Lucas built a model to show the effect of human population growth on an aquifer that supplies water for a growing city. Recent measurements show that the levels of water in the aquifer are dropping at a rate that correlates with population growth. Lucas placed a tub under a stream of water until the tub filled and began to overflow. Then he used a water pump to begin removing water from the tub. At first he pumped slowly, and the water still overflowed. Then he pumped harder until, eventually, the water level in the tub began to go down. What is represented by the pump in this model?