Quoted from: Yourek, Matt, Erin S. Brooks, Dave J. Brown, Matteo Poggio, and Caley Gasch. "Development and application of the soil moisture routing (SMR) model to identify subfield-scale hydrologic classes in dryland cropping systems using the Budyko framework." Journal of Hydrology 573 (2019): 153-167.  https://doi.org/10.1016/j.jhydrol.2019.03.030 

SMR is a distributed, grid-based water balance model well-adapted to regions with steep-sloping land and shallow soils. It was developed as a spatially explicit management tool to inform management decisions addressing nonpoint source pollution from dairy farms in the Catskill Mountains of New York (Frankenberger et al., 1999, Walter et al., 2000). As a management tool, it was developed to be a relatively simple model with few parameters and requiring minimal calibration (Brooks et al., 2007, Frankenberger et al., 1999). This 3D model simulates both vertical water fluxes within a grid cell (e.g. rainfall, snow accumulation and melt, evapotranspiration, deep percolation below the root zone) as well as net lateral subsurface flow between cells. Surface runoff occurs in the landscape whenever the accumulated volume of soil water in a grid cell exceeds available pore space (i.e. saturation excess runoff). Surface runoff is not explicitly routed overland to the outlet of the catchment but rather is assumed to reach the watershed outlet within the model timestep. Infiltration excess runoff is not directly simulated by the model, which makes the model best suited to regions where the soil infiltration capacity is greater than the typical rainfall intensity (Boll et al., 1998, Frankenberger et al., 1999, Mehta et al., 2004). Subsurface lateral flow out of a grid cell is simulated using Darcy’s equation with hydraulic gradient equal to the land slope and routed to up to eight neighboring cells following the approach described in Brooks et al. (2007). Variability in soil characteristics with depth (e.g. bulk density and lateral saturated hydraulic conductivity) are represented by explicit functions. The soil water balance in SMR for each grid cell (Eq. (5)) consists of positive fluxes: precipitation (P) and incoming lateral flow (Qin), and negative fluxes: actual evapotranspiration (ETA), deep percolation (DP), outgoing lateral flow (Qout), and surface runoff (RO). Positive and negative fluxes balance to equal change in soil water storage (ΔS/Δt).

ΔS/Δt=P+Qin-ETA-DP-Qout-RO

Additional details about model development and functioning can be found in the literature (Frankenberger et al., 1999, Brooks and Boll, 2005, Brooks et al., 2007).