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Watershed Hydrology, a Hawaiian Prospective; Groundwater Ali Fares, PhD Evaluation of Natural Resource Management, NREM 600 UHM-CTAHR-NREM

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Presentation Outline Infiltration – Definition & theory Green-Ampt, Horton & Philip equations – Infiltration measurement – Infiltration as affected by: Soil type

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Aquifers

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Aquifer Properties

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Water flows from area of high energy to low energy A unit of water has energy due to 3 factors: – Elevation – Pressure – Velocity (not important for groundwater)

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Total Energy is expressed as feet of “head” Head is equivalent to a column of water so many feet high (convertible to pressure by multiplying by the weight of water)

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Darcy ’ s Law a)French engineer experimented with water flowing through sand in mid- 1800s b)He found that the amount of flow was – i) proportional to the area through which the flow occurred, and was – ii) proportional to the hydraulic gradient (the change in head per length of flow path), and – iii) related to the nature of the material through which the flow was occurring

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Hydraulic Conductivity Darcy’s K, a velocity, is known as Hydraulic conductivity Darcy’s K is actually a function of – the liquid as well as – the porous media (sand, gravel, clay, etc)

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Darcy’s Law can be used to predict total flow if the hydraulic gradient and other factors are known Transmissivity: hydraulic conductivity times the thickness of the aquifer (Kb)

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Subsurface classifications

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Water content of an Unconfined Aquifer porosity sets the maximum volume available for water below the water table porosity (n) = Specific yield (Sy - the water the aquifer will release) + Specific retention (Sr - the water not released)

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Surface water & Groundwater Interactions A “gaining” stream: receives water from the groundwater (the groundwater discharges to the surface) – The water table is higher than the stream bed A “losing” stream recharges the groundwater – The water table is lower than the stream bed

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Effect on the Water Table of Pumping from a Well a “Cone of Depression” is created in the water table (potentiometric surface) when a well is pumped at a sustained rate This can result in a shift in direction of flow as the potentiometric surface is changed sub-surface flow patterns are a function of basin thickness, water table gradient, and geologic complexity ( mixed hydraulic conductivities)

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Darcy’s Law can be used to predict the drop in level of the water table due to pumping from a well, or system of wells

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Drop Down Under steady state conditions at a given pumping rate, and if K is known Theis in the 1930s predicted the drawdown in the water table at any given time while the Cone of Depression is expanding Values of W(u) can be looked up in Table 9.1 for calculated values of u

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Capture Zone of Wells The zone of influence of a well is determined by the slope of the potentiometric surface (water flows down “energy”) Dimensions of the capture zone can be calculated if the characteristics of the aquifer are known (thickness, K, hydraulic gradient, etc.)

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