Irrigation-induced changes in potential evapotranspiration in southeastern Turkey: Test and application of Bouchet's complementary hypothesis

Since the early 1980s the southeastern part of Turkey has experienced major land use changes because of increasing irrigation. To determine the influence of large-scale irrigation on surface hydrologic fluxes, we investigated feedback mechanisms between the land surface and atmosphere within the framework of Bouchet's complementary relationship between potential and actual evapotranspiration. Using 23 years of meteorological observations of temperature, humidity, wind speed, and radiation within the advection-aridity model (which is built on the complementary theory), we found that potential evaporation has decreased over 50%, from about 14 mm d(-1) to about 7 mm d(-1), paralleling the increase in irrigated acreage over the same period. Pan evaporation data show a similar trend. The observed decline in potential evaporation is a direct result of decreases in wind speed and, to a lesser degree, increases in humidity. A comparison between reference values of evapotranspiration obtained from irrigation water use and evapotranspiration estimates from the advection-aridity method show a reasonable correlation (root-mean-square error=1.1 mm d(-1)), especially for large values of both variables. These results suggest that the rapid expansion of irrigation in southeastern Turkey has strongly modified the lower atmosphere in a way that appears consistent with Bouchet's complementary theory.