Science Briefs

Irrigation and 20th Century Climate

Photo of irrigation in Idaho

Figure 1: Lateral-moving pipe irrigation in southeast Idaho. (Image: US Dept. of Agriculture, Natural Resources Conservation Service)

For many regions, the rapid expansion of irrigation during the twentieth century has significantly altered the hydrologic cycle and energy budget at the land surface. Evidence suggests that current irrigation significantly alters climate in some areas, with the magnitude of the climate response depending on the spatial extent of irrigation and the degree to which a region's climate regime is linked to its land surface processes. While current irrigation may cause significant climatic changes at the regional scale, it is not known when and where these effects became significant during the 20th century.

One of the important direct climatic effects of irrigation is the reduction of surface air temperature through shifts in the Bowen ratio from sensible to latent heating. Irrigation effects on climate may also be indirect, especially in monsoon regions where alteration of the thermal contrast between land and ocean may produce changes in monsoon circulation and the accompanying climatic variables. Could irrigation-related cooling have "masked" the global-warming signal in certain regions during the twentieth century?

Four maps of global irrigation patterns

Figure 2, above: Global distribution of gross irrigation (mm/season) for boreal winter (December-February) and summer (June-August). Left column: Mean for early 20th century (1905-1925); Right column: Mean for late 20th century (1980-2000).
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In a recent study, we estimated the effects of the expansion of irrigation on 20th-century climate. We attempted to apply irrigation realistically in space and time to the land surface component of a global atmosphere general circulation model, the GISS ModelE, allowing the model to compute explicitly the water and energy dynamics of the land surface.

The irrigation data is drawn from a new reconstruction of global hydrography for the 20th century. We use this dataset in combination ModelE to examine, for the first time, the impact of irrigation on climate over the course of the 20th century.

Iime series line plots of regional irrigation, temperature anomaly and precipitation anomaly

Figure 3, above: Regional time series of boreal summer (JJA) irrigation, ensemble-mean temperature anomalies, and ensemble-mean precipitation anomalies for Western North America (130°W-100°W, 30°N-50°N), India (68°E-88°E, 8°N -36°N), and China (98°E-122°E, 22°N-42°N).
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Early in the century, irrigation is primarily localized over Southern and Eastern Asia, leading to significant cooling in boreal summer (June-August) over these regions. This cooling spreads and intensifies by century's end, following the rapid expansion of irrigation over North America, Europe, and Asia. Irrigation also leads to boreal winter (December-February) warming over parts of North America and Asia in the latter part of the century, due to enhanced downward longwave fluxes from increased near surface humidity. Precipitation increases occur primarily downwind of the major irrigation areas, although precipitation in parts of India decreases due to a weaker summer monsoon.

Irrigation begins to significantly reduce temperatures and temperature trends during boreal summer over the northern hemisphere mid-latitudes and tropics beginning around 1950; significant increases in precipitation occur in these same latitude bands. These trends reveal the varying importance of irrigation-climate interactions and suggest that future climate studies should account for irrigation, especially in regions with unsustainable irrigation resources.

Reference

Puma, M.J., and B.I. Cook, 2010: Effects of irrigation on global climate during the 20th century. J. Geophys. Res., 115, D16120, doi:10.1029/2010JD014122.

Contact

Please address all inquiries about this research to Dr. Michael J. Puma.