Back to Basics: The Water Cycle September 2017

As we know from the 2017 State Water Plan, Texas faces a water supply shortage and will not have enough water to meet the state's needs should a repeat of the drought of record occur in the next 50 years.

No problem, we'll just create more water!

If only it were that simple. Unfortunately, we are limited to the water that is available to us now—the same water that our ancestors drank and the dinosaurs swam in thousands of years before them. There is no new water.

This month, in recognition of back to school, we're going back to the basics: the water cycle. As most of us learned in school, the water cycle is a natural process that continuously moves water throughout the earth, from the oceans and plants up to the sky and back down again. But the water cycle may not be something we all actively think about when we ask the question, "Where does our water come from?"

How does the water cycle play a role in Texas' water supply? And what are we doing to make the best use of this limited resource?

Let's start with evaporation, the change in state of water from a liquid to a gas. Evaporation from oceans, seas, lakes, rivers, and the land surface accounts for about 90 percent of the water in the atmosphere. Transpiration accounts for most of the remaining 10 percent. Transpiration is the process by which water in plants is transferred as water vapor to the atmosphere. Plant roots take up liquid water from the soil and transpire that water as vapor through openings in their leaves.

Evaporation and transpiration rates increase as the temperature increases and humidity decreases—for example, East Texas evaporation rates are much lower than the dry High Plains and Panhandle. Just how much water evaporates from Texas lakes each year? Monthly and annual gross rates are available through the TWDB's Surface Water Division webpage. Lake evaporation data are available from 1954 through 2016. Evaporation can also reduce the amount of water that is available to our rivers and streams.

After water evaporates, the next step in the water cycle is condensation. Condensation occurs when water vapor in the air changes to a liquid or a solid. Condensation is responsible for the formation of clouds, which makes precipitation possible. Precipitation is the act of water in various forms falling to the earth as rain, snow, sleet, fog, or hail. Precipitation data from 1940 through 2016 are also available through the TWDB's Surface Water Division webpage.

Most precipitation falls back into the oceans, but when it falls onto land, it flows as surface runoff over the landscape and can enter rivers and streams, flowing toward the ocean. Runoff also soaks into the ground (called infiltration), replenishing aquifers and the freshwater supply that is stored within—although the rate of recharge varies by aquifer. The Edwards Aquifer recharges quickly, for example, whereas the Ogallala Aquifer recharges very, very slowly. You can view groundwater levels in wells that tap Texas aquifers at Water Data for Texas through the TWDB's interactive Groundwater Data Viewer.

Some groundwater stays close to the land surface and can seep back into surface water bodies (lakes and rivers) or emerge at the land surface through springs or seeps. Transpiration continues as plant roots absorb shallow groundwater.

Regardless of how or where precipitation falls, it all eventually keeps moving through the water cycle. But if water is continuously moving throughout the cycle, how does drought occur?

Texas finds itself perpetually stuck between instances of drought and instances of flood. Drought refers to insufficient water, in the form of precipitation, soil moisture, or streamflow, to meet water demand. It can occur when precipitation fails to arrive in a typical rainy season (like May and June in most of Texas). Drought that occurs due to a lack of precipitation is called meteorological drought. The lack of precipitation can lead to hotter temperatures and more evaporation in the short term, resulting in a reduction in soil moisture. Drought that occurs due to a lack of soil moisture is called agricultural drought. If the reduction in precipitation and increase in evaporation continues, a reduction in streamflow and lake or groundwater levels can occur, causing a hydrological drought.

When drought conditions kick in, decreased precipitation leads to reduced water levels or flows in lakes and rivers (and some aquifers), which can negatively impact aquatic ecosystems. Vegetation dies because there is less moisture in the soil, which results in less transpiration, less condensation, and less precipitation. Rain showers become fewer or nonexistent, and the cycle continues until the prevailing weather pattern changes and we receive precipitation.

If you'd like to keep an eye on drought's whereabouts in Texas, check out the TWDB's Water Weekly, a one-page report that includes a map or two and an easily digestible update on drought conditions and water-related issues across the state. It is published at the beginning of each week.

No matter the amount of precipitation Texas receives, we will always face drought-induced water challenges. That's why we must conserve the water we have and use it wisely. Making simple, daily conservation choices that add up over time can make a real difference. Turn good water choices into habits, and then they'll become as natural as the water cycle.

At the end of the day, it is up to us as individuals and as communities to take responsibility and ownership over this finite resource that we all rely on. Texas' history of drought contributes to why we diligently plan for water resources needed by the state and proactively implement the water management strategies identified in the 2017 State Water Plan. Taking action now will help ensure we have enough water in our lifetime and that of future generations.