This will be a familiar scenario if you farm in western Nebraska 10 years ago, or Mendota five years ago, or even Paso Robles today. There is only so much water that comes from the sky each year, and when it arrives is not necessarily when the plants need it, so a farmer draws from groundwater to meet the demand. Given a fixed water resource, a farmer faces the challenge of allocating the water that is available to them and using it in an optimal way: for the plants that are most valuable per unit of water (not for wheat when you can grow tomatoes) and at the moments where the added water contributes most to yield (not when it’s raining).
This return on fixed water is known as its shadow value, or the amount of money you would make if you had just a little bit more of it. Land has a shadow value too, it’s the rent you should be willing to pay to farm another acre (see a great series
of blog posts
around Mike Preiner and the team at Granular’s work on this topic). For decades, scientists have thought plants behave in the same way: they regulate their water use by means of tiny valves called stomata to optimally use a fixed resource.
In reality we know that wells drawing from the ground are pulling from a shared aquifer, so there is no such thing as saving water for later on your land when your neighbors are busy pulling water for their own land. Likewise with plants, we know that roots are busy stealing water and nutrients from their neighbors, so the idea that they are saving some water for later when their return is greater is absurd on the face of it. So why did this theory persist for so long?