Changes in Moisture Estimates at Different Soil Profile Layers: Role of an Advanced SMA-based Runoff Method in Continuous Hydrologic Model-Structure

The objective of this study is to introduce a soil moisture accounting (SMA) based runoff estimation method in the continuous semi-distributed Soil and Water Assessment Tool (SWAT) model. The new approach re-conceptualizes the conventional SCS curve number method by deriving runoff equation as a continuous derivative of time. The proposed approach directly relates the volume of direct runoff to the soil moisture storage level after a certain storage threshold is satisfied. The underlying SMA notion hypothesizes that all the rainfall accumulates in the soil profile until its storage during the simulation time-step becomes equal to the antecedent store level plus the lumped amount of initial abstraction. Therefore, higher the soil moisture storage, higher the fraction of rainfall to be converted into runoff. Both the existing CN method in SWAT and the proposed method are tested over three watersheds in Indiana with different drainage areas and landuse characteristics. Results show that at deeper layers in the proximity of maximum plant rooting depths, moisture storage simulated by the SMA approach is significantly higher compared to the conventional CN method. This variation can be as large as 1 cm particularly in late summer at depths around 10 - 24 inches from the surface. At the top soil cover as well as in the intermediate layers, the difference between the two methods is minimal. Accordingly, evaporation from soil layers increases in the SMA-based method, without changing total potential evapotranspiration. All these changes have resulted into an overall reduction in surface runoff volume invariably for HRUs of all landuse types, maintaining the net daily water balance in both the methods unaltered. With some exceptions, uncalibrated streamflow outputs from SWAT with the new SMA approach being incorporated, are able to better represent the observed gauge data by lowering the unusual peaks produced in the existing CN based method. In case of medium to low flow regimes, the SMA approach is likely to amplify the flow slightly towards the observed value ranges.