Streamflow, or runoff, is the amount of water that is not retained as soil storage or evaporated through evapotranspiration. The water balance principle states that total water inputs to an ecosystem must equal total water outputs. Precipitation and changes in storage contribute to inputs, while evapotranspiration and runoff constitute outputs, with the equation P + ΔS = ET + R representing the water balance. The water balance can also be expressed as inflows - outflows = change in storage.