Method of Calculation

WET WEATHER

Wet weather surface water flows within the Los Angeles River originate mainly from precipitation falling upon the watershed.  Other sources of water within the watershed include irrigation, groundwater upwelling, and wastewater discharges, which are usually classified as nuisance or dry-weather flows.

The fate of this water ends up:

  • Surface Water Discharge: stormwater that runs off into rivers and the ocean
  • Incidental Groundwater Recharge: stormwater that passively infiltrates into the ground through permeable surfaces into potable aquifers.
  • Artificial Groundwater Augmentation: stormwater that is infiltrated into groundwater basins via stormwater facilities; and
  • Evapotranspiration and other losses: stormwater that is used by plants or evaporated directly or infiltrated into perched aquifers or aquifers that are considered non-potable and thus are not used.

The Loading Simulation Program in C++ (LSPC) model, adapted for the Los Angeles Department of Water and Power’s Stormwater Capture Master Plan (LADWP, 2015) was used to model the flows within the Los Angeles River watershed. The LSPC model provided an analysis of the existing distribution of the average annual, wet year, and dry year flows in the river. The wet and dry year determinations were determined by quantifying the total precipitation over the watershed and the contributing portions of each of the watersheds for each water year between 1987 and 2011. The model includes precipitation from 71 rain gages, therefore the precipitation in the model represents a good representation of rainfall over the entire watershed and contributing area. The wettest year was Water Year (WY) 2005 (October 1, 2004 through September 30, 2005) and the driest year was WY 2007 (October 1, 2006 through September 30, 2007)

The table shows the total flow in the Los Angeles River for the average annual year, a wet year, and a dry year at the six different reaches as identified by the Los Angeles Regional Water Quality Control Board (LA RWQCB) as depicted in the accompanying map.

  • Outlet of Reach 6: entry to Sepulveda Reservoir
  • Outlet of Reach 5: Sepulveda Dam
  • Outlet of Reach 4: confluence with Verdugo Wash
  • Outlet of Reach 3: confluence with Arroyo Seco
  • Outlet of Reach 2: confluence with Compton Creek
  • Outlet of Reach 1: mouth of LA River

 

DRY WEATHER

The flow at each river mile within the Los Angeles River was estimated during dry weather to provide an estimate for dry weather flow. Flows into each river segment were estimated and summed to create a mass balance and to determine flowrate of dry weather runoff released downstream. Incoming flows include flow from upstream segments of the river, incidental urban runoff entering through storm drain outlet, wastewater discharges from the DC Tillman, Burbank, and LA-Glendale wastewater treatment plants (WWTP), and groundwater upwelling. Outflows include evaporation or evapotranspiration and flow to downstream. Evaporation and incidental urban runoff were applied over the whole river, the wastewater discharges were applied only at three outfall locations, and groundwater upwelling flows were applied only within the soft bottom reaches where upwelling has been studied before. These flow inputs and outputs are summarized in the dry weather flow table.