Passage I

A geologist conducted three studies to evaluate the rate at which water enters and leaves a lake and its effect on lake volume. Water entering the lake is called inflow and water leaving is called outflow. The studies were conducted over a five-year period.

Study 1

The geologist measured two major sources of water inflow: rainfall and inflowing streams. Measurements of each were taken regularly over the five-year period. From these measurements, the geologist calculated that the annual averages totaled 260 × 10⁶ cubic meters of water per year (m³/yr). The results are in Table 1.

Study 2

Water leaves the lake naturally by outflowing streams, evaporation, and infiltration into the rocks below the lake. Measurements of these three processes were taken regularly over the five-year period. In addition, a water company pumps water out of the lake to supply several communities. The water company gave the geologists measurements of the pumping rate during the investigation period. A total outflow of 300 × 10⁶ m³/yr was calculated. The average outflow for each process is in Table 2.

Study 3

The geologist installed a water depth recorder at the center of the lake and took regular monthly readings during the five-year period. These readings were averaged each year. The lake level was found to be decreasing at an average rate of 10 cm per year.