The MUSLE, or Modified Universal Soil Loss Equation (Williams, 1975), is a mathematical model used in soil science and hydrology to estimate soil erosion. It was developed as an extension and modification of the original Universal Soil Loss Equation (USLE). The MUSLE takes into account factors such as land use, topography, soil erodibility, and climate conditions to predict the potential erosion rate in a particular area. The modifications to the original USLE equation changed the formulation to calculate erosion from surface runoff instead of precipitation. The other components of the original formulation remained the same. The method works best in agricultural environments where it was developed. However, some users have adapted it to construction and urban environments.
\begin{aligned}
&\text { Sed }=11.8\left(Q_{\text {surf }} \times q_{\text {peak }}\right)^{0.56} \times K \times L S \times C \times P\\\\
&\begin{aligned}
& \mathrm{ Sed }=\text { Sediment Yield per Event (metric tons) } \\
& \mathrm{O}_{\text {surf }}=\text { Surface Runoff Volume }\left(\mathrm{m}^3\right) \\
& \mathrm{q}_{\text {peak }}=\text { Peak Runoff Rate }\left(\mathrm{m}^3 / \mathrm{s}\right) \\
& \mathrm{K}=\text { Soil Erodibility Factor } \\
& \mathrm{LS}=\text { Topographic Factor } \\
& \mathrm{C}=\text { Cover and Management Factor } \\
& \mathrm{P}=\text { Support Practice Factor }
\end{aligned}
\end{aligned}
Required Parameters
Parameters that are required to utilize this method within HEC-HMS include the erodibility factor, topographic factor, cover factor, and practice factor.
A tutorial using the MUSLE in an event simulation can be found here: TBD.
A Note on Parameter Estimation
The Threshold parameter was introduced as an independent variable to segment storm events for continuous simulation. It establishes the lower boundary for direct runoff flow rate, marking the commencement of a sediment flow event when the direct runoff exceeds this threshold. Conversely, the event concludes when the direct runoff drops below the specified threshold. This parameter assumes particular significance in the calibration process, especially for continuous simulations.
