As so often is the case, one does not question why things work until they no longer do. For example, you flip a wall switch, but the light doesn’t come on. Your first reaction is to change the light bulb, but if that doesn’t work, you might check the electric breakers. If that doesn’t work, perhaps you call an electrician.

The scenario was much the same for drought watchers throughout Texas as they observed reservoir levels fluctuate and decline from 2011 through early 2015. With most reservoirs full and passions a little less heated, now is a good time to take a closer look at just what happened during the drought and at how reservoirs work.

Those that began to ask where the water in our Brazos basin reservoirs went, learned that many Texas reservoirs were actually built to store water for use and that much of this use occurs and is more noticeable during times of drought. The drought of the 1950s taught Texans a hard lesson that not having water stored when it is needed can be disastrous for farms, communities and industry alike. The state as a whole had not seen dry conditions similar to the 1950s until 2011 when rainfall seemed to skirt the majority of the state and the water supply stored in reservoirs began to disappear. Outrage followed in some cases, and while several communities were concerned as they counted the number of days that water would still be available for drinking, farming, and generation of electricity, areas previously accustomed to relatively full lakes began to fume over perceived local economic effects and declining recreational opportunities caused by lower lake levels.

So, the question most posed to the Brazos River Authority was: Where does all the water go?

Water leaves a reservoir in three primary ways:

1) it can be pumped from the reservoir (for a beneficial purpose such as municipal water supply, production of electricity, agricultural irrigation, industrial or mining use);

2) it can be released downstream to meet these same needs, to pass excess inflow when the reservoir is full, or to provide for environmental flows within the river that benefit the fish and wildlife; or

3) it is lost to evaporation.

As reservoir levels continued to decline, water accounting came into question. Explanations of water measurement in acre-feet, questions regarding water contracting and the real benefits of different forms of water use were scrutinized. However, interestingly, it’s the evaporative loss that is often the source of confusion.

Unlike pump station meters that record amounts moving through a pipeline and streamflow gages that measure water moving down a watercourse, evaporation is less tangible. You normally can’t see or feel evaporation; yet, science and physical measurements tell us the water cycle is continuous, and water levels continue to decline even when pumps are silent and dam gates are closed.

Evaporation is measured on a daily basis at BRA reservoirs. It’s basically the opposite of a rainfall measurement. Instead of measuring the increase in the water level like you do in a rain gage, you measure the decrease in the water level from an evaporation pan. On a hot, windy summer day, the decrease (evaporation) can be around ½ inch per day. The evaporation depth is then multiplied by the surface area of the lake to determine the daily volume lost to evaporation.

The Brazos River Authority began publishing a water use and reservoir accounting summary beginning with calendar year 2014. For some, the results were both startling and unexpected.

The report notes that more water evaporated from the reservoirs than was used for water supply. Total reservoir evaporation in 2014 was about 412,000 acre-feet whereas total water use by all BRA customers was about 256,000 acre-feet. The report also noted that 45 percent of the water used by BRA customers was for municipal purposes, 49 percent was for industrial use (including power plant cooling for generation of electricity), 5 percent was for agricultural irrigation, and 1 percent was for mining (including fracking).

During the drought, the BRA reservoir system reached low points between 60 and 70 percent full in 2011, 2013, and 2014. Lake Proctor was impacted the most and reached a low point of about 28 percent full by early 2015.

Just to the northwest of Lake Proctor in the upper portion of the Brazos River basin, the watershed feeding Possum Kingdom Lake and Lake Granbury experienced some of the driest conditions in the state, leaving the flow into the reservoirs at the lowest levels in recorded history.

As the lack of rainfall dragged on, continued water use and evaporation along with little to no inflow to replenish and refill the reservoirs resulted in low levels not seen in over 40 years at Possum Kingdom Lake and never experienced at Lake Granbury. One of the results was the closing of boat ramp access and unusable lakeside docks.

The reservoirs were built to supply water during times like this, and while water use continued from the lakes through the drought, the cause of declining reservoir levels lay more with the lack of replenishment than the use itself.

Since there are few natural spring-fed lakes in Texas, Texas reservoirs depend entirely on rainfall to be replenished. And, not just how much rain falls – but where it falls as well.

“One of the questions that comes up a lot is how much rain will it take to refill the lakes or keep them full,” said BRAs Brad Brunett. “That seemingly simple question is impossible to answer because it depends on so many variables such as how dry it is before the rain occurs, how large of an area the rain covers, and how fast it falls. A widespread three-inch rain that falls in a short time over saturated ground can generate a tremendous amount of runoff whereas a three-inch rain that occurs over 3 days over a small dry area may produce none.”

With the advent of 2015s El Niño weather phenomenon, drought conditions became a very bad memory for most of the state. It was a lesson that, like the drought of the 1950s, will not soon be forgotten - but might in the future be better understood.