Possible Increase of Air Temperature by Irrigation

Irrigation cools near surface air temperature by increasing evapotranspiration from wetter soil. However, elevated evapotranspiration can also increase atmospheric albedo and enhance the local greenhouse effect via increased atmospheric water vapor. Their net effects on daily air temperature remains controversial. Here we show that in several considered regions, Northwest India and Central Valley of California, irrigation could result in warmer air temperature if night-time warming is stronger than daytime cooling by irrigation. During the daytime, air temperature reduces through evaporative cooling and reduced solar radiation from increased atmospheric albedo outweighing the local greenhouse effect. At night-time, the increased atmospheric water vapor by irrigation tends to make a stronger local greenhouse effect that increases night-time temperature. Our results highlight the possible increase of air temperature by irrigation and the importance of considering sub-daily processes when assessing the impact of irrigation on daily air temperature and temperature related socioeconomic phenomena.