But suppose you did try to put large numbers of cows on land like this. They would eat the fields bare, after which they would simply starve, unless you invested in heroic levels of irrigation and used heavy industrial fertilizers. After these interventions, you would discover an inconvenient consequence: A lot of your fertilizer would pass right through your animals, washing back onto the land in the form of copious floods of urine, which the porous soil would not retain. Your groundwater and local rivers would end up doused with nitrates. We know this for a fact, because we tried it. We are still trying it.

The plains of Canterbury are the most extreme example of a land-use conversion wave that swept through New Zealand from the 1990s onward. The country had been known as a sheep-farming nation. But over the past three decades, for a range of reasons — the collapse of wool prices, the rise of China as a market for dairy products , a lack of central or local government regulation — the move to dairy farming has had the air of a rural-sector gold rush. Canterbury provides a disastrous case study in the consequences for our freshwater systems.

The river you can see winding its way through Edoras in the Jackson films is the Rangitata, one of numerous river systems running east from the Southern Alps across the Canterbury Plains. Like other rivers in the area, the Rangitata now provides the source water for multiple large-scale irrigation schemes. To distribute this water, sprinkler systems are needed. Most farmers have opted for pivot irrigators. These irrigators have rotating arms half a mile long or more, making them excellently efficient at getting a lot of water onto a lot of land with minimal human supervision. The only problem is, you need the land to have no trees on it.

Canterbury is regularly afflicted by Nor’Westers: famously hot, dry winds that can suck moisture out of the soil. Over the course of the 19th and 20th centuries, thousands of protective tree shelterbelts were planted around the local fields and carefully nursed to maturity. To run the pivot irrigators, most of these tree belts had to be cut down, so the Nor’Westers are once again free to drink their fill. This detail sums up the ecological insanity of our dairy conversions: To irrigate the plains to the level cow farming requires, we have carefully ensured they will dry out whenever the Nor’Westers blow, so that we can drain still more water from our increasingly damaged rivers.

Because of the vast numbers of cows we insist on cramming onto our fields, we have also ensured that irrigation alone is not enough to keep them fed. Despite importing large quantities of ecologically unsustainable palm kernel extract from Southeast Asia as a feed supplement, we mostly pasture our cows on grass. This requires ultra-fertile soil, which requires high nitrogen content, which requires constant inputs of fossil-fuel-derived fertilizer. The nitrogen ends up as a concentrate in the constant flood of cow effluent that washes through Canterbury’s thin soil into the region’s aquifers and rivers.