Irrigation Has Dramatic Effects on the Local Climate in Central Asia


Central Asia is highly agrarian. Due to sparce precipitation, most of the croplands rely on irrigation. Naturally a question arises--How does irrigation affect the regional climate in Central Asia? The answer is still unclear so far.

To answer this question, the research group of Prof. FENG Jinming from Institute of Atmospheric Physics at the Chinese Academy of Sciences and Dr. WU Liyang from Henan University modified the irrigation module in the region climate model, the Weather Research and Forecasting model (WRF) and carried out abundant numeric simulations to assess the effects of irrigation on the local surface air temperature, precipitation, surface heat fluxes, etc. The related research article is published in JGR-Atmosphere.
"An irrigation module has been coupled to the WRF model since version 4.2 in 2020. However, it has two shortcomings: it uses fixed time interval for irrigation, such as irrigating every five days, while the actual irrigation is flexible." Lead author Dr. WU said. "The other problem is that irrigation occurs in the whole model grid, which is also inconsistent with the reality. There are always multiple land use types in a model grid. Only croplands need irrigation, and other land use types do not need irrigation."
Therefore, their study has made two corresponding improvements to the original irrigation module. One is to use a dynamical irrigation strategy, in which irrigation is triggered by the water deficiency of soil. The other is to use a sub-grid irrigation strategy, that is, irrigation only occurs in the cropland part of a model grid. Compared with the experiment without irrigation, the simulated surface temperature, surface air temperature and soil moisture in the experiment with the dynamical plus sub-grid irrigation strategy are closer to the observations. 
The study found that irrigation significantly increased the latent heat flux and meanwhile decreased sensible heat flux, which in turn led to a significant decrease in surface temperature and daily maximum temperature. Soil evaporation and crop transpiration contributed roughly equally to the increased latent heat flux. The study also found that irrigation led to higher daily minimum temperature. 
"It's because irrigation increases soil moisture and soil thermal conductivity increases accordingly. More energy is therefore transferred to soil during the daytime, and then released at the nighttime, heating the surface atmosphere." Said Prof. FENG, the corresponding author of the study.
A highlight of this study is that the numerical simulations indicate that irrigation leads to increased precipitation in the mountainous areas in central Asia. There are two main irrigation areas in central Asia, the Fergana Basin and the northern slope of the Tianshan Mountain in Xinjiang, both of which are mountain-basin terrains, where upslope winds that move from valley to hillside prevail during the daytime. Irrigation leads to an increase in the water vapor in the basin. The upslope wind during the daytime brings the abundant water vapor above the basin to the mountains, and the water vapor climbs up the hillside, and condenses to form precipitation.
"This improved irrigation module can be further used to predict the impacts of future climate change on the irrigation amount in central Asia, which has important reference value for the future water resources allocation in this region." said Prof. FENG. 
Wu, L., Feng, J., Qin, F., and Qiu, Y., 2022: Regional climate effects of irrigation over Central Asia using Weather Research and Forecasting model. Journal of Geophysical Research: Atmospheres, 127, e2021JD036210.
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