time at which the groundwater limit is exceeded.
All sustainability indicators are based on dynamic balances and they enable screening and comparing of different agro-ecosystems without having to know all processes in detail.
A study of the same arable farming systems that were used to illustrate the static and dynamic balances is used here to illustrate the sustainability indicators (Moolenaar et al. 1997b). In Table 33.3 the values for the (critical) soil cadmium contents, the (critical) sus-tainability factors and the (critical) sustainability times are presented for the four systems.
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