Hydrology

Precipitation

forms of moisture that reach the ground from the atmosphere(clouds).

USUAL FORMS OF PRECIPITAITON

rainfall. snowfall, hail, glaze, rime, and ice pellets

DRIZZLE

fine sprinkle of numerous water droplets of size less than 0.5 mm, and intensity less than 1mm/h. The drops are so small that they appear to float in the air

RAIN

rainfall is used to describe precipitation in the form of water drops of sizes larger than 0.5 mm

6 mm

maximum size of a raindrop OF RAIN

GLAZE

rain or drizzle comes in contact with cold ground at around 0°C.

SLEET (RIME)

frozen raindrops of transparent grains

HAIL

showery precipitation in the form of irregular pellets or lumps of ice.

HAIL

Size more than 5mm. Sp. Gravity = 0.8, occur in violent thunderstorms in which vertical currents are very strong.

FRONT

It is the interface between two distincy air masses. Under certain favorable conditions when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of a front.

CONVECTIVE PRECIPITATION

In this type of precipitation a packet of air which is warmer than the surrounding air due to localized heating rises because of lesser density.

OROGRAPHIC PRECIPITATION

The moist air masses may get lifted-up higher altitudes due to the presence of mountain barriers and consequently undergo cooling, condensation, and precipitation.

RAINFALL CHARACTERISTIC SIZE AND SHAPE

Drops vary in size from the tiny cloud droplets (measuring less than 0.1 mm in diameter) to the large drops associated with heavy rainfall, and reaching up to 6 mm in diameter.

INTENSITY

Amount of rain in a certain time (depth/time).

DURATION

Time at which rain occurs

DEPTH (d)

Volume of rain over an area (d = Vol/A) d = i*t

HIGH INTERNSITY STORMS

have a larger drop size than low intensity storms

depth to which rainfall water would stand

Precipitation is expressed in terms of the ________________on an area if all the rain were collected on it.

RAIN GAGE SETTING:

The ground must be level and inthe open and the instrument must present a horizontal catch surface. The gauge must be set as near the ground as possible to reduce wind effects. The instrument must be surrounded by an open fenced area of at least 5.5m x 5.5m. No object should be nearer to the instrument than 30m or twice the height of the obstruction.

RAINGAGE NETWORK

n flat regions of temperate, Mediteranean and tropical zone (according to WMO): 1. Ideal - 1 station for 600 - 900 km ^2 Acceptable - 1 station for 900 - 3000 km ^2

1 station for 600 - 900 km ^2

Ideal RAINAGE NETWORK

1 station for 900 - 3000 km ^2

Acceptable RAINAGE NETWORK

1 station for 100 - 250 km ^2

In mountainous regions of temperate, Mediterranean and tropical zones, Ideal

1 station for 250 - 1000 km ^2

In mountainous regions of temperate, Mediterranean and tropical zones,, Acceptable

1 station for 1500 - 10 000 km ^2

In arid and polar zones, RAINGAGE NETWORK

1 station for 25 km ^2 (depending on the feasibility)

In Islands, RAINGAGE NETWORK

NORMAL ANALYSIS

If the normal annual precipitations at various station ae within about 10% of normal annual precipitation at station X:

Evaporation

movement of water to the air from sources such as the soil, canopy interception, and water bodies.

Transpiration

movement of water within a plant and the subsequent loss of water as vapor through stomata of the leaves.

Evapotranspiration

The total amount of water loss in the form of water vapors into the atmosphere from the surface of the soil, canopy interception, water bodies as well as from the aerial parts of plants in a process known as

15%

Evapotranspiration is responsible for______________of the atmosphere's water vapor

Transpiration

Involves various processes like vapor pressure, osmotic pressure, and diffusion

Potential Evapotranspiration

It is the measure of the ability of the atmosphere to remove water from the surface through the processes of evaporation and transpiration assuming no control of water supply

Actual Evapotranspiration

It is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration.

Temperature, Humidity, Water Available, Colour of Surface, Wind Speed

Environmental Factors

Stomatal Resistance

Plants regulate transpiration through adjustment of small openings in the leaves called stomata. As stomata close, the resistance of the leaf to loss of water vapor increases, decreasing to the diffusion of water vapor from plant to the atmosphere

Geographical Factors

Evapotranspiration rates are also dependent upon geography, an area's latitude and climate. Regions on the globe with the most solar radiation experience more evapotranspiration. Evapotranspiration rates are also highest in areas with a hot and dry climate. Evapotranspiration is lesser at higher altitude.

Soil Characteristics

Soil capillary character Water table depth If soil mositure content has high value, evapotranspiration is also high in value.

1) Lysimeter experiment 2) Field experimental plots 3) Soil moisture depletion studies 4) Water balance method

The principal methods for direct measurement of evapotranspiration are:

Lysimeter

special watertight tank containing a block of soil and set in a field of growing plants. The plants grown in the lysimeter are the same as in the surrounding field. Evapotranspiration is estimated in terms of the amount of water required to maintain constant moisture conditions within the tank measured either volumetrically or gravimetrically through an arrangement made in the lysimeter. Lysimeter studies are timeconsuming and expensive

Field Experimental Plots

Measurement of water supplies to the field and changes in soil moisture content of the field plots are sometime more dependable for computing seasonal water requirement of crops than measurement with lysimeters which do not simulate field conditions. The seasonal water requirements are computed by adding measured quantities of irrigation water, the effective rainfall received during the season and the contribution of moisture from the soil. Field water balance may be expressed by the following relationship:

The soil moisture depletion method

is usually employed to determine the consumptive use of irrigated field crops grown on fairly uniform soils when the depth to the ground water is such that it will not influence the soil moisture fluctuation within the root zone.

Seasonal consumptive use (Cu = Σu)

calculated by assuming consumptive use values of each sampling interval. A correction is made by adding PET values for accelerated water loss for the intervals(s) just after irrigation and before soil moisture sampling.

Water Balance Method

It is also called the continuity equation or conservation equation. In other words, it is the balance of the input and output of water within a given area taking into account net changes in storage.

Water Balance Method

Mass inflow - Mass outflow = Change in Inflow Storage

Water Balance Method, density of inflow and outflow is the same,

Volume inflow - Volume outflow = Change in Storage

water budget equation

P - ( R + G + E + T ) = ΔS

Water Balance Method

Precipitation - Runoff = Evapouration

Significance of Evapotranspiration

Plays a major role in precipitation. It is the most significant component of the hydrologic budget. A thorough knowledge of evapotranspiration is very important for planning and adjucating the distribution of water resources.

INFILTRATION

the process of water entry into the soil through the earth's surface. The water at the soil surface can originate from rain, snowmelt or anthropogenic activities.

INFILTRATION

is the flow of water across the ground surface into the soil. This process provides a critical link between surface hydrologic processes and subsurface soil and groundwater systems

INFILTRATION

the physical process involving movement of water through the boundary area where the atmosphere interfaces with the soil. The surface phenomenon is governed by soil surface conditions. Water transfer is related to the porosity of the soil and the permeability of the soil profile

percolation test

is the movement of water through the soil itself.

PERCOLATION

the movement of water though the soil, and it's layers, by gravity and capillary forces

PERCOLATION

Water that is in the zone of aeration where air exists is called vadose water. Water that is in the zone of saturation is called groundwater

PERCOLATION

Water that is in the zone of aeration where air exists is called vadose water. Water that is in the zone of saturation is called groundwater

water table

The boundary that separates the vadose and the saturation zones is called the

SEEPAGE

It is the process of seeping by which a liquid leaks through a porous substance

SEEPAGE

Water that has seeped or oozed through a porous soil

PRECIPITAION

form of water, liquid or solid, that falls from the atmosphere to the Earth's surface. It includes rain, snow, sleet, and hail and is a crucial component of the Earth's water cycle. Precipitation plays a vital role in providing freshwater to ecosystems and human activities, replenishing water sources, and influencing weather patterns.

BASE FLOW

steady and continuous flow of water in a river or stream, primarily sourced from groundwater seeping into the watercourse. It represents the portion of a stream's flow that is not directly influenced by recent precipitation events but is sustained by the slow release of groundwater into the stream. In the hydrological cycle, base flow is related to infiltration because it is a product of water percolating through the ground, recharging aquifers, and subsequently feeding into rivers and streams, contributing to their overall flow

SOIL CHARATERISTICS

physical and chemical properties of the soil that influence its behavior and suitability for various purposes. Soil properties such as texture, structure, porosity, and organic matter content can affect the rate at which water can penetrate the soil, with well-structured, loamy soils generally allowing for better infiltration, while compacted or impermeable soils hinder the process, potentially leading to surface runoff and erosion.

SOIL SATURATION

condition in which soil pores are completely filled with water, leaving no room for additional water to infiltrate or percolate into the soil.

LAND COVER

physical material at the surface of Earth. Land covers include grass, asphalt, trees, bare ground, water, etc.

Slope of the land

essentially the gradient or incline of the land. A steep slope refers to a sharp incline; a gentle slope refers to a slight incline

Evapotranspiration

includes water evaporation into the atmosphere from the soil surface, evaporation from the capillary fringe of the groundwater table, and evaporation from water bodies on land. Evapotranspiration also includes transpiration, which is the water movement from the soil to the atmosphere via plants

HORTON'S INFILTRATION METHOD

an empirical technique used to estimate the rate at which water infiltrates into the soil during a rainfall or precipitation event.

HORTON'S INFILTRATION METHOD

Developed by American hydrologist Robert E. Horton in the early 1940s, this method provides a simple and practical way to estimate infiltration when detailed soil data and instrumentation are not available. Horton's infiltration equation is based on empirical observations and is commonly used in hydrology and civil engineering for quick and approximate estimations of infiltration rates.

Horton infiltration capacity curve

The capacity decreases with time and ultimately reaches a constant rate, caused by filling of soil pores with water, which reduces capillary suction

Horton's Infiltration Method:

F = Fc + ( F0 - Fc )e^-kt

Cumulative Infiltration Volume

F = Fc(t) + ( Fo - Fc/k ) ( 1 - e ^(kt)

GREEN-AMPT INFILTRATION METHOD

widely used in hydrologic modeling to account for loss from infiltration.

GREEN-AMPT INFILTRATION METHOD

was initially developed for ponded infiltration into a homogeneous soil with a uniform initial water content and the water wetting front movement in the soil was governed by Darcy's law.

nitial water content (dimensionless), saturated water content or porosity (dimensionless), wetting front soil suction head (inch), and hydraulic conductivity (in/hr). Some literatures and software list three (3) parameters by combining initial water content and saturated water content as a single parameter - initial water content deficit (saturated water content - initial water content

Generally four (4) parameters are required for Green-Ampt infiltration method

GROUNDWATER

groundwater

infiltrates through soil and water table

saturated zone (phreatic zone)

the zone also known as groundwater soil all pores of the soil are filled with water

Aquifer

saturated formation of earth material

Aquitard

only seepage is possible

aquiclude

high porosity, low permeability

Aquifuge

neither porous or permeable

water table

top surface of saturated zone

Groundwater recharge

infiltration migration percolation, groundwater replaced

Groundwater Discharge

groundwater flow into stream, escape at the surface

Capillary fringe

region above water table, water rises due to capillary forces

Aquifer

geologic unit, store and transmit water

aquifer contamination

manmade product, gasoline, oil road salt and chemical get into groundwater

porosity

percentage of volume occupied by voids

Permeability

transmission property of media, inter

Movement of groundwater

exceedingly slow typical rate of movement

centimeters per day

Movement of groundwater

Darcy

french engineer

darcys law

permeability remains uniform

Hydraulic Gradient

water table slope

Hydraulic head

vertical difference

Q=KA(h1-h2)/d

Formula for Darcys Law

Groundwater Withdrawal

extracting groundwater from aquifer

Lowering the water table

cone of depression circular lowering of water table

Compaction and Surface Subsidence

aquifer rock are no longer saturated with water

karst sinkholes

underground water dissolving large volume of soluble rocks.