Watershed Hydrology Practice Exam

Think of the watershed water budget as a balance of water fluxes. Over many years, the amount of water stored in the watershed (in soil, groundwater, snow, etc.) doesn’t keep accumulating; it tends to return to a baseline when you average across years. Because of that, the storage-change term is neglected in the long-term annual budget. The main pieces then are the input of water from precipitation and the outputs from evapotranspiration and runoff. Precipitation brings water into the system, evapotranspiration removes water to the atmosphere, and runoff transports water away from the watershed. When you average over a long period, the equation simplifies to precipitation roughly balancing evapotranspiration plus runoff, with storage changes averaging to zero. So the term representing changes in storage is not an important component for long-term average flux budgets.