What causes the phenomenon of runoff in irrigation?

Runoff in irrigation primarily occurs when excess water is applied to the soil beyond its absorption capacity. When the amount of water introduced exceeds the soil’s ability to hold or infiltrate that water, the surplus begins to flow over the surface, leading to runoff. This situation can arise from overly strong irrigation practices or heavy rainfall conditions when the soil is already saturated.

In contrast, insufficient irrigation that leaves soil dry would not create runoff; rather, it would result in inadequate moisture for crops. Likewise, improper drainage systems contribute to waterlogging rather than runoff unless they fail to manage excessive water correctly. Lastly, while high evaporation rates impact soil moisture levels, they do not directly result in runoff, as evaporation is a loss of water rather than a movement of water across the surface. Therefore, the correct understanding of runoff is fundamentally connected to the application of excess water that the soil cannot absorb.