Elements of The Hydrologic Cycle

Precipitation

Precipitation is the release of water from the atmosphere to reach the surface of the earth.The term ‘precipitation’ covers all forms of water being released by the atmosphere (snow, hail,sleet and rain).Precipitation is the major input of water to a river catchment area.It needs careful assessment in hydrological and hydrogeological studies.

Occurence and Types of Precipitation

The ability of air to hold water vapour is temperature dependent (Davie, 2008): the cooler the air the less water vapour is retained.If a body of warm, moist air is cooled then it will become saturated with water vapour and eventually the water vapour will condense into liquid or solid water (i.e. water or ice droplets).The water will not condense spontaneously.There need to be minute particles present in the atmosphere, called condensation nuclei.Upon condensation nuclei the water or ice droplets form.The water or ice droplets that form on condensation nuclei are normally too small to fall to the surface as precipitation.They need to grow in order to have enough mass to overcome uplifting forces within a cloud.

There are three conditions that need to be met prior to precipitation forming (Davie, 2008):

• Cooling of the atmosphere
• Condensation of the vapor onto nuclei
• Growth of the water or ice droplets

There are three major types of precipitation:

• Convective precipitation
• Orographic precipitation
• Cyclonic precipitation

Convective precipitation

Heated air near the ground expands and absorbs more water moisture.The warm moisture-laden air moves up and gets condensed due to lower temperature, thus producing precipitation.This type of precipiation is in the form of local whirling thunder storms.

Orographic precipitation

The mechanical lifting of moist air over mountain barriers, causes heavy precipitation on the windward side of the mountain.

Cyclonic precipitation

The uneven heating of the earth surface by the sun results high and low pressure regions.Air masses move from high pressure regions to low pressure regions, and this motion produces precipitation.If warm air replaces colder air, the front is called a warm front. If cold air displaces warm air, its front is called a cold front.

Measurement of Precipitation

The precipitation is usually expressed as a vertical depth of liquid water.Rainfall is measured by millimetres (mm), rather than by volume such as litres or cubic metres.The measurement of precipitation is the depth of water that would accumulate on the surface if all the rain remained where it had fallen. Snowfall may also be expressed as a depth of liquid water.

For hydrological purposes it is most usefully described in water equivalent depth.

Water equivalent depth is the depth of water that would be present if the snow melted.

For hydrological analysis it is important;

• to know how much precipitation has fallen,
• and when this occurred.

Precipitation at different locations in the terrain is recorded using two main types of rain gauges:

• non-recording rain gauges
• recording rain gauges.

Non-recording Rain Gauges

The non-recording rain gauge consists of a funnel with a circular rim    and a glass bottle as a receiver.

The cylindrical metal casing is fixed vertically to the masonry foundation with the level rim    above the ground surface.

The rain falling into the funnel is collected in the receiver and is measured in a special measuring glass graduated in mm of rainfall. Usually, rainfall measurements are made at 08.00 hr and at 16.00 hr .During heavy rains, it must be measured three or four times in the day.Thus the non-recording rain gauge gives only the total depth of rainfall for the previous 24 hours.

Recording Rain Gauges

A recording type rain gauge has an automatic mechanical arrangement consisting of:

• a clockwork,
• a drum with a graph paper fixed around it
• and a pencil point, which draws the mass curve of rainfall.

This type of gauge is also called self-recording, automatic or integrating rain gauge.

From this rainfall mass curve;

• the depth of rainfall in a given time,
• the rate or intensity of rainfall at any instant during a storm,
• time of onset and cessation of rainfall, can be determined.

There are three types of recording rain gauges:

• Tipping bucket rain gauge
• Weighing type rain gauge
• Float type rain gauge

Tipping bucket rain gauge

Tipping bucket rain gauge consists of a cylindrical receiver 30 cm diameter with a funnel inside.

Below the funnel there are a pair of tipping buckets.The buckets pivoted such that when one of the bucket. Tipping bucket type rain gauge (after Raghunath, 2006). receives a rainfall of 0.25 mm it tips and empties into a tank below, while the other bucket takes its position and the process is repeated. The tipping of the bucket actuates on electric circuit which causes a pen to move on a chart wrapped round a drum which revolves by a clock mechanism.

Weighing type rain gauge

In weighing type of rain gauge, when a certain weight of rainfall is collected in a tank, it makes a pen to move on a chart wrapped round a clockdriven drum.

The rotation of the drum sets the time scale while the vertical motion of the pen records the cumulative precipitation

Float type rain gauge

In float type rain gauge, as the rain is collected in a float chamber, the float moves up which makes a pen to move on a chart wrapped round a clock driven drum.

When the float chamber fills up, the water siphons out automatically through a siphon tube kept in an interconnected siphon chamber. The weighing and float type rain gauges can store a moderate snow fall which the operator can weigh or melt and record the equivalent depth of rain.The snow can be melted in the gaugeitself (as it gets collected there) by a heating system fitted to it or by placing in the gauge certain chemicals (Calcium Chloride, ethylene glycol, etc).

Areal Mean Precipitation

Point precipitation: It is the precipitation recorded at a single station.

For small areas less than 50 km2, point precipitation may be taken as the average depth over the area. In large areas, a network of precipitation gauge stations (meteorological stations) has to be installed. As the precipitation over a large area is not uniform, the average depth of    precipitation over the area has to be determined.Areal mean precipitation is the average precipitation of a large area (basin, plain, region etc.) for a specified time period (year, month etc.).

Areal mean precipitation is determined by one of the following three methods:

• Arithmetic mean (average) method
• The isohyetal method
• Thiessen polygon method

Mean precipitation amounts of the precipitation gauge stations for the common (same) time period are used in the application of these methods, because the length of the observation period for each station may be different.

Arithmetic mean (average) method

It is obtained by simply averaging arithmetically the amounts of                  precipitation at the individual precipitation gauge stations (meteorological stations) in the drainage area.

Pave = ∑ Pi / n                          (2.1)

Pave = average depth of precipitation over the area

∑ Pi = sum of precipitation amounts at individual precipitation gauge stations

n = number of precipitation gauge stations in the area

This method is fast and simple and yields good

estimates in flat country (Raghunath, 2006):

• if the gauges are uniformly distributed,
• and if the precipitation at different stations do not vary very widely from the mean.

The isohyetal method

In this method; the precipitations measured at gauging sites (meteorological stations) are plotted on a suitable base map, and the lines of equal recipitation (isohyets) are drawn giving consideration to orographic effects and storm morphology.

An isohyetal map shows lines of equal precipitation drawn the same way a topographic contour map is drawn. An isohyetal map has a precipitation interval between isohyets-10 mm, 25 mm, 50 mm, etc.

The average precipitation between the succesive isohyets (P1, P2, P3,…) are taken as the average of the two isohyetal values.

These averages are; weighted with the areas between the isohyets (a1, a2, a3, …), added up, and divided by the total area of the basin which gives the average depth of precipitation over the entire basin.

Pave = ∑ * (Pi +Pi+1)/2 ] ai / A                     (2.2) ai = area between the two

successive isohyets Pi and Pi+1

A = total area of the basin.

Thiessen polygon method

This method attempts to allow for non-uniform distribution of gauges by providing a weighting factor for each gauge (Raghunath, 2006).

The stations are plotted on a base map and are connected by straight lines.

Perpendicular bisectors are drawn to the straight lines, joining adjacent stations to form polygons.

Each polygon area is assumed to be influenced by the precipitation gauge station inside it.

P1, P2, P3, …. are the precipitations at the individual stations,

and a1, a2, a3, …. are the areas of the polygons surrounding these stations (influence areas).

The average depth of precipitation for the basin is given by

Pave = ∑ Pi ai / A          (2.3) A = total area of the basin.

The results obtained are usually more accurate than those obtained by simple arithmetic averaging.

The gauges (stations) should be properly located over the catchment to get regular shaped polygons.

Evaporation and Transpiration

The process through which water is transferred from the Earth surface (land surface, free water surfaces, soil water, etc.) to the atmosphere is called evaporation. During evaporation process the latent heat of evaporation is taken from the surface of evaporation. Therefore evaporation is considered as a cooling process. Evaporation from land surface, free water

surfaces, soil water, etc.    are of great importance in hydrological and meterological studies because it affects (Usul, 2001):

• the capacity of reservoirs,
• the yield of river basins,
• the size of pumping plants,
• the consumptive use of water by plants, etc.

Transpiration defines the water loss from plants to atmosphere through the pores at the surface of their leaves. In the vegetation covered areas it is almost impossible to differentiate between evaporation and transpiration. Therefore, the two processes are lumped together and referred to as evapotranspiration.

Evaporation

The rate of evaporation and evapotranspiration vary depending on:

• meteorological (atmospheric) factors influencing the region,
• and on the nature of the evaporating surface.

The factors effecting the rate of evaporation (and also evapotranspiration) are:

1. Solar radiation
2. Relative humidity
3. Air temperature
4. Wind
5. Atmospheric pressure
6. Temperature of the liquid water
7. Salinity
8. Aerodynamic characteristics
9. Energy characteristics

Measurement of evaporation

The most common method for the measurement of evaporation is using an evaporation pan.

This is a large pan of water with a water depth measuring instrument.

This device allows to record how much water is lost through evaporation over a time period.

At a standard meteorological station the evaporation is measured daily as the change in water depth. An evaporation pan is filled with water, hence the open water evaporation is measured. A standard evaporation pan, called a Class A evaporation pan is 122 cm in diameter and 25.4 cm deep.

Empirical coefficients (pan coefficient) are applied to estimate the evaporation from larger water bodies (lake, dam resservoir etc.) using measured pan evaporation.

The values of the pan coefficient for Class A evaporation pan ranges between 0.60-0.80, and 0.70 is used as an annual average.

Evaporation Estimation Methods

The difficulties in measuring evaporation using meteorological instruments has led to much effort being placed on estimating evaporation.

There are different methods to estimate evaporation:

1. Water budget method
2. Energy budget method
3. Emperical equations (Thornthwaite, Penman, Penman-Monteith, etc.)

Water budget method

A simple approach to determine evaporation involves the maintenance of a water budget.

Continuity equation can be written in the following form to determine evaporation (E) for a certain period:

E=(∆S+P+Qs) – (Qo+Qss)

∆S: Change in the storage, P: Precipitation,

Qs: Surface inflow, Qo: Surface outflow,

Qss: Subsurface outflow (seepage)

Energy budget method

To determine the evaporation from a lake energy budget can be used.

E=(Qn+Qv-Qo) / ρ.Le (1+R)

Qn: Net radiation absorbed by the water body, Qv: Advected energy of inflow and outflow,

Qo: İncrease in energy stored in the water body, ρ : Density of the water,

Le: Latent heat of vaporization,

R: Ratio of heat loss by conduction to that by evaporation.

Emperical equations (Thornthwaite, Penman, Penman-Monteith, etc.)

Emperical equations are based on measured meteorollogical variables (parameters).

Precipitation, solar radiation, wind speed and relative humidity values are used in estimation of the evaporation by these equations.

Using these equations it is possible to make good estimation of evaporation from lakes for annual, monthly, or daily periods.

Transpiration

Transpiration by a plant leads to evaporation from leaves through small holes (stomata) in the leaf.

This is sometimes referred to as dry leaf evaporation.

Various methods are devised by botanists for the measurement of transpiration. One of the widely used methods is measurement by phytometer (Raghunath, 2006).

A phytometer consists of a closed water tight tank with sufficient soil for plant growth with only the plant exposed.

Water is applied artificially till the plant growth is complete.

The equipment is weighed in the beginning (W1) and at the end of the experiment (W2).

Water applied during the growth (w) is measured and the water consumed by transpiration (Wt) is obtained as

Wt = (W1 + w) – W2

Evapotranspiration

Evapotranspiration (Et) is the total water lost from a cropped (or irrigated) land due to evaporation from the soil and transpiration by the plants.Potential evapotranspiration (Ept) is the evapotranspiration from the short green vegetation when the roots are supplied with unlimited water covering the soil. It is usually expressed as a depth (cm, mm) over the area.

The following are some of the methods of estimating evapotranspiration (Raghunath, 2006):

• Tanks and lysimeter experiments
• Field experimental plots
• Evapotranspiration equations as developed by Lowry-Johnson, Penman, Thornthwaite, Blaney-Criddle, etc.
• Evaporation index method.

Infiltration

Water entering the soil at the ground surface is called infiltration. It replenishes the soil moisture deficiency and the excess water moves downward by the force of gravity. This process is called deep seepage or percolation, recharges groundwater and builds up the ground water table.

The maximum rate at which the soil in any given condition is capable of absorbing water is called its infiltration capacity.

Infiltration (f) often begins at a high rate (20 to 25 cm/hr), and decreases to a fairly steady state rate (fc) as the rain continues, called the ultimate fp (=1.25 to 2.0 cm/hr)

The infiltration rate (f) at any time t is given by Horton’s equation

(Raghunath, 2006): f = fc + (fo – fc) e–kt

fo = initial rate of infiltration capacity

fc = final constant rate of infiltration at saturation

k = a constant depending primarily upon soil and vegetation e = base of the Napierian logarithm

t = time from beginning of the storm

The infiltration depends upon:

• the intensity and duration of rainfall,
• weather (temperature),
• soil characteristics,
• vegetal cover,
• land use,
• initial soil moisture content (initial wetness),
• entrapped air in the soil or rock,
• and depth of the ground water table.

Determination of the Infiltration

The methods of determining infiltration are:

• Infiltrometers
• Observation in pits and ponds
• Lysimeters
• Artificial rain simulators
• Hydrograph analysis

REFERENCES

• Prof.Dr. FİKRET KAÇAROĞLU, Lecture Note, Muğla Sıtkı Koçman University
• Davie, T., 2008, Fundamentals of Hydrology (Second Ed.). Rutledge, 200 p.
• Raghunath, H.M., 2006, Hydrology (Second Ed.). New Age Int. Publ., New Delhi, 463 p.
• Usul, N., Engineering Hydrology. METU Press, Ankara, 404 p.