**4. Conclusions**

Undoubtedly, groundwater flow problems involve a number of ambiguities and uncertainties, making the use of differential equations even more difficult to solve. However, nowadays, the opportunity is given through the fuzzy partial differential equations theories to include these uncertainties in the final calculations and to provide more accurate results supporting the sustainability of groundwater as well as the researchers and engineers to make better decisions and plannings.

This work presents an innovative analytical solution in the nonlinear Boussinesq equation, which describes the groundwater's unsteady flow. According to the results, the proposed solution completely coincides with the Runge–Kutta method results used as a reference solution for comparison reasons in order to prove the accuracy and reliability of the proposed analytical solution. The volume membership function (Figure 3) could support decision-makers and planners with a higher degree of confidence than the previous years, thanks to the possibility theory.

**Author Contributions:** Conceptualization, C.T.; methodology, C.T. and N.S.; software, C.T.; validation, K.P. and C.E.; writing—original draft preparation, C.T.; writing—review and editing, N.S. and C.E.; visualization, C.T.; supervision, C.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
