Adds initial information in ELM user guide

components/elm/docs/user-guide/index.md

@@ -1 +1,91 @@
-start of the ELM User's Guide
+# ELM User's Guide
+
+This User's Guide describes how to set up and run ELM.
+
+## Table of Conents
+
+1. [Steps to build and run ELM](#steps-to-build-and-run-elm)
+    1. [Scientifically supported compsets](#scientifically-supported-compsets)
+    2. [Supported grids](#supported-grid)
+    3. [Model spin-up for pre-industrial condition](model-spin-up-for-pre-industrial-condition)
+2. [Customizing runs](customizing-runs)
+    1. [Changing monthly output file](changing-monthly-output-file)
+    2. [Saving additional output files](saving-additional-output-files)
+
+## Steps to build and run ELM
+
+A step-by-step instruction on how to run fully coupled E3SM can be found [here](https://acme-climate.atlassian.net/wiki/spaces/DOC/pages/2309226536). Here we describe running ELM driven by atmospheric forcings provided via the data atmosphere (DATM) model for configurations that are used in the E3SM water cycle v3 campaign.
+
+The water cycle campaigns of E3SM v1 and v2 used ELM's satellite phenology mode (SP-mode) in which a prescribed leaf area index is used in ELM. However, the E3SM v3 water cycle campaign uses an interactive phenology by including an active biogeochemistry (BGC) cycle in ELM. Additionally, a parameterization of sub-grid topographical effects on solar radiation is included within ELM.
+
+### Scientifically supported compsets
+
+The land-only compsets are referred to as "I"-compset and are supported for the following time periods: pre-industrial (1850) and historical transient (20TR). Additionally, multiple atmospheric forcing datasets can be used to drive the ELM simulations. The supported compsets are:
+
+1. `I1850CNPRDCTCBCTOP`: Climatological pre-industrial using Qian atmospheric forcings
+2. `I1850CRUCNPRDCTCBCTOP`: Climatological pre-industrial using CRUNCEP atmospheric forcings
+3. `I1850GSWCNPRDCTCBCTOP`: Climatological pre-industrial using GSWP atmospheric forcings
+4. `I20TRCNPRDCTCBCTOP`: Historical ELM simulation using Qian atmospheric forcings with time varying greenhouse gas forcing and land use, land cover dataset (year 1850-2014).
+5. `I20TRCRUCNPRDCTCBCTOP`: Historical ELM simulation using CRUNCEP atmospheric forcings with time varying greenhouse gas forcing and land use, land cover dataset (year 1850-2014).
+6. `I20TRGSWCNPRDCTCBCTOP`: Historical ELM simulation using GSWP atmospheric forcings with time varying greenhouse gas forcing and land use, land cover dataset (year 1850-2014).
+
+***Should we limit the number of DATMs that are supported? What about SSP compsets? Need to make sure all the compset listed are included within E3SM master.***
+
+### Supported grid
+
+The `r05_r05` is the supported grid resolution for performing offline ELM simulation.
+
+### Model spin-up for pre-industrial condition
+
+***Add notes on how to spin-up the model.***
+
+## Customizing runs
+
+Few useful changes to `user_nl_elm`
+
+### Changing monthly output file
+
+ELM by default outputs monthly history file in `*elm.h0.**.nc` files
+that include many variables (>200). At times, many of the default output
+variables may not be of interest, thus one could remove all default variables
+(via `hist_empty_htapes`) and only include select variables (via `hist_fincl1`)
+to the monthly history files by
+
+```fortran
+   &elm_inparm
+     hist_empty_htapes = .true.
+     hist_fincl1 = 'TG', 'TV', 'FSA'
+   /
+```
+
+#### Saving additional output files
+
+ELM can output additional history files (such as `*elm.h1.*.nc`, `*elm.h2.*.nc`)
+that have different temporal averaging (e.g. daily, hourly, every model timestep) via
+`hist_nhtfrq` where
+
+- `-24` corresponds to daily average
+- `-1` corresponds to hourly average
+- `0` corresponds to monthly average
+- `1` corresponds to each model time step
+
+The number of time slices in these additional files can be controlled
+vai `hist_mfilt`.
+
+```fortran
+   &elm_inparm
+     hist_fincl2 = 'TG'
+     hist_fincl3 = 'TV'
+     hist_fincl4 = 'TG', 'TV', 'FSA'
+     hist_nhtfrq = 0, -24, -1, 1
+     hist_mfilt  = 12, 30, 24, 48
+   /
+```
+
+Using the above-mentioned settings:
+
+- Each `*.elm.h1.*.nc` will include 30 daily average values of `TG`
+- Each `*.elm.h2.*.nc` will include 24 hourly average values of `TV`
+- Each `*.elm.h3.*.nc` will include 48 values of `TG`, `TV`, and `FSA` at
+  each model time step, which is typically is 30 min.
+