Aiming at explicit description of temporal meaning of causal relations generated by qualitative reasoning systems, this article proposes a causal time ontology which defines a set of general time concepts in qualitative models, called causal time scales. Each of them associated with a modeling technique represents a temporal granularity and/or an ontological viewpoint. They allow us to specify temporal performance of the reasoning engines and to identify a general causal reasoning scheme together with sophisticated feedback analysis. Lastly, we present a causal time resolution required to derive causal relations in fluid-related systems and a reasoning system satisfying it.