Project Engineer Dr. Nirjhar Shah recently published two articles that describe and explore the need for improvements to the modeling techniques currently utilized for shallow water-table environments to incorporate the dynamic interaction of the surface and groundwater processes due to shallow water-table conditions. The article entitled “Variability in Specific Yield under Shallow Water-table Conditions” was published in the ASCE Journal of Hydrologic Engineering and the second entitled, “Observations of Long Term Air Entrapment Affecting Runoff and Water-table Fluctuations” was published in the International Journal of Water.
Both of these articles are extensions of Dr. Shah’s dissertation which involves enhancing modeling conceptualization for shallow water-table environments, such as in the South Eastern United States. These articles are intended for people working in the hydrologic and hydraulic modeling field and illustrate that processes such as variability in specific yield and air entrapment have to be studied in greater detail such that their impact can be fully understood.
The articles were born out of a need to revisit the processes that cause or impact water-table fluctuations. This was determined because for shallow water-table environments it becomes very difficult to model the water-table fluctuations using traditional methodologies. Model parameters have to be tweaked sometimes to values which may depart significantly from the widely accepted parameter’s value. The objective was to make sure that the modeling conceptualizations that are used currently are sufficient to describe the physical processes. Dr. Shah has provided summaries of both articles and the results of the findings for this publication.
For further exploration of the full length articles they can be found at the following websites:
- http://scitation.aip.org/dbt/dbt.jsp?KEY=JHYEFF&Volume=14&Issue=12#TECHNICAL PAPERS
- http://www.inderscience.com/offer.php?id=28723
Variability in Specific Yield
Specific yield is a modeling parameter which is defined as the volume of water released from pumping down an aquifer by a unit head per unit area of the aquifer. From a groundwater modeling perspective it is used to determine water-level fluctuations in an aquifer in response to a given volume of water added or removed. Traditionally, specific yield has been used as a fixed value in groundwater flow models. This study seeks to elucidate the variability in specific yield due to natural processes such as evapotranspiration and recharge as well as anthropogenic processes such as pumping. It was found that the magnitude of specific yield was not only a function of process causing water-level fluctuation but also the antecedent water-level conditions. For shallow water table environments it was found that assumption of constant specific yield can yield highly erroneous results. For deep water-table environments, however, this variation in specific yield value is not pronounced and a constant value of specific yield results is a reasonable estimation of water table fluctuations.
Long Term Air Entrapment
In shallow water-table environments during high intensity rainfall, air, which is present in soil voids, gets entrapped between the advancing water/wetting front from the top and the water-table at the bottom. As the wetting front moves downwards it compresses this entrapped air thereby increasing its pressure. The increase in air-pressure not only impedes the advancement of the wetting front (i.e. reduces infiltration) but also causes the water levels in the observation wells to artificially rise due to the pressure differential created by the entrapped air. This study aims at quantifying the impact of air-entrapment on infiltration as well as the observed water-table for varying antecedent conditions. Numerical modeling as well as simple spreadsheet based analyses was carried out to understand the air pressurization effects in detail.
What impact will these articles have?
In the field of modeling, elucidation of specific yield behavior will shed light on the fact that the current modeling approaches using constant specific yield are inadequate and may be erroneous in the water-table estimation in shallow water- table environments. Additionally, the article shows the steps that need to be taken to develop modeling conceptualizations to incorporate this behavior in the existing models to make them more capable of representing shallow and dynamic water-tables. In addition, understanding the effects of air-pressurization will help people performing hydrologic modeling to look at the observed data set and make an informed decision about what data set to use for calibration. It should also prevent modelers from artificially tweaking modeling parameters to make the model match the observed water–table during high intensity rainfall.
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Nirjhar Shah, PhD is a Project Engineer for AMEC-BCI's Water Resource team and has been instrumental to the study of wetlands and quantification of wetland hydroperiods. He has been part of continual research for and development of the Integrated Hydrologic Model (IHM), particularly for the vadose-zone component that is being developed by the University of South Florida (USF) and Tampa Bay Water to model the hydrology of the Tampa Bay region. His modeling research has been part of more than 10 journal and national conference publications. |
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