Water Use by Crops Study Notes

Water Use by Crops

Overview

  • Focuses on the significance of water in crops, particularly in Saskatchewan.

Importance of Water for Plants

  • Water as Essential Nutrient
    • Water is crucial for plant growth.
    • Unlike other nutrients that are stored in plant tissues, most water is not retained.
  • Water Movement
    • Moves from soil to roots, then through stems to leaves.
    • Evaporation primarily occurs through stomata on leaf surfaces.

Plant Available Water (PAW)

Definition

  • The amount of water available for plant growth is defined as:
    Plant Available Water (PAW)=Field Capacity (FC)−Permanent Wilting Point (PWP)\text{Plant Available Water (PAW)} = \text{Field Capacity (FC)} - \text{Permanent Wilting Point (PWP)}

Key Concepts

  • Field Capacity (FC)

    • The total amount of water that soil can retain without losing it due to gravitational pull.
    • Represents water held after excess has drained away.

  • Permanent Wilting Point (PWP)

    • The specific soil moisture level at which plants can no longer extract water effectively, leading to wilting.

Calculation of PAW

  • PAW quantifies the volume of water that can be readily absorbed by plants.
  • The availability of this water varies based on soil texture.

Soil Water Dynamics

Water Retention

  • In soil, various pore sizes dictate water retention:
    • Large pores may hold air and some water, but little water is available for plant growth once gravitational water is lost.

Soil Moisture Conditions

  • Classification of soils includes:
    • Pore Size: Affects porosity and water retention.
    • Textural Variations: Create differences in available water as demonstrated in a soil moisture graph.

Soil Moisture by Texture

Graphical Representation

  • The graph outlines the relationship between field capacity, wilting point, and percentages of water available across different soil textures:
    • Categories include: Sand, Sandy Loam, Loam, Silt, Clay, and Clay Loam.

Available Water by Soil Type

  • Summary of available water when soil is at field capacity (inches of water per foot of soil):
    • Coarse – Sandy Loam: 1.0 inches
    • Medium – Loam, Clay Loam: 1.5 inches
    • Fine - Clay: 2.0 inches
    • Example: A loam soil at a depth of four feet provides:
    • 4 ft×1.5 inches=6 inches of available water4\, \text{ft} \times 1.5\, \text{inches} = 6\, \text{inches of available water}

Crop Water Utilization

Water vs. Rain

  • Analogy: Soil water functions like a bank deposit; it remains stored until needed by plants.
    • The contribution of six inches of soil water is equivalent to six inches of rainfall.
  • Surface water is subject to evaporation, whereas water below six inches remains until utilized by roots for crop production.

Crop Yield and Moisture Interaction

  • Estimation of yields for various crops affected by available moisture using coefficients:
    • Relation established between spring soil moisture levels and predicted rainfall from May to July.

Water Use Efficiency (WUE)

Variations by Soil Zone

  • Efficiency of water use varies depending on the soil zone:

    • An additional inch of moisture yields greater benefits in the Black soil zone compared to the Brown soil zone.
    • Drier regions experience higher temperatures, thereby increasing water demand.

  • Yearly Variations:

    • Wetter years usually have lower temperatures, resulting in reduced water requirements.
    • Conversely, dry years can elevate water demands because of increased heat.

Temperature Effects on Wheat Water Use

  • The relationship between temperature and water consumption for wheat is detailed:
    • At 10-15°C: Water Use = 0.10 inches/day
    • At 16-21°C: Water Use = 0.15 inches/day
    • At 21-27°C: Water Use = 0.20 inches/day
    • At 27-32°C: Water Use = 0.25 inches/day
    • At 32-37°C: Water Use = 0.30 inches/day

References

  • Les Henry: Henry's Handbook of Soil and Water
  • Saskatchewan Polytechnic