Question

What is the relationship between porosity and specific yield?

• A. Porosity is greater than specific yield
• B. Specific yield is greater than porosity
• C. Porosity and specific yield are equal
• D. Porosity and specific yield are independent of each other

Hint:

• Porosity ($n$) is the total pore space.
• Specific Yield (S$_y$) is the part of pore water that drains by gravity.
• Specific Retention (S$_r$) is the part of pore water retained against gravity.
• $n = S_y + S_r$.
• Since $S_r \ge 0$ (and usually $S_r>0$ for water-bearing materials), then $S_y \le n$.
• This means porosity is generally greater than specific yield.

Answer 1

The Correct Option is A

To determine the relationship between porosity and specific yield, we need to define both terms and understand their physical meaning.

1. Understanding the Concepts:

- Porosity (n): The ratio of the volume of void spaces to the total volume of a rock or soil sample. It represents the total amount of space available to hold water.
- Specific Yield (Sy): The ratio of the volume of water that an aquifer releases by gravity drainage to the total volume of the aquifer. It represents the amount of water that can be drained from the aquifer due to gravity.

2. Physical Meaning:

Porosity accounts for all the void spaces, including those that are too small or poorly connected to allow water to drain freely. Specific yield, on the other hand, only accounts for the water that can actually drain from the aquifer. Some water is held in the pores due to surface tension and capillary forces and cannot be drained by gravity.

3. Relationship Between Porosity and Specific Yield:

Since specific yield only accounts for the drainable water, and porosity accounts for all void spaces (including those that hold water that won't drain), porosity is always greater than specific yield. The difference between porosity and specific yield is called specific retention (Sr), which is the water retained against gravity.

Final Answer:

Porosity is greater than specific yield.