Assessment of Long-term spatiotemporal soil moisture variation in the lower Mahanadi River basin: a hydrological modeling based approach

Soil moisture is recognized as most essential climatic variable and it has a great impact on many environmental processes such as hydrological, ecological climatological, geomorphological, and other natural processes. Its spatiotemporal dynamics are controlled by many factors such as soil properties, topography, vegetation types, management, solar radiation, and meteorological conditions. The past catchment-scale soil moisture studies are concerned only in the surface soil dynamics, which may not yield reasonable knowledge on the plant water uptake and subsequent irrigation planning. Hence, the present study conceptualised the Soil and Water Assessment Tool (SWAT) model to understand the variability in the catchment-scale soil moisture across multiple vertical depths for better improvement in regional crop planning. The present study analyzes the seasonal fluctuation in hydro-climatic variables (rainfall, soil moisture content, and evapotranspiration) in the Lower Mahanadi basin. The model was calibrated and validated using stream discharge data for the period 2005 to 2012 and 2013 to 2020, respectively. The model's performance was evaluated using the R2, NSE, and PBIAS values and found to be 0.80, 0.74, and 12.7% during calibration and 0.79, 0.80, and 4.9% during validation periods, respectively. The seasonal spatiotemporal variation in hydro-climatic variables were assessed for the sixteen years period (2005-2020), depicting that variation in the soil moisture and evapotranspiration was solely dependent on the land use, soil characteristics, and local climatic conditions of the study area. The mean seasonal soil moisture content values were found to be 34.23, 30.24, 49.89, and 42.39 mm for the winter, pre-monsoon, monsoon, and post-monsoon periods, respectively. The observed mean seasonal values of rainfall for the winter, pre-monsoon, monsoon, and post-monsoon periods are 32.79, 153.83, 1131.76, and 157.87 mm, respectively. Certainly, the seasonal SMC dynamics showed that there is sufficient amount of soil moisture available during the post-monsoon period, which can be utilized to cultivate the second crops in rice fallow area during the rabi season. Overall, the result showed that the SWAT is capable of investigating the spatiotemporal variation in soil moisture content at catchment-scale with utmost accuracy.