When heavy rainfall occurs during sowing and harvesting, large machinery cannot enter the fields for long periods of time, growth is delayed and severe loss of harvests occurs. In fact, an examination of yield fluctuations between 1950 and 2000 (yield as a percentage of that in the previous year) has shown that, since around the 1970s and early 1980s, yields of corn, soybeans and cotton, which grow in the summer, have fluctuated greatly, whereas yields of wheat, which grows in a different season, have fluctuated relatively little (Rosenzweig and Hillel, 2008). From this it can be surmised that, although water issues were not necessarily the sole cause, the production of major grains in the United States began to destabilize in the 1970s and early 1980s, when global warming was not very pronounced, and that this trend will become even more pronounced in the future. This aquifer underlies 111.4 million acres (174,000 square miles) in parts of eight US states: Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas and Wyoming.