The registration for the Database for Hydrological Time Series for Inland Waters (DAHITI) website provides access for water level time series, surface area time series and volume time series of inland waters.

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Creative Commons Lizenzvertrag
All data sets provided by "Database for Hydrological Time Series for Inland Waters" (DAHITI) are maintained and created by the "Deutsches Geodätisches Forschungsinstitut der Technischen Universität München" (DGFI-TUM) and are licensed under a Creative Commons Attribution 4.0 License.
Terms of Use

All products are distributed in the hope that they will be useful, but DGFI-TUM provides the data “as is” without warranty, expressed or implied, as to the use or appropriateness of the data and products, and there are no warranties of merchantability or fitness for a particular purpose or use.
It is only allowed to use DAHITI products for free by private persons, educational institutions and non-profit organizations.
It is not allowed to use DAHITI products for commercial purposes
If you are using products of DAHITI for your own work you agree to cite the used products as it is described on the associated product page.