IRRIGATION AND DRAINAGE ENGINEERING (10 CREDITS) By: Ir. NAHAYO Déogratias Lecturer, Civil Engineering Department COURSE GOALS This course has two specific goals: (i) To introduce students to basic concepts of soil, water, plants, their interactions, as well as irrigation and drainage systems design, planning and management. (ii) To develop analytical skills relevant to the areas mentioned in (i) above, particularly the design of irrigation and drainage projects. 08/02/2017 Irrigation and Drainage Engineering 2 Course Outline 08/02/2017 Introduction to irrigation and drainage Basic Soil-Plant-Water Relations. Irrigation Water Requirements, Sources, quantity and quality of irrigation water: Irrigation planning, scheduling and efficiencies. Design of irrigation structures. Design of drainage structures. Irrigation and Drainage Engineering 3 Course Objectives On Completion of this course, students should be able to: i. Understand the basic soil-plant-water parameters related to irrigation ii. Understand how to estimate the quantity of water required by crops iii. Be able to plan and design irrigation and drainage projects. iv. Design channels and other irrigation structures required for irrigation, drainage, soil conservation, flood control and other water-management projects. 08/02/2017 Irrigation and Drainage Engineering 4 Course Assessment i. ii. iii. 08/02/2017 Assignments CAT Final Exam : 20 % : 40% : 40% Irrigation and Drainage Engineering 5 Reading Materials (i) James, L.G. (1988). Principles of Farm Irrigation System Design. John Wiley, New York. (ii) Chin, D.A.. (2000). Water Resources Engineering, Prentice Hall, New Jersey. (iii) Journal of Irrigation and Drainage Engineering, American Society of Civil Engineers. (iv) Course comprehensive note book and other handouts and tutorial sheets. 08/02/2017 Irrigation and Drainage Engineering 6 Prerequisites Basic Soil Science/Physics Plants Water Plant/Soil/Water Relations 08/02/2017 Hydraulics Hydrology General Engineering Principles Irrigation and Drainage Engineering 7 Lecture I: Introduction to Irrigation and Drainage Engineering Three basic requirements of agricultural production are soil, seeds and water. In addition, fertilizers, insecticides, sunshine, suitable atmospheric temperature, and human labour are also needed. Among all of them water appears to be the most important requirement of agricultural production. The application of water to soil is essential for plant growth and it serves the following functions: 08/02/2017 Irrigation and Drainage Engineering 8 Functions of application of water to the soil for plant growth i. ii. iii. iv. v. vi. vii. It supplies moisture to the soil essential for the germination of seeds, and chemical and bacterial processes during plant growth. It cools the soil and the surroundings thus making the environment more favorable for plant growth. It washes out or dilutes salts in the soil. It softens clods and thus helps in cultivation operations. It enables application of fertilizers. It reduces the adverse effects of frost on crops. It ensures crop success against short-duration droughts. 08/02/2017 Irrigation and Drainage Engineering 9 Definitions of Irrigation and Drainage In several parts of the world, the moisture available in the root-zone soil, either from rain or from underground waters, may not be sufficient for the requirements of the plant life. This deficiency may be either for the entire crop season or for only part of the crop season. Irrigation: the application of water to the soil to supplement natural precipitation and provide an environment that is optimum for crop production. 08/02/2017 Irrigation and Drainage Engineering 10 Definitions of Irrigation and Drainage…… Irrigation water delivered into the soil is always more than the requirement of the crop for building plant tissues, evaporation, and transpiration. In some cases the soil may be naturally saturated with water or has more water than is required for healthy growth of the plant. This excess water is as harmful to the growth of the plant as lack of water during critical stages of the plant life. 08/02/2017 Irrigation and Drainage Engineering 11 Definitions of Irrigation and Drainage…… This excess water can be naturally disposed of only if the natural drainage facilities exist in or around the irrigated area. In the absence of natural drainage, the excess water has to be removed artificially. Drainage: Artificial removal of the excess water is termed drainage which, in general, is complementary to irrigation. 08/02/2017 Irrigation and Drainage Engineering 12 Methods of Irrigation In general, there are many methods of applying water to the field. However, in irrigation practice there are three basic methods namely: Surface irrigation: basin irrigation furrow irrigation border irrigation Sprinkler irrigation Drip irrigation 08/02/2017 Irrigation and Drainage Engineering 13 OBJECTIVES OF IRRIGATION To Supply Water Partially or Totally for Crop Need To Cool both the Soil and the Plant To Leach Excess Salts To improve Groundwater storage 08/02/2017 To Facilitate continuous cropping To Enhance Fertilizer ApplicationFertigation Irrigation and Drainage Engineering 14 Impact of irrigation on human environment Impact Positive Negative Engineering Improvement of the water regime of irrigated Danger of waterlogging and salination of soils, rise in soils. ground water table. Improvement of the micro climate. Changing properties of water in reservoirs. Deforestation Possibility provided for waste water use and of area which is to be irrigated and with it a change of the disposal water regime in the area. Retention of water in reservoirs and possible Reservoir bank abrasion. multipurpose use thereof. Health Securing increased agricultural production and Possible spread of diseases ensuing from certain types of thus improving the nutrition of the population. surface irrigation. Recreation facilities in irrigation canals and Danger of the pollution of water resources by return reservoirs. runoff from irrigation. Possible infection by wastewater irrigation, new diseases caused by retention of water in large reservoir. Social and Culturing the area. Increasing the social and Colonization of the irrigated area. Displacement of Cultural Aesthetic 08/02/2017 Political cultural level of the population. Tourist interest population from retention area. Necessity of protecting in the area of the newly-built reservoir. cultural monuments in inundated areas. New man-made lakes in the area. Project architecture may not blend with the area. Irrigation and Drainage Engineering Increased self-sufficiency in food, thus lesser 15 SOIL CONSTITUENTS 08/02/2017 Mineral Material: Sand, clay and silt Organic matter - very valuable Water Air Irrigation and Drainage Engineering 16 MINERAL COMPONENTS Except in the case of organic soils, most of a soil’s solid framework consists of mineral particles. They are variable in size and composition. They can vary from small rock particles to colloids. 08/02/2017 Irrigation and Drainage Engineering 17 MINERAL COMPONENT CONTD. The mineral can be raw quartz and other primary materials – coarse fractions which have not changed from parent material) They can also be silicate clays and iron oxides formed by the breakdown and weathering of less resistant minerals as soil formation progressed. These are called secondary minerals. 08/02/2017 Irrigation and Drainage Engineering 18 MINERAL CONSTITUENTS AFNOR DIN ROCKS > 2 mm > 2 mm SAND 0.05 to 2 mm 0.02 to 2 mm SILT 0.002 to 0.05 mm 0.002 to 0.02 mm CLAY < 0.002 mm 08/02/2017 Irrigation and Drainage Engineering < 0.002 mm 19 SAND COMPONENT Visible to the Naked Eye and Vary in Size. They are tenacious when rubbed between Fingers. Sand Particles do not Adhere to one another and are therefore not Sticky. 08/02/2017 Irrigation and Drainage Engineering 20 SILT AND CLAY COMPONENTS Silt Particles are smaller than sand. The silt particles are too small to be seen without a microscope. It feels smooth but not sticky, even when wet. Clays are the smallest class of mineral particles. They adhere together to form a sticky mass when wet and form hard clods when dry. 08/02/2017 Irrigation and Drainage Engineering 21 SOIL TEXTURE Relative proportions of the various soil separates (sand, silt and clay) in a soil. Terms such as sandy loam, silty clay, and clay loam are used to identify soil texture. Soil Components are separated using Mechanical Analysis, Sieving for Sand and Rate of Settling in Pipette for Silt and Clay. 08/02/2017 Irrigation and Drainage Engineering 22 SOIL TEXTURE CONTD. From the mechanical analysis, the proportions of sand, silt and clay are obtained. The actual soil texture is determined using the Soil Textural Triangle e.g. for a Soil with 50% sand, 20% silt and 30% clay, the texture is Sandy Clay Loam. Arranged in the increasing order of heaviness, there are 12 soil textures namely: sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay and clay. 08/02/2017 Irrigation and Drainage Engineering 23 08/02/2017 Irrigation and Drainage Engineering 24 COLLOIDAL MATERIAL The smaller particles (< 0.001 mm) of clay and similar sized organic particles) have colloidal properties and can be seen with an electronic microscope. The colloidal particles have a very large area per unit weight so there are enough surface charges to which water and ions can be attracted. These charges make them adhere together. Humus improves the water holding capacity of the soil. 08/02/2017 Irrigation and Drainage Engineering 25 WATER 08/02/2017 Quantity of water in a soil as determined by its moisture content does not give a true indication of the soil ‘wetness’. A clay soil, which on handling feels dry, can be at the same moisture content as a sandy soil, which feels wet. A plant will have less difficulty extracting water from a sandy soil than from a clay soil at the same moisture content. Irrigation and Drainage Engineering 26 SOIL WATER CONTD. There is need for a soil ‘wetness’ which reflects the ease or difficulty of extraction of water from the soil by the plant. The Concept of Soil Water Potential is therefore used in Soil/Plant/Water Relations 08/02/2017 Irrigation and Drainage Engineering 27 Mechanism of Soil Water Movement The flow of water in any hydraulic system, including the soil-plant-water system, takes place from a state of higher to one of lower potential energy. The steepness of the potential gradient from one point in the system reflects the ease with which water will flow down the potential gradient between the points. 08/02/2017 Irrigation and Drainage Engineering 28 Components of Soil Water Potential As in any other hydraulic system, the total potential (or total hydraulic head) in the soil-water system is made up of a number of distinguishable components. Some of these are as follows: Gravitational Potential: Reflects gravitational forces on the soil water. 08/02/2017 Irrigation and Drainage Engineering 29 Components of Soil Water Potential Contd. Pressure Potential: This is positive when greater than atmospheric pressure, and negative when below atmospheric. Negative pressure potential (or tension, or suction) is also known as the matric potential. It is characteristic of soil water above a free water surface. 08/02/2017 Irrigation and Drainage Engineering 30 Components of Soil Water Potential Contd. Osmotic Potential: reflects the effect of solutes in soil water, in the presence of a semipermeable membrane The total potential of soil water at a point is the sum of all the components of potential, which are acting. Note that the movement of water in the soil is slow, so kinetic energy is neglected. 08/02/2017 Irrigation and Drainage Engineering 31 Soil Water Potential and Soil Water Content If a water pressure less that atmospheric (usually referred to as suction) is applied to a saturated soil, some water will drain off until equilibrium is reached. At this state of equilibrium, the total potential of the soil water relative to a free water surface at the same elevation will be negative. Its value is known as the soil suction or matric suction since it is equal to the negative pressure potential of the soil water. 08/02/2017 Irrigation and Drainage Engineering 32 Soil Water Potential and Soil Water Content…. As the pressure potential is reduced ( i.e. suction increased) more water is removed from the soil. The relationship between suction and actual water content is referred to as soil water characteristic. Soil Water Potential is normally measured by tensiometers (matric potential), hanging water column (sand box) and pressure chamber. 08/02/2017 Irrigation and Drainage Engineering 33 08/02/2017 Irrigation and Drainage Engineering 34 Methods of Measuring Soil Water Content By Feel: This is by far the easiest method. Assessment by feel is good for experienced people who have sort of calibrated their hands. The type of soil is important. Gravimetric Method: This is equal to: Mw Mass of Water Pm M s Mass of Dry Solids 08/02/2017 Irrigation and Drainage Engineering 35 Gravimetric Method Contd. Weigh wet soil in a container, put in oven at 105 oC for about 48 hours; weigh again and obtain the weight of water by subtraction. A good soil should have moisture contents between 5 and 60% and for peat or organic soils, it can be greater than 100%. 08/02/2017 Irrigation and Drainage Engineering 36 Methods of Measuring Soil Water Content …… (iii) Volumetric water content, Pv. This is equal to: Vw Volume of Water Pv Vs Va Vw Total Volume of Undisturbed Soil Sample Recall that volume = mass/density i.e. Mw Dw Mw Pv and Pv x D sin ce Dw 1 b Ms Ms D b 08/02/2017 and Drainage Engineering Pv Pm x D Irrigation where D is the bulk density of the soil b b 37 Soil Bulk Density Bulk Density, Db is defined as the mass of a unit volume of dry soil. This includes both solids and pores. i.e. bulk density = Ms/V ; Ms is the mass of dry soil and V is the total volume of undisturbed soil. The major method of measuring bulk density in the field is to collect a known volume of undisturbed soil (V) in a soil core, and drying it in the oven to remove all the water to obtain Ms. 08/02/2017 Irrigation and Drainage Engineering 38 Methods of Measuring Soil Water Content Contd. (iv) Neutron Probe: It consists of a probe lowered down a hole in the soil. A box (rate meter or rate scalar) is at the top. Within the probe is a radioactive source e.g. beryllium (435 years life span). Close to the source is a detector. The source emits fast neutrons, some of which are slowed down when they collide with water molecules (due to hydrogen molecules). A cloud of slow neutrons (thermal neutrons) build up near the probe and are registered by the rate meter or rate scalar which measures the number of slowed down neutrons. 08/02/2017 Irrigation and Drainage Engineering 39 NEUTRON PROBE Fig. 1.3: Diagram and Photograph of Neutron Probe in Use The method is quick but very expensive. It is also dangerous since it is radioactive and must be used with care. 08/02/2017 Irrigation and Drainage Engineering 40 Methods of Measuring Soil Water Suction i) Electrical Resistance Unit: This consists of a porous body with two electrodes embedded into it. The porous body when buried equilibrates with the soil water and the readings are obtained through the resistance meters attached to the electrodes. Resistance units are measured and the instrument needs to be calibrated against matric suction or volumetric moisture content (Pv). Various porous bodies needed are gypsum, nylon or fibreglass. The instrument is relatively cheap but it takes a long time to equilibrate or react e.g. 48 hours. The method is insensitive in wet soils <0.5 bars. It measures from 0.5 to 15 bars and more. 08/02/2017 Irrigation and Drainage Engineering 41 ELECTRICAL RESISTANCE UNIT Figure 1.4 Portable meter and resistance blocks used to measure soil moisture. (Courtesy Industrial Instrument, Inc.) 08/02/2017 Irrigation and Drainage Engineering 42 Methods of Measuring Soil Water Suction… ii) Tensiometer: Tensiometer operates on the principle that a partial vacuum is developed in a closed chamber when water moves out through the porous ceramic tip to the surrounding. A vacuum gauge or a water or mercury manometer can measure the tension. The gauge is usually calibrated in centibars or millibars. After the porous cup is put in the soil, the tensiometer is filled with water. Water moves out from the porous tip to the surrounding soil (as suction is more in the soil). A point is reached when the water in the tensiometer is at equilibrium with the soil water. The reading of the gauge is then taken and correlated to moisture content using a calibration curve. 08/02/2017 Irrigation and Drainage Engineering 43 08/02/2017 Irrigation and Drainage Engineering 44 Soil Water Equilibrium Points In a soil, which is completely saturated, large pores are filled with what is called gravitational water because it can drain out under gravity. It drains out so fast that it is not available to the crops. The time of draining out varies from one day in sandy soils to four days in clay soils. 08/02/2017 Irrigation and Drainage Engineering 45 Soil Water Equilibrium Points….. Field Capacity (FC): This is the amount of water a well-drained soil contains after gravitational water movement has materially ceased. It is taken as the water content after 48 hours the soil has been subjected to heavy rainfall or irrigation sufficient to cause saturation. Field capacity can also be determined by finding the moisture content when suction is 1/3 bar for clay and 1/10 bar for sand. There still remains the water held loosely between the soil particles by surface tension at field capacity. This is called capillary water and is the main source of water for plant growth. Plants continuously take this up until there is no more water available for crop growth and wilting occurs. 08/02/2017 Irrigation and Drainage Engineering 46 Soil Water Equilibrium Points….. Permanent Wilting Point (PWP): This is the soil moisture content at which crops can no longer obtain enough water to satisfy evapotranspiration needs. The plant will wilt and may die later if water is not available. Water tension of soil at PWP is generally taken as 15 bars. For field estimation, a crop is planted and when it wilts, the moisture content is the PWP. This technique requires personal judgment and prone to mistakes. 08/02/2017 Irrigation and Drainage Engineering 47 Soil Water Equilibrium Points….. Available Water (AW): This is the water available to crops. It is the water content at field capacity minus that at permanent wilting point. Readily Available Water (RAW): This is the level to which the available water in the soil can be used up without causing stress in the crop. For most crops, 50 to 60% available water is taken as readily available. 08/02/2017 Irrigation and Drainage Engineering 48 Typical Soil Water Equilibrium Points Field Permanent Capacity (FC) Wilting Point (PWP) (By Weight) (By Weight) Available Readily Water (AW) Available Water = 0.5AW Clay 45 30 15 7.5 Clay Loam 40 25 15 7.5 Fine Sand 15 8 7 3.5 Sand 8 4 4 2.0 08/02/2017 Irrigation and Drainage Engineering 49 Available Water in the Soil Saturated •Excess water 100% available Field Capacity Readily Available Water Available Water Wilting Point Oven dry 08/02/2017 •Little reserve available and plants stressed 0% Available •No water available Irrigation and Drainage Engineering 50 1.5.7 DEFINITION OF SOIL WETNESS Soil Wetness can be described as: a) By Mass (Pm): This is the gravimetric system. b) By Volume (Pv): This is the volumetric system. It is given as: Pv = Pm x Dry bulk density ( Db). c) By Equivalent Depth: This is expressed in depth eg. in mm. This is normally used in irrigation engineering. d = Pm . Db . D where d is the equivalent depth of water applied (mm); Pm is the moisture content by mass (fraction or decimal); D is the root zone depth (mm). In this case, Db is the specific gravity of the soil, which is dimensionless. It has the same units in g/cm3. The unit of d51 is 08/02/2017 as bulk density when Irrigationexpressed and Drainage Engineering therefore determined by the unit of the root zone depth, D Table: Effective Rooting Depth (mm) of Some Crops Crops Fruits Effective Rooting Depth(mm) 750 Lucerne 1200 Cotton 900 Maize, small grains, wheat 600 Most Vegetables 300 08/02/2017 Irrigation and Drainage Engineering Source: Hudson’s Field Engineering 52 Infiltration of water Infiltration is the entry of water into the soil. It is a very important variable in irrigation design since it shows the rate at which water can move into the soil mass to replenish the root zone. Infiltration rate of a soil is the maximum rate at which water will enter the soil mass through the surface. Infiltration rates into soils depend on soil texture and structure, density, organic matter content, hydraulic conductivity (permeability) and porosity. 08/02/2017 Irrigation and Drainage Engineering 53 Infiltration of water…. As wetting time increases, the infiltration rate decreases and usually approaches a constant value, which in the case of heavy clays may be zero. A general equation for the Infiltration rate (I) is the Kostiakov (1932) equation: I = (a Tn ) mm/hr. Where: a and n are constants and T is the elapsed wetting time 08/02/2017 Irrigation and Drainage Engineering 54 Methods of Measuring Infiltration Irrigation is practiced mainly in three ways: By flooding the whole surface of the soil surface; By Flooding part of the surface and By Sprinkling. The method used influences the measured intake rate of water into the soil. When designing irrigation systems, the method used for measuring the soil infiltration rate should simulate, as far as possible, the mechanism of water intake during the application. 08/02/2017 Irrigation and Drainage Engineering 55 Infiltration Measurement For Flooded Irrigation For Flooded irrigation (border strip and basin), a double infiltrometer is normally used. This consists of two concentric cylinders, the inner about 0.4 m diameter, the outer 0.5 m. Water is maintained at the same level in each cylinder, 25 mm above the soil surface, or more if the water level is likely to be higher during irrigation. The water infiltrating from the outer ring prevents lateral seepage by the water from the center cylinder. By measuring the rate at which the water is added to the center cylinder, the infiltration rate can be found. 08/02/2017 Irrigation and Drainage Engineering 56 Double Ring Infiltrometer 08/02/2017 Irrigation and Drainage Engineering 57 Infiltration measurement for Furrow Irrigation For flood irrigation (furrow), in addition to the usual factors affecting infiltration, the intake of water depends on the spacing and shape of the furrow. The difference between inflows and outflows of water flowing through hydraulic flumes placed at different distances of test furrows represent the total infiltration. Furrow dimensions are used to obtain the infiltration rates. 08/02/2017 Irrigation and Drainage Engineering 58 Infiltration measurement for Sprinkler Irrigation The mechanism of infiltration under sprinkler irrigation is different from the surface methods. There is no head of water above the soil surface and the effect of sprinkler drops on the soil tends to form soil pans on the surface, reducing infiltration rate. The ideal method of measuring infiltration rates for sprinkler irrigation is to use sprinklers at various rates of spraying. Water could be sprayed into infiltrometers to obtain a small head of water and the intake rate found as described earlier. 08/02/2017 Irrigation and Drainage Engineering 59 Thank You For Your Attention 08/02/2017 Irrigation and Drainage Engineering 60