US Irrigation Study Values Avoided Land Conversion at 6.86 Gigatonnes

A study published online September 14 in the Proceedings of the National Academy of Sciences estimates that United States irrigation raises crop production by 3 to 39 percent across crop types. The authors calculate that this output provides at least $31.9 billion in additional revenue.

The researchers combined estimates of irrigation's direct greenhouse gas emissions with crop productivity data and a spatially explicit global economic model. Their question was how the world might replace production if United States irrigated output were removed.

In the model, irrigation's productivity benefit avoids the net conversion of about 32 million hectares of forest and 7 million hectares of other natural vegetation. The modeled land balance also includes 35 million fewer hectares shifted to grassland and 4 million fewer hectares shifted to cropland.

The resulting gross land sparing benefit is estimated at 6.86 gigatonnes of carbon dioxide equivalent. The study places current direct annual emissions from United States irrigation at 0.019 gigatonnes, making the modeled gross benefit equivalent to roughly 363 years of those annual emissions.

The transmission mechanism is higher output from existing farmland. If irrigation disappeared and demand remained, lower yields would push production toward other regions and create pressure to convert carbon storing ecosystems into agricultural land.

That comparison does not make irrigation emission free. Pumping energy and other direct sources remain part of the production footprint, and the study's result strengthens the case for efficient systems and lower carbon power rather than treating current practices as costless.

The 6.86 gigatonne figure is a modeled counterfactual, not a forecast of an imminent land conversion event. It also is not a complete judgment on groundwater depletion, river flows, habitat or competition for scarce water, which require separate local analysis.

For production policy, the study argues against evaluating irrigation only at the pump. Climate accounting should include both direct emissions and the land use consequence of yield changes, while water management must still respect the physical limits of each basin.