Processes acting on the formation of groundwater flow

Groundwater is subsurface water which fills totally the pores and cavities of the lithosphere (saturated zone). It comprises about 97 percent of the fresh water on earth. Although the groundwater aquifers can be regarded as a huge water reservoir the groundwater table shows a distinct pressure gradient through which the water in this reservoirs is subjected to continuous flow and exchange processes with rivers, lakes and the unsaturated zone. Up to 60 to 80 % of the longterm flow volume in rivers is a result of groundwater exfiltration. Compared to interflow and surface runoff this flow process is much slower and can take years before the infiltrated precipitation water can reach the river. Due to these retained flow processes the exfiltration of groundwater into the rivers persists even during long periods of no rain. Thus the groundwater is responsible for the dry-water-flow in rivers (also named basic river flow). But at flood fast changes of exfiltrated groundwater can be also observed. They are not due to lateral flow processes through the aquifer. In confined aquifers a distant rise of the groundwater table through infiltrated precipitation water can increase the groundwater pressure close to the river instantly similar to the principle of communicating tubes in Hydraulics. Then old groundwater along the river will be exfiltrated instantly into the river while new groundwater will follow later from the distant aquifer regions.

In urban areas the groundwater is affected by anthropogenic changes in the same way as the interflow. The sealing of the surface will reduce the groundwater recharge. Thus the overall amount of groundwater will be less in urban watersheds. The impact is a lowering of the average groundwater table which reduces the exfiltration of groundwater into the river.Especially in dry periods rivers with urban watershed show less runoff or even fall totally dry.

Discharge hydrograph with the indicated base flow

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Module
Hydrology of Floods
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TUHH

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