Summary: The biggest direct climate impact with farmland irrigation has to do with the energy used to pump and distribute water. However, efficient use of fresh water is a very important goal by itself, since in many regions there is barely enough water to meet the needs of agriculture, cities and ecosystems.
Replacing flood irrigation methods by sprinklers and drip irrigation reduces water and greenhouse gases by 25% and 40% respectively (Drawdown).
Growing crops in the appropriate climate region avoids over-use of water. For example, Washington State became a leading producer of apples because valleys on the east side of the Cascades have a perfect climate and rainfall for apple trees. In California’s Central Valley, on the other hand, the dairy industry has stimulated production of alfalfa, one of the thirstiest crops around (Slate).
Goal: Planting of region-appropriate crops, using low-water-use varieties if they are available. Efficient use of water by local agriculture.