A fictional annual catchment budget

A simulated basin of 100 square kilometers receives 700 millimeters of precipitation in one year. Actual evapotranspiration is supplied as 450 millimeters and measured storage increases by 20 millimeters. The exercise assumes no net groundwater exchange across the chosen boundary and no imported or exported water. All quantities refer to the same year and area. A student subtracts potential evapotranspiration instead of the supplied actual value, while another assumes that a positive storage change is an additional water input. The class must account for every stated term, convert a depth to volume and explain why an annual total alone cannot predict a flood peak. These are fictional accounting observations and not a forecast or engineering design for an actual basin.