MEASUREMENTS OF HYDROLOGICAL VARIABLES

Hydrological Variables

Precipitation

Evaporation

Transpiration

Wind Velocity

Humidity

Radiation

Evapotranspiration

Precipitation

Precipitation is any form of water that falls to the Earth's surface. Different forms of precipitation include rain, snow, sleet, hail and drizzle.

How Precipitation Is Measured Standard rain guage

Weighing precipitation gauge

Tipping bucket rain gauge

Optical rain gauge

Weather station

Radar

Standard Rain Guage

Often have a funnel opening into a cylinder guage

Come in a variety of shapes and sizes

Calculate the rainfall (in mm) by dividing the volume of water collected by the area of the opening of the cup.

Standard Rain guage

Placement of Rain Gauges

Gauges are affected by wind pattern, trees and the gauge itself, therefore it is important to have the gauge located and positioned properly.

1m above ground level is standard.

2 to 4 times the distance away from an isolated object (such as a tree or building).

Rainfall Depth

The Rain Gauge

The cross-sectional of measuring tube is

1/10 the cross-sectional area of the

outer tube.

So, 10 mm of rain falling into the outer tube lookslike it fills the measuring tube 10 cm high.

Measurement is 10 mm!

Monthly Climatic Report

Evaporation

Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere.

OR

Process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy (ASCE, 1949).

Methods For Estimating Evaporation

Water budget methods

Energy budget methods

Mass transfer techniques (e.g., Meyer, Thornthwaile-Holzman)

Combination of energy budget and mass transfer methods (e.g.,Penman)

Energy Budget Method

Net energy advected (net energy content of incoming and

outcoming water -

Ee

Total solar

radiation - Rt

Reflected solar

radiation - Rr

Energy used for evaporation

(latent heat)- Ee

Sensible heat loss from the water body to the

atmosphere - Hn

Energy stored - Es

Net long-wave radiation exchange between the atmospere and the water

body- R1

Energy Budget Method

12 day-cm

calgRHERREE nertas

Energy Budget Method

Characteristics:

Most accurate method (evaporation is a function of the energy state of the water system).

Difficult to evaluate all terms.

Energy balance equation has to be simplified.

Empirical formulas are used.

Water Budget Method

Surface runoff - Qr

Subsurface runoff - Qs

Inflow- Q

Outflow- Q0

Evaporation- E

Subsurface seepage losses- Qd

EEQQQQQPt

Sdsr 0

Precipitation - P

Water Budget Method

Characteristics:

Simple

Difficult to estimate Qd and Qs

Unreliable, accuracy will increase as Δt increases

Transpiration

Transpiration is the evaporation occurring through plant leaves (stomatal openings).

Transpiration is affected by plant physiology and environmental factors, such as:

Type of vegetation

Stage and growth of plants

Soil conditions (type and moisture)

Climate and weather

How Transpiration is Measured

Capillary tube

Movement of meniscus is measured over time

Wind Velocity

Wind velocity can be measured by a device called anemometer.

Two types of anemometers

Velocity anemometers

Pressure anemometers

Anemometer

Anemometer

Anemometer

The air flow past the cups in any horizontal direction turning the cups.

The no. of turns is proportional to the average wind speed.

The needle show the direction of the wind.

Humidity

Humidity is the amount of water vapor in the air.

Types of humidity are:

Absolute humidity

Relative humidity

Specific humidity

Humidity during rain

Sling psychrometer is used to measure humidity.

Sling Psychrometer

Hygrometer

Hygrometer is used for measuring relative humidity.

Psychrometer is the simplest form of hygrometer.

Radiation

Solar radiation entering Earth’s atmosphere consists of direct, diffuse, and albedo radiation.

Radiation

How Radiation is Measured

Radiation can be measured by:

Pyrheliometer

Pyranometer

Pyrheliometer

Pyranometer

Evapotranspiration

Combined “loss” of water vapor from within the leaves of plants (“transpiration”) and evaporation of liquid water from water surfaces, bare soil and vegetative surfaces.

Evapotranspiration Measurement Evapotranspiration can be measured by:

Lysimeter

Evapo-transpiration

Transpiration

Evaporation

Rain

Runoff

Drainage

Root ZoneWater Storage

Irrigation

Below RootZone

Lysimeter

Lysimeter

Lysimeter

Evapotranspiration Report

Weather Station

A weather station is a facility with instruments which can measure

Temperature

Humidity

Wind velocity

Barometric pressure

Precipitation

Weather Station

References

http://www.wikipedia.org/

Research papers

Thanks