Add promise

docs/concept/equations.qmd

@@ -103,6 +103,8 @@ $$
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 Hence the reference used for computing the relative error is the average of the total inflow and total outflow of the basin (which are both non-negative).
 
+The default tolerances are $0.001 \text{ m}^3$ for the balance error and $0.01$ for the absolute error, which should not be exceeded for realistic models.
+
 In extreme cases where storage rate is many orders of magnitude smaller than the storage itself, these computations can have floating point truncation errors which can lead to large relative errors.  This is however only when the storage is roughly $10^{15}$ or more times bigger than the storage rate.
 
 ### Example calculation