Final answer:
Antecedent moisture is the water already present in a drainage basin before new precipitation, including the soil's moisture and any surface water. It's a key variable in understanding how much water will be available for runoff, infiltration, and evaporation.
Step-by-step explanation:
The amount of water already present in a drainage basin, known as antecedent moisture, refers to the water that exists in the soil and surface before a new precipitation event. This includes moisture left from prior rainfall, snowmelt, or other sources.
Antecedent moisture affects the amount of water that will run off into streams and rivers, infiltrate the soil, or evaporate.
To estimate the amount of evaporation, we can reference a scenario where, with constant solar energy input of 120 watts per square meter, the amount of water that evaporates can be determined.
One square meter, receiving a steady input of 120 W, and considering that each millimeter of water on that square meter equates to a liter or a kilogram, allows us to calculate the volume of water drawn off in a year.
Considering the energy required to evaporate water is 2.45 MJ/kg (or 2450 J/g), we calculate the evaporation rate in millimeters per year.