Which method is the least water-efficient for irrigation?

Surface irrigation is considered the least water-efficient method among the options provided. This method involves the direct application of water onto the soil surface, allowing it to flow over the field by gravity. While surface irrigation can be effective in certain conditions, it is often associated with significant water loss due to evaporation and deep percolation, particularly if the field is not graded appropriately or if the soil conditions are not favorable.

The efficiency of surface irrigation can be further reduced by factors such as uneven land, excessive runoff, and the inability to control the amount of water applied uniformly across the entire field. This can lead to either waterlogged conditions or inadequate moisture in certain areas, ultimately affecting crop yield.

In contrast, drip irrigation, sprinkler irrigation, and subsurface irrigation are more efficient methods that target water application directly to the plant root zones. Drip irrigation minimizes water loss through evaporation and runoff by delivering water directly to the soil at a slow rate. Sprinkler systems can distribute water more uniformly across larger areas, while subsurface irrigation allows for water delivery below the soil surface, further reducing evaporation and surface runoff.