This exercise is designed to evaluate the students' understanding of both the hydrologic cycle and the water budget (mass balance) equation. In my course, the exercise is the students' first exposure to models in the course. While the exercise may seem basic, students gain experience in creating conceptual models and then generating mathematical models from the conceptual model. The exercise provides students with an introduction (or refresher) to some basic Excel formulas. Finally, the exercise can be modified to include more "what if" scenarios that require critical thinking and analysis from the students.