Soil Moisture and Ground Water

Introduction

All the surface as well as atmospheric water are the part of water cycle and in rare case minor amount of groundwater enter from anotrher origins.the water which has been out of the contact with atmosphere for appreciable geologic period called as connate water.water from magma is called magmatic water and water at deep separation is called plutonic water.water from shallow depth called as volcanic water and water fom from magmatic or cosmic origin and have not been part of hydrosphere is called juvile water and water associated with rocks during metamorphis called as metamorphic water.

different types of dating like radioactive,tritium arears and age dating techniques have revelled groundwater age from few years to several hundred of years.

Definations of Porosity

The water bearing capacity with in the pore space,voids,pore of rock or soil is called Porosity.Normally the portions of rock or soil occupied by groundwater instead of solid mineral matter which consists voids,interstices spaces,pores and pores space.The interstices serve is also known as water conduits are the fundamental importance to study groundwater.Typically they are having different shape,size,distribution and irregularity.interstices are found in different igneous and metamorphic rocks created by geological processes governing the origin of geologic formation are called as primary interstices.secondary interstices are developed after the formation of rocks which include joints,fractures,solution openings,opening formed by plants and Animals .on the basis of size interstices can be classified into capillary,supercapillary and subcapillary. capillary interstices are sufficently smaller so surface tension forces hold water within them.supercapillary interstices are larger compare to the capillary and subcapiillary interstices so the water is hold by adhesive forces.

Dependings on connection of interstices they are classified as communicating or isolated.The Porosity of rock or soil can be measured by contained by interstices or voids expressed as the ratio of volume of interstices to total volume.

If A is porosity

A=Vv/V =Vt-Vs/Vt

Where Vv=volume of interstices

Vs=volume of solids

V=Total bulk volume

Porosity can be expressed as

A=Pm-Pd/Pm =1-Pd/Pm

Pm=dendity of minerals particles(grain density)

Pd=bulk density

The amount of interconnected pore space available for fluid flow is known as effective Porosity.It is expressed as ratio of interconnected interstices to total volume.The two different porosities mainly for unconsolidated porous media and many consolidated rocks are identified.primary and secondary porosity are associated with original and secondary interstices.

Soil classification

The unconsolidated geologic materials are classified according to size and distributionnormally.The evaluation of distribution of size is accompolised by mechanical analyisis based on particle or grain size.This classification includes sieving particles coarser than 0.05mm and measuring rates of settlement for smaller particles in suspensions.thus the results obtained are plotted particle size distribution graph.The percentage of finer scale on ordinates shows percentage of matter than that of the given particle size by dry weight basis.The effective particle size is 10 percent finer than value(vd) and distribution of particle is characterised by uniformity coefficient vc

Vc=d60ld10

d60=60 percent finer than value

The uniform material have low uniformity coefficient than the well graded material(alluvium

Soil Classification Chart(Morris and johnson45)

clay <0.004

silt 0.004-0.062

very fine sand 0.062-0.0125

fine sand 0.0125-0.25

medium sand 0.25-0.5

coarse sand 0.5-1.0

very coarse sand 1.0-2.0

very fine gravel 2.0-4.0

fine gravel 4.0-8.0

medium gravel 8.0-16.0

coarse gravel 16.0-32.0

very coarse gravel 32.0-64.0

The relative portions of sand,silt,and clay present in particle size analysis called texture of soil which can be expressed as soil textural triangle.

Forces acting on Groundwater

The retentative property of soil is influenced by the surface area it occupies which depend on particle sizes.shape and type of clay or another minerals present.specific surface refers to area per unit weight of materials which can be expressed as m2/g.the relative method for measuring specific surfaces are based retention of polar organic moleculeas ethylene,glycol which is related to absolute value derived from statistical calculations.mostly clay particles contributes greatest amount of surface area than consolidated particles.most non swelling clay such as kaolinite have external surface and exhits more specific surfaces and swelling clays such as montmorillonite and vermiculite have internal and external surfaces the yield specific surface value about 800m2/g.

Vertical Distribution of Groundwater

The subsurface water can be divided into two zones.mainly zone of aeration which partially water and air are contained in fixed ratio and zone of saturation are interstices are filled with water under hydrostatic pressure.mostly in land masses a single zone of aeration overlies asingle zone of saturation and extends upward to groundwater.In zone of areation vadose water occurs which can further divided into soil water zone,intermediate vadose zone and capillary zone.The saturation extends slightly above water table due to capillary attraction where the water present there have lessatmospheric pressure.water present in zone of saturation is commonly known as ground water and it can also said as phreatic water

Zone of Aeration :

The further description is given below

soil Water zone: The soil water zone only exists at less than saturation except temporarily when excessive water reaches ground from the overflow by rainfall or irrigation .the zone extends from groundwater passing through major water root zones.the thickness varies from the vegetation and the soil types.the water is supplying moisture to roots,agricuturists and soil moisture distributionand movement.the amount of water in soil water zone depends on recent exposure of soil to moisture.in hot and arid condition .water vapour equillibrium tends to established between anbient air and surface of fine grained and a thin films of moisture develop khown as hygroscope water which remained absorbed on the surfaces,for coarse grained materials additional moisture is avilable,water formed liquid rings surrouding contact between grains.the water is hold by surface tension forcea and also called as capillary water.this is also known as gravitational water which drains through soil under influence of gravity.

Intermediate Vadose zone; The intermediate vadose zone extends from lower part of soil water zone and upper part of the capillary zones.the thickness varies from 0 to 100m.under deep water table condition.the zone serves as connection between zone near ground water with water table through which water moving vertically downward.the temporary water migrates downwards as gravitational water by hydroscopic or capillary forces.

capillary zones: The capillary zones lies below vadose zone and up to limit of capillary rise of water.the material containing innmerable pores of wide rangr of size and upper boundary of zones look like jagged with observed microscopically.the areas above water table consists of pores with capillary water and only few smallest connected pores contains water lifted above water table.

Available water: The retained and absorbed water by soil is withdrawn by plants during period of rainfall and irrigation.The water holding capacity is called available water which is range of plant available water and moist end is called field capacity and dry end is called wilting point.the field capacity is the amount of water held in soil after wetting and subsequent drinage.the wiling point is the water contents of soil whenplant growing in soil reduced permanent wilted conditions.

Zone of Saturation:

The zone which lies below the zone of Aeration is called zone of Saturation in which groundwater fills all of interstices and porosity provides direct measures of water contained per unit.the water can be drainaged or pumped from zone of saturation.the surface tension and molecular forces hold the surface water.