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Watershed Hydrology

NREM 662

Ali Fares, Ph.D.

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1. Understanding the components ofhydrologic processes

2. Understanding the quantity and availability

of water

3. Understanding the quality of water4. Understanding the impacts of land use

and forest management practices on

water resources

5. Understanding the most basic concepts ofhydrologic monitoring

6. Utilizing hydrologic information resources

to solve real problems

Aspects of this course

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Watershed Hydrology

Physical Hydrology

Watershed Processes

Human Impacts on Water Resources

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Basic Definition

HYDROLOGY is the science of water that is

concerned with the origin, circulation, distribution

and properties of waterof the earth.

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Basic Definition

FOREST HYDROLOGY, RANGE

HYDROLOGY, WILDLAND HYDROLOGY is the

branch of hydrology which deals with the effects

of land management and vegetation on thequantity, quality and timing of water yields,

including floods, erosion and sedimentation

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Basic Definition

WATERSHED, or CATCHMENT, is a

topographic area that is drained by a stream,

that is, the total land area above some point on

a stream or river that drains past that point.

The watershed is often used as a planning or

management unit. Natural environment unit.

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Basic Definition

RIVER BASIN is a largerland area unit that,

although comprised of numerous sub

watersheds and tributaries still drains the entire

basin past a single point. Land use,management and planning is often diverse and

complex. River basins, like theAmazon and

Mississippi may drain an ocean or inland sea.

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Basic Definition

WATERSHED MANAGEMENT is the process

ofguiding and organizing land and other

resource use on a watershed to provide desired

goods and services without affecting adverselysoil and water resources.

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Oahus Watersheds

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Ala Wai Canal Watershed

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Mississippi RiverBasin

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Why Watershed Approach?

Watersheds are among the most basic units ofnatural organization in landscapes.

The limits of watersheds are defined by

topography and the resulting runoff patterns ofrainwater.

The entire area of any watershed is thereforephysically linked by the flow of rainwater runoff.

Consequently, processes oractivities occurringin one portion of the watershed will directlyimpact downstream areas (land or water).

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When detrimental activities like clear-cut

deforestation occur, negative impacts are

carried downstream in the form of eroded

sediments or flooding. Poor agricultural land management activities like

excess fertilizer application convey negative

impacts to downstream areas in the form of

eutrophication and possible fish kills.

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Water is the fundamental agent that links all

components (living and non-living) inwatersheds, and watershed managementgenerally revolves around water as a centraltheme.

A significant portion of the course will bedevoted to examining the pathways andmechanisms by which water moves from theatmosphere, to the watershed surface and

subsurface, into and out of biologicalcommunities, and ultimately downstream to theocean or subsequent river reach.

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Recognizing that enhanced interactions between

seemingly separate systems and organismsoccur within watershed areas, both scientistsand progressive-thinking resource managershave, in recent years, called for management

programs to be organized at the watershedlevel.

By working in concert with nature in this way, wemight manage resources in an integrative

fashion that avoids some of the many pastfailures that were brought by not recognizing orconsidering the larger-scale impacts of any onemanagement decision.

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Watershed Interactions

Cover

crops,

vegetation

Waterways,

channels

Riparian

buffer zones

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WS Management Strategies & Responses to

Problems

Watershed management involves: Nonstructural (vegetation management) practices

Structural (engineering) practices

Tools of WS management Soil conservation practices Land use planning

Building dams

Agroforestry practices

Protected reserves Timber harvesting

Construction regulation

The common denominator or integrating factor iswater

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WATERSHED MANAGEMENT PRACTICES

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WATERSHED MANAGEMENT PRACTICES

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Integrated WS Management

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Watershed Water Cycle

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Impacts of Management

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WSM: a global perspective

Practices of resource use & management

do not depend solely on the physical &

biological characteristics of WS

Economical, social, cultural & political

factors need to be fully integrated into

viable solutions.

How these factors are inter-related can

best be illustrated ?

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WSM: a global perspective

Land & water scarcity: is the major

environmental issue facing the 21st century

Demands > supplies (17%

) Next 25yrs 2/3 pop. water shortage

Land scarcity forest cut

Desertification Hydrometeorological extremes, role of

WSM

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Are these disasters preventable ?

Different approaches may be needed:

Modifying Nat. Sys.

Modifying Hum. Sys.

A combination

Bio-engineering & vegetative measures alongwith structures to have some control overextreme hydro-meteorological events

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Components of hydrologic cycle

Location % of total

Oceans (salt water) 97.5

Fresh water 2.5

Icecaps and glaciers 1.85

Groundwater 0.64

Lakes, rivers, soil, atmosphere 0.01

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Components of hydrologic cycle

Precipitation

- rain, snow, fog interception

Runoff- surface, subsurface

Storage

Evaporation- soil, plants, water surface

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Uses of the hydrologic cycle (HC)

One of the uses of the HC is in the estimationof surface storage.

Storing and transferring a sufficient quantity

of water has been one of the major problems. What volume of water is stored in a surfacereservoir/soil and how does the volume changeover time? What causes the water supply to bedepleted or increased?

How are the storage and releases managed?

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Watershed Water Cycle

Based on the conservation of mass:

Input output = change in storage P + R + B - F - E - T = S

volumes are measured in units m3, L, ac-ft, f3, gal,

or in & cm over the watershed area

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Water Depth

We have to use the same units; thus we

have to remove the area from our

calculation

We need to convert volume into unit

depth; thus whats water depth:

Water depth (d) = Volume of water (V) /

Surface of the field (A)

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Conversion

1 acre-foot = 1317.25 m3

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Problem 1

Suppose there is a reservoir, filled with

water, with a length of 5 m, a width of10

m and a depth of 2 m. All the water from

the reservoir is spread over a field of1

hectare. Calculate the water depth (which

is the thickness of the water layer) on the

field.

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Answer1

Surface of the field = 10 000 m2

Volume of water = 100 m3

Formula:

d = v/a =100 / 10,000 = 0.01 m = 10 mm

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Problem 2

A water layer1 mm thick is spread over a field of

1 ha. Calculate the volume of the water (in m3),

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Answer 2

Given

Surface of the field = 10 000 m2

Water depth =1

mm =1

/1

000 = 0.001

m Formula: Volume (m) = surface of the

field (m) x water depth (m)

Answer

V = 10 000 m2 x 0.001 m

V = 10 m3 or10 000 liters

Documents

08 22 Lecture