Understanding Tile Drain Drainage Coefficient

A tile drain is a type of subsurface drainage system used in agricultural land to remove excess water from the soil profile. This is important for maintaining optimal soil aeration for crop growth, especially after irrigation or heavy rainfall. The effectiveness of a drainage system is quantified using a measure called the drainage coefficient.

What is Drainage Coefficient?

The drainage coefficient represents the rate at which excess water must be removed from a drainage area to protect crops from damage due to waterlogging. It is essentially the required capacity of the drainage system. It indicates how much water depth needs to be removed over the entire drainage area within a specific time period.

Units of Drainage Coefficient

The drainage coefficient is typically expressed as a depth of water per unit of time over the drainage area. Common units include:

• Centimeters per day (cm/day)
• Millimeters per day (mm/day)
• Inches per day (in/day)

For example, a drainage coefficient of 1 cm/day means that the drainage system is designed to remove a depth of 1 cm of water from the entire drainage area over a period of 24 hours.

Analyzing the Options for Drainage Coefficient Units

Let's examine the given options:

• Option 1: Cm of water removed per second. While this is a rate of flow (volume per time, or depth per area per time), expressing the *drainage coefficient* in cm/sec is not standard for agricultural drainage design. This unit implies a very high removal rate, which is not typical for expressing the overall capacity needed over a large field area.
• Option 2: Percentage of applied water which intercepted by this drain. This describes the efficiency of the drain in collecting applied water, not the required removal capacity (the drainage coefficient). Drainage coefficient is an absolute rate based on climate, crop needs, and soil type, not a percentage of input.
• Option 3: Cm of water depth removed from the drainage area per day. This unit, centimeters per day (cm/day) over the drainage area, directly matches the standard definition and common expression of the drainage coefficient in agricultural engineering. It specifies the required depth of water removal from the entire land area per day.
• Option 4: Percentage of stored water which intercepted by this drain. Similar to option 2, this relates to the efficiency of the drain concerning stored water, not the required capacity rate represented by the drainage coefficient.

Based on the standard definition and common practice in agricultural drainage, the drainage coefficient for a tile drain laid below cropped land to remove excess irrigation water is typically expressed as a depth of water removed from the drainage area per unit of time, commonly per day.

Therefore, "Cm of water depth removed from the drainage area per day" is the correct way to express the drainage coefficient.