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Saturday, June 1, 2019

Review of infiltration in agriculture

Review of infiltration
Infiltration: is the process by which water enters the soil through the soil surface and becomes soil water
(mm)
Infiltration rate: is the rate at which water enters the soil surface. It is a unit of mm/h or m/s which is
synonymous to unit of a velocity. Infiltration capacity: is the maximum rate at which water enter the soil at a given time. It is higher at the
beginning of the infiltration and then decreases steadily with time until a constant infiltration rate is
reached. This is called basic infiltration or steady state
Infiltration rate (mm/h)
 Cummulative infiltration: is the total amount of water that enetered the soil profile during
infiltration test (mm). instruments for measuring infiltration rate are:
(1) Ring infiltrometer
(2) Tension infiltrometer
 Presence of impeding layer inside the soil profile: layer which can impede flow of water always
retard movement of water in the soil and this will reduce infiltration rate.  Application rate: if the rainfall intensity or application rate during irrigation is less than the
saturated hydraulic conductivity or infiltration rate for a deep homogenous soil, infiltration will
continue indefinitely at a rate equal to the rainfall intensity without ponding of water on the soil
surface and no run off will occur. This means that the application rate of water during irrigation
should be less than the infiltration rate of the soil so that infiltration can occur quickly without
run off
I = aT
b + f
Factors affecting infiltration rate
1. Initial water content: dry soil has high infiltration rate at the beginning of the infiltration but
decreases as the soil wets up down the soil profile.

2. Time from the set of rainfall or irrigation: infiltration rate is relatively high at first but decreases
over the tine and eventually reached the constant rate called steady state infiltration rate. 3. Hydraulic conductivity of soil: the higher the saturated hydraulic conductivity of the soil is, the
higher the infiltration rate of the soil and vice versa
4. Soil surface condition: when the soil surface is highly porous and open structure, the initial
infiltration will be higher than that of a uniform soil but the final infiltration rate remain constant
is it is limited by the lower conductivity of the transmission zone beneath the soil surface. The
surface crust acts as a hydraulic barrier and impeding. Where I= cumulative infiltration (mm)
T: elapsed time (min)
a, b, and f are soil constants that depend on the intake family of soil
f is usually taken as 7. We shall discuss this later in boarder design. Other terms that are important in irrigation
1. Water intake: is the movement of irrigation water from the soil surface into and through the soil. 2. Percolation: is the downward movement through the saturated or nearly saturated soil in response
to the force of gravity. 3. Seepage: is the downward movement and lateral movement of water in the soil from a source of
supply such as reservoir, river, canal to another area. 4. Deep percolation: this occur when water is added more than the water holding capacity of the soil
(field capacity) excess water will be lost. Soil profile beyond root zone by percolation. This losses
by percolation is called deep percolation
5. Irrigation frequency (irrigation interval) is the number of days between successive irrigation
during the period without rainfall. It depends on the consumptive use rate of the amount of
available water in the crop root zone. It is the function of crop, soil and climate. This means that
sandy soil must be irrigated more often than fine textured deep soil like clay. 6. Irrigation period: is the number of days that can be allowed for applying one irrigation to a given
design area during the peak consumption. Use period of crop being irrigated. Irrigation period
must not be greater than the irrigation frequency to prevent water stress. Irrigation water requirement
Irrigation requirement: is the total amount of water that must be supplied by irrigation to a disease free
crops, growing a larger field with adequate soil water and fertility for achieving full production potential
under a growing environment (Doorenbos and Pruitt, 1977) as reported by Michael (2008)
Irrigation water requirement involves determination of crops consumptive use (CCU) and water which a
soil can hold during irrigation. Consumptive use: is the sum of evapotranspiration and water that is used by plants for its
metabolic activities such as photosynthesis, transport of minerals, digestion of plant food, plant growth and structural support. The actual water that is used for metabolic processes is insufficient (very small)
because it is less than 1% of evapotranspiration. Therefore, consumptive use is approximately
evapotranspiration (ET) or assumed to be equal to evapotranspiration. Let us look at evapotranspiration. Evapotranspiration involves two terms: (1) evaporation (2) transpiration
Evaporation: is the process by which soil water changes from liquid state to vapour (gas) and it
occurs through the soil surface. Transpiration: is the process by which water leaves the living plant and enter the atmosphere
Evapotranspiration (ET): is the quantity of water transpired by plant (leaves) the plant tissue
during its growth and retained in plant tissue and moisture evaporated from the surface of the soil. Evapotranspiration is the basic water requirement in planning any irrigation project. Water must be
supplied at the required quantity and at appropriate time to avoid reduction in crop yield or total
destruction of the crop at the critical stage of crop growth. The unit of evapotranspiration is mm/day. The
average daily water use rate during the few days of the highest consumptive use (CU) of the sea is called
peak use rate. Peak period of consumptive evapotranspiration (ET) can be determined using the following
methods:
1. Lysimeter
2. Blanney-criddle formula
3. Pan- evaporation
4. Penman equation
5. Thornthwaite formula
6. Penman- monteith formula
7. FAO penman- monteith formula and other methods
1. LYSIMETER: this instrument is used for direct measurement of evapotranspiration. It is a tank
containing soil, data logger and a particular crop growing on it which Etc is to be measured. Lysimeter is isolated from the direct contact with the soil surface. It must have outlet for drainage
of excess water from the bottom of the tank (lysimeter) due deep percolation
Types of lysimeter
1. Weighing type lysimeter
2. Non-weighing lysimeter
Weighing lysimeter is usually more accurate in measuring water lost due to evapotranspiration than the
non weighing lysimeter
A sketch of lysimeter for ET measurement


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