6.1 Understanding water cycle

PRESENTER: All the water on Earth today, every drop, is all the water there has ever been on the planet. Fresh water is actually, millions of years old. The same water flowing in a continuous loop. Falling as rain and snow from clouds to the Earth's surface. Running in rivers. Pooling in ponds. Flowing from faucets. Irrigating crops. Traveling through plants. Generating power. Eventually, evaporating into the air and condensing into clouds, again.

ANNA MICHALAK: Why is there life on Earth? And the reason there is life on Earth is because Earth has this perfect water cycle.

PRESENTER: The water cycle, so simple even small children understand the basics. Yet, so complex, the most advanced Earth scientists-- hydrologists, geologists, and biogeochemists-- are studying every part and process.

MARTHA CONKLIN: The water cycle is fascinating. It's something that's around us all the time. And yet, we don't really understand it.

PRESENTER: How to summarize what is known about the water cycle? With two words-- flows and stores. The water cycle is a series of flows of water between various water stores, or storages-- clouds in the atmosphere.

TOM HARMON: There's always a little bit of water in the atmosphere. We talk about relative humidity. It's a humid day. It's a dry day. Either way, there's water-- sometimes, a little. Sometimes, a lot.

PRESENTER: There's a lot of water in the ocean-- 70% of all the water on Earth. In the ice sheets and glaciers, 2/3 of all the freshwater on Earth. In the snow packs atop mountains like the Sierra Nevada. In the Great Lakes. In rivers and streams. In reservoirs and watersheds. In wetlands. In the soil. In and on plants and trees rooted in the soil. And beneath the soil, in water tables and underground aquifers like the Ogallala-High Plains, which runs underneath parts of eight states, from South Dakota to Texas. All this storage is temporary. Water in all its forms is always in flux and always moving. And there is a name for every kind of movement in the water cycle, starting with precipitation.

ANNA MICHALAK: Precipitation is the process of water falling onto the surface of the Earth. You could have precipitation in many forms-- rain, snow, hail.

PRESENTER: Rain is falling water in liquid form. Snow, ice, hail, and sleet are falling water in solid or frozen form. Fog and mist, falling water in gas or vapor form. Precipitation that falls directly onto the oceans becomes part of Surface Ocean and can be churned by wave and wind action into ocean currents. Rain and snow that falls directly on rivers and streams becomes one part of stream flow. Rain that falls onto land takes a different path to the river. As does the snow and ice that falls and collects on mountain tops when temperatures warm.

MARTHA CONKLIN: When snow melts, some of it runs through the snow pack and goes into small streams, tributaries that feed into large rivers.

PRESENTER: What about that precipitation that falls on and over land? Some is intercepted by vegetation-- plants and trees. TOM HARMON: Like you might imagine, someone in the game of football intercepting a pass, these are raindrops trying to come to the ground and the leaves on the tree intercept them before they hit the ground.

PRESENTER: And the precipitation that does hit the ground, it can run off if the ground is hardscape, covered with asphalt or concrete, or if the soil is too wet or saturated to absorb more water, like an over-soaked sponge. Otherwise, precipitation infiltrates the soil surface, percolates into the ground.

TOM HARMON: Think of it as the water percolating through your coffee grounds in the morning. Gravity continues to pull it downwards so it will move through.

PRESENTER: Through the topsoil. Into spaces between soil and rock particles. Down to bedrock and further into fractures. Into deep underground aquifers. Even ground water here is moving sideways or laterally, discharging toward a river, lake, or the sea. Generally, the deeper the flow, the slower the flow.

MARTHA CONKLIN: Some of that fractured water might take a very long time-- thousands to millions of years-- to get out.

PRESENTER: And how does water get back out into the atmosphere? It evaporates. It's turned from a liquid into a gas or vapor, by the heat of the sun.

ANNA MICHALAK: If you put a bit of water into a bowl and you set it outside on a sunny day, it's going to disappear. It's still water, it's just in the form of a gas rather than the form of a liquid.

PRESENTER: Water evaporates from every wet surface, even from wet air. Some rain and snow evaporates into the air while falling. Water evaporates through our respiration and perspiration. And from plants through transpiration. Trans means through or across. Plant roots draw up groundwater. And plants pull that water up through their stems into the leaves and then, release them back out through evapotranspiration. Evapotranspiration-- a spelling bee worthy term for evaporation from soil and water surfaces, plus transpiration from plants. Evaporated water molecules are tiny enough to flow into the air. Mix with smoke and dirt particles in the atmosphere. Cool. Condense into visible masses of water vapor-- clouds. Winds move clouds into colder air, water droplets collide and merge, grow bigger and heavier until they are so heavy, they fall again, as rain or snow, sleet or hail. Precipitation, collection, runoff, interception, infiltration, percolation, discharge, transpiration, evaporation, condensation-- the water cycle

This animation shows multiple connections within the hydrological cycle, including those with human controlled systems. It gets kinda bulky in the end so I will go step-by-step.

Let's start with ocean. The ocean is the biggest reservoir on the Earth, which accounts for about 96.5% of global water reserve. The ocean exchanges water with the atmosphere. Through evaporation and precipitation. Atmospheric water vapor accounts for only 1000th of a percent of the global water. But because the surface of the evaporating ocean is so huge, they exchange is extremely important as transport mechanism.

Now, let me push this ocean box to the side and clear up space for some other storages and elements of the cycle. We can identify a few main types of continental water storages. Surface water, rivers and streams, lakes, and some snow and ice masses. Snow and ice, by the way, represent quite a significant storage. They contain more water than all other service storage is altogether. Although storages are fed by the precipitation fluxes, rainfall or snow fall, and they also release some water back to the atmosphere. Snow eventually can melt and feed some water to the surface freshwater system. Then surface water flows to the streams and rivers through collection and surface runoff and reverse channel water to the more stationary storage lake or flow directly to the ocean. This is quite well-known schema of natural water exchange, which you may have heard in basic earth science class. But let's go a little bit further and add some more details here.

Some of the rain can be intercepted by plants and that water eventually drips and flows down over with some delay. Plants also release some water to the atmosphere through transpiration and evaporation. Okay, The next step is very important. Considerable amount of surface water infiltrates into the soil from where it can be taken by plants. This process fosters biomass production. The rest of it percolates down to the groundwater and groundwater can actually migrate and exchange with the surface water storage is through the base flow.

Groundwater is another huge reservoir, which may contain either fresh or salty water. Some of the aquifers may have connections to juvenile resources. In other words, the origin of this water can be related to the magmatic processes in the interior. So we put that into the picture. The next link, some of the groundwater in juvenile water reacts with rocks and is present in the mineralized form. For example hydrated minerals. And finally, here we have the geothermal water, which can be connected to both juvenile water pricing or dehydration of minerals on the high temperature and pressure. In some spots, geothermal water can just charge to the surface or to the ocean floor. This closes the loop from the downside. So this is the natural water cycle without humans in the picture.

But what if we put our water utilization system right in the middle here? That includes agricultural, domestic, and industrial use of water. And those can be connected to an artificial water storage. So where does that usable water come from? Can be extracted from the aquifers or taken directly from the surface reservoirs, for example agriculture can use some diverted streams for irrigation. All those users produce some wastewater for sure and that wastewater can be treated and returned to the environment. Can be either discharged to streams or sprayed onto soils, or even can be re-inject it to the aquifers. Some of the treated wastewater can be re-used, which makes it closed recycling loop here. Agricultural irrigation is another way for the water to return to the natural cycle. Now we understand that rain can fall on the human-made environment as well. And after this indirection, we have some stormwater produced. This storm water is also considered some kind of wastewater which can be treated or not treated depending on the locality. But either way, storm water contributes to the discharge flux from the human water utilization system to the nature.

Then we add another discharge curve here, showing that some of the wastewater sometimes goes back to the environment without any treatment. This is not good, but it happens. Can we extract water from other resources if groundwater is not available? Sure. Here we put the additional arrows to show extraction of water from rivers and lakes and exchange fluxes with the ocean certainly also take place. Some locations direct circulation of geothermal water. There's also a viable option. People can use it for heating the homeschool, for example. I'm sure this schematic can be developed further and more storages and fluxes can be identified. But let's stop here. You see this boundary zone here between the naturally balanced part of the cycle and human control part of the cycle is where the potential problems occur. Mismatch in physical and chemical fluxes can throw the system off balance and lead to water shortages, ecological crisis, pollution, and all other negative consequences. So to ensure global and local sustainability of water resources, we need a number of special technologies that would help make these two systems more compatible. Some technologies are used at the extraction phase, for example drinking water purification. Those make sure the water is sufficient and safe for human consumption. Other technologies are used at the discharge path. for example wastewater treatment. Those are designed to mitigate pollution and make sure that we're not depleting our natural reservoirs too soon.

Finally, some technologies help control water loops within the human domain, for example water distribution and collection. The bottom line of this demonstration is that the development of novel technologies for water treatment and monitoring is very important for providing smooth interaction between the sphere of human activity and the environment. The ultimate sustainability goal here is to re-install the water balance locally and globally. And to be sure that this most important and irreplaceable resource is conserved for centuries to come.