In-depth Notes on Irrigation and Drainage Engineering

Module 1.1: Introduction to Irrigation and Drainage

  • Irrigation Definition: Application of water to soil for supplying moisture essential for plant growth (Israelsen, 1979).

Purpose of Irrigation

  • Water Addition: Supplies essential moisture for plants.
  • Crop Insurance: Provides insurance against drought.
  • Soil Cooling: Cools soil and atmosphere to boost growth.
  • Salt Dilution: Washes out salts in soils.
  • Softening Soil: Softens tillage pans and clods.
  • Nutrient Dissolving: Dissolves nutrients from soil/fertilizers.

Drainage

  • Definition: Natural or artificial removal of excess water from land to lower field water levels.

Purpose of Drainage

  • Early Planting: Facilitates early plowing and planting.
  • Growing Season Extension: Lengthens the crop-growing season.
  • Soil Moisture Enhancement: Increases available moisture and nutrients by enhancing root-zone depth.
  • Soil Ventilation: Enhances soil ventilation.
  • Erosion Reduction: Decreases soil erosion and gullying through better infiltration.
  • Bacterial Growth: Creates favorable conditions for soil bacteria growth.
  • Salt Leaching: Removes excess soil salts.
  • Temperature Increase: Promotes higher soil temperatures.
  • Health Improvement: Improves sanitary and health conditions.

Irrigation and Drainage as a Science

  • Expands beyond just water application; includes water sources, delivery systems, and drainage channels.

Importance of Irrigation and Drainage in the Philippines

  • Variable Rainfall: The average annual precipitation is about 100 inches, unevenly distributed.
  • Necessity for Irrigation: Critical during dry months due to unpredictable weather.
  • Drainage Needs: Essential in areas with excessive runoff from heavy rains and poor irrigation practices.

Module 1.2: Basic Soil-Water-Plant Relations

  • Water Extraction: Plants extract water through roots and transpire it to the atmosphere.
  • Moisture Movement: Depends on climatic and soil conditions as well as plant characteristics.

Soil Properties and Water Relations

  • Soil Texture: Particle size affects water retention capacity.

    • Classifications include Gravel (> 2.00 mm), Sand (0.05-2.00 mm), Silt (0.002-0.05 mm), Clay (< 0.002 mm).

  • Soil Structure: Arrangement of soil particles affects moisture movement.

    • Types include Granular (ideal for irrigation), Blocky (prone to clogging), Columnar, Prismatic, etc.

  • Bulk Density (Bd): Ratio of dry soil weight to its volume, generally ranging from 1.0 to 1.6 g/cm³.

  • Porosity (n): Volume of pore spaces compared to total soil volume, significant for understanding water availability.

    • Formula: n = \frac{VAW}{VT} \times 100

Soil Moisture Retention

  • Field Capacity (FC): Amount retained against gravitational force; varies by soil type.
  • Permanent Wilting Point (PWP): Soil moisture tension when plants can no longer extract water, typically around 15 atm.

Crop Water Requirements (CWR)

  • Factors influencing CWR include crop type, soil type, meteorological data, and water management strategies, especially in irrigation scheduling configurations.

Crop Coefficient (Kc)

  • Indicates the relationship between actual and potential evapotranspiration.
  • Varies by crop and growth stage.

Irrigation Scheduling

  • Determines the timing and quantity of irrigation needed to optimize water use and maximize crop yield, including critical stages in crop development.

Module 2: Evapotranspiration

  • Evaporation: The process through which water turns into vapor; influenced by various environmental factors.
    • Factors include temperature, humidity, wind speed, and surface type.

Measurement Methods for Evapotranspiration

  1. Direct: Using lysimeters to gauge crop water loss.
  2. Indirect: Utilizing meteorological data to estimate using empirical formulas, including Penman-Monteith.

Water Measurement Structures

  • Structures for measuring flow include weirs, flumes, and orifices; vital for managing water allocation in irrigation systems.

Sample Problems and Exercises

  • Various sample problems are provided throughout to demonstrate calculations involving soil properties, moisture content, and irrigation requirements, facilitating better understanding of practical applications in irrigation and drainage engineering.