Properties of Water

Water is a unique liquid that plays a major role in plant growth and soil processes. Some of its important properties are:

• Latent heat of vaporization: 580 Cal/g at 0°C (Heat required to convert water into vapour without changing temperature.)
• Heat of sublimation: 680 Cal/g at 0°C (Heat needed to change ice directly into vapour.)
• Heat of fusion: 80 Cal/g at 0°C (Heat required to melt ice into water.)
• Highest density: at 4°C (That means water is heaviest at this temperature.)
• Expansion on freezing: Water expands by 9% in volume when it freezes.
• Dielectric constant: 80 (This high value makes water a good solvent for many substances.)
• Specific heat: 1 Cal/g (Water can absorb or release a lot of heat without large temperature changes.)
  ➤ It has the highest specific heat among all known substances except liquid ammonia (1.03 Cal/g).

Water Logging

Definition: Waterlogging occurs when all soil pores (both large and small) are filled with water either temporarily or permanently.

Disadvantages (Demerits):

• Reduces ODR (Oxygen Diffusion Rate) → roots get less oxygen.
• Reduces Eh (Redox Potential) → affects soil chemical reactions.
  • Well-aerated soil: 400–700 mV
  • Waterlogged soil: –250 to –300 mV
• Causes denitrification losses (loss of nitrogen as gas).
• In submerged rice fields, sulphates (SO₄²⁻) are reduced to sulphides (HS⁻), which form hydrogen sulphide (H₂S) gas — toxic to rice roots and causes Akiochi disease.

Approaches of Irrigation Scheduling

Irrigation scheduling means deciding when and how much water to give crops.
There are different scientific methods to plan irrigation:

1. Transpiration Ratio Approach (Absolute Approach)

• It is the ratio between the amount of water transpired by a crop and the dry matter produced.
• Shows how efficiently a crop uses water.

2. Soil Moisture Deficit Approach

• Irrigation is given when soil moisture decreases by a certain percentage.
• For maize and wheat: irrigate after 25% depletion of soil moisture.
• For drought-resistant crops: irrigate after 50% depletion of soil moisture.

3. Climatological Approach (IW/CPE Ratio)

• Also called the Soil Moisture Index Approach.
• IW = depth of irrigation water applied
• CPE = cumulative pan evaporation
• The IW/CPE ratio shows the level of soil moisture stress.
  ➤ Lower the ratio → more water stress.
• This method is scientific but less practical for farmers.

Critical Stage Approach

• Certain stages in crop growth are very sensitive to water shortage.
• If water is given at these stages, crop yield increases significantly.

Critical Stages and IW/CPE Ratio for Major Crops

Percentage of Irrigated Area and Water Requirement of Major Field Crops