...The science has been clear for the better part of a century. Drawing groundwater from near a stream can suck that stream dry. In turn, using all the water in streams and rivers lessens their ability to replenish the aquifers beneath them. Farmers who drill new wells to supplement their supplies with groundwater are often stealing water from their neighbors who hold rights to the rivers above them. This understanding has been the foundation of the U.S. Department of the Interior’s water accounting for decades, and was used by the U.S. Supreme Court to decide one of the most significant water contests in history.
Yet California and Arizona — the two states water experts say are facing the most severe water crises — continue to count and regulate groundwater and surface water as if they were entirely separate.