Question

The soil hydraulic head is expressed by

• (D) only.
• (A) and (C) only.
• (B) and (C) only.
• (C) only.

Hint:

Remember the three potential bases:
- Energy/Mass $\rightarrow$ $\text{J/kg}$ (dimensions $\text{L}^2\text{T}^{-2}$).
- Energy/Volume $\rightarrow$ $\text{Pascal}$ (dimensions $\text{M L}^{-1}\text{T}^{-2}$).
- Energy/Weight $\rightarrow$ $\text{Head (meters)}$ (dimension $\text{L}$).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Soil water potential represents the potential energy state of soil water.
This potential can be expressed in three different ways depending on the reference base chosen:
- Energy per unit mass ($\text{J/kg}$ or $\text{m}^2/\text{s}^2$).
- Energy per unit volume (Pressure, $\text{Pa}$ or $\text{N/m}^2$).
- Energy per unit weight (Head, $\text{m}$ or $\text{cm}$).

Step 3: Detailed Explanation:
Let us analyze how each statement relates to hydraulic head:
Statement (A): "Potential energy per unit mass of soil water"
This potential is represented by $\phi$ and has units of $\text{J/kg}$.
It is commonly used in thermodynamic derivations but is not called the hydraulic head.
Thus, Statement (A) is incorrect.
Statement (B): "Potential energy per unit volume of soil water"
This potential is represented by $P$ and has units of pressure ($\text{N/m}^2$ or $\text{Pa}$).
It is commonly measured using tensiometers or pressure plates.
Thus, Statement (B) is incorrect.

Step 3: Detailed Explanation (Continued):
Statement (C): "Potential energy per unit weight of soil water"
Dividing potential energy ($m \cdot g \cdot h$) by the weight of water ($m \cdot g$) gives:
\[ H = \frac{m \cdot g \cdot h}{m \cdot g} = h \]
The resulting unit is a simple unit of length (such as meters or centimeters).
In soil physics and hydrology, this is called the "hydraulic head" ($H$).
Expressing potential as a head is highly practical because it can be measured directly in the field as the height of a water column in a well or piezometer.
Thus, Statement (C) is correct.
Statement (D): "Height of standing water on the soil surface"
This is only a specific component of the pressure head (ponding head), not the total hydraulic head of the soil profile.
Thus, Statement (D) is incorrect.

Step 4: Final Answer:
The soil hydraulic head is expressed by potential energy per unit weight of soil water (Statement C only).
Therefore, the correct option is (D).