October is typically a rainy month here in Texas. But the return of the La Niña weather outlook has helped solidify the Climate Prediction Center's forecast for below-normal rainfall and above-normal temperatures through the end of the year.
This past summer, the El Niño-Southern Oscillation (ENSO), a climate pattern of changing sea surface temperatures in the central and eastern tropical Pacific Ocean, was neutral. When ENSO conditions are neutral, the weather could tip in either direction. But, during the La Niña phase, the water temperatures are cooler, typically resulting in warmer and drier weather for Texas.
With La Niña coming back for the fall and winter season, Climate.gov states that "We’ve already seen one likely effect of La Niña this year—a more active Atlantic hurricane season, with nearly twice as many storms as average so far this year. But the most substantial La Niña effect on North American rain, snow, and temperature happens during winter. In summary, La Niña winters tend to be drier and warmer across the southern third of the U.S., and cooler in the northern U.S. and Canada.”
Consequently, the Climate Prediction Center's Seasonal Drought Outlook shows that drought conditions will likely develop or persist in most of Texas for the rest of 2021. But what does this mean for the Brazos River Authority’s water supply system?
As of Oct. 14, slightly more than half of the Brazos River basin was experiencing drought conditions, with the BRA water supply system at 96% full. Now that dry conditions have found their way back into the basin; the BRA is publishing two-month reservoir projections that will be updated at the beginning of each month until drought conditions decrease.
As seen below, the reservoir projection graphics represent the potential for reservoir capacity and drawdown. The first chart depicts a “worst-case scenario” with lake levels under extremely dry conditions with minimum inflows and high evaporation levels. With this reservoir projection, the BRA water supply system would be 91% full by Nov. 30, 2021.
The second chart projects lake levels under average conditions. With normal inflows and normal evaporation rates, the chart shows that the water supply system would be 97% full by Nov. 30, 2021, close to the basin’s current conditions. Each of the charts also shows the projected elevation levels for the 11 reservoirs within the system.
It is important to note that these reservoir projections are based on historical data, not weather forecasts.
“With climate predictions and even the La Niña, the forecasts are just probabilities,” said Chris Higgins, a senior hydrologist at the Brazos River Authority. “We don’t change our operations based on predictions. [The climate predictions] are mostly useful to be aware of in case those conditions do develop so that we are not caught off guard. The reservoir projections that we put out are based on historical hydrology, not based on any type of weather forecast.”
The reservoir projections depict what could occur in good and bad conditions regarding drought. However, what could end up happening could be better, worse, and right in the middle of the two.
“The projections are there to show “what if” scenarios—what if the La Niña does bring very dry conditions, etc.,” Higgins said. “These are not a forecast for where we think conditions are going to be; it’s what conditions could be under these different scenarios.”
Even if dry conditions are present during the fall and early winter, reservoir levels typically stabilize during the fall months due to two main factors: shorter, cooler days that cause less evaporation and decreased water usage. Vegetation that requires frequent watering during the warmer months go dormant in the fall and winter, meaning that homeowners will water their lawn and gardens less and the trees, grass, shrubs and weeds that line the rivers and reservoirs will require less water.
“As vegetation goes dormant, it also helps reduce evaporative effects on our lakes and rivers,” Higgins said.
It also appears that this La Niña event will be a weakened occurrence and potentially end by spring of 2022.
"There are some warmer subsurface temperatures that ought to be reaching the surface in a few months, so any La Niña is likely to be gone by early next year," state climatologist John Nielsen-Gammon said in our most recent newsletter. "Thus, the most likely dry period is October through December."
Even if the La Niña persists and strengthens into early 2022, these projections will help both the BRA and the public prepare for both best- and worst-case scenarios.
You can view these charts by clicking here, where the reservoir projections will be updated at the beginning of each month. The weekly drought monitor for the Brazos River basin is also available on this page.