Hydrologic systems are generally analyzed by using mathematical models. ‘ These

models may be empirical, statistical, or founded on known physical laws. They may

be used for such simple purposes as determining the rate of flow that a roadway

grate must be designed to handle, or they may guide decisions about the best way to developa river basin for a multiplicity of objectives.

The choice of the model should betailored to the purpose for which it is tobe used. In general, 'the simplest modelcapable of producing informationadequate to deal with the issue should bechosen

Unfortunately, most waterresources systems of practical concernhave physical, social, political,environmental, and legal dimensions;and their interactions cannot be exactlydescribed in mathematical terms.

For the most part, mathematical models are designed to describe the way a

system's elements respond to some type of stimulus( input). For example, a model

of a groundwater system might be developed to demonstrate the effects on groundwater storage of various schemes

for pumping.

These equations are mathematical models of the hydrologic budget.

This figure can be considered a pictorial model of the rainfall-runoff process.

• Precipitation- the process of separating a solid substance from liquid.

• Stream Flows • Evaporation- changing from liquid to gas• Soil Moisture• Snow Fields • Sedimentation- the natural process in which

material is carried to the bottom of a body of water and forms a solid layer

• Transpiration – the act of or process or an instance of transpiring

Hydrologic data are needed to describe:

• Infiltration- to pass into or through by filtering or permeating

• Water Quality• Air• Soil• Water Temperature• Other Variables Or Components Of

Hydrologic Systems

Sources of data are numerous, with theU.S. Geological Survey being the primary onefor stream flow and groundwater facts. TheNational Weather Service (NOAA or NationalOceanic and Atmospheric Administration) isthe major collector of meteorological data.Many other federal state, and local agenciesand other organizations also compilehydrologic data.

In the Philippines we have the National Meteorological and Hydrological Services to manage the data. Philippines is one of

the members of the (WMO) World Meteorological Organization who also

monitors data.

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Stream and river flows are usually recorded as:• cubic meters per second( m3/sec)• cubic feet per second (cfs)• second-feet (sec-ft)

Ground water flows and water supply flows:

• gallons per minute• hour or day (gpm, gph, gpd), • millions of gallons per day (mgd)

Flows used in agriculture or related to water storage:• acre-feet (acre-ft)• acre-feet per unit time• inches (in.)• centimeters (cm) • depth per unit time• acre-inches per hour (acre-in./hr)

Volumes are often given as gallons, cubic feet, cubic meters, acre-feet, second-foot-days, and inches or centimeters

• An acre-foot is equivalent to a volume of water 1 ftdeep over 1 acre of land (43,560 ft3).

• A second-foot-day( cfs-day,s fd) is the accumulated volume produced by a flow of 1 cfs in a 24-hr period.

• A second-foot-hour (cfs-hr) is the accumulated volume produced by a flow of 1 cfs in 1 hr.

• Inches or centimeters of depth relate to a volume equivalent to that many inches or centimeters of water over the area of concern.

• In hydrologic mass balances, it is sometimes useful to note that 1 cfs-day = 2 acre-feet with sufficient accuracy for most calculations

• Rainfall depths are usually recorded in inches or centimeters whereas rainfall rates are given in inches or centimeters per hour.

• Evaporation, transpiration, and infiltration rates are usually given as inches or centimeters depth per unit time.

To

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It is true that humans cannot exist without water; it is also true that water, mismanaged, or during times of deficiency (droughts), or times of surplus (floods),

can be life threatening. Furthermore, there is no aspect of environmental concern that does not relate

in some way to water. Land, air, and water are all interrelated as are water and

all life forms. Accordingly, the spectrum of issues requiring an understanding of hydrologic processes

is almost unlimited.

As water becomes more scarce and as competition for its use expands, the need for improved water management will grow. And to provide water for the world's expanding population, new industrial

developments, food production, recreational demands, and for the preservation and protection

of natural systems and other purposes, it will become increasingly important for us to achieve a

thorough understanding of the underlying hydrologic processes with which we must contend

This is the challenge to hydrologists water resources engineers, planners, policymakers,

lawyers, economists, and others who must strive to see that future allocations of water are

sufficient to meet the needs of human and natural systems.