Irrigation & Drainage Systems: Module 1

• Module 1 introduces irrigation and drainage systems.
• This is covered in ABE 115, Irrigation and Drainage Engineering.

Objectives

• Discussing the relationships between soil, water, and plants which include:
  • Soil and Water
  • Plant and Water
  • Soil-Plant-Water
• Listing soil properties that influence water usage and plant growth
• Demonstrate how to measure soil moisture content
• General irrigation terminologies will be covered

Content

• Basic Soil-Plant-Water Relations will be discussed.
• Mechanical and Physical Properties of Soil will be explained.
• Measurement of Soil Moisture Content will be demonstrated.
• General Irrigation Terminologies will be covered

Pre-Test

• Soil bulk density refers to the weight of dry soil divided by the total soil volume.
• Soil texture refers to the relative sizes of soil particles in a given soil.

Basic Soil-Plant-Water Relationship

• Soil acts as a medium for water storage and movement.
• The physical properties of soil influence water retention, infiltration, and drainage.
• Water is essential for plant growth and serves as a carrier for nutrients and provides hydration for metabolic processes.
• Soil-Plant-Water Relationships relate the properties of soil that affect the movement, retention, and use of water.
• Soil-Plant-Water Relationships can be divided into:
  • Soil-water relation
  • Soil-plant relation
  • Plant-water relations

Mechanical and Physical Properties of Soil

• Soil serves as a plant nutrient storehouse and a habitat for bacteria, an anchorage for plants, and a moisture reservoir needed by the plant.
• Soil is a complex mass of mineral and organic particles.
• Soil has chemical, microbiological, mechanical, and physical properties that determine its functions.
• The amount of water the soil can hold depends on its physical properties.
• Soil Profile refers to the different soil layers from top to bottom.
• Movement and retention of water in the soil column is important in irrigation and drainage.
• Factors determining the physical characteristics of soil include, but are not limited to:
  • Soil texture
  • Soil structure
  • Bulk density
  • Soil porosity
• These factors affect the interaction between soil, water, and air, which affects the decision-making process of irrigation engineering.

Soil Composition

• Soil composition consists of Air, Water, Mineral Matter, and Organic Matter.

Soil Texture and Soil Structure

• Soil texture and soil structure are the two most important physical properties of soil.
• Soil texture refers to the relative proportion of various particle size groups of minerals.
• Soil structure refers to the manner in which soil particles are arranged in groups or aggregates.

Soil Texture

• Soil texture is based on different combinations of sand, silt, and clay particles.
• Mechanical analysis in the laboratory determines the various particle size groups of minerals.
• Soil textural classes include:
  • Sandy soils
  • Loamy soils
  • Clayey soils

Importance of Soil Texture

• The effect of soil texture can be beneficial or detrimental to plants.
• High clay content in the subsoil is usually desirable.
• High silt and clay content may retard root growth due to relative resistance to root penetration.
• Sandy soil has low water-holding capacity (WHC).
• Clay soil has high water-holding capacity (WHC).
• Clayey soil is more fertile than sandy soil.
• Soil Aeration can be classified as
  • Aggregated (high)
  • Non-Aggregated (low)
  • Sandy (high)

Soil Structure

• Soil structure influences the rate at which water and air move through the soil and affects penetration and soil nutrition.

Maintaining and Improving Soil Structure

• Some of the important steps include:
  • Plowing below compacted layers, not at the same depth each year
  • Allowing ample time for soil and air interaction post-plowing before pre-planting irrigation or seedbed preparation
  • Returning all possible organic matter to the soil
  • Practicing crop rotation with legumes, cash crops, and fibrous-rooted crops
  • Reducing cultivation and tillage operations

Desirable vs Undesirable Soil Structure

• A desirable structural condition implies:
  • High permeability to water
  • Satisfactory water infiltration and retention capacities
  • Easy root penetration
  • Resistance to compaction from farm implements
• An undesirable structure implies:
  • Slow permeability of air and water
  • Resistance to root extension
  • Prevalence of anaerobic conditions

Soil Compaction

• Compaction occurs on farm soils when modern implements like rubber-tired wheels and disks are used
• Compaction should be minimized while meeting seed-bed preparation and weed control needs.
• Compaction reduces porosity, infiltration, and water-holding capacity.
• Compaction can be increased by:
  • Traffic
  • Tillage
  • Mechanical or hydraulic impact
  • Chemical changes (sodium, calcium)

Soil Tilth

• Soil tilth refers to the physical condition of the soil in relation to plant growth and ease of tillage.
• Soil Tilth depends partly on granulation and granule stability evaluated as:
  • Good
  • Fair
  • Poor
• Evaluation is based on:
  • Ease of working the soil
  • Rate at which soil takes in water.
• Soils with good tilth are:
  • Mellow
  • Crumbly
  • Easily worked
• They readily take up water when dry.
• Soils with poor tilth are:
• Hard
• Cloddy
• Difficult to work
• They take up water slowly and run together when wet.
• Good soil tilth can be developed and maintained on most soils using good soil management.

Real Specific Gravity

• Real specific gravity is the ratio of the density of a single particle to the density of a volume of water equal to the volume of the soil particle.
• This is also known as particle density.
• Soils with low organic matter have an average of 2.65.
• Soils with high organic matter content have between 1.5 to 2.0.

Soil Particle Density

• Soil Particle Density measures weight per unit volume of soil particles excluding pore spaces.
• Mineral soils typically range from 2.6 to 2.75 g/cc.
• Organic matter ranges from 1.2 to 1.7 g/cc
• Particle Density (PD) is calculated as: weight of solid (Msolid) / volume of solids (Vsolid)

Apparent Specific Gravity (As)

• Refers to the ratio of the weight of a given volume of dry soil, which includes air space, to the weight of an equal volume of water.
• It can be calculated as: Bulk Density of soil (g/cc) / Density of water (g/cc)
• Bulk Density is the oven-dry weight (ODW) of a unit volume (V) of soil including the pore spaces.
• Bulk Density (BD) is calculated as: ODW (g) / V(cc)

Soil Bulk Density

• Uncultivated soil typically has a bulk density of 1.0 to 1.6 g/cc.
• Compaction increases bulk density (BD).
• It helps estimate soil weight per unit area

Soil Bulk Density and Porosity

• Soil bulk density and porosity reflect the size, shape, and arrangement of particles and voids.
• Bulk density and porosity indicate suitability for root growth and soil permeability.
• They are important for the soil-plant-atmosphere system.
• A low bulk density (BD) of <1.5 g/cm3 is desirable for optimal air and water movement through the soil.

Soil Porosity

• Soil Porosity (n) refers to the percentage of soil volume occupied by pore spaces.
• Soil Porosity can be calculated via:
• n = 100 – (BD/PD x 100)
• n = 100 – (1- (As/Rs))
• n = Vp/V
• Volumetric moisture content Pv = Vpore/Vsoil
• Saturation is the portion of the pore space filled with water. Saturation S = Vwater/Vsoil
• Soil porosity varies with texture, shape of individual particles, soil structure, amount of organic matter (OM), and degree of compaction.

Permeability

• Permeability – How fast water can move into/through the soil
• Measured in inches/hour
• Higher in dry soil; lower in wet soil
• Higher in soils with larger pore spaces such as:
  • Sands
  • Loams
  • Amended soils
  • Potting mix
• Lower in soils with smaller pore spaces such as:
  • Silts
  • Clay
  • Compacted soil or soil layers
• Saturated permeability for clay soils can be <0.06"/hour.
• Saturated permeability for silt loam soils can be 0.2"/hour.
• Decreased permeability occurs in situation with:
  • Compaction
  • Rainfall
  • Irrigation
  • Increased Moisture Content
  • Decreased Organic Matter
  • Increased Density

Assignments

• Assignment #01 and #02
  • Identifying the mechanical and physical properties of soil that influence water usage and plant growth.

Assignment #02

• Calculate the weight of 1 hectare furrow slice to a depth of 15 cm if the soil BD is 1.4 g/cc.
• Also identify the percentage of porosity if PD is 2.65 g/cc.
• A fresh soil sample weighs 300 g and has a volume of 150 cc - If the oven-dry weight is 200 g, what is BD?

Problem Set #01

• A cylindrical soil sampler takes samples from a 4-inch diameter with a 10-inch height.
  • An oven-dried sample weighs 2470 grams
  • Calculate the soil's bulk density (BD)
• A 2000 mL cylinder is pushed into soil without compressing or disturbing it.
  • The weight within the cylinder is 4000 grams before drying and 3200 grams after.
  • Calculate moisture content of the soil
  • Assume a real specific gravity (RSG) of 2.50
  • Calculate the soil bulk density
  • Calculate the porosity
• Three soil samples weigh 185, 190, and 210 grams before drying.
  • Afterwards, they weighed 153, 158, and 174 grams.
  • Determine average moisture content (dry basis)
  • If the Apparent Specific Gravity is 14.1 what is the moisture content in volume basis?

Soil Moisture Content

• Infiltration is a process in which water enters the surface strata of the soil and moves downwards to join the ground water reservoir.
• A groundwater reservoir is a soil mass saturated with water.
• The soil mass above the ground water reservoir’s surface is unsaturated.
• A water table is the surface of a ground water reservoir.
• Infiltration rate is the lowering of the water surface in centimeters per hour (cm/hr).
• Permeability is the property that allows water to move through soil.
• Dyne: force to accelerate a mass of one-gram at one-centimetre per second squared.
• Soil-Water is classified into:
• Hygroscopic, Unavailable
• Capillary, Available
• Gravitational, Superflous

Wilting Point

• Wilting Point: Drooping plant leaves due to lack of water wherein the rate of transpiration exceeds water absorption by roots.
• Capillary water storage reduces the rate at which plant roots can absorb water
• Permanent Wilting Point (PWP): Moisture content at which nearly all plants wilt and do not recover

Field Capacity

• Upper Limit of Water Retentivity
• This is the amount of moisture retained in the soil after it has been drained which it typically takes 72 hours after heavy rain or irrigation.
• Moisture retention ranges from 0.1 to 0.3 of an atmosphere.
• The moisture content between field capacity (FC) and permanent wilting point (PWP) is known as available soil moisture for plant growth.
• Example: Water was applied to the field having clay loam as soil type with the following data, find out field capacity (FC) of the soil.
• Depth of root zone = 1.5 m
• Existing water content = 3%
• Dry density of soil = 1.5 g/cm^3
• Quantity of water applied to the field = 600 m^3
• Water loss in evaporation = 10%
• Area of the field plot = 1000 m^2
• Assume that no water is lost due to deep percolation

Problem Set #02 - Soil Moisture Content - February 11, 2025

• Calculate the depth of available soil moisture in clay loam with the following data:
  • FC = 27%
  • PWP = 13%
  • Dry Density = 1.5 g/cm^3
  • Root Zone Depth = 1.23 m
• Also, find out how much water is applied on a 6 ha farm if it is in permanent wilting point with the following data:
• 80 cm Rooting Depth
• Volumetric moisture: 0.15 (permanent wilting), 0.32 (field capacity)

PAES Note Making

• A well-structured and effective reviewer is necessary as the deadline is February 18, 2025
• Should be an A5 size (5.82 X 8.26 inches)
• Define each term clearly, write simple and concise definitions
• Important terms should be highlighted in colours or bold
• Examples should be included for understanding
• Notes need to be brief but informative

PAES Review Note

• PAES Review Note must include:
  • PAES 216 – General Irrigation Terminologies (February 18, 2025)
  • PAES 217 – Irrigation Water Requirements (February 25, 2025)
  • PAES 218 – Open Channels – Design of Main Canals Laterals and Farm Ditches (March 04, 2025)
  • PAES 209 – Conveyance Systems – Performance Evaluation of Open Channels – Determination of Seepage and Percolation by Ponding Method (March 11, 2025)
  • PAES 220 – Conveyance Systems – Performance Evaluation of Open Channels – Determination of Conveyance Loss by Inflow-Outflow Method (March 18, 2025)
  • PAES 221 – Design of Canal Structures – Road Crossing, Drop, Siphon and Elevated Flume (March 25, 2025)
  • PAES 222 – Design of Basin, Border and Furrow Irrigation Systems (April 01, 2025)
  • PAES 223 – Design of a Pressurized Irrigation System – Part A: Sprinkler Irrigation (April 08, 2025)
  • PAES 224 – Design of a Pressurized Irrigation System – Part B – Drip Irrigation (April 15, 2025)
  • PAES 231 – Groundwater Irrigation – Shallow Tubewell (April 22, 2025)
  • PAES 232 – Wastewater Re-use for Irrigation (April 29, 2025)