Roughly 70% of our planet is covered by water, of which 97% is contained in our oceans, with the remaining 3% being freshwater.
Surface water is water in a river, lake or freshwater wetland. Surface water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation, evapotranspiration and groundwater recharge.
In order to quantify surface water, generally water level, is measured to determine volumetric storage capacities such as lakes or drinking water reservoirs, or the basis of flood warnings on river systems. Discharge is also required which is a function of measuring velocity and area.