Why does the water rise?

Why does water climb up through soil and building materials?

Capillary rise is the upward vertical movement of groundwater through a material's pore structure, driven by force of capillarity [2]. In ordinary terms, water clings to the walls of tiny spaces and pulls itself upward, much as it would climb a narrow straw. The narrower the pores, the more effectively this pull can lift water against gravity.

This matters in the real world. In building materials, capillary rise is the route by which groundwater can move up into the pore structure of walls and foundations [2]. In agriculture, the same process can supply water to crops from a shallow water table below the surface; one modeling study examined how capillary water rise contributes to corn water regimes in Portugal [4]. The mechanism is the same in both cases, but the consequences differ: in a wall it can mean dampness, and in a field it can mean a hidden water supply for plants.

Why do oceans and lakes rise?

At the scale of a sea or a lake, rising water is a matter of accounting: the level goes up when the water coming in exceeds the water going out. Groundwater extraction fits into this balance because water pumped from underground does not simply disappear. One study quantified how much of the extracted groundwater ends up contributing to sea-level rise, treating it as part of the net flow of water from land to ocean [1]. In other words, water that was stored beneath the land is moved to the sea, and the sea responds.

The Great Lakes show the same balance at a smaller scale. Their levels are governed by net basin supplies, meaning the water delivered by precipitation and runoff minus what is lost to evaporation [5]. When those supplies run high, the lakes rise; when evaporation and dry conditions dominate, they fall. The study notes that unusually high air and water surface conditions were associated with water level rise, following an earlier period of unusually low levels from 1998 to 2013 [5]. The lake level is not set by a single factor but by the net result of several competing flows.

Why does water rise relative to the land at some coasts?

Along low-lying deltas, the water level can rise relative to the land for two reasons at once. One is eustatic rise, the global rise of sea level itself; the other is subsidence, the sinking of the land. A study of the Ebre Delta in Spain describes water level rise driven by both eustatic rise and subsidence, and links it to wetland loss, coastal erosion, and saltwater intrusion [3]. This is an important distinction for anyone living on a delta: the water does not have to rise much on its own if the ground beneath you is going down.

The Ebre Delta case also shows why the answer to "why does the water rise?" depends on the setting. In a brick, the cause is capillary pull through pores [2]. In the open ocean, it is the net addition of water from land [1]. In a lake, it is the balance of precipitation, runoff, and evaporation [5]. At a delta, it is the combination of sea-level rise and sinking land [3]. The same phrase, rising water, describes several distinct processes, and the right explanation depends on which one you are looking at.

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