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The water balance is also referred to as a water budget. Developing water budgets is a fundamental activity in the science of hydrology. According to the US Geological Survey: [4] An understanding of water budgets and underlying hydrologic processes provides a foundation for effective water-resource and environmental planning and management.
The total volume of water in rivers is estimated at 2,120 km 3 (510 cu mi), or 0.49% of the surface fresh water on Earth. [2] Rivers and basins are often compared not according to their static volume, but to their flow of water, or surface run off. The distribution of river runoff across the Earth's surface is very uneven.
Volumetric water content, θ, is defined mathematically as: = where is the volume of water and = + + is equal to the total volume of the wet material, i.e. of the sum of the volume of solid host material (e.g., soil particles, vegetation tissue) , of water , and of air .
The liquid water content (LWC) is the measure of the mass of the water in a cloud in a specified amount of dry air. It is typically measured per volume of air (g/m 3 ) or mass of air (g/kg) (Bohren, 1998).
The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. [1] The curve number method was developed by the USDA Natural Resources Conservation Service , which was formerly called the Soil Conservation Service or SCS — the ...
Mass flow rate is defined by the limit [3] [4] ˙ = =, i.e., the flow of mass through a surface per time .. The overdot on ˙ is Newton's notation for a time derivative.Since mass is a scalar quantity, the mass flow rate (the time derivative of mass) is also a scalar quantity.
Where τ is the shear stress, S is the slope of the water, ρ is the density of water (1000 kg/m 3), g is acceleration due to gravity (9.8 m/s 2). [14] Shear stress can be used to compute the unit stream power using the formula = Where V is the velocity of the water in the stream. [14]
A discharge is a measure of the quantity of any fluid flow over unit time. The quantity may be either volume or mass. Thus the water discharge of a tap (faucet) can be measured with a measuring jug and a stopwatch. Here the discharge might be 1 litre per 15 seconds, equivalent to 67 ml/second or 4 litres/minute. This is an average measure.