Is the streamflow within the Brazos River basin really that much different during the winter than the rest of the year?
Well, it depends. And it depends on a lot of factors.
The meteorological winter ends March 1. But of course, Punxsutawney Phil, the world’s most famous weather-prognosticating groundhog, decided winter will stick around for a while longer.
OK, but first, what is streamflow? Streamflow is simply a term that refers to the amount of water flowing in a river. That streamflow is measured by gages placed throughout our waterways by the United States Geological Survey, abbreviated USGS and formerly simply known as the Geological Survey. And the one thing constant about streamflow is that it’s constantly changing, and the main influence is precipitation, whether that’s on the area or from runoff within the watershed. Streamflow is measured in cubic feet per second.
According to the USGS, there are many natural and human-induced mechanisms that influence a streams’ flow, including natural and human-induced measures. From the amount of rainfall added to a river to the amount taken out to provide drinking water for cities, flows are never entirely stable.
The USGS gages help shine some light on what normal streamflow looks like for the Brazos River.
“The discharge in any month is really driven by the amount of precipitation we get,” said Aaron Abel, Brazos River Authority water services manager. “For much of the basin, pretty much all the basin, the winter months we don’t get all that much rain because the atmosphere is cooler, and we just don’t get the heavier rainfall we get from March to June.”
Since the Brazos River watershed extends from the Texas-New Mexico border all the way down to the Gulf of Mexico, the climatological difference is vast.
So, while the whole basin typically gets less rain in winter months, the upper basin, from about Possum Kingdom Lake north, has a lower streamflow than the central and lower basin, Abel said.
For instance, the streamflow at South Bend on the Brazos River, north of Possum Kingdom Lake, in any given January is likely to be 10% to 20% less than the flow typically seen on a mean discharge in June, Abel said.
“There are some years, like 2018, we had significant rainfall really throughout most of the fall and winter,” Abel said. “In that period of time, we had a gate open at Possum Kingdom Lake for like a month or so. There can be some anomalies in all this.”
Meanwhile, the gage on the Brazos River near Richmond shows that on any given January, the streamflow averages 50% to 65% of the monthly mean discharge during the spring, Abel said.
During winter months, reservoirs release less water than when it’s raining constantly as the lake’s elevation rate drops, and there is no need to push through floodwaters.
Rain certainly hasn’t been in abundance for the Brazos River basin as of late.
The U.S. Drought Monitor released Feb. 17, 2022, showed that 88% of the Brazos River basin is still experiencing a certain level of drought.
Over the last two months, most of the state of Texas has experienced above-normal temperatures, and it was relatively dry, Abel said. And while rain isn’t typical in Texas during the winter months, it does happen. Many parts of the basin are seeing an up to 4-inch rain deficit for this time of year, he said.
Take a further look back, and over the past six months, there are areas upstream of Possum Kingdom Lake that have received 8 inches less rain than what’s normal this time of year, Abel said. And the watershed upstream of Lake Belton, despite recent rainfall, is still 12 inches below normal.
La Nina is still dominating weather conditions and is likely to continue into the spring, according to the National Weather Service Advanced Hydrologic Prediction Service. A La Niña forms when waters in the tropical Pacific Ocean are much cooler than normal, which typically results in warmer and drier weather. Overall, above-normal temperatures and below-normal precipitation are expected across the National Weather Service West Gulf River Forecast Center area for the next 30-60 days, which will increase the severity of drought conditions across the region.
Drought is like the frog in the water analogy, Abel said. The premise of the story is that if a frog is suddenly put in boiling water, it will jump out. But if the frog is slowly placed in tepid water and then brought to a boil slowly, the frog will not perceive the danger and will be cooked to death.
“You enter into it so slowly, you may not even realize you’re in a drought until you look around,” Abel said.