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There are many variables related to runoff generated in ELM/CLM. These fluxes are then passed to MOSART with different names, need to be very clear what do they mean.

The hydrologic processes for CLM 4.5 are clearly represented below. I got it from Gautam but don’t know if he made it. (more here for CLM 5).

CLM  hydrology

Basically, the runoff generated by ELM has three parts: 1) surface runoff, 2) subsurface runoff, and 3) glacier, wetland, and lakes (GWL) for water balance.

1) Surface runoff is qflx_surf + qflx_h2osfc_surf at column level or qflx_rofliq_qsur_grc + qflx_rofliq_qsurp_grc at grid level. The first term is called “surface runoff”, which is excess water from soil saturation (QOVER in ELM output); the second term is called surface water runoff, which is the outflow from surface water storage such as pond (QH2OSFC in ELM output). It is very confusing by looking at the two names in the output.

2) Subsurface runoff is qflx_drain + qflx_qflx_drain_perched at column level or qflx_rofliq_qsub_grc + qflx_rofliq_qsubp_grc at grid level. The first term is called “sub-surface drainage” (QDRAI in ELM output); the second term is called “perched wt drainage” (QDRAI_PERCH in ELM output). These two terms are pretty straightforward.

3) GWL runoff is the glacier, wetland and lakes water balance residual component which is not routed in MOSART

The sum of the three terms is QRUNOFF in ELM output file. These three terms are passed to MOSART for routing and named QSUR_LIQ, QSUB_LIQ, and QGWL_LIQ in MOSART’s output file. The fluxes that enters MOSART channels for routing in E3SM V1 are the sum of QSUR_LIQ and QSUB_LIQ where QSUR_LIQ = ELM-QOVER+ELM-QH2OSFC; QSUB_LIQ = ELM-QDRAI+ELM-QDRAI_PERCH.

some other highlights/comments

  • In default, irrigation is added directly on the ground, bypassing the canopy.
  • There’s a elevation standard deviation term (STD_ELEV) in ELM surface data to describe the microtopology in the grid, which is used to dynamically calculate the fraction of surface water storage (FH2OSFC) over the grid.
  • CLM 4.5 history field names can be found here.
  • In current hydrology scheme, there’s a pre-defined term qflx_flood taking flood flux from the river model (RTM) and re-applied it on top of the soil, this can be re-activated when coupling ELM and MOSART in terms of flood inundation. Not sure if we can directly add inundated water into H2OSFC here.