This paper demonstrates the utility of the Qualitative Reasoning approach for hypothesis testing in the domain of ecology regarding the functioning of 'black box' systems. As a test case, we refer to the study performed by Mangin (1969) with scale models to investigate the hidden mechanism of the Fontestorbes fountain, a spring that exhibits a periodic flow situated in the south of France. In our approach, a Qualitative Reasoning method (and hence a qualitative model) is used to test the 'siphon-hypothesis', which traditionally explains the oscillations of the flow rate of a periodic spring by the principle of filling and emptying an underground reservoir through a siphon action. Parts of the simulation results show that the hypothesis is qualitatively accurate; in particular the model produces a cyclic behaviour that matches with the observed one. However, the qualitative model also exhibits a contradictory behaviour (true negative) that challenges the hypothesis consistency. The causal account of this true negative denotes and explains a flaw in the siphon-hypothesis. The paper concludes that, with the Qualitative Reasoning method, models can be constructed for hypothesis testing. Such models should generate the desired behaviour as a first and necessary step to support the viability of the hypothesis. However, the occurrence of unexpected behaviours provides information that challenges the hypothesis, and may lead to having to discard it.