1. Irrigation Engineering CE-301
TOPIC –INFILTRATION AND INDICES

2. Preface- Introduction to Infiltration Some related Definitions
Principle behind infiltration
Infiltration in waste water collection
Factors affecting Infiltration

3. Hydrology..
Hydrology means the science of water . It is the science which deals with occurrence , circulation ,and distribution of the water on the earth and it’s atmosphere.

5. Infiltration When water falls on a given formation ,a small part of it is first of all absorbed by the top thin layer of soil ,so as to replenish the soil moisture deficiency. Thereafter ,excess water moves downward ,where it is trapped in the voids and becomes ground water. This process ,whereby the water enters the surface strata of the soil and moves downward towards the watertable ,is known as infiltration.

6. Terms related to Infiltration-
Infiltration capacity
Infiltration rate
Soil moisture
Field capacity
Equivalent moisture

7. Infiltration capacity- The ground water stored in the under-ground soil depends mainly upon the number of voids present in the soil ,which ,in turn ,does not depend upon the size of the soil particles but rather upon the arrangement ,sorting ,shape and degree of compaction . Therefore different soils will have different no. of voids and hence ,different capacity to absorb water. The maximum rate at which a; soil in any given condition is capable of absorbing water is called its infiltration capacity . denoted by f.
Infiltration rate-
It is evident that the rain water will enter the ground soil at a full capacity rate (f) only during the period when rainfall exceeds the infiltration capacity. When the rain intensity is less than the infiltration capacity, the prevailing infiltration rate is approximately equal to the rainfall rate. Hence, the actual prevailing infiltration may be equal to or less than the infiltration capacity. This actually prevailing rate at which the water will enter the soil at any given time is known as the infiltration rate.

8. If the rainfall intensity(p) exceeds the infiltration capacity(f) ,the difference is called the excess rainfall rate(pe).This excess water is, first of all ,accumulated on the ground as surface detention(D) and then flows overland into the streams.

9. Soil moisture The water below the watertable is known as the ground water and the water above the watertable is known as the soil moisture.
The region above the watertable is devided into 3 zones:
1. Capillary zone
2. Intermediate zone
3. Soil zone

10. Extending above the water table , a distance usually ranging from about 0.3 to 3m ,depending upon texture, is a zone the moisture content is maintained practically constant by capillary.
Extending down from the ground surface is the soil zone , which is defined as being the depth of overburden that is penetrate by the roots of vegetation.
Throughout this root zone ,the m.c. varies tremendously, ranging from a partly saturated state during and immediately following the periods of heavy rains to a minimum content after a spell of long continued drought.

11. ground surface soil zone intermediate zone soil moisture
water table zone
capillary zone
ground water zone
The zone between the capillary zone and the soil zone is called the intermediate zone. Throughout this zone, except during the period of ground water accretion from rainfall , the amount of water contained within any given space is nearly constant throughout the year.
In many cases, the height of capillary fringe (or zone) may be more and may extent up to soil zone. In this case, there will be no intermediate zone.

12. Field capacity
Immediately after a rain, when all the gravity water has drained down to the water table, a certain amount of water is retained on the surface of the soil grains by molecular attraction. This is known as pellicular water. This water cannot be easily removed. The degree of its resistance to the movement is expressed by the surface tension. When this water, which any soil can retain indefinitely against the force of gravity, is expressed in terms of depth of water (as of it were spread on the basin), then it is known as the field capacity.
A certain portion of this pellicular water which can be easily abstracted by the root action of vegetation is called the available moisture.
The remainder is called the hygroscopic water, which is unavailable moisture.
The soil can extract water from the soil till the wilting point is reached. Therefore the available moisture is the moisture between the field capacity(upper limit) and the wilting point (lower limit).

13. The wilting point is that moisture content at which permanent wilting takes place.
The hygroscopic water is therefore, the m.c. of the soil, just after the soil has wilted up.
The depth of the water required to bring the soil m.c. up to its field
capacity, is called the field moisture deficiency or soil moisture deficiency.
Equivalent moisture
Just as the field capacity is the water retained by a saturated soil after acted upon by gravity, similarly the equivalent moisture is the water retained by a saturated soil after being centrifuged by a centrifugal force of 1000 times that of gravity. Therefore, it is slightly less or at the most equal to the field capacity.

14. Infiltration in waste water collection-
Waste water collection system consist of a set of lines, junctions and lift stations to convey sewage to a waste water treatment plant. When these lines are compromised by rupture, cracking or tree root invasion, Infiltration/inflow of storm water often occurs. This circumstance can lead to a sanitary sewer overflow ,or discharge of untreated sewage to the environment.
Infiltration in waste water collection-

15. Why is infiltration a concern??
Proper management of the soil can help maximize.
infiltration and capture as much water as allowed by a
specific soil type
If the water infiltration is restricted or blocked water does not enter the soil and it either ponds on the surface or runs off the land thus ,less water is stored in the soil profile for use by plants.
Run off can carry soil particles and surface applied fertilizers and pesticides off the field.

16. Factors influencing infiltration
Texture
Crust
Compaction
Aggregation and structure
Water content
Frozen surface
Organic matter
Pores