 meaning that the process of infiltration is important from the point of view of fusing fire irrigation, goes into the soil and how much of it will flow over the soil. But the infiltration rate is equipment also which can be used for finding out the infiltration rate. What you do is you absorb the depth of water which has been lost in a period of time. That will give you the infiltration rate. Now what is important to understand is what is, what are the various factors which influence this infiltration rate. If you look in nature the soil is dry, moisture tension at a very high level, it will be able to attract the moisture with lot of uhh force because when you look at the moisture availability in soil, the moisture is trapped in the soil, pores in the form of uhh the water under tension okay. Now this surface tension is because of two things, one is the tension between the soil particles and water which is which is the force of a design, another force which is working which is the tension between the water particles, water molecules that is known as the cohesion. These are when cohesion forces they are responsible for the moisture availability in the soil. Because of these forces the moisture tension keeps on varying depending on what is the level of moisture availability in the soil. Look at the various factors, the major factors which influence the infiltration rate. The soil texture is one, then the soil moisture content and the soil structure, these are the three major factors which influence the infiltration rate. For example in the case of uhh soil texture if the soil is coarse soil, the pore spaces will be large enough, they will let the water move in the downward direction at a rate which is much higher in comparison to those soils which are fine soils. In the case of fine soils, your pore spaces will be will be much smaller and they will be creating hindrance to the movement of the moisture. So the infiltration rate will depend largely on the the type of soil which we are using uhh to know the infiltration rate. Similar to the soil moisture content we have just seen that the moisture availability in the soil is dependent on the adhesion and the cohesion forces when the moisture content increases, these forces the moisture tension also reduces. So when the soil is at a higher moisture tension, it will have the capability of attracting more more water into it and that will in turn increase the infiltration rate. On the other hand when the moisture tension is much less or you can say when the moisture content of the soil is higher then the the infiltration rate will be lower. And the soil structure also influences the infiltration rate because the soil is granular soil is uhh uhh is structured in a granular fashion, the infiltration rate will be higher on the content if the structure of the soil is massive, the infiltration rate will be influenced it will reduce, okay. Later on we will again come back to these aspects when we deal with the infiltration process in detail, right now at this stage I will like you to have a basic understanding of what the influence of these individual factors which influence the soil and water relationships. It is also important to look at the various conditions of soil moisture and when we talk of the soil moisture condition the first thing which we which we must know about is the soil moisture content, this is one term which we must have the clearest understanding about what do we mean by soil moisture content. Soil moisture content is nothing but is the level of availability of the moisture in the soil with respect to the depth it can be expressed in various manners, you can express in terms of uhh the depth of moisture per unit depth of soil or you can express in the form of uhh percentage uhh by volume, how much of uhh percentage of soil volume is occupied by the water. Now this is one depiction where if we assume that there is a block of dry soil and you add its depth is 1 meter, its area is also square meter and if you add a depth of water of 150 millimetres that water will penetrate into the soil and it will increase its moisture content. So in this uhh moisture content either you can say that is 150 millimetres per meter depth of soil or if you want to express this in terms of the the volume you will have to find out how much is the volume of soil if it is 150 millimetres this much is the amount of water into 1 meter to 1 meter, the amount of water is 0.15 cubic meters 1 cubic meter of soil that is what it is it amounts to. So in terms of uhh percentage it will be 15 percent, this much depth is equivalent to 15 percent moisture content per volume because most of the time when you will talk in terms of uhh the moisture availability and that is what we are interested in. When we talk of irrigation we must know that where the irrigation water is going is it going and getting stored into the soil or is it going as a loss is anything which is not being used by the plants is a loss. So our total aim is to to ensure that what is our water is being supplied through irrigation it must be stored effectively into the soil by effectively we mean it must be litron consumed by the crop. Soil moisture content can be at any time and quite a useful thing to to define some of the properties of the soil in terms of the soil moisture content. So in uhh literature you will find that there are some soil moisture conditions which have been characterized which have been defined in terms of the soil moisture content which signify the various levels of uhh the the soil moisture availability and those definitions we will look at they they those definitions are very important in terms of uhh the usability of water in terms of the availability of that water which has been stored subsequently by the crop and these these conditions are the condition of saturation, field capacity and the permanent melting point. They are also known as the soil moisture equilibrium equilibrium point. They will also be termed as soil moisture equilibrium points in literature. Let us try to look at these conditions of these levels one by one. What is the saturation level? The saturation level of the soil is that level when all the pore spaces are filled up with water. So if you if you supply lot of water to the soil either it can be in the form of natural rainfall or it can be in the form of artificial irrigation. So the end of that moisture supply you will find that the soil can come up to the saturation level. Just uhh a sort of depiction here sorry this is for the field capacity. In the case of saturation if I take the help of this this slide this was the soil structure. That is the level of saturation. Now this level of saturation cannot stay for long unless you assume that you keep on supplying the moisture or the rainfall continues for a very long period or the depth of water which has formulated on the top of the ground and the moisture availability is continuous. Otherwise the moment the moisture supply is stopped you will find that all the excess water which can be drained out by the force of gravity that will be drained out of the soil and the moisture content of the soil will reduce. At that stage when all the moisture the excess moisture which can be drained out of the soil under the force of gravity it has been drained out that level is known as or that moisture content is known as the field capacity level. This depiction this is what has been shown that you have if this is the total soil column and in this soil column you have at the saturation level the total column was filled with water. When you have let the water get drained under the force of gravity then all that water has gone down and may be it has joined the ground water table it has gone down the this particular level of the profile or the column of soil. Once it has come out of this well which is attained after that prolonged drainage and that the time required for such drainage will vary from soil to soil. In the coarse soils the time requirement will be much less the draining will be quite fast but in the fine soils in comparison it will require a longer period to let the water drain out at the from the soil due to the gravitational forces but might vary from 24 hours to 72 hours that can be the order of magnitude. Once that has happened the level of the moisture content which is prevailing at that time that is what is known as the field capacity level. In the permanent wilting point which is the third important soil moisture condition is that level which when attained would not allow any further consumption of the soil moisture by the crop. So if you have a if you have a crop the crop is consuming water from the soil through the evapotranspiration we will try to discuss that later what what do we mean by evapotranspiration what evapotranspiration is a combination of evaporation and transpiration which is the consumption of the moisture by the plants plus the evaporation activity which is taking place. That consumption can be only allowed up to a level which is known as the permanent wilting point. So if the moisture content reaches that level of permanent wilting point then the crop will wilt. There will be a permanent damage to the crop in terms of its yield and it cannot revive even if you supply water at that level it would not be able to revive. So in other words you can see that this is the the lowest level up to which the crop might be in a position to use water but this is not the level of moisture up to which you can afford to go. Though at even that stage if you look at this picture this was the total moisture at the level of saturation up to this level it drained out and the remaining was the field capacity level the content at the field capacity level and the crop could only utilise the water between this level and up to this level but by reaching here at this level which is the permanent wilting point the crop has damaged itself and the yield is affected you might not get anything out of the crop. Though there is still some water left in the soil that water is the water which cannot be used by the crops. There is still some moisture available in the soil which is there but it cannot be used by the crops. So from the angle of knowing what is the level of moisture availability in the soil these moisture conditions are very essential they are very useful and we will discuss the various aspects related to these moisture conditions in the next class.