r_lod
r_lod is a console variable available in all 
Source games. It forces a specific LOD model globally. A higher value makes models have fewer polygons (i.e. less detailed).
Usage
Syntax: r_lod <integer>.
The default value is "-1" which allows the engine to compute a models level of detail (LOD) normally, typically based on distance.
Setting a number from 0 and above forces all rendered models to use that specific LOD, and disables all automatic level of detail computations. Level 0 will render the base model, and a higher number like 4 will reduce the level of detail.
Model rendering will be clamped to their maximum level of detail level, some models may have only 2 levels, some may have no LODs, but most have 4. Example: r_lod 20 will attempt to set all models to use a very low LOD: 20. Most models do not have 20 level of detail swaps, so most models will clamp to the closest available number, typically 4.
Note:While forcing r_lod to 0 gives better visual quality, it also negatively affects your framerate (FPS), as it completely disables the engine LOD calculation system (including distance based systems).
Bug:Level of detail distance computation is disabled in 
Half-Life 2: 20th Anniversary Update, both by setting r_lod to 0 on Model Quality High, and internally the distance calculation seems to be disabled no matter what setting you pick.