Water Management & Irrigation Agronomy

Physics of water infiltration in soils:

• According to Poiseuille's law of capillary flow, water flux through cylindrical soil pores is proportional to the square (or fourth power) of pore radius ($Q \propto r^4$). Thus, increasing soil pore size increases infiltration rate parabolically/geometrically.
• Physical water properties (temperature, viscosity, surface tension) directly affect viscosity and flow rate; wetting depth decreases hydraulic gradient over time, reducing infiltration rate. Hence, statement D only is scientifically valid.

Plant available water definition:

• Available water capacity (AWC) is defined as the capillary water retained in the soil profile between Field Capacity (FC, tension of $0.33\text{ bar}$ or $1/3\text{ atm}$, or $0.1\text{ bar}$ in sands) and the Permanent Wilting Point (PWP, tension of $15.0\text{ bar}$ or $15\text{ atm}$).
• Water held below $0.33\text{ bar}$ is gravitational water that drains freely; water held at tensions $\gt 15\text{ bar}$ is hygroscopic water unavailable to plants.

Definitions of water types:

• Green water: Soil moisture derived directly from rainfall used by rainfed crops, grasslands, and forests (A-II).
• Fossil water: Deep ancient non-renewable aquifers tapped for agriculture and municipal use (B-III).
• Grey water: Domestic wastewater without fecal contamination (from showers, sinks, washing machines), easily reused for kitchen gardening and lawns (C-I).
• Black water: Wastewater containing fecal matter/sewage, requiring extensive treatment to remove pathogens and heavy metals before any agricultural reuse (D-V).
• Virtual water: The embedded volume of water required to produce goods traded internationally (E-IV).