Water Use by Crops

Water is essential for plant growth.
Other nutrients may be retained within the plant, but most water is not retained.
Water moves continuously from the soil to the roots, through the stems, and evaporates at the leaf surface, primarily through the stomata.

Definition of Plant Available Water (PAW):
- PAW is defined mathematically as follows:
  PAW = FC - PWP
- Where:
- Field Capacity (FC): The maximum amount of water soil can retain without losing water due to gravitational forces. It is the amount of water remaining after excess water has drained away.
- Permanent Wilting Point (PWP): The point at which plants can no longer extract water from the soil.
PAW represents the amount of water available for plant uptake, which is the difference between the field capacity and the permanent wilting point.
The amount of plant available water varies significantly with soil texture.
Reference Source: FAO (fao.org), Chapter 2 - Soil and Water.

Soil pores can contain water or air:
- When water is present, gravitational water can be lost.
- Larger soil pores hold air and water, while smaller pores retain more water, which is essential for plant growth.

Soil moisture levels and conditions vary by texture, affecting water retention and availability.
The following represents typical soil textures and their relationships with water retention:
- Sandy Loam:
- Lower moisture retention due to larger pore sizes.
- Loam:
- Moderate moisture retention, well-balanced texture for crop growth.
- Clay Loam:
- Higher moisture retention due to smaller pore sizes.
- Clay:
- Highest moisture retention but can lead to drainage issues.

Available Water When Soil is at Field Capacity (inches of water per foot of moist soil):
- Coarse (Sandy Loam): 1.0 inches
- Medium (Loam, Clay Loam): 1.5 inches
- Fine (Clay): 2.0 inches
- Example Calculation:
- For a loam soil at field capacity to the 4-foot level:
  - 4 ext{ ft} imes 1.5 ext{ inches/foot} = 6 ext{ inches of available water}

Six inches of available water from soil is analogous to six inches of rainfall concerning crop growth.
Water stored in soil is like "money in the bank"; it remains available until the plant needs it.
Surface water can evaporate, but moisture below the six-inch depth will remain until utilized by roots for crop production.

Crop yield can be estimated based on coefficients related to soil moisture and precipitation probabilities for various crops and soil zones.
Typically, crops will utilize all available soil moisture in most years.
Available Crop Moisture Equation:
- ext{Spring Soil Moisture} + ext{Probable Rainfall (May to July)} = ext{Crop Available Moisture}

Water use efficiency varies by soil zone; it is defined as how effectively crop yield correlates with moisture availability.
An inch of moisture is more influential on crop yield in the Black soil zone compared to the Brown soil zone.
Regions with less water availability (drier areas) require more water due to higher temperatures.
Year-to-year variations are observed:
- In wet years: Lower temperatures decrease water needs for optimal yield.
- In dry years: Higher temperatures increase water demands for achieving the same yield.