What is the relationship between evapotranspiration and groundwater fluctuations? How does this relationship affect recharge analysis?

I would like to further add my thoughts for your evaluation:

Evaporation relates to groundwater in two ways:

The first including vegetable intercepting precipitation and recharge the unsaturated zone soil moisture, which is then evaporated. This process does not directly involve groundwater from the saturated zone.

The second occurs after precipitation has recharged the groundwater. Groundwater then evaporates directly to the surface (part of bare soil evaporation), or is absorbed by vegetation and transpired. Groundwater can also evaporate into the soil, recharging soil moisture, which is subsequently absorbed by vegetation and transpired or directly evaporated. This process represents a groundwater sink.

When analyzing groundwater recharge, we need only to consider the first process. Further, if we are interested in the lag of groundwater response to precipitation, we should focus on the interception of precipitation by vegetation in the first process. This is because the lag in groundwater response to precipitation recharge is influenced by many factors, including soil moisture content. Soil evaporation, on the other hand, is an indicator of soil water content. If soil evaporation is removed from the precipitation recharge calculation, it may lead to inaccuracies in recharge estimates, as soil water content from soil evaporation is sourced from multiple processes, including past groundwater recharge.

Given that in areas with a clay cover, it can take months for groundwater to receive recharge from precipitation, does the process of groundwater evaporation through the soil and to the land surface also occur over a similar timescale, lasting for months?

Am I correct in this understanding? I would appreciate your feedback.

Yes, evapotranspiration is a groundwater sink. If the groundwater table is very shallow (near the ground surface) groundwater evaporation will take place since evapotranspiration is evaporation from water bodies (physical process) and transpiration (biological process) from plants.

Hi, Wondmyibza Tsegaye Bayou :

Thank you for your interest in my query. But may not exactly correspond to my worried: my thought was, would there be a month-level lag in this evaporation? Like, for example, I found that actual evaporation exists with a one-month lag for potential evaporation (from gleam). So, shouldn't soil evaporation and vegetation transpiration from groundwater sources have a lag time relative to groundwater? For example, groundwater rises to a peak, and then there is a lag of days/weeks/months for soil evaporation and vegetation transpiration to rise to a peak?

I think it is correct to speak not about evaporation, but about the flow of groundwater up the soil profile under the influence of capillary forces, as well as about the transpiration process.

re Xiaoran comment "in some common response analyses, recharge is considered as the net of precipitation minus evaporation,..."

No, this is imcomplete water balance. Must account also for (a) interception losses above the ground surface by vegetation (b) wetting of the vadose zone above the water table (must occur before groundwater recharge can occur), and (c) epheemeral lateral surface flow (i.e. overland flow) and ephemeral lateral shallow subsurface (above water table) flow of bulk water that occurs in many areas; a large portion of such lateral flow may empty directly into rills and streams without ever recharging groundwater.

re Xiaoran comment "Does evaporation cause groundwater to fall, or does rising groundwater lead to increased evaporation?"

This is not either/or; both processes can occur.

Bare soil evaporation is the process of upward movement of water (in liquid and/or vapor phase) in soil profile due to suction (matric potential) gradient (liquid phase) or vapor pressure gradient (vapor phase); with these moisture gradients in the soil profile being maintained by evaporation/vapor loss from the soil surface. In this way, water from the water table is transported up to ground surface and into the atmosphere; which may cause the groundwater table to fall (depending also on the balance of bulk lateral/sub-lateral recharge and discharge within the groundwater system)

A rising groundwater table will generally lead to an increase in the rate of evaporation/vapor loss from the ground surface; since the magnitude of both matric potential gradients and vapor phase concentration gradients will, on average, be larger.

The answer depends on the aquifer. There will be minimal effect on deep aquifers, but as the questioner points out, shallow aquifers less than about 2 m below the surface that can be reached by plants will be significantly affected. The source of the water will play a major role. Is the groundwater coming from higher elevations, or is it located in a river valley and is an alluvial aquifer. Alluvial aquifers will likely show less effect from ET as they quickly recharge, but shallow aquifers relying on recharge from upslope areas will likely be slower to recharge and hence show more response to deep rooting plants.

I may add to what already mentioned above.

The causes of groundwater or water table level fluctuations are rainfall and evapotranspiration (the process of evaporation from soil and plants and transpiration from plants). As the water table approaches the ground surface due to rainfall, evaporation from groundwater increases with the rate mainly depends on weather and soil hydraulic conductivity which becomes a dominant factor affecting evaporation rate as the depth to the water table increases. In this case, capillarity supplies water to the soil surface from subsoil; capillary rise limit is greater in fine textured soils (e.g. clayey soils) compared to coarse textured soils (e.g. sandy soils). Beyond the capillary rise limit, upward soil moisture transport can occur in the vapor phase up to the depth of several meters.

Transpiration rate becomes equivalent to the uptake of water by roots submerged below the rising water table. Transpiration rate fluctuates depending on vegetation type, weather, with the climate change affecting the density of stomata in addition to stomata opening and closing.

Accordingly, maximum water table drawdown may occur during hot and windy days (high evapotranspiration) compared to small variations occurring in water table level during cool and cloudy days (low evapotranspiration). Water table rises sharply after rainfall on high water table vegetated land. For diurnal water table fluctuation, maximum groundwater discharge occurs, normally, near midday when the air temperature is at its highest level; during the night, water table level rises indicating recharge in excess of discharge.

A useful simple approach for describing direct groundwater recharge by rainfall is the water balance concept. In this concept, direct groundwater recharge by rainfall is assumed directly proportional to the difference between rainfall and evapotranspiration

Using the groundwater fluctuation method assumes that groundwater changes in a shallow aquifer are caused by evapotranspiration only , and this method has been widely used to estimate ETg in riparian and wetland areas.

The relationship between evapotranspiration (ET) and groundwater fluctuations is an essential consideration in hydrology, as it directly influences groundwater recharge analysis. Here's how they are related and how this relationship affects recharge analysis:

Relationship Between Evapotranspiration and Groundwater Fluctuations

Impact on Recharge Analysis

Practical Implications

By accurately incorporating ET effects into recharge analysis, water resource managers can better predict and manage groundwater resources under varying climatic and land use conditions.

Evapotranspiration (ET) removes water from the soil and atmosphere, thereby reducing the amount of water available for groundwater recharge. Also, increased ET during dry periods can lower groundwater levels.

Concerning

Impact on Recharge Analysis,

Recharge analysis involves detailed consideration of the interactions between ET, precipitation, runoff, and other hydrological processes to provide a comprehensive understanding of groundwater recharge.