Pea plant clones are given different amounts of water for a 3 week period. The first pea plant receives 400 mL a day. The second pea plant receives 200 mL a day. The third pea plant receives 100 mL a day. The fourth pea plant does not receive any extra water; the plant only receives natural ways of receiving the water. The height of the pea plants is recorded daily. What is the independent variable in this example?