| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 105 === The expansion of irrigated agriculture is one of the major land use changes, which influences local and regional climate significantly. The impact of irrigation on the local precipitation has been investigated substantially, but no agreement has been reached yet. Though previous studies have examined the influence of irrigation on many aspects, few have considered the impact of irrigation area size, which might also be an influencing factor on irrigation-induced climate change. In this study, WRF model is used to explore how the local irrigation effect changes with irrigation area size from 20 × 20 km2 to 600 × 600 km2 with geometric increases in the Great Plains region. The results show that the magnitude of irrigation-induced climate change varies with the area size of irrigation. Both the cooling and moistening effects from irrigation practices are enhanced with increasing irrigation area size: the tendency of near-surface temperature difference and low-level water vapor difference with the irrigation area changes are -0.0012 ℃/km and 0.0014 mm/km, respectively. The precipitation change has an ascending tendency with area size, i.e., 0.001 mm/day/km, and such phenomenon is supported by the analyses of energy and water budget, thermodynamic structure, and circulation changes. The lateral moisture divergence, which contributes to moisture accumulation at local irrigated area, is critical to the water vapor tendency along with irrigation area size, highlighting the importance of horizontal variability to irrigation effects. The notion of geostrophic adjustment is applied to explain the dynamic changes over the irrigated area. In the larger size of irrigation simulations, geostrophic adjustment is mainly achieved by adjusting the wind field, benefiting moisture accumulation at local irrigated region. This study suggests that the irrigation area scale is necessary to be considered when examining the impact of irrigation on the local climate.
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