Sub-irrigation
Sub-irrigation is a method of irrigation where water is delivered to the field through ditches spaced at a certain distance apart. The water is allowed to seep into the ground and maintain the water table at a specific height, ensuring that water is available to the crops through the capillary fringe. This method of irrigation is also known as seepage irrigation.

Explanation:
Sub-irrigation is a type of irrigation system where water is supplied to the crops from below the ground surface. It is commonly used in areas where there is a high water table or where there is limited water availability. This method of irrigation has several advantages, including better water distribution, reduced evaporation losses, and improved water-use efficiency.

Water Delivery and Spacing:
In a sub-irrigation system, water is delivered to the field through ditches that are spaced about 30 meters apart. These ditches act as conduits for the water to seep into the ground. The spacing between the ditches ensures that the water is evenly distributed across the field, allowing for uniform irrigation.

Water Table and Capillary Fringe:
The purpose of sub-irrigation is to maintain the water table at a specific height, which ensures that the water is available to the crops through the capillary fringe. The capillary fringe is the zone of soil above the water table where water is drawn upward due to capillary action. This zone provides a continuous source of moisture to the crops, ensuring their water requirements are met.

Advantages of Sub-Irrigation:
1. Improved Water Distribution: Sub-irrigation allows for better water distribution as the water seeps into the ground and reaches the plant roots directly.
2. Reduced Evaporation Losses: Since the water is supplied from below the ground, there are fewer evaporation losses compared to other irrigation methods.
3. Improved Water-Use Efficiency: Sub-irrigation can help improve water-use efficiency as it provides water directly to the plant roots, reducing wastage.
4. Suitable for Water-Logged Areas: This method is particularly suitable for areas with a high water table or water-logged soils, where other irrigation methods may not be effective.

Conclusion:
Sub-irrigation is a method of irrigation where water is delivered to the field through ditches, spaced at a certain distance apart. The water seeps into the ground, maintaining the water table at a specific height, and ensuring water availability to the crops through the capillary fringe. This method offers advantages such as improved water distribution, reduced evaporation losses, and improved water-use efficiency, making it a suitable option for areas with high water tables or water-logged soils.