Picture a coastal landscape after a storm. Water sits in the ocean, in the air as water vapour, on leaves, in the soil, in streams, and deep underground. Those places are the system’s containers. At the same time, water moves between them as evaporation, precipitation, flow through soil, and river discharge. This course treats the water cycle as a set of linked containers and connecting pipes, except the pipes are physical processes that speed up or slow down depending on weather, vegetation, and terrain.
One clean way to keep the story straight is to separate what water is stored in from how water moves. Stores are where water can accumulate. Flows are the transfers between stores. With that split, evapotranspiration, infiltration, and runoff stop being vocabulary words and become the main pathways that connect land and atmosphere.
Take a look at a labeled landscape version of that idea.
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A store is a place where water can be held for some time without immediately leaving. In this course, the key stores are the ocean, the atmosphere, snow and ice when present, water on the land surface, soil moisture in the unsaturated soil, rivers and lakes, and groundwater in an aquifer. A store can be thin and fast, like a puddle, or thick and slow, like groundwater. What makes it a store is not its size but that it can change its amount.
A flux is a flow rate of water from one store to another. It answers how much per time. The everyday translation is that a flux is a pace, not a pile. If a store is like an inventory on a shelf, a flux is like the restocking or selling rate.
This inventory view matters because a single storm can feed multiple fluxes at once. Some rainfall becomes runoff into a channel within minutes. Some infiltrates and may not reappear in a river for weeks or years. Some returns to the atmosphere quickly through evapotranspiration. The landscape is not just a map. It is a set of pathways with very different time scales.
A store has an amount of water in it. A flux changes that amount. The simplest bookkeeping statement is that a store rises when inflows exceed outflows, and it falls when outflows exceed inflows. You do not need calculus to use that idea. Over a chosen time interval, you can think in averages.
Suppose a patch of soil receives rainfall for one day. If more water enters the soil than leaves it during that day, soil moisture increases. If more leaves than enters, it dries. The same logic works for a river reach, a lake, or the atmosphere above a region.
Time scale controls which stores matter. On the scale of minutes to hours, the important stores are water on the surface, in the vegetation canopy, and in small channels. Over days to seasons, soil moisture becomes central because it buffers rainstorms and feeds plants. Over years to decades, groundwater and snowpack can dominate because they release water slowly and remember past climates. A common mistake is to talk about the water cycle as if every part responds on the same clock. It does not.
Tip: When you feel lost in a water cycle diagram, ask two questions. Where is the water stored right now, and what process can move it next on the time scale you care about.
Evaporation and precipitation are familiar, but three pathways control how the land reshapes a rainfall input before the atmosphere sees it again. They are evapotranspiration, infiltration, and runoff. Each names a distinct physical process with distinct consequences for flooding, drought, and ecosystems.
Explore these three pathways in a concrete set of physical scenes.