Regional temperature change over the Huang- Huai- Hai Plain of China: the roles of irrigation versus urbanization

Irrigation and urbanization, two widely occurring land-use/land-cover changes, have important influences on regional climate, especially on temperature. The effect of irrigation and urbanization on temperature is separately documented in several studies. However, there are few studies analysing the combined effects of irrigation and urbanization on temperature. In this study, changes in surface temperature were analysed in relation to irrigation and urbanization on the Huang-Huai-Hai Plain of China from 1955 to 2007. To better characterize the combined effects of these two processes on temperatures, long-term weather observations are used along with irrigation and urbanization data sets. The results indicated that irrigation had a significant cooling effect of 0.17-0.20 degrees Cdecade(-1) on average daily maximum temperature of the hottest 1, 5, and 30d of each year on the Huang-Huai-Hai Plain. Compared with the reference conditions, irrigation also indicated a cooling effect of 0.12 degrees Cdecade(-1) on summertime daily maximum temperature. In contrast to its effect on maximum temperature, irrigation appeared to induce a warming effect of 0.43 degrees C/decade on average daily minimum temperature of the coldest 1d of each year. Where irrigation interfaced with urbanization, the urbanization warming of extreme daily maximum temperature seemed to only partly counteract the irrigation cooling effect. The findings of this study deepen our insight into the effects of irrigation and urbanization on temperature dynamics, and the combined implications for regional climate change. Further efforts to understand irrigation and urbanization effects on climate should not only use observations, but should also be coupled with dynamic land-use and regional climate models to understand the complex processes and controlling mechanisms.