Your question is very general and it is difficult to give specific answers, but I will try to offer some pointers. For example I cannot determine whether you are interested in time to failure, general system reliability or both. Also you indicated that the failure could be driven by a number of factors, such as weather, accidents or interferences.
Each of these potential causes of failure is different. For example, weather, as a natural hazard, may be considered as time dependent - being related to the risk of a storms over a period of time. The others could simply be due to random or unpredictable causes; therefore each of these factors may require a different probability density form.
In any case, the first step is to determine the form of mass or density function that will be used to model the failure of each sensor. For time to failure, a density function often used is the Hazard function or one of its derivatives. In simple terms the Hazard function is also called a bathtub function as the failure risks are typically much higher in early-life, decrease in mid-life and increase again at end-of-life. Human life can be modelled with this form of a risk profile.
The manufacturer may offer useful data as the Hazard function is commonly used in product reliability assessment. For example, the risk of failure of light bulbs is modelled well with a Hazard function. In your case, a paper illustrating the application of the Hazard function to natural hazards is found here.
Proper assessment of the risk of flooding due to weather requires consideration of the rainfall intensity associated with a, say, 100-year storm, for example, and the associated flooding levels from the characteristics of the watershed and it's unit hydrograph.
Consideration of the forms associated with accidents or interferences and how they may be combined is important. If the events are statistically independent then the overall result may be computed as a product of the probabilities. Otherwise dependency effects must be considered.
If you want to assess the reliability of the entire system - this is again another rather large subject. I have set out several alternative techniques in this thread.