The process through which water is transferred from the surface of the Earth (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:

• 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.

The water returns to the atmospherein vapor form, not via a single mechanism, but through three distinct processes.

• the first process involves the fraction of water intercepted by vegetation before reaching the ground,
• the second is the transpiration of plants,
• and the third is the evaporation of gravitational water.

A portion of the precipitation falling on the vegetation covered land may be retained by plants. This portion is called interception.

This portion generally evaporates back to the atmosphere without reaching the ground surface.A very small amount of the water retained on the plants falls on the ground from the leaves. This portion is named as throughfall.

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

Evaporation begins with the movement of molecules of water.Inside a mass of liquid water, the molecules vibrate and circulate in random fashion.This movement is related to the temperature: the higher the temperature, the more the movement is amplified.

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. Depth of water
9. Aerodynamic characteristics
10. Energy characteristics

Solar radiation

Solar radiation is a driving force of weather and climatic conditions, and consequently, of the hydrological cycle.Solar radiation supplies the energy necessary for the liquid water molecules to evaporate.

Solar radiation affects

• the atmosphere,
• the hydrosphere
• and the lithosphere

At the time of evaporation, thermal energy (i.e. sensible heat) is transferred into latent energy.Latent heat (energy) is the heat either absorbed or released during a phase change from ice to liquid water, or liquid water to water vapour.When water moves from liquid to gas this is a negative flux (i.e. energy is absorbed). During the opposite phase change (gas to liquid) positive heat flux occures (i.e. energy is released).

Relative humidity

For a certain temperature and air pressureit is possible to specify the maximum amount of water vapour that may be held by the parcel of air.

The saturation deficit is the difference between the saturation vapor pressure e_S and the actual vapor pressure e_a.

This deficit (e_s-e_a) can also be described in relation to the concept of relative humidity H_r, H_r = (e_a / e_s). 100

Relative humidity is the relationship between the quantity of water contained in an air mass and the maximum quantity of water the air mass can hold.

H_r = (e_a / e_s). 100

The ability of the air to absorb more water vapor decreases as the humidity of air increases, so the rate of evaporation becomes slower.

Air temperature

Temperature is closely linked to the rate of radiation. Radiation itself is correlated directly to evaporation. It follows, then, that there is a relationship between evaporation and the temperature at the evaporating surface. The rate of evaporation is, in particular, a function of increasing temperature.Near the ground, air temperature is heavily

influenced by

• the nature of the land surface
• and the amount of sunshine.

The total amount of water vapour that may be held by a parcel of air is temperature and pressure dependent.

The temperature of air has double effect on evaporation:

• It increases saturation vapor pressure, which means increasing the saturation deficit.
• On the other hand, high temperature implies that there is energy available for evaporation.

Wind

As the liquid water vaporizes from a water body, land surface, or soil, etc.the air adjacent to these environments will become vapor saturated. For the continuation of evaporation, this saturated air should be removed. In other words atmospheric mixing has to occure.

The wind plays an essential role in the evaporation processbecause, it replaces the saturated air next to an evaporating surface with a drier layer of air. The removal of the saturated air (atmospheric mixing) is carried out by the wind.If the wind speed is zerothe parcel of air will not move away from the evaporative surface and will be saturated with water vapour.

In general, a 10% change in the wind speed causes 1-3% change  in the evaporation amount when the other meteorological factors are the same.

Atmospheric pressure

Atmospheric pressure, is expressed

• in kilopascals (kPa),
• in millimeters of mercury (mm Hg)
• or in millibars (mb).

It represents the weight of a column of air per unit of area. An increase in atmospheric pressureprevents the movement of molecules out of water. The rate of evaporation increaseswhen atmospheric pressure decreases .It may be an important factor where there is an elevation difference of more than a few thousand meters.

Temperature of the liquid water

Molecular motion in the water is temperature dependent. When the temperature of the liquid water is high, molecular motion is fast. In this case the number of molecules leaving the water body will also be high, resulting an increase in evaporation.

If the temperature of the evaporating water is high, it can more readily vaporize. Thus evaporation amounts are high in tropical climates and tend to be low in polar regions. Similar contrasts are found between summer and winter evaporation quantities in mid-latitudes.

Salinity

The salinity (total dissolved solids) refers to all ions (cations and anions) dissolved in the water. The salinity of the water adversely affects evaporation. A 1% increase in salt concentration causes a 1% decrease in evaporation. A similar relationship exists with other substances in solution, because the dissolution of any substance brings about a decrease of vapor pressure. This drop in pressure is directly proportional to the concentration of the substance in solution.

Depth of water

The depth of a body of water plays a determining role in its capacity to store energy. The main difference between a shallow waterbody and a deeper one is that the shallow water is more sensitive to seasonal climatic variations. A shallow waterbody will be more sensitive to weather variations depending on the season.Deeper waterbodies, due to their thermal inertia, will have a very different evaporation response.

Aerodynamic characteristics

The aerodynamic characteristics of the surface such as

• roughness,
• texture of the material on the surface (fine or coarse materials),
• or size of the surface

also affect the amount of the evaporation.

Energy characteristics

The reflection coefficient (albedo) of the surface defines the energy characteristics of the surface.If this coefficient (albedo) is high, a larger portion of the incoming radiation will be reflected, and then evaporation will be lower from that surface.

REFERENCES

• Prof.Dr. FİKRET KAÇAROĞLU, Lecture Note, Muğla Sıtkı Koçman University
• Davie, T., 2008, Fundamentals of Hydrology (Second Ed.). Rutledge, 200 p.
• Musy, A., Higy, C., Hydrology. CRC Press, 316 p.
• Newson, M., 1994. Hydrology and The River Environment. Oxford Univ. Pres, UK, 221 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.

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.

Hydrogeology is a branch of geology that deals with the study of the distribution, movement, and quality of water in the subsurface. Hydrogeology is concerned with understanding the occurrence, movement, and storage of groundwater in aquifers, which are geological formations that contain water. Hydrogeologists study the properties of rocks and sediments that control the movement of water, the interaction between groundwater and surface water, and the impact of human activities on the quality and quantity of groundwater resources. Hydrogeology is an interdisciplinary field that draws on geology, physics, chemistry, mathematics, and engineering to address a wide range of environmental, geological, and engineering problems.

Water is a precious natural resource. Without water there would be no life on Earth. Two-thirds of our body is composed of water by weight.

Water supplies are also essential in supporting food production and industrial activity. The most important factor that determine the density and distribution of vegetation is the amount of the precipitation (Fetter, 2001).

Agriculture can flourish in some deserts, but only with water either pumped from the ground or imported from other areas (Fetter, 2001).

Civilizations have flourished with the development of reliable water supplies, and then collapsed as their water supplies failed (Fetter, 2001).

A person requires 3 liters (L) of potable water per day to maintain the essential fluids of the body (Fetter, 2001).

Primitive people in arid lands existed with little more than this amount as their total daily consumption

In New York City the daily per capita water usage exceeds 1000 L; much of this is used for industrial, municipal, and commercial purposes (Fetter, 2001).

The over-exploitation of groundwater by uncontrolled pumping can cause some problems (Hiscock, 2005):

• detrimental effects on neighbouring boreholes and wells,
• land subsidence,
• saline water intrusion,
• and the drying out of surface waters and wetlands.

Uncontrolled uses of chemicals and the careless disposal of wastes on land cause groundwater pollution (Hiscock, 2005).

Major sources of groundwater pollution :

• agrochemicals,
• industrial and municipal wastes,
• tailings and process wastewater from mines,
• oil field brine pits,
• leaking underground storage tanks,
• leaking pipelines,
• sewage sludge,
• and septic systems

Scope of Hydrogeology

Hydrogeology is the scientific study of the properties, distribution, and movement of groundwater in the Earth’s subsurface. It is an interdisciplinary field that combines elements of geology, hydrology, chemistry, physics, and engineering. The scope of hydrogeology includes the following:

1. Study of groundwater occurrence and availability: Hydrogeologists study the occurrence, distribution, and availability of groundwater in the subsurface. They use various techniques such as geophysical surveys, drilling, and well logging to explore the subsurface.
2. Groundwater flow and transport: Hydrogeologists study the flow and transport of groundwater in the subsurface. They use numerical models to predict the direction and rate of groundwater flow, and to simulate the transport of contaminants in groundwater.
3. Aquifer characterization: Hydrogeologists characterize the properties of aquifers, which are geologic formations that store and transmit groundwater. They study the hydraulic properties of aquifers such as hydraulic conductivity, transmissivity, and storage coefficient.
4. Groundwater quality: Hydrogeologists study the quality of groundwater, including its chemical composition and the presence of contaminants. They use various techniques to sample and analyze groundwater, such as pumping tests, slug tests, and well logging.
5. Groundwater management: Hydrogeologists play a key role in the management of groundwater resources. They use their knowledge of hydrogeology to develop strategies for sustainable use and protection of groundwater resources. This includes designing well fields, managing groundwater recharge, and controlling groundwater contamination.
6. Interaction of groundwater with surface water: Hydrogeologists study the interaction of groundwater with surface water, such as rivers, lakes, and wetlands. They use their knowledge of hydrogeology to understand the role of groundwater in maintaining the flow of surface water and to develop strategies for managing water resources in a sustainable manner.

Hydrogeological investigation

Hydrogeological investigation is the process of studying the properties and behavior of water in the subsurface. It involves the use of various tools and techniques to gather data about the hydrogeological system, such as the geology and hydrology of an area, the quantity and quality of groundwater, and the potential for water resource development and management.

Hydrogeological investigation is important in many applications, such as in the planning and design of groundwater supply systems, the identification of potential water sources for mining or industrial operations, the assessment of environmental impacts related to groundwater, and the evaluation of potential impacts of climate change on groundwater resources.

Hydrogeological investigation may involve a range of activities, such as geological mapping, hydrological data collection, aquifer testing, water quality analysis, and computer modeling of groundwater flow and transport. The results of hydrogeological investigations can be used to make informed decisions about the sustainable use and management of groundwater resources.

There are several steps involved in a hydrogeological investigation, including:

1. Defining the study area: The first step in a hydrogeological investigation is to define the area of study, including the location and boundaries of the study area.
2. Collecting data: The next step is to gather information about the geology, hydrology, and hydrogeology of the study area. This may include collecting data on the geology of the area, the surface and subsurface hydrology, and the groundwater resources.
3. Analyzing data: The collected data is then analyzed to understand the occurrence and movement of groundwater in the study area. This may involve analyzing geological and hydrological data, as well as data on the quality and quantity of groundwater resources.
4. Developing a conceptual model: Based on the data collected and analyzed, a conceptual model of the groundwater system in the study area is developed. This model helps to understand how groundwater moves through the subsurface and how it is affected by various factors.
5. Testing and refining the model: The conceptual model is then tested and refined through further data collection and analysis, in order to improve the understanding of the groundwater system.
6. Reporting findings: The final step in a hydrogeological investigation is to report the findings of the study, including any recommendations for the management and use of groundwater resources in the study area.

Hydrogeology and Human Affairs

ydrogeology is closely tied to human affairs in many ways. Here are a few examples:

1. Water supply: One of the most important applications of hydrogeology is to assess and manage groundwater resources for water supply. Hydrogeologists investigate and characterize aquifers, estimate recharge rates and groundwater flow, and develop models to predict how aquifers will respond to different pumping scenarios. This information is used by water managers to make decisions about water allocation, well placement, and pumping rates.
2. Contaminant transport: Hydrogeologists also play a key role in assessing and managing groundwater contamination. They investigate the movement of contaminants in groundwater, assess the potential for contaminants to reach drinking water sources, and develop strategies to remediate contaminated sites. Hydrogeological investigations are often part of environmental site assessments for industrial sites, landfills, and other contaminated sites.
3. Land use planning: Hydrogeology is important in land use planning, particularly in areas where groundwater resources are vulnerable to contamination or overuse. Hydrogeological investigations can identify areas that are suitable for certain types of development (e.g. residential, industrial, agricultural), as well as areas that should be protected from development to maintain groundwater resources.
4. Climate change: Hydrogeology is also important in understanding the impacts of climate change on groundwater resources. As patterns of precipitation and evapotranspiration change, groundwater recharge rates and groundwater flow patterns are likely to be affected. Hydrogeological investigations can help to predict how aquifers will respond to these changes and identify areas that are particularly vulnerable to drought or other impacts.

Overall, hydrogeology is an important field that contributes to our understanding of water resources and their interaction with human activities.

History of Hydrogeology

The history of hydrogeology dates back to ancient civilizations, such as the Greeks and Romans, who were interested in the origin and movement of groundwater. The first recorded scientific investigation of groundwater was conducted by Leonardo da Vinci in the 15th century. He proposed that the movement of water through the Earth was driven by the sun’s heat and gravity.

During the 18th and 19th centuries, significant advances were made in the field of hydrogeology. Scientists began to develop theories on groundwater flow and the relationship between surface water and groundwater. The development of new technology, such as drilling equipment and pumps, allowed for the construction of wells and the measurement of groundwater levels. This led to a better understanding of the quantity and quality of groundwater resources.

In the 20th century, hydrogeology became increasingly important for water resources management and environmental protection. The development of new techniques, such as geophysical surveys and computer modeling, allowed for more accurate and efficient groundwater exploration and management. Today, hydrogeologists play a crucial role in ensuring the sustainability of groundwater resources and protecting the environment from contamination.

Hydrologic Cycle

The water on our planet Earth is found in three phases, as solid, liquid and gas.

The hydrologic cycle, also known as the water cycle, is the process by which water moves through the earth’s systems. The cycle includes the following steps:

1. Evaporation: The process by which water changes from a liquid to a gas, usually from the surface of oceans, lakes, and rivers or from the ground.
2. Transpiration: The process by which water is absorbed and released into the atmosphere by plants.
3. Condensation: The process by which water vapor in the atmosphere cools and changes back into liquid form, forming clouds.
4. Precipitation: The process by which water falls from the atmosphere in the form of rain, snow, sleet, or hail.
5. Infiltration: The process by which water seeps into the ground and is absorbed by soil and rock.
6. Runoff: The process by which water that does not infiltrate the ground flows over the surface of the earth, eventually making its way to streams, rivers, and oceans.

The hydrologic cycle plays a crucial role in regulating the amount and distribution of water on the earth’s surface and in the ground, which is important for sustaining life and supporting various ecosystems.

Evaporation of the water from the surface waters (sea, lake, and river) and land surface and transpiration from vegetation produces clouds.

When suitable meteorological conditions arises, precipitation occurs as rain, snow, etc., and falls on land or surface water bodies.

A portion of the precipitation falling on the vegetation covered land may be retained by plants. This portion is called interception.

This portion generally evaporates back to the atmosphere.

A very small amount of the water retained on the plants falls on the ground from the leaves. This portion is named as through fall.

Precipitation that falls on the land surface enters various pathways of the hydrologic cycle.

The part of the precipitation reaching the ground surface first wets the soil and rocks.

Some water may be temporarily stored on the land surface as ice and snow or water in puddles. This is known as depression storage.

Some of the rain or melting snow drains across the land to a stream channel, lake, or sea. This is termed overland flow or surface flow.

If the surface soil or rock is porous, some rain or melting snow will seep into the ground. This process is called infiltration.

A portion of the infiltrated water is stored in the vadose zone (or zone of aeration).

The soil and rock pores in the vadose zone contain both water and air.

The water in the vadose zone is called vadose water.

At the top of the vadose zone is the belt of soil water.

Some parts of the waters stored in depressions, vadose zone, and flowing as overland flow evaporates.

The plants use the soil water, and then transpire as vapour to the atmosphere by a process called transpiration.

Evaporation form the land surface, water bodies, and transpiration by plants are lumped together as evapotranspiration.

The water entering the soil or rock may move laterally in the vadose zone above the groundwater table towards lower elevations.

This water is called interflow or subsurface flow.

Part of the infiltrated water; may reach the groundwater table by percolation, recharge groundwater storage.

Then the water moves there horizontally becoming groundwater flow (or base flow).

Surface, subsurface and groundwater flows eventually reach sea lake, and stream and from there evaporate back to the atmosphere.

At some depth, the pores of the soil or rock are saturated with water.

The top of the zone of saturation is called the water table (or groundwater table).

Water stored in the zone of the saturation is known as groundwater.

Groundwater moves as groundwater flow through the rock and soil layers of the earth.

Groundwater discharges as a spring or as seepage into a pond, lake, stream, river, sea, or ocean.

The figure shows the major reservoirs and the pathways by which water can move from one reservoir to others.

Magmatic water is contained within magmas deep in the crust.

If the magma reaches the surface of the earth or the ocean floor, the magmatic water is added to the water in the hydrologic cycle (Fetter, 2001).

Hydrologic processes rarely operate completely uninfluenced by human activities; in other words human activities cause changes in these processes.

The main activities that result in modifications in the hydrologic processes are;

• artificial precipitation,
• modifications in the vegetative cover (afforestation, deforestation, change in vegetation type),
• urbanization,
• construction of dams on the rivers,
• irrigation,
• drainage,
• abstraction of groundwater and surface water.

Global Distribution of the Water

The water in the whole Earth is in equilibrium.Saline water in the oceans

accounts for 97.25%.Land masses and the atmosphere therefore contain 2.75%.Ice caps and glaciers hold 2.05%Groundwater to a depth of 4 km accounts for 0.68%,Freshwater lakes 0.01%,Soil moisture 0.005%, Rivers 0.0001%, and biosphere 0.00004%

About 75% of the water in land areas is locked in glacial ice or is saline.

The remaining quarter of water in land areas, around 98% is stored underground.

Only a very small amount of freshwater available to humans and other biota.

Taking the constant volume of water in a given reservoir and dividing by the rate of addition (or loss) of water to (from) it enables the calculation of a residence time for that reservoir.

The time that a water molecule spends in the ocean and sea more than 4 000 years.

Lakes, rivers, glaciers and shallow groundwater have residence times ranging between days and thousands of years.

Groundwater residence times vary from about 2 weeks to 10 000 years, and longer.

A similar estimation for rivers provides a value of about 2 weeks.

Basin Properties

Basin properties refer to the physical, geological, and hydrological characteristics of a watershed or river basin that influence the quantity and quality of water available within it. Some important basin properties include:

1. Size and shape: The size and shape of a basin determines the area from which water is collected and the quantity of water that can be stored within it.
2. Topography: The topography of a basin determines the direction of flow of water and affects the rate of surface runoff.
3. Geology: The geology of a basin determines the type of rocks and soils that are present, which can affect the storage and movement of groundwater.
4. Soil characteristics: Soil characteristics such as texture, structure, and permeability affect the rate of infiltration of water into the ground and the amount of water that can be stored in the soil.
5. Vegetation cover: Vegetation cover affects the rate of infiltration and evapotranspiration, which are important processes in the hydrologic cycle.
6. Climate: Climate plays a major role in the hydrologic cycle, with temperature, precipitation, and evapotranspiration rates affecting the amount and distribution of water within a basin.
7. Land use: Land use changes, such as urbanization or deforestation, can have a significant impact on the hydrologic cycle by altering surface runoff, infiltration rates, and evapotranspiration.

References

• Prof.Dr. FİKRET KAÇAROĞLU, Lecture Note, Muğla Sıtkı Koçman University
• Domenico, P.A., Schwartz, F.W., 1990. Physical and Chemical Hydrogeology. John Wileyand Sons, USA, 824 p.
• Fetter, C.W., 2001. Applied Hydrogeology (Fourth Ed.). Prentice Hall, USA, 598 p.
• Hiscock, K., 2005, Hydrogeology. Blackwell Publishing, 389p.
• Younger, P.L., 2007, Groundwater in the Environment. Blackwell Publishing, 318 p.
• Usul, N., Engineering Hydrology. METU Press, Ankara, 404 p.
• Newson, M., 1994. Hydrology and The River Environment. Oxford Univ. Pres, UK, 221 p.