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Why must land usually be graded before using surface irrigation?

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Large wheeled irrigation sprinkler sprays water over green crops in a field, with mountains and cloudy sky in the background. — Irrigation Specialized irrigation equipment watering crops in a field.

Land grading is essential for surface irrigation because it ensures the existence of uniform slopes across the field.1

In surface irrigation, water is distributed directly over the soil surface, either by flooding the entire area or by running water through furrows between crop rows. Because this method relies on gravity to move water across the land, the field must be shaped to provide a consistent gradient. This grading allows the water to flow evenly and effectively across the intended area, preventing uneven distribution and ensuring that excess water can be properly managed or removed to avoid waterlogging. Unlike other methods, such as sprinkler irrigation, which can operate on uneven terrain, surface irrigation requires this land preparation to function satisfactorily.21