Introduction to Hydrology

Introduction to Hidrology

Introduction to HidrologyHidrology: Science that deals with all phases of the earth water. It focuses on the study of the components of the hydrologic cycle

Hidrologic Cycle: Circulation of water between the earth and the atmosphere, by changing the state in this closed system.1Residence time expresses how fast something moves through a system in equilibrium. It is the average time a substance spends within a specified region of space, such as a reservoir. It is a measure of the average age of the water in that reservoir.For example, the residence time of water stored in groundwater, as part of the water cycle, is about 5,000 years. A common method for determining residence times is to calculate how long it would take for a region of space to become filled with a substance.

2Examples

Atmospheric (precipitation)Volume = 12900 km3Flow Rate= 577000 km3/day

Surface (rivers)Volume = 2120 km3Flow Rate= 44700 km3/day

Groundwater (fresh)Volume = 10530000 km3Flow Rate= 2200 km3/year

3In the Troposphere

T0 is the surface temperature (z0=0)a is the ambient lapse rate (L-1)4Hydrostatic Pressure DistributionAir parcel

Force Balance

Zero acceleration, motionless air or constant velocity

Taking:

HYDROSTATIC PRESSURE DISTRIBUTION, if ra were constant

if ra is different to constant, IDEAL GAS EQUATION

5IDEAL GAS EQUATION

Ma is the molecular weight of air (around 29 g/mol)R is the universal gas constant (8.314 J/mol K 0.082 l atm/mol K)

Knowing that:

and, with

So,

Integrating

PP0Z6Density Profile

rr0ZStable ProfileSo: heavier air (at the bottom) gets warmed up by higher temperatures and gets pushed up by higher pressures. When it ascends, it gets cold off by lower temperatures, gets heavier and them comes down.

Air coolsAir warmsLighterHeavierIf surface air (cool) is polluted, this creates poor air quality problems, since pollutants are unable to escape Common problem in cities such as Mexico, LA (flat, clear skies)Atmospheric HumiditySpecific Humidity (w)

Relative Humidity (r)

Vapor Pressure (Pv), partial pressure of water vapor in humid air (mixture of vapor +dry air)

Where:

8Knowing

Also,

Note: = function of temperature Thermodynamic Tables

Links humidity to water pressure

9Raudviki, 1979

Rainfall Potential10Rainfall Empirical RelationshipsMain mechanism of air-mass lifting are:

Frontal lifting, warm air is lifted over cooler air by frontal passage (cyclonic or frontal storm) and the zone where the warm and cold zone meet is called a front. (South Florida)

Orographic lifting, warm air rises as it flows over hills or mountains. (Seattle, Washington)

Convective lifting, air rises as by virtue of being warmer and less dense than the surrounding air, convective storms or thunderstorms. (central United States with moist summers short duration)

11 Rainfall is measured by government agencies using: Rain gages

Standard rain gage, 20.3 cm diameter funnel that passes water into a cylindrical measuring tube, the whole assembly is placed within an overflow can.

Measuring tube has a cross-sectional area one-tenth that of the collector funnel, a 2.5 mm (0.1 in) rainfall will occupy a depth of 25 mm (1 in) in the collector tube12 Radar Doppler (NexRAD)

13Empirical RelationshipsIntensity-Duration-Frequency (IDF) CURVEStatistics of the rainfall measurements are typically used, the most common form are the IDF curves, which express:

relationship between the intensity in a rainstorm and the averaging time (duration), with each relationship having a probability

Data required to create IDF curve:

Record of rainfall measurements in the form of the depth of rainfall during fixed intervals of time, Dt, typically on the order of 5 minutes14