Soil Moisture Constants'

1. Saturation Capacity:

It may be define as the total water content of a soil when 100 % of the pores of soil are filled with water, that means it is the maximum water holding capacity of soil.

2. Filed Capacity:

It is the amount of water content held in the soil after excess water gets drained off due to gravity and rate of downward movement has decreased or it can be defined as the bulk water content retained in the soil at particular hydraulic head. Field capacity is characterized by measuring water content after wetting a soil profile, covering it and monitoring the changing soil moisture in the profile. When the rate of changing soil moisture is very small, the water content at that point in said to field capacity. 

3. Permanent Wilting Point:

Water content at which plant is no longer able to extract water from the soil for its growth. at permanent wilting point, limits of water are held very tightly and hence plant roots are not able to extract sufficient water for their growth.

4. Readily Available Moisture: 

It is the portion of available water which can be most readily extracted by the plants, generally readily available moisture is 75% of the available moisture.

5. Available Moisture:

It is the difference in water content of the soil between field capacity and permanent wilting point.

6. Leaching:

It is the phenomena of application  of excess irrigation water in order to avoid building up of salinity in the soil.

7. Soil Moisture Tension:

Soil moisture tension can be defined as force per unit area that must be exerted by plant roots in order to extract water from the soil.

8. Osmotic Pressure:

When two salt solution differing in concentration are separated by a semipermeable membrane water moves from the solution of lower concentration to that  of higher concentration, the force with which the water moves across such a membrane is called osmotic pressure. Plant growth is main function of soil moisture stress. It can be drastically reduced by controlled leaching. 

9. Soil Moisture Stresses:

Soil moisture stresses is the sum of soil moisture tension and osmotic pressure of salt solution is soil.

Set Back Distance

Set back distance is the clear distance required from center line of the curve to the obstruction on the inner side of the curve to provide adequate sight distances,

Two different cases of calculating setback distance,

Case-1: When length of a horizontal curve is greater than the sight distance (Lc > SD)

• For Single Lane:

Let M is the setback distance and sight distance is measured along the center line, then the setback distance from the center line of road is given,

But we know that R is the radius of a circle then the length of an arc l subtending an angle of α degree at the center is,

l = R.α

length of arc = SD

R.α = SD

by dividing the above equation by 2 ,

 

but α is in degree so, 

• For Multi Lane:

Let 2d is the total width of a lane, then setback distance from the center line of road having only two lane is given,

Now if we have to calculate the setback distance from the center line of inner lane, then it will be given as, For a two lane road, 

and 

Case-2: When the length of horizontal curve is less than the sight distance (Lc < SD)

• For Single Lane:

AC - Setback distance from the center line of the road,

hence,

M = MAB + MBC

so, 

for MBC, consider a triangle PQR 

therefore setback distance from center line of road is, 

but α is in degree so,

• For Two Lane:

 

Setback distance is given as,

M = MAB + MBC

therefore,

and

setback distance from center line of inner lane is, 

and