In the Spotlight: Measuring and Forecasting L.A.’s Water Supply

To better understand the full picture of the work that goes into measuring and then forecasting the City’s water supply, it is helpful to start from the beginning.

Snow surveying, or the measuring of snow depths to determine spring and summer water runoff, began in California’s Sierra Nevada mountain range in 1906 with the work of Dr. James Church of the University of Nevada at Reno. In 1910, the first permanent snow courses as we now know them were set up and measured in the Tahoe Basin. Hydrographers placed markers on the ground at specific locations and returned on a regular basis to measure the amount of snow at the site, and the water content in the snow.

Snow samples are collected by using a series of aluminum tubes that are about 1.5 inches in diameter and about 30 inches long and can be screwed together. Hydrographers screw enough tubes together to reach from several feet above the surface all the way down through the snowpack to the ground. On March 8 at the Rock Creek survey, Hydrographers had to use four tubes screwed together! The tubes and snow core are then weighed.

In 1925, Hydrographers from LADWP visited Dr. Church to observe his techniques. Taking what they had learned, LADWP crews established snow courses and began regular snow surveys at four Eastern Sierra basins: Cottonwood Canyon, Big Pine Canyon, Rock Creek and Mammoth Pass. Today, these four locations continue to provide valuable data.

From January to April every year, 13 hydrographers led by Chief Hydrographer Ben Butler collect snowpack and water content data including rainfall totals and streamflow volumes. Engineers Scantlin and Ushijima then use that information to calculate runoff and water supply projections. The final, highly anticipated survey is performed annually on April 1. The full measurement and forecasting process is overseen by Manager of Aqueduct Operations and Conservation Eric Tillemans.

Here is where the fun part starts. Scantlin and Ushijima take all of the data and run their “Runoff Forecast Model,” which predicts the monthly volume of water that travels down from the Eastern Sierra mountain slopes and into the Owens River, for an April 1 through March 31 year. Next, they take those volumes and input them into a monthly “Aqueduct Operations Model.” This model incorporates the other LADWP Eastern Sierra groups’ estimates of groundwater pumping, as well as various irrigation, recreational, environmental and other water uses, along with assessing potential water losses (e.g. evaporation) within the system. The final number is the amount of water that will be delivered to the City.

All of the above is completed by an annual Water Supply symposium meeting in late April. The comprehensive monthly “Aqueduct Operations Model” is then distilled into a simplified daily model that keeps track of the actual flows versus what was forecasted, allowing Scantlin and Ushijima to adjust operations as the year progresses.

Scantlin and Ushijima were forecasting the very real possibility of breaking the 2017 record (wettest year in the last 50 years), and possibly the 1969 historic year. Earlier this month, Ushijima reported the final April 1 snowpack results as 296 percent of normal, making 2023 the wettest year on record!

Interested? Visit www.ladwpeasternsierra.com to view the final April 1 snowpack report.

Photos by Chris Corsmeier.