Irrigated crops contribute to about 40 percent of food production worldwide. As 90 percent of freshwater consumption is used to support irrigation, understanding and modeling the effects of irrigation is important for planning of water and land resources. PNNL scientists incorporated into the ACME Land Model an irrigation module that considers both irrigation water sources and irrigation methods. Researchers then conducted a set of global numerical experiments to explore the pathways through which irrigation affects the land surface water balances. Results showed that irrigation has large effects on terrestrial water balances—especially in regions with extensive irrigation. Such effects depend on the irrigation water sources; irrigation fed by surface water leads to decreases in runoff and water table depth, while groundwater-fed irrigation increases water table depth, with positive or negative effects on runoff depending on the groundwater pumping intensity. Irrigation effects also depend significantly on the irrigation methods. Flood irrigation applies water in large volumes within short durations, resulting in much larger effects on runoff and water table depth than drip and sprinkler irrigation. Differentiating the irrigation water sources and methods is important not only for representing the distinct pathways of how irrigation influences the terrestrial water balances, but for estimating irrigation water use efficiency. Specifically, groundwater pumping has lower irrigation water use efficiency due to higher recharge rates that reduce root zone soil moisture. Different irrigation methods also affect water use efficiency, with drip irrigation the most efficient followed by sprinkler and flood irrigation. This study highlights the importance of explicitly accounting for irrigation sources and methods, which are the least understood and constrained aspects in modeling irrigation water demand, water scarcity, and irrigation effects in Earth system models.