Introduction
Drainage water refers to the water that is collected and carried away through drains or sewer systems from various sources such as rainwater, household wastewater, or industrial wastewater. While drainage water is typically considered as waste and often discharged into rivers or oceans, it can also be put to beneficial use, such as for irrigation.

Explanation
1. Non-conventional source
The correct answer is option 'B' - Non-conventional source. This is because drainage water is not a conventional or typical source of water for irrigation. Conventional sources usually include freshwater sources like rivers, lakes, or groundwater wells. Drainage water, on the other hand, is an unconventional source that is usually considered as waste or dirty water.

2. Surface water source
Option 'A' - Surface water source is not the correct answer. Surface water generally refers to water sources such as rivers, lakes, or streams that are on the Earth's surface. While drainage water may eventually reach surface water bodies if not properly managed, it is not considered a direct surface water source.

3. Dirty water
Option 'C' - Dirty water is partly correct but not the complete answer. Drainage water can indeed be considered dirty water as it may contain pollutants, contaminants, or impurities from various sources. However, the term "dirty water" is not specific enough to categorize drainage water as a source for irrigation.

4. Groundwater
Option 'D' - Groundwater is not the correct answer. Groundwater refers to the water that is stored beneath the Earth's surface in aquifers or underground formations. Drainage water is different from groundwater as it primarily comes from surface runoff or wastewater sources.

Conclusion
In conclusion, drainage water is considered a non-conventional source for irrigation. While it may be categorized as dirty water due to its potential contaminants, it is not a direct surface water source or groundwater. The use of drainage water for irrigation can be a beneficial practice, as it helps in recycling water and reducing the demand for conventional freshwater sources. However, proper treatment and management of drainage water are essential to ensure its suitability for irrigation and prevent negative environmental impacts.