Root Zone Soil Moisture dynamics from Terrestrial Water-Energy Coupling across Indian Agroecological Regions

Understanding root zone soil moisture dynamics across seasons is essential for improving land–atmosphere coupling estimates and drought monitoring.  Seasonal variability of soil moisture thresholds – the effective critical point (S_WT) and effective wilting point (S_TD) and response (Λ) of root zone soil moisture dynamics to surface soil moisture observations are derived by adjusting the low pass filter parameter (Λ) across India's agroecological zones. Results show that S_WT varies substantially across seasons and landscapes, controlled by soil texture, evaporative demand, and infiltration, while S_TD remains more physiologically constrained with limited spatial variability. We find that the driest regions (northwest India) exhibit the lowest thresholds (<0.12), while humid, forested zones (northeast India, Western Ghats) maintain the highest thresholds (~0.25–0.30) due to deeper soils and persistent vegetation. Seasonally, threshold (S_WT) are lowest in the dry winter/premonsoon (JF–MAM) in arid areas and rise sharply during the monsoon (JJAS) when soils recharge, reaching maxima (> 0.30) under dense canopies. The analysis reveals that arid/semi-arid zones have very strong positive feedback in dry seasons ( Λ ≈ 0.8–1.0, m  ≈ 8–10), whereas humid/coastal regions remain largely decoupled (Λ ≈ 0.2–0.5, m ≈2–4) year-round. Correlation with SMAP Level-4 demonstrates strong agreement, with high values (>0.75) across most regions. Areas with lower agreement align with complex terrains and dense vegetation where vertical signal propagation is less coherent. Seasonal variation in the transitional slope (m) and LP filter parameter (Λ) further reveals dynamic coupling regimes that govern evapotranspiration control. These findings emphasize the need to incorporate seasonally adaptive thresholds and infiltration parameters in land surface modeling to better represent ecohydrological processes and surface flux feedbacks.