Ask an expert: Common questions about reservoir data Posted on July 15, 2026

In the second installment of this two‑part Ask an Expert series, hydrologist Dr. Mark Wentzel addresses some common questions about the reservoir data featured on the Water Data for Texas dashboard. The dashboard, managed by the Texas Water Development Board (TWDB), brings together real‑time information on reservoirs, aquifers, rainfall, and evaporation, giving Texans an accessible way to track water supplies and understand evolving water supply conditions. To find out more about the types of reservoir data available on Water Data for Texas, read our first installment in this series entitled Ask an expert: Reservoir data on Water Data for Texas.

Why do reservoirs in East Texas tend to refill annually, while those in West Texas may only refill every few years?

All water supply reservoirs store water during times of plenty for use in times of scarcity. They behave similarly to a bank account where you deposit paychecks and gradually draw down funds as needed. In that analogy, rainfall in East Texas is like a steady job, consistently contributing to your account. East Texas reservoirs typically decline in summer when rainfall drops, but—except in extreme drought—they recover later in the year. Toledo Bend Reservoir on the Texas‑Louisiana border shows this pattern: since first filling in 1968, it has declined every summer but recovered before the next, except in drought years such as 1996, 2006, 2011, and 2012.

Rainfall in West Texas is much less consistent and varies considerably from year to year. As a result, inflow to reservoirs occurs less frequently than in East Texas, though inflows can be quite large when they do occur. In the banking analogy, West Texas resembles a commission‑based job that pays well when you hit a target but may include long periods with no pay. O.H. Ivie Reservoir in Central Texas reflects this pattern: although it has filled several times since completion in 1989, it has had only a few periods of quick gains followed by long declines since it last filled in 1997—typical of reservoirs in the more arid parts of the state.

Why might 50 percent capacity be good news for Lubbock but concerning for Fort Worth?

Fort Worth, farther to the east where rainfall and surface water are more plentiful, is exclusively reliant on surface water to supply its needs. Since 1990, the contents of reservoirs that supply Fort Worth have averaged 91 percent of capacity with a range from 61 percent during severe drought to 100 percent during wet periods. A drop to 50 percent of capacity would represent a new, nearly 40-year low for Fort Worth’s reservoirs, indicating severe drought conditions.  

Lubbock, farther to the west where rainfall and surface water are less abundant, relies on both surface water and groundwater to meet its needs. The contents of surface water reservoirs for this system have averaged 39 percent of capacity since 1990 with a range from less than 10 percent full during severe drought to 75 full percent during the best conditions. Storage of 50 percent of capacity would represent relatively plentiful conditions for this system at any time of the year.

Why do reservoir conditions lag behind drought conditions—both when entering and recovering from drought?

There is a natural lag between rainfall on the landscape, runoff to streams and rivers, and eventual inflow to reservoirs. Droughts begin when conditions shift from normal or wet to dry, and reservoirs are usually full or relatively high at the onset. It can take weeks or months for reduced rainfall to show up as reduced inflow, especially in basins where groundwater strongly supports streamflow. Evaporation losses and water withdrawals also contribute to declining reservoir volumes, and depending on when drought begins, lower winter evaporation and demand can delay noticeable impacts. Drought restrictions may slow volume declines, but as drought persists, reservoirs will drop.

During drought recovery, landscape conditions and streamflow usually rebound faster than reservoir levels. After prolonged drought, upstream soils are dry, and several inches of rainfall may be needed before runoff begins or groundwater aquifers start to recharge. Additional moisture is often required to re-wet stream channels and replenish aquifers enough for inflows to return to normal. Even then, reservoir elevations may take weeks to rise. Recovery can take months in humid East Texas and years in the arid regions of West and South Texas. Still, a large rainfall event over or upstream of a reservoir can refill it in just a few days.

Are there any upcoming advancements or new technologies in reservoir data collection or sharing?

Advancements in remotely sensed data (via satellites, radars, aircraft, and other platforms) and ever-increasing computing power are creating opportunities to collect and share more detailed reservoir data. For example, reservoir evaporation and precipitation data for Texas are provided by the TWDB every month at a coarse scale across the state—in grids of one-degree latitude by one-degree longitude, or roughly 69 miles by 58 miles—on Water Data for Texas. This data comes from ground-based physical measurements from nearly 4,000 rain gages and roughly 90 evaporation pans across Texas and neighboring states.

In recent decades, remotely sensed weather data has made it possible to provide daily and sub-daily weather data for grids of four kilometers by four kilometers (2.5 miles by 2.5 miles) or smaller across the continental United States. Examples include Gridded Surface Meteorological data (gridMET) and Real-Time Mesoscale Analysis (RTMA). Researchers from Texas A&M University and the Desert Research Institute have used this fine-scale weather data to generate estimates of daily precipitation and evaporation across Texas. Results are displayed on Water Data for Texas for both areas of the state and for individual reservoirs.

As technology advances, remotely sensed data may also make it possible to monitor ungauged lakes and reservoirs in Texas. The National Aeronautics and Space Administration, U.S. Department of Agriculture, and research partners have demonstrated the capacity to monitor the elevation and extent of large water bodies (including several in Texas) using existing satellite data. Additional satellite platforms with more precise measurement capabilities may make it possible to extend this monitoring capability to smaller lakes and reservoirs.

This article is posted in Reservoirs / Water Data .