What is "reservoir trapping" in response matrix modifications?

This term is explained in the following article:

Here we present phosphorus response matrix modifications that adjust for 1) point-source discharges, 2) reservoir trapping, and 3) instream processes. (Li, S., Wallington, K., Niroula, S., & Cai, X. (2022). A modified response matrix method to approximate SWAT for computationally intense applications. Environmental Modelling & Software, 148, 105269.)