Understanding the soil water dynamics during excess and deficit rainfall conditions over the Core monsoon zone of India

TL;DR

This study analyzes soil water dynamics in India's core monsoon zone over two decades, highlighting the differences in surface and subsurface soil moisture behavior during excess and deficit monsoon conditions, with implications for understanding climate variability.

Contribution

It provides a comprehensive analysis of soil moisture variability, memory, and surface-subsurface coupling during monsoon extremes, emphasizing the prolonged subsurface moisture memory and its role in climate responses.

Findings

Subsurface soil moisture has twice the memory timescale of surface moisture.

Evapotranspiration varies significantly between excess and deficit monsoon years.

Surface-subsurface soil moisture coupling differs markedly during wet and dry extremes.

Abstract

Observations of soil moisture (SM) during excess and deficit monsoon seasons between 2000 to 2021 present a unique opportunity to understand the soil water dynamics (SWD) over core monsoon zone (CMZ) of India. This study aims to analyse SWD by investigating the SM variability, SM memory (SMM), and the coupling between the surface and subsurface SM levels. Particularly intriguing are instances of concurrent monsoonal extremes, which give rise to complex SWD patterns. Usually, it is noted that a depleted convective activity and persistence of higher temperatures during the pre-monsoon season leads to lower SM, while monsoon rains and post-monsoon showers support the prevalence of higher SM conditions. The long persistent dry spells during deficit monsoon years enhances the Bowen ratio (BR) due to the high sensible heat fluxes. On the other hand, the availability of large latent heat flux during excess monsoon and post-monsoon seasons tends to decrease the BR. This enhancement or reduction in BR is due to evapotranspiration (ET), which influences the SWD by modulating the surface subsurface SM coupling. The surface and subsurface SM coupling analysis for CMZ exhibits significant distinction in the evolution of wet and dry extremes. SM variations and persistence time scale is used as an indicator of SMM, and analysed for both surface and subsurface SM observation levels. Evidently, subsurface SM exhibits remarkably prolonged memory timescales, approximately twice that of surface SM. Furthermore, we dissect SWD linked to wet and dry extremes by analysing annual soil water balance (SWB). Our findings reveal augmented (diminished) ET during deficit (excess) years, subjected to a higher (lower) number of break events. In essence, our study underscores the significance of surface-subsurface SM observations in unravelling the intricate tapestry of SWD.