Abhinav Dengri, Yiwen Mao, Tomohito J. Yamada
Received 12 October, 2023
Accepted 11 January, 2024
Published online 30 October, 2024

Abhinav Dengri¹⁾, Yiwen Mao²⁾, Tomohito J. Yamada²⁾

1) Graduate School of Engineering, Field Engineering for the Environment, Hokkaido University, Japan
2) Faculty of Engineering, Hokkaido University, Japan

Soil moisture is a key variable in land-atmosphere interactions. This study investigates the bimodal distribution of boreal summer surface soil moisture. We first identify the geographical locations of global bimodal hotspots using a Gaussian Mixture Model (GMM), with parameter estimation using the Expectation-Maximization algorithm. Subsequently, we explore the soil moisture dynamics in bimodal areas, focusing on both natural climate variability and alterations due to anthropogenic activities, notably irrigation. Bimodal hotspots were detected in mid-north India, the western Sahel, and the central United States – regions also recognized as key land-atmosphere interaction zones. However, bimodality in the central United States is dependent on the temporal scale, with greater bimodality at finer temporal resolutions. The transition from dry to wet conditions during the season emerged as the principal driver for bimodality in mid-north India and the western Sahel. In contrast, in the central U.S., interannual variations play a more significant role in inducing bimodality. Moreover, the influence of irrigation on soil moisture distribution exhibits regional differences. In the central U.S., irrigation practices diminish bimodality by reducing inherent soil moisture variability. However, in mid-north India, irrigation during this period coinciding with monsoonal rainfall, did not markedly alter bimodality. These insights enhance our understanding of soil moisture dynamics in crucial land-atmosphere interaction regions and underscore the interplay between natural and anthropogenic influences.

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