Climate Fact: Crops and Cooling
In Brief: More irrigation likely drove a 3.6 degree Fahrenheit decline in average summertime daily maximum temperatures between 1934 and 2002 at weather stations in heavily farmed areas of California.
Over the past few centuries, large parts of North America were converted from native ecosystems to agricultural and urban landscapes. Agricultural land now covers about 17 percent of America’s surface. One of the most intensively farmed areas is California, which grows about half of America’s domestically consumed produce. As the number of farms in California grew between 1934 and 2002, the amount of irrigated land grew as well. At weather stations located in the State’s agricultural centers, the irrigated proportion of the surrounding landscapes grew from 22.4 to 62.2 percent. Large irrigation systems, which provide vital water to crops in California’s Mediterranean climate where almost no rain falls in the summer, can increase evaporation and cloud cover, ultimately having a cooling affect. Compared to other California weather stations where large increases in irrigation did not occur, daily maximum temperatures decreased by 3.6 degrees Fahrenheit between 1934 and 2002. Because daily minimum temperatures showed little trend at these stations during this period, the average daily temperature range, or the difference between the daily maximum and daily minimum temperature, became smaller.
Seasons: Spring, Summer
Source: Lobell, David B., and Celine Bonfils. “The Effect of Irrigation on Regional Temperatures: A Spatial and Temporal Analysis of Trends in California, 1934-2002.” Journal of Climate 21 (2008): 2063-2071.
