The Impact of Rainfall on Soil Moisture Variability in Four Homogeneous Rainfall Zones of India during Strong, Weak, and Normal Indian Summer Monsoons

This observational study mainly examines the impact of rainfall on Indian soil moisture (SM) variability in four homogeneous rainfall zones (i.e., central India (CI), northwest India (NWI), south peninsula India (SPIN), and northeast India (NEI)) as defined by India Meteorological Department (IMD) during strong, weak, and normal Indian summer monsoons (ISMs), which are determined regionally for each homogeneous rainfall zone separately. This study uses the daily gridded (0.25 degrees x 0.25 degrees) rainfall data set provided by IMD and the daily gridded (0.25 degrees x 0.25 degrees) SM combined product version 06.1 from European Space Agency Climate Change Initiative (ESA CCI) over the period 1992-2020. Results reveal that monthly and seasonal mean SM in NWI, CI, and SPIN are overall higher during strong than during weak ISMs. The daily SM and its dependence on rainfall appear to be region-locked in space and phase-locked in time: Strong correlation and large response to rainfall generally occur in most parts of SPIN and NWI during June (June-July) of strong (weak) ISMs where SM values are relatively small; Weak correlation and small response generally occur in CI and NEI in July-September (August-September) of strong (weak) ISMs where SM values are relatively large. The phase-locked feature is associated with the features of ISMs. The region-locked feature is presumably associated with the local features, such as soil and vegetation types and/or environmental conditions. Both region-locked and phase-locked features cause regional distinct features in SM persistence.