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While irrigation is often the only way to farm at all in dry regions, the agricultural land in wet regions is irrigated to increase yields. Depending on the prevailing climate, technologies and methods for irrigation vary from the damming of water on surfaces (e.g. paddy fields) to the precise application of water through pipes and nozzles.

The influence of irrigation on local the climate has been the focus of numerous investigations. However, the sheer amount of water emitted into the atmosphere suggests that irrigation also has an influence on the global climate. However, only few studies have investigated the impact of irrigation on this scale. Dr. Philipp de Vrese, Dr. Stefan Hagemann and Prof. Martin Claussen, all scientists in the department “Land in the Earth System“ at the MPI-M, have examined the impact of Asian irrigation-based agriculture on other regions for the first time. They have used the MPI´s climate model which includes the land model JSBACH. For these simulations, an irrigation scheme was implemented into the land model. The authors have focused on the irrigation-based agriculture in South Asia because the world´s biggest share of irrigation systems is located there due to rice cultivation.

They found that irrigation can directly influence the climate in regions several thousand kilometers away. The effects of irrigation in Asia occur not only locally but also take a leap across the sea to Africa. Up to 40 % of the present-day precipitation in some of the arid regions of East Africa can be related to irrigation-based agriculture in Asia.

Underlying mechanisms – How Asian irrigation influences the East African rain
The authors have studied the underlying mechanisms and have identified the advection of water vapor as well as alterations of the Asian monsoon by irrigation as key mechanisms. The water vapor originates from evapotranspiration that is increased by irrigation. The term evapotranspiration refers to the sum of water transpiration via the leaf surfaces of plants, and water evaporation via water surfaces and soils.

At the onset of boreal spring (February-March), the evaporative moisture flux originating from irrigation in South Asia is already pronounced. Due to the resulting increase in evapotranspiration, the low-level atmospheric humidity rises. During that time, the low-level winds in the Arabian Sea are flowing in a southwesterly direction. Consequently, a large fraction of the atmospheric water vapor originating from irrigation in South Asia is advected across the Arabian Sea towards Africa´s east coast, where the humidity increases distinctly. Between April and May the winds turn and the moisture advection from Asia ceases. Instead, water vapor from regions in the Middle East gets transported to East Africa. In some of the dryer areas of East Africa the increase in precipitation corresponds to more than a third of the simulated annual mean precipitation. Furthermore, irrigation in Asia results in a weakening of the monsoon, which is responsible for a large part of the precipitation in Southeast Asia in summer.
Thus, irrigation does not only lead to an increase in precipitation in remote regions, but can also reduce it.

Impacts
The increase in precipitation leads to increased evapotranspiration and an associated cooling of the land surface. The additional atmospheric water vapor affects cloud formation. The mean cloud cover in the lower troposphere increases in parts by more than 7.5 %, which reduces incoming solar radiation at the surface. In combination, these two effects induce a surface cooling in remote regions of up to 0.5 Kelvin.

The weakening of the monsoon reduces the precipitation in some regions in South East Asia, which leads to a decreased evapotranspiration and consequently to a lower associated cooling of the earth's surface. This can, in some cases, reduce the precipitation in summer by up to 2 mm per day and lead to a temperature rise of up to 2 ° C.

Conclusion
The authors have limited their investigations to the remote impacts in East Africa, Southeast Asia and southern China because here the underlying mechanisms are most clearly visible. However, the performed simulations indicate that remote effects can also be found in other regions. For example, water vapor from irrigated areas is advected into Russia, where it influences the near-surface climate, and irrigation in southern Europe may have a strong effect on the atmospheric moisture content and surface temperatures in Western Europe and Scandinavia.

Even with significant advances in irrigation technology, the amount of water used for irrigation is likely to increase by almost 50 % (relative to 2000) until the year 2050, as irrigation-based agriculture is necessary to meet the food demand of a growing world population.

While irrigation affects the hydrological cycle and the land surface energy budget by the redistribution of water, it is in turn influenced by changing climate conditions and declining water resources. It is likely that a future extension or decline of irrigated areas, due to increasing food demands or declining fresh water resources, will also affect precipitation and temperatures in remote regions.

Paper
Philipp de Vrese, Stefan Hagemann, Martin Claussen (2016). Asian Irrigation, African Rain: Remote Impacts of Irrigation. Geophysical Research Letters. doi: 10.1002/2016GL068146

Contact

Dr. Philipp de Vrese
Phone: +49 40 41173 138
Email: philipp.de-vrese@mpimet.mpg.de

Dr. Stefan Hagemann
Phone: +49 40 41173 101
Email: stefan.hagemann@mpimet.mpg.de

Prof. Dr. Martin Claussen
Phone: +49 40 41173 226 (Assistant Sylvia Houston)
Email: martin.claussen@mpimet.mpg.de

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http://onlinelibrary.wiley.com/doi/10.1002/2016GL068146/abstract Link to paper