How we get the numbers
The Texas Water Development Board obtains lake levels from partner organizations including the United States Geological Survey (USGS), International Boundary Water Commission (IBWC), United States Army Corps of Engineers (USACE), United States Bureau of Reclamation (USBR), and Lower Colorado River Authority (LCRA). These organizations maintain gauges at the reservoirs that automatically measure water surface elevation and transmit the data back to the organization. We then collect data from these organizations via the internet. Where possible, we provide a link to the original data source in the "Additional Information" section of each individual reservoir page.
Reservoir gauges provide near real-time measurements, but we poll the data from the organizations and update our web site throughout the day only approximately every six hours. Using this data, we calculate averaged water level values for the day and display this on our website. Occasionally, gauges can become damaged or malfunction leading them to provide no data or incorrect data. If a gauge stops reporting, measurements may be unavailable on our web site for short periods of time. If a gauge reports incorrect data, we may report this for a short period of time on our web site until we can identify the problem and make corrections to or remove the data. Because we poll data only about every six hours and provide only daily average information, the information we provide on our web site may not be adequate where near real-time or un-averaged data is needed.
In most cases, we translate water surface elevation measurements into estimates of water volume by comparing the measurements to "Elevation-Capacity Curves" (or "rating curves"). These rating curves are produced from surveys and bathymetric models of the reservoir that indicate the reservoir storage volume corresponding to different water surface elevations. When available, we provide these tables on each individual reservoir page in the "Elevation-Capacity Curves" section. In a number of monitored reservoirs for which we don’t have rating curves (e.g. Lake Amistad), we obtain storage volume data directly from the sources.
The amount of water stored in a reservoir is displayed as "Reservoir Storage". Reservoirs are designed and authorized to hold up to a certain amount of water under normal operations. This is referred to as the "Conservation Capacity". The water surface elevation corresponding to the Conservation Capacity is the “Conservation Pool Elevation”. Reservoirs with flood storage function are also designed to hold additional amount of water to prevent downstream flooding, referred to as the "Flood Pool". When the water surface elevation is above the Conservation Elevation, it lies in the flood pool. Some reservoirs have a Flood Pool that changes seasonally to provide additional capacity during rainy seasons. This results in variations in the Conservation Capacity and Conservation Pool Elevation. For some reservoirs, there may also be a level below which water cannot be effectively used (for example, the elevation of the lowest intake pipe). Water below this point is referred to as "Dead Pool". Conservation Storage is calculated by taking the Reservoir Storage and subtracting Flood Pool storage (if it is above Conservation Elevation) and Dead Pool storage. Percent Full is calculated from the ratio of Conservation Storage to the Conservation Capacity.
Information about reservoirs is aggregated to collections of reservoirs that are useful for planning and other purposes. These include statewide totals, planning area totals, river basin totals, municipal area totals, and climate region totals. Daily reservoir storage, conservation storage and conservation capacity of individual reservoirs are added together to produce totals for each of these collections. If data for a reservoir on a particular day is unavailable due to equipment malfunction or a gap in the period of record, then an estimated daily value is calculated by linearly interpolating between the closest available daily values or by using the last valid reading.
Some special conditions apply to reservoirs that are shared with other states or Mexico. See the footnotes on individual reservoir pages or aggregation pages for details on how those conditions affect the numbers.
If you would like additional information on our methodology, please contact us.