My back-of-the-napkin calculation tells me that watering an acre of alfalfa uses the same amount of water as having a nine-month shower with an inefficient flow

My back-of-the-napkin calculation tells me that watering an acre of alfalfa uses the same amount of water as having a nine-month shower with an inefficient flow. If a Pigovean tax of 1c/gallon was applied to California, an 8-minute shower would cost about 20-40c more while an acre of alfalfa would cost an extra $20,0000.