outlook+water: Not much benefit of July rains to the drouthy parts of the state

SUMMARY:

• Drought conditions declined to 20% of the state (D1–D4); statewide reservoir storage declined slightly to 77.0% full, about five percentage points below normal for this time of year.
• La Nada is here with a 70% chance of La Niña arriving in August-September-October.
• Drought is expected to remain in the western part of the state and most of the Hill Country through October.

I wrote this article on August 2, 2024.

We’ve had a lot of rain over the past month along the Gulf Coast and on the eastern part of the Edwards Plateau, with much of these areas seeing more than eight and even more than 10 inches (Figure 2a). So how have the drouthiest parts of the state, namely the Rio Grande Valley and the Hill Country, benefitted? In short, not much.

The biggest benefactor was Lake Travis, which saw 78,000 acre-feet of runoff pour into its basin. Good news! The bad news is that the reservoir remains less than 50% full (Figure 5d). Regardless, a journey to a full conservation pool requires many raindrops, and these raindrops certainly helped. Other area reservoirs did not do so good. Lake Buchanan saw nothing, Canyon Lake increased by 1.4 percentage points (from a record low of 56.3% full to 57.7% full, a record low for this time of year; Figure 1a), Medina increased from 2.3% full to 3.6%, while Falcon Reservoir saw no improvement.

Barton Springs saw a bump but is nearly back to pre-bump levels (Figure 1b), Comal and San Marcos springs saw nothing, and Las Moras Springs sprung back to life (Figure 1c). Like reservoirs, any bump in springflow is appreciated as it is either a step toward recovery or a delay in impacts.

Figure 1a: Reservoir storage in Canyon Lake since 2022 compared to statistics (median, min, and max) for statewide storage from 1990 through 2023 (graph from Texas Water Development Board).

Figure 1b: Flow in Barton Springs over the past 30 days (graph from USGS).

Figure 1c: Flow in Las Moras Springs over the past 30 days (graph from USGS).

Over the past month, we’ve had the usual more rain in the east and northeast (with large areas receiving more than 8 inches of rain) and less rain in the west (less than a quarter of an inch), although the eastern part of the Edwards Plateau did well (Figure 2a). The percent-of-normal rainfall over the last month shows the Gulf Coast and the eastern part of the Edwards Plateau receiving two to four times or more of normal rainfall over the past month (Figure 2b). However, a large part of the state, including North-Central Texas and the Big Bend area, received less than half (or less) of normal in July (Figure 2b).

The amount of the state under drought conditions (D1–D4) decreased from 25% four weeks ago to 20% today (Figure 3a), with drought improvements in all parts of the state that are still under drought (Figure 3b). Extreme drought or worse increased from 2% of the state four weeks ago to 3.4% today, with exceptional drought peeking back in at 0.3% (Figure 3a). In all, 42% of the state is abnormally dry or worse (D0–D4; Figure 3a), up from 39% four weeks ago.

The North American Drought Monitor, which runs a month behind current conditions, shows drought in New Mexico, Oklahoma, and Mexico, among other places across the continent (Figure 4a). The Rio Grande Basin north of Elephant Butte Reservoir is in long-term drought (Figure 4a). The Rio Conchos basin in Mexico, which confluences with the Rio Grande just above Presidio and hosts the largest tributary to the Lower Rio Grande, remains in extreme to exceptional drought (Figure 4a).

Conservation storage in Elephant Butte Reservoir—an important source of water for the El Paso area—decreased from 17.2% four weeks ago to 11.8% more recently (Figure 4c), about six percentage points below median storage. 

Combined conservation storage in the Amistad and Falcon reservoirs increased to 19.7% full from 19.2% four weeks ago, almost 40 percentage points below normal for this time of year (since 1990) and the lowest in at least the past 33 years for this time of year (Figure 4d).

Basins across the state continue to host flows over the past week below historical 25^th, 10^th, and 5^th percentiles (Figure 5a). Statewide reservoir storage is at 77.0% full, down about 190,000 acre-feet from 77.6% four weeks ago and about six percentage points below normal for this time of year since 1990 (Figure 5b). Recent rains have not topped up most of the reservoirs in the central part of the state, although Lake Buchanan is now more than 77% full (Figure 5c).

The reservoir marked in orange in the Dallas-Fort Worth area (the lemon in a sea of blueberries) is New Terrell City Lake, which dropped from full in early March (Figure 5c). I couldn’t discern the cause, but I suspect it’s for maintenance on the reservoir and/or dam.

My (drumroll, please) “Hydrograph of the Month” is Lake Travis, which has seen a slight bump from 41% full to 48% full over the last month, although it continues to be about 35 percentage points lower than normal (Figure 5d).

Sea-surface temperatures in the Central Pacific that, in part, define the status of the weather phenomenon known as the El Niño Southern Oscillation are under neutral conditions, and sea-surface temps continue to drop (Figure 6a). Projections suggest a transition to La Niña conditions in mid-to-late summer (Figure 6a, b). This month’s projection is similar to last month’s except for a cooler La Niña.

The official probabilities, a month newer than the projections in the graph, give La Nada conditions a ~45% chance for La Niña conditions arriving July-August-September, a drop from a 65% chance last month (Figure 6b). The chances for La Niña increase to 70% for August-September-October and nearly 80% in September-October-November (Figure 6b). Accordingly, we remain under a La Niña Watch.

The U.S. Seasonal Drought Outlook through August projects drought persistence in West and Far West Texas, with drought remaining in the Hill Country (Figure 7a).

 The three-month temperature outlook projects (surprise, surprise) above-average temperatures for the state (Figure 7b), while the three-month precipitation outlook favors normal conditions for the vast majority of the state (Figure 7c).