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## Watershed Hydrology, a Hawaiian Prospective; Groundwater

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**Watershed Hydrology, a Hawaiian Prospective; Groundwater**Ali Fares, PhD Evaluation of Natural Resource Management, NREM 600 UHM-CTAHR-NREM**Presentation Outline**• Infiltration • Definition & theory • Green-Ampt, Horton & Philip equations • Infiltration measurement • Infiltration as affected by: • Soil type**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)**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)**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**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)**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)**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)**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**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)**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**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**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.)