INTRODUCCION

What is Hydrology?

It can simply be summarized that hydrology is the detailed scientific study of water.

Scope of Hydrology

The study of hydrology is multi-disciplinary. The hydrologic cycle is the central theme of the study of hydrology.

Hydrological studies involve the application of scientific knowledge and mathematical principles to solve water related problems such as quantity, quality and availability in society.

Most of the basic concepts and processes studied in hydrology have their roots in the hydrologic cycle.

Hydrologic cycle

The hydrologic cycle can be considered a closed system for the earth because the total amount of water in the cycle is fixed even though its distribution in time and space varies. There are many sub-cycles within the worldwide system, which are generally open-ended. It is these subsystems that give rise to the many problems of water supply and allocation that confront hydrologists and water managers.

The entire process in the hydrologic cycle can be divided into five parts, which include: condensation, precipitation, infiltration, runoff and evaporation. The hydrologic cycle is very complex. This is because all biological lives depend on water. However, it is necessary, in spite of its complexities to look at the major path-ways or processes in the movement of water through the cycle.

THE HYDROLOGIC BASIN

Definition of basin

An area of land that drains water, sediment, and dissolved materials to a common outlet.

Watershed are separated by drainage divides.

Can be any shape and size.

The watershed is the basic unit of all hydrologic analysis and designs. Any watershed can be subdivided in to a set of smaller watersheds. Usually a watershed is defined for a given drainage point. This point is usually the location at which the analysis is being made and is referred to as the watershed “outlet”.

Drainage Area (A)

The area of watershed is also known as the drainage area and it is the most important watershed characteristic for hydrologic analysis. It reflects the volume of water that can be generated from a rainfall. Once the watershed has been delineated, its area can be determined, either by approximate map methods or by GIS.

Perimeter of a basin (km)

Is the outer boundary of the watershed that enclosed its area. It is measured along the divide between watersheds and may be used as an indicator of watershed size and shape.

Watershed length (L)

Increases as the drainage increases. L is important in hydrologic computations. L defined as distance measured along the main channel from the watershed outlet to the basin divide. L is measured along the principal flow path.

A & L

Both measures of watershed size; they may reflect different aspects of size. A - Indicate potential for rainfall to provide a volume of water; L - used in computing time parameter - measure of travel time of water through a watershed.

Delimitation of a basin

Boundary of a watershed consists of the line drawn across the contours joining the highest elevations surrounding the basin.

A common task in hydrology is to delineate a watershed from a topographic map.

The water divide is the line linking the points of greatest height between two drainage basins, and separating their surface runoffs. It delimits the entire catchment area which is drained by the whole of a river network.

When defining watershed boundaries and stream channels it is important to remember that water flows from high elevation to low elevation, and in a simple sense, perpendicular to contour line.

CHARACTERISTIC CURVES

The hypsometric curve

The hypsometric curve (area–altitude relation) of a watershed has been used since the 1950s to describe the distribution of watershed area with elevation, usually as a proportion of area above each proportion in elevation. A related analysis is between watershed area and stream gradient.

A hypsometric curve is a graphical representation showing on the abscissa the basin areas situated above various altitudes. If necessary, the basin areas can be given as percentages of the total. The hypsometric curve has also been termed the drainage-basin relief graph (Vladimirescu, 1978). Being a global representation, such a curve has the disadvantage of containing little or no information about certain significant relief features, in particular slope discontinuities, platforms and scarps, which are not at the same altitude throughout the basin area (Baulig, 1959).

They permit determination of the mean altitude of a drainage basin or region.

REPRESENTATIVE INDICES

Stream order

The stream order is a measure of the degree of stream branching within a watershed. Each length of stream is indicated by its order (for example, first-order, second-order, etc.). A first-order stream is an unbranched tributary, a second-order stream is a tributary formed by two or more first-order streams. A third-order stream is a tributary formed by two or more second-order streams and so on. In general, an nth order stream is a tributary formed by two or more streams of order (n-1) and streams of lower order. For a watershed, the principal order is defined as the order of the principal channel.

Form Factor (Ff)

Form factor (Ff) is defined as the ratio of the basin area to the square of the basin length, using the following equation.

Where:

R f=Au

Lb2

Au=basin area (km )

Lb=Basinlength(km)

This factor indicates the flow intensity of a basin of a defined area (Horton, 1945). The smaller the value of the form factor, the more elongated will be the basin. Basins with high form factors experience larger peak flows of shorter duration, whereas elongated watersheds with low form factors experience lower peak flows of longer duration.

Circulatory Ratio (Rc)

Circulatory ratio obtained from the ratio of basin area (Au) to the area of a circle (Ac) having equal perimeter as the perimeter of drainage basin.

Where:

Rc=Au

Ac

It is influenced by the length and frequency of streams, geological structures, land use/ land cover, climate and slope of the basin.

The high value of circularity ratio shows the late maturity stage of topography.

Elongation Ratio (Re)

Schumm’s 1956 used an elongation ratio (Re) defined as the ratio of diameter of a circle of the same area as the basin to the maximum basin length.

Where:

Rl=Dc

Lbm

Dc=Diameter of ˚having same

areaas the given drainagebasin(km)

Lbm=Maximumlength(km)

The value of Re varies from 0 (in highly elongated shape) to unity i.e. 1.0 (in the circular shape).Thus higher the value of elongation ratio more circular shape of the basin and vice-versa. Values close to 1.0 are typical of regions of very low relief, whereas that of 0.6 to 0.8 are usually associated with high relief and steep ground slope (Strahler, 1964).These values can be grouped as,

Elongation ratio Shape of basin<0.7 Elongated

0.8-0.7 Less elongated0.9-0.8 Oval

>0.9 Circular

The circular basin is more efficient in run-off discharge than an elongated basin (Singh and Singh, 1997).

Drainage Density (Dd).

Drainage density has long been recognised as topographic characteristic of fundamental significance. It reflects the landuse and affects infiltration and the basin response time between precipitation and discharge. Drainage basin with high Dd indicates that a large proportion of the precipitation runs off. On the other hand, a low drainage density indicates the most rainfall infiltrates the ground and few channels are required to carry the runoff (Roger, 1971). Dd is considered to be an important index; it is expresses as the ratio of the

total sum of all channel segments within a basin to the basin area i.e., the length of streams per unit of drainage density.

Where:

Dd=Ls

A

Ls=Total lenght of all streamchannels∈thebasi n

A=Area od thebasi n

Compactness factor

The compactness coefficient C c is defined as the radio of the perimeter of a circle whose area is a equal to the area of the basin. That is

C c=P

√4πA

P=perimeter of the basin ( km)

A=Area of basin(km2)

It may be observed that the circularity ratio is nothing but the reciprocal of the square of the compactness coefficient.

Rc=1

C c2

Thus the circularity ratio and the compactness coefficient are not truly independent parameters.

SLOPE OF A BASIN

Slope (S)

There are many ways of defining the slope of a basin. A simple way of obtaining it is to divide the difference between the elevations of the highest point on the basin perimeter and the basin outlet by the distance between these two points.

Slope of the main channel

It is usually calculated as the elevation difference between the endpoints of the main flow path divided by the length.

Where:

S=HL

S=slopeof themainchannel .

H=height difference of the ends of

the channel .(km)

L=channellength .(km)

The elevation difference may not necessarily be the maximum elevation difference within the watershed since the point of highest elevation may occur along a side boundary of the watershed rather than at the end of the principal flow path. If there is significant variation in the slope along the main flow path, it may be preferable to consider several sub-watersheds and estimate the slope of each.

Nash and Shaw (1966) have suggested the equation for determining channel slope as:

S=2∑ LiZ i

¿¿¿

Where Li is the distance along the main stream between successive contours and Zi is the average elevation above the outlet for each reach length Li (Fig. 4.2, Schulz and Lopez, 1974). Reich (1962) and Laurenson et al. (1963) described the slope quantity as the slope of a straight line joining the elevation of the outlet on the profile of the main stream with the average elevation of the actual stream profile. The average main channel slope can be developed by drawing a straight line (Fig. 4.2) such that the area under the line is equal to the area under the profile diagram (hypsometric curve).

Wu (1963) obtained a mean slope of the main channel by studying topographic maps.

Wu (1963) used the method developed by Taylor and Schwarz (1952) to determine the mean slope as:

Smean=( 11

√ S1+ 1

√S2+ 1

√S3+…+ 1

√Sn

)2

Where ‘n’ represents the number of reaches of equal length, and S1 to Sn are the slopes of each small reach.

PROBLEMS OF APPLICATION

Delimitation of a basin

The example map below shows an illustration of contour lines, topography, stream channel network, and drainage divide. The following are the steps used to delineate watershed boundaries and stream networks:

To trace the boundary, start at the outlet & then draw a line away on the left bank, maintaining it always at right angles to the contour lines. (The line should not cross the drainage paths)

1. Continue the line until it is above the headwaters of the stream network. Return to the outlet and repeat the procedure with a line away from the right bank.

2. Two lines should join to produce the full watershed boundary.

3. Use of GIS (Geographic Information System) popular and has facilitated much of the work of hydrologists.

4. The use of DEMs (Digital Elevation Models) in particular has made watershed delineation a smooth procedure.

¿Qué es la hidrología?

Simplemente se puede resumir que la hidrología es el estudio científico detallado de agua.

Ámbito de Hidrología

El estudio de la hidrología es multidisciplinario. El ciclo hidrológico es el tema central del estudio de la hidrología.

Estudios hidrológicos implican la aplicación de los principios del conocimiento científico y matemáticas para resolver problemas relacionados con el agua, tales como la cantidad, calidad y disponibilidad de la sociedad.

La mayor parte de los conceptos básicos y procesos estudiados en hidrología tienen sus raíces en el ciclo hidrológico.

Ciclo hidrológico

El ciclo hidrológico puede considerarse un sistema cerrado para la tierra porque la cantidad total de agua en el ciclo se fija a pesar de su distribución en el tiempo y en el espacio varía. Hay muchos sub-ciclos dentro del sistema en todo el mundo, que son generalmente de composición abierta. Son estos subsistemas que dan lugar a los muchos problemas de abastecimiento y distribución del agua que enfrentan los hidrólogos y gestores del agua.

El proceso entero en el ciclo hidrológico se puede dividir en cinco partes, que incluyen: condensación, precipitación, infiltración, escorrentía y la evaporación. El ciclo hidrológico es muy complejo. Esto es porque todas las vidas biológicas dependen del agua. Sin embargo, es necesario, a pesar de sus complejidades a mirar las principales maneras de caminos o procesos en el movimiento del agua a través del ciclo.

Definición de cuenca

Un área de tierra que drena el agua, los sedimentos y materiales disueltos a una salida común.

Cuencas están separadas por divisiones de drenaje.

Puede ser de cualquier forma y tamaño.

La cuenca es la unidad básica de todos los análisis hidrológicos y diseños. Cualquier cuenca se puede subdividir en un conjunto de cuencas pequeñas. Por lo general, una cuenca hidrográfica se define por un punto de drenaje dado. Este punto es generalmente la ubicación en la que se está realizando el análisis y se conoce como la cuenca "salida".