The Impact of Irrigation on Near-surface Air Temperature and the Occurrence of Heatwaves
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2023-03
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Melbourne Authors
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Infrastructure Engineering
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PhD thesis
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Open Access
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© 2023 Huazhen Li
Abstract
Irrigation is an important human activity that accounts for 70% of all freshwater withdrawals globally and plays an important role in changing regional-to-global climate and hydrological cycles. Irrigation cools near-surface air temperature by increasing evapotranspiration from wetter soil. However, additional atmospheric vapour from elevated evapotranspiration can also increase atmospheric albedo and enhance the local greenhouse effect. The net impact of these individual influences varies diurnally. This study examines the comprehensive influence of irrigation under a range of environmental and climate conditions using a coupled Earth system model. Furthermore, freshwater shortage and overexploitation of groundwater caused by increasing climate variability and groundwater extractions mean that farmers may face significantly reduced water available for irrigation in the near future over many regions around the world. Investigating the impact of stopping local irrigation is a unique contribution of this study.
Chapter 3 examines the local impact of irrigation on daytime and night-time air temperature under two different assumptions of irrigation status over surrounding areas by using SCAM (The Single Column Atmosphere Model). Among several considered regions, Northwest India and Central Valley of California, irrigation could result in warmer air temperature when night-time warming is stronger than daytime cooling by irrigation. This chapter highlights 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.
While Chapter 3 focuses on a local scale land-atmosphere interaction with an impact of irrigation, without a full implementation of lateral water transport caused by irrigation, Chapter 4 further expands the irrigation-induced land-atmosphere processes taking the lateral transport of water and energy into consideration to provide a better understanding of the impact of stopping local irrigation on near-surface air temperature globally. The Community Earth System Model (CESM) is used in Chapter 4 to explore the effects of stopping irrigation in an area on near-surface air temperature and heatwaves or heat extremes at the local and global scale. Overall, evaporative cooling is a dominant factor in local areas in response to local irrigation over Northwest India. However, given our results over North America in November, December, January, and February, wind flow anomaly with increased humidity and increased downward longwave radiation contributes to winter warming, which is likely related to irrigation over remote areas in Northwest India. Therefore, the possible temperature impacts from irrigation practices over remote regions are equally important as irrigation over local regions.
Heatwave events have been occurring more frequently in recent decades. However, the connection between irrigation and hot extremes has not been thoroughly examined yet. In Chapter 5, we use CESM to simulate several ideal scenarios of different regional irrigation practices to examine the sensitivity of heatwaves occurrence/severity to regional irrigation controls. Our results indicate a significant yearly and seasonal response of heatwave metric to both global irrigation and regional irrigation controls when the influence of relative humidity is considered in combination with air temperature to determine the severity of heatwaves. Overall, the global heatwave frequency exhibits different responses to global and regional irrigation control highly depending on the choice of heat stress indices and criteria considered therein. Some remote heatwave impacts by the regional irrigation control are also observed, indicating global implications of regional-scale irrigation practices.
Main findings of this research could provide useful insights on the impacts of irrigation practice on global heatwaves, assisting in sustainable management of global irrigation and possible results in human-induced extreme hydrological variability in the future.
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Keywords
irrigation, near surface air temperature, heatwave, greenhouse gas, global climate model CESM