Monday, May 13, 2019


Irrigation Irrigation is the artificial application of water to soil in order to bring soil moisture content to field capacity level and to supplement water available from rainfall for the purpose of crop production. Irrigation may involve conservation of water during the rainy season by construction of dam (barrier) across a stream for water storage or using the existing river as a source of water and pumping of water to field through pipe, hose or canal for distribution of water to crop on the farm Objectives or purpose of irrigation 1. Toaddwater to soil that is essential for plant growth 2. Tocool the soil and atmosphere in order to provide a favourable condition for plant growth 3. Towashout or dilute harmful salt in the soil by leaching. 4. Tosoften the tillage pan (hardpan) 5. Toreduce the hazard of frost in temperate region like USA and Canada. 6. Toprovide crop insurance against short dry draught. Hard pan: is a hard soil layer formed in the profile (below the soil surface) due to compaction of the soil when machine (tractor) is frequently used in the soil. The hardpan reduces the permeability of the soil to water thereby causing water-logging of the area and hinder the root penetration of the crop through the soil. Frost: Freezing of soil water in the temperate region and accumulation of frozen vapour on plant can injure the plant. Problems of irrigation Irrigation is beneficial to mankind because it allows crop production all year round. On the other hand, irrigation have some disadvantage particularly in the area where irrigation project is sited. It has some environmental impact like; Soil degredation, flooding and causing certain diseases to where the project is sited if not properly planned. Developed countries like USA, England, Germany, Holland, Australia normally pay more attention to soil degredation and they normally carry out environmental impact assessment (EIA) of an irrigation project. Soil salinity : excessive irrigation over a long period of time can increase the accumulation of salt in the soil profile and this render the soil unproductive for crop production Destruction of soil structure and removal of top soil that is rich in plant nutrients by run off during surface irrigation.

Flooding: it can occur during a heavy rainfall causing overflow of water from irrigation dam or failure of dam can destroy lives and properties. Water in the dam, canal channel are breeding areas for mosquito tsetsefly, water snail and blackfly that can cause certain diseases to man Forms of soil moisture or soil water 1. Gravitational water: is the water that moves freely into through and out of the soil under the influence of gravity. It is the excess water above the field capacity. It is in excess of hygroscopic water and capillary water. 2. Capillary water: is the principal source of water for plant growth. It is in excess of hygroscopic water which is held by molecular attraction. It dissolves the mineral nutrients in the soil for plant growth at a reasonable pressure. 3. Hygroscopic water: this is a thin film of water held tightly to the soil particles and it is not normally available for plant growth. This water cannot be removed by plant. The suction pressure is greater than 31 bars. 1 atm of water= 1036cm or 76.39cm of mercury. Soil water constant  Saturation capacity: this occurs when all the micro and macropores are filled with water. The soil is said to be under saturation capacity or maximum water holding capacity. The tension of water at saturation capacity is almost zero and it is equal to free water surface. Field capacity and permanent wilting point were formerly considered soil water constant but recently found imprecise soil water constant but they are quantitatively useful terms.  Field capacity: it is the moisture content after the drainage of gravitational water has becomes very small and the moisture content is relatively stable which can store water temporarily for plant use. It occurs 2-3 days after the soil has been properly wetted by rain or irrigation. If water content is determined at this period, it is said to be at it’s field capacity. Field capacity depends on soil texture, organic matter in the soil, layers of impediment in the soil profile, evapotranspiration and depth of wetting in the soil profile. For instance, a coarse than fine textured soil. In fine textured soil, the rate of internal drainage is low and it continues for many days after irrigation. Therefore, soil texture influences the field capacity. Soil layer that can impede or restrict flow of water in the soil profile will increase the field capacity. Organic matter when added to the soil increases the water holding capacity of soil and this will modify the field capacity. N.B: Percentage of organic matter in the soil vary from place to place and from time to time. When evapotranspiration is high, more water is removed from the soil after irrigation which will reduce the field capacity.

Frankly speaking, there is no particular day(s) or depth and which internal drainage of water in the soil ceased. Moisture tension of field capacity is between (1/10 and 1/3) bar.  Wilting point: this is also known as permanent wilting point or percentage. It is the water content of soil at which plants cannot absorb enough water to meet evapotranspiration requirement. Below this point, plant is wilted unless water is added to the soil before this point. At this point, films of water is held tightly around soil particles that the roots cannot remove water rapidly to prevent wilting of the plant leaves. The moisture tension of the soul at permanent wilting point is between 7 to 32 atm but 15 bar is commonly used. Wilting point depends on soil texture, climatic condition and crop types. in contrast to the above definition, there is no point at which plant cannot absorb water but it depends on plant type. For instance, xerophytes can absorb water below 15 bars and sunflower is another crop that can absorb water below the wilting point. Again plant can wilt during sunny day when the evapotranspiration demand is higher than the rate of water absorbed even where there is enough water in the soil.  Available water: Physical firms of water and soil water constant Field capacity Available water this is the amount of water that can be released between field capacity and permanent wilting point which can be used by plant. It varies with soil texture. Fine textured soil have high water holding capacity and more water between field capacity and permanent wilting point. Coarsed texture soil have low available water because more water is drained after irrigation. If the soil water content approaches the wilting point during the period of high atmospheric demand for water at flowering and pollination stage, the crop yield will be affected. Available water can be used to know when to irrigate, which usually ranges from 25-50% depletion of the available. Readily available water(RAW) is 75% of available water. The diagram below is used to illustrate the physical firm of water and soil water constants. Gravitational water High drainage Wilting point Unavailable water Capillary water Low drainage . . .. . … …. .. . .. . . . . . .. . . .. .. . .. . . . . . . . . .. . . . Hygroscopic water Nodrainage  Unavailable water : this is the water held tightly to soil particles with suction pressure (tension) greater than 15 bar and cannot be taken by the root of the growing plants

Measurement of soil moisture content Moisture content (MC): is the percentage of water present in a given sample of soil Mathematically: air = Ma, water = Mw soil solid= Ms MC=Mw/Ms(d) x 100 (1) MC=Mw/Mt x100 (2) Where: Mw=mass of water expelled from the soil (g) Ms(d) = mass of soil solid/mass of ovendry soil(g) Mt =mass of wet soil or total mass (g) MC=moisture content(%) Method of measuring MC are: 1. Gravimetric method: this is the most common method for determining MC in the developing countries (Nigeria). It involves weighing a sample of wet soil on a sensitive weighing balance as M2, empty can on weighing balance as M1 and the put the wet soil in an electric oven at 1050c for 24 hours or until a constant mass is obtained as M3 MC= M2–M3 M3–M1 4 x 100 unit in % The MC obtained is percentage and must be converted to volumetric moisture content by multiplying MC with Pb/Pw Pb= bulk density (g/cm3) Pw=density of water (g/cm3) Pb/Pw = apparent specific gravity of soil Bulk density: is the mass of oven dry soil per given volume of the soil sample. Acore sampler which is a cylinder is used and pressed through the soil to obtain the soil sample Volume of core sample = Vc V=πd2h d Pb =Ms(d) Vc Pb =4MdD π d2h h = 4(M3-M1) Core sample Ground surface π d2h Moisture content in volumetric M.C (m3/m3) = M.Cx(Pb/Pw) = M.C x Pb/Pw unit m3/m3 Example: A core sampler of internal diameter 5.6cm and 7.5 cm high is forced through the soil profile. The soil sample is put into an evaporating can of 29a. the can and wet soil weighed 331g on a sensitive weighing balance. The wet soil is then put in an electic oven for 24 hours at 1050c. the oven dry soil weighed 281g after 24 hours calculate: 1. Moisture content of the soil.

Bulk density 3. Volumetric moisture content (Take Pw to be 1g/cm3) Solution: V=πd2h 3,142 x (5.6)2 x 7.5 4 1. MC= M3–M2 M3–M1 MC=19.84% 2. Pb=Md Vc 4 x 100 =M3-M1 = 184.75 331 –281 = 50 281 –29 252 x 100 =281–29 Vc =1.364g/cm3 = 1364kg/m3 Or Pb =4(M3 –M1) = 4(281–29) π d2h 184.75 = 1.364g/cm3 3.142(5.6)2 x 7.5 3. Toconvert MC in %tovolumetric MCinvolumetric = MC x Pb/Pw Pw = 1g/cm3 = 19.84 x 1.364 = 0.2706m3/m3 100 Order methods for measuring moisture content: 1. Gamma ray attenuation: Reginato(1974) developed a dual energy gamma ray technique by which changes in both soil water and bulk density can be measured. 2. Neutron moisture meter: this determine moisture content instantly at in situ. This principles is based on the measurement of the number of hydrogen nuclei that are present in a unit volume of soil. Radium-beryllium and americum have been used but recently americum is commonly used. The fast neutrons are entitled into soil and gradually loses energy by collision with various atomic nuclei of hydrogen. Hydrogen is present almost in the entire soil water and it is effective element in the soil that can slow down nuetrons. The degree of slowing down of the neutrons is a measure of the soil water content. Tensiometer : is an instrument for measuring soil water potential. It consists of a porous ceramic cup that is filled with water and connected to a vacuum gauge (may be connected to a mercury manometer) to show changes in soil water content by pressure changes. In operation, the instrument is put in the soil to cover the ceramic cup or to a desired depth. Water moves in and out of the ceramic cup with changes in soil-water. This changes the pressure of the vacuum gauge which can read on the vacuum gauge. Tensiometer is measured in bar. It is effective only in the wet soil between 0-0.8 bar. Below this potentials. It is unreliable.

Tensiometer 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 = aTb + 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 growthand 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. FAOpenman-monteith formula and other methods non weighing lysimeter 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 crop Tank containing soil Removable or adjustable stand when it is not in used will rest on soil surface Ground surface Asketch of lysimeter for ET measurement.

Operating principle of lysimeter Water is added to the crop daily or every two days interval according to the irrigation interval. The lysimeter is weighed after the irrigation and reweighed before the next irrigation action in order to determine the amount of water containing crop due ET. The difference or reduction in the weight is the water lost due to evaporation. Depth of water lost can be determined from the following equations Recall that Pw=Ma Where: Pw = density of water (g/cm3) Vw Vw=Mw Pw ETc = Vm Ab Ma=mass ofwater loss (g) Vw=volume of water loss due to ET(m3 or L) ETc = crop evapotranspiration Ab =area of the container ETc = crop evapotranspiration in mm/day Non-weighing lysimeter: This may be usually floating method using liquid. Reduction in the volume of water is due to water lost. 2. EVAPORATIONPAN The pan that is use for measuring the evaporation rate is called Class A evaporation pan or simple class A pan. The pan has a diameter of 1.2m and a depth of 250mm (0.25m) and it is usually placed in 150mm above the ground on a wooden support as shown in the following figures. Evaporation should be located in open space which must not be under vegetation. Water is filled to 200mm level to touch the fixed point gauge. Operation principles or operation 1. Fill the evaporation pan to the level of the pointer on the first day. 2. On the second day after 24 hours. Use a 1000c (1 litre) to add the water to the evaporation pan continuously to reach the arrow pointer. 3. Determine the total volume that added to reach the pointer. 4. Determine the area of the evaporation pan (A = πd2 ) 4 5. Divide the volume of the water added by the area to get the depth of the water lost due to evaporation 6. Thedepth of water lost per day is the evaporation occurred per day (mm/day) i.e Epan = Volume Area of the pan To convert the evaporation that occurred per day to evapotranspiration ET is by multiplying by a factor called pan coefficient which varies from 0.70 to 0.75 depending on the relative humidity (RH) of the location. Other factors influencing the pan coefficient are the location of the pan,surrounding of the ground cover, the distance of the place of measurement from the pan and the climate particularly the RH. Reference evapotranspiration (ET0) from class A evaporation pan is ET0 = Kp.Epan Where ET0 = reference evapotranspiration (mm/day) Kp=pan coefficient Epan = pan evaporation (mm/day) Subtract the depth of water added by rainfall from evaporation pan Class-A evaporation pan or evaporation pan Evaporation pan Evaporation pan with the arrow pointer.

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