**1. Introduction**

The term 'engineering' makes reference by definition, to the voice 'to produce'. Engineering can be considered as part of the technical sciences that put together the general and abstract scientific knowledge and the technology, being that as human activity, it is present in the design, construction and testing; maintenance, conservation and operation of structures, machinery (fixed and mobile), installations and systems [1].

Forensic activities are primarily oriented to the establishment of the origin and causes of a fact; that is precisely the reason why engineering should be involved is such activities, giving birth by this to what it is known as forensic engineering.

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It is true that we are used to hear about the application of forensic science in crimes and felonies. In fact, even the dictionary definition for forensic science leads us to the application of scientific practices within a legal process. This can be translated into the participation of highly speci‐ alized professional or criminologists, who do research, seek and locate evidence—many times laboratory based—that can provide conclusive proofs about a fact that needs to be clarified. Sometimes, the evidence cannot be seen at first glance and a much deeper exam is needed in order to determine the true causes of an event. Even engineering or hydrology in particular have not been mentioned so far, a lot of the concepts described seem to fit very well in these disciplines. At the end, any forensic investigation has the objective of establishing how an undesired event happened and eventually, what to do in order to prevent that it happened again.

Forensic engineering has evolved to an interdisciplinary approach, considering another specialty such as anthropology, sociology and economy among others. It can be state that the general purpose of forensic engineering is to determine or clarify the causes of failure of certain system with the objective of improve the designs or assist in the operation procedures.

Common losses or failures that forensic engineers usually address include among others, damages to structures caused by natural phenomena such as thunderstorms, hurricanes, storm surge, saline intrusion, fires, etc. In fact, the need of research in this field is associated to a combination of climate conditions, topographic and physiographic conditions and resilience of the population: for example, coastal zones in poverty and a high potential of hurricanes incidence are highly susceptible to suffer. Nevertheless, some of these fields have rarely been addressed by forensic analysis, missing out on the advantages it could provide investigations on this topic.

All of these lead to derive from the environmental and earth sciences and forensics, the socalled forensic hydrology. Even though the term is often associated to the different quantitative components of the water cycle, forensic hydrology also includes topics as water pollution and contamination, floods, droughts, other water-related resources and water infrastructure operation among others. There will be occasions in which forensic hydrology would help to prevent or at least reduce severe damages and in some others, it would lead to better water management, improve its use and allocation.

The adjective forensic applied to geosciences appeared around 1980s [2], but few investigations have been developed since then. Derived from cases associated to pollution of water or soil, researchers started to talk about forensic geochemistry or forensic geology, as a synonym of the process to describe the use of specific techniques to find the sources of the pollution.

Some 20 years later, the growing concern on nature protection we could see the beginning of environment sciences forensics. On the other hand, the so-called engineering forensics, do research on materials, products, structures or components that fail or do not work in the way they were designed, generating or contributing to the collapse or failure of a civil work.

It is in the intersection of environment science forensic and engineering forensics where we can locate forensic hydrology with the main objective of determine the probable cause of an event or the human sources that contribute to increase damages or even human losses. This type of analysis can be seen as a useful tool through which it is possible, in a full objective way, to determine the real causes, natural or human induced that made a natural phenomenon cause a disaster.

Forensic hydrology could be referred to the topics of water pollution, but also to floods, droughts or even water management, among others. In the case of floods, for example, forensic hydrology would help to identify responsibilities in a particular event and to plan for actions in order to prevent future damages. In the case of droughts, forensic hydrology allows to systematically address the effects caused by the phenomena, both immediate and in the midterm, and also evaluate the measures that were taken in order to prevent future problems.

Precisely in the topic of extreme phenomena, forensic hydrology can be directly applied as we said to the case of flood or drought. In the case of flooding events causing material damages and even loss of human lives, forensic analysis can provide results that help to clarify respon‐ sibilities but specially to prevent that future disasters happen again. Even drought is a phenomenon much more complex to address, because its geographic scope is bigger and the impacts produced can extend a lot in time, forensic analysis can help to determine the main aspects and the locations that are more urgent to focus on.

Forensics hydrology procedures will give us the reconstruction of the event in order to determine what really happened, what factors contributed, what failed, who were the main actors that have something to blame about the damages, etc. In the case of floods, for example, forensic analysis should include principles for hydrometeorology, hydrology, hydraulics, social and political sciences, with the help of technology for modelling and simulating.

In this chapter, the topics of hydrological extremes, both floods and droughts are presented. In the first case, a methodological guide based on Ref. [3], is provided, while in the second a more general methodology is sketched. This investigation will provide important information for the new forensic hydrologists, a discipline with great future, especially in the context of climate change, which will increase the magnitude, and frequency of extremes.
