Hydrology is the science that studies the waters that lie on the ground, both above and below the surface, their formation, circulation and distribution over time and in space, their biological, physical and chemical properties and their interactions with the environment, including those with living beings. Hydrology provides answers to questions such as water management, land-use planning and designing of hydraulic works (dams, river banks, sewers, aqueducts, etc).

The term Citizen Science (literally, from the English, the science of citizens) indicates that complex of activities linked to scientific research involving citizens in collaboration with scientists or under the guidance of professional scientists and scientific institutions. Citizen Science projects actively involve citizens in scientific research that generates new knowledge or understanding. Citizens can act as contributors, collaborators, or project leaders and play a significant role in the project. Both professional scientists and citizens involved benefit from taking part in a Citizen Science project. Advantages can include the availability of more data, the publication of search results, learning opportunities, personal enjoyment, social benefits, satisfaction for contributing to providing scientific evidence, for example: finding answers to issues of relevance local, national and international, and through these, have the opportunity to influence on sectoral policies. Citizen Science is considered a research method like any other, with limits and margin of error that must be considered and kept under control, however, unlike traditional research methodologies, Citizen Science provides opportunities for broad public involvement and democratization of science. Citizen Science projects and data are publicly available and, where possible, the results are published in a format of open access ［5］.

Citizen Science methodologies have already been applied to various hydrological research projects. There are many examples of volunteers gathering data on weather data, such as rainfall and temperatures, as well as some experiments focussed on level data collection. For example the US project "Community Collaborative Rain, Hail and Snow – CoCoRaHS” ［3］ for the collection of rainfall data began in 1998 in the United States of America. Cithyd is inspired by the project "Crowdhydrology" for the widespread collection of level data through volunteers ［6］. Cithyd has introduced some important changes and additions in the orignal project, including the use of QR codes for fast data entry via smartphones, station monitoring and verification of measurements through users with the role of keeper. Cithyd was developed from 2015 and officially presented at the international conference "Citizen observatories for water management" held in Venice in June 2016 ［1］.

In Italy there are several institutions that are responsible for collecting and publishing environmental data, including rainfall, temperature, wind speed and direction, and also level anad flow measurements in rivers. At regional level these institutions are generally the Regional Agencies for Environmental Protection (ARPA). The data may be published in real time or periodically after validation process. A list of the main sources of hydrological and / or meteorological data, grouped by region, is available on the unofficial Italian Hydrological Data Portal, developed thanks to a thesis at the University of Brescia http://meteoniardo.altervista.org/portale-dei-dati-idrologici-italiani.html ［verified on 15/09/2017］

Hydrometric level at a given river site is the difference between the water surface of the river and an altimetric reference point, which may be the average level of the river sea or hydrometric zero, that is, a chosen conventional point (eg the top of the parapet of a bridge or the plane of the road), or the point where the zero value is read on the staff gauge. The zero of the staff gauge is the altitude (above the sea level) that was agreed for that staff gauge. Through the hydrometric zero, it is possible to express the water level measurement in meters above sea level.

Staff gauge is a graduated pole or board that is placed in or beside a watercourse and from which it is possible to measure directly the height of the water surface relative to a known datum elevation Nowadays, more complex measuring instruments are used, which allow automatic reading and sending of data.

The flow rate of a watercourse is defined as the volume flowing into the time unit through a given cross section. The principles of hydraulics teach that it is possible to establish a relationship between the flow rate in a certain water cross section and the water level elevation. The validity of the relationship over time also depends on the stability of the cross section geometry, which is generally not guaranteed due to artificial or natural causes (such as erosion and deposition of sediments). There are some instruments that simultaneously measure the water level and surface velocity, in order to estimate the flow into the river.

The rise of the water level at a given river site depends on various factors, slope and shape of the riverbed, phenomena tha generate the flood (eg precipitation, melting of snow, maneuvering of the dam regulating organs, etc.). However, it is generally true that the catchement area affects the rate of growth of the water levels. For this reason, in small streams, the level can grow rapidly, within a few hours or even a few minutes, while in large rivers the water level rises (or drops) generally more slowly. The floods that occur on small rivers are therefore more instantaneous, more difficult to predict and often more dangerous tha flood in large rivers. In the information associated with the Cithyd measurement points we included the catchment area so citizens can easily understand the different behavior of the rivers in relation to the area.

Cithyd is a scientific research project, conducted strictly on the basis of the experiences reported in scientific publications related to similar projects. The development of the platform was preceded by a careful analysis of the available bibliography, published on the main scientific journals in the field of hydrology and citizen science. The quotes of the main bibliographic references used are included in brackets in the frequently asked questions page. They are referred to the following papers:

1. Balbo A. and Galimberti G. (2016): Citizen hydrology in River Contracts for water management and people engagement at basin scale, Proceedings of COWM2016 - International Conference on Citizen Observatories for Water Management, Venice, 7-9 June 2016
2. Buytaert W. et al (2014): Citizen science in hydrology and water resources: opportunities for knowledge generation, ecosystem service management, and sustainable development, Frontiers in earth science, Volume 2, Article 26, 1-21.
3. Cifelli R. et al (2005): The Community Collaborative Rain, Hail, and Snow Network: Informal Education for Scientists and Citizens. Bull. Amer. Meteor. Soc., Vol. 86, 8(Aug), 1069-1077.
4. Davids J.C., Van De Giesen N. and Rutten, M. (2017): Environmental Management 60: 12.
5. European Citizen Science Association (2015): 10 Principles of Citizen Science, https://ecsa.citizen-science.net
6. Lowry C. S. and Fienen M. N. (2012): CrowdHydrology: Crowdsourcing Hydrologic Data and Engaging Citizen Scientists, Ground Water, 1-6

Cithyd papers contained in the "Documents" section offer additional references for further reading.