Annual precipitation varies greatly from east to west across the semi-arid US Great Plains where precipitation is the primary factor affecting yield. Dryland farmers would have a better means of understanding the applicability of cropping systems research done in one part of the Great Plains to their specific location if they were aware of the rate of change of precipitation with east-west direction at their latitude.
In a paper recently published in Agricultural & Environmental Letters, David Nielsen presents the quadratic relationship between latitude and the east-west rate of change in precipitation for the U.S. Great Plains. That rate of change is nearly constant (8.5 km for a 10 mm change or 13 miles for an inch change in annual precipitation) between 31oN and 38o N. Further north, however, the east-west gradient increases curvilinearly, reaching a maximum of 42 km for a 10 mm change or 66 miles for an inch change at 49o N on the U.S.−Canada border. The quadratic relationship serves as a decision support aid to help farmers determine the applicability of research results that may have been acquired many miles east or west of their locations with significantly different annual precipitation.