An isolated 3-hour storm occurred over a catchment. Given the –index and rainfall data, compute the total rainfall excess (rounded off to one decimal place).
Official Solution
Correct Option: (1)
Catchment 1: 15% area
cm/hr
Rainfall: 0.4, 2.5, 1.6 cm
Excess (negative → 0):
Weighted excess:
Catchment 2: 35% area
cm/hr
Rainfall: 0.8, 3.0, 2.1 cm
Excess:
Weighted:
Catchment 3: 50% area
cm/hr
Rainfall: 0.6, 2.6, 1.9 cm
Excess:
A fully penetrating well is installed in a homogenous and isotropic confined aquifer. The aquifer has uniform thickness of 16 m and hydraulic conductivity of 25 m/d. Water is being pumped out from the well at a constant rate of 0.1 m /s till steady state condition is reached. If a drawdown of 3.5 m is observed at a distance of 75 m from the well then the drawdown at a distance of 150 m from the well will be ....................... (in m, rounded off to two decimal places).
Official Solution
Correct Option: (1)
Step 1: Governing equation. For a confined aquifer, the Thiem equation for steady state drawdown is: where, - = drawdown at distance (m) - = pumping rate (m /s) - = hydraulic conductivity (m/s) - = aquifer thickness (m) - = radius of influence - = distance from well (m). Step 2: Ratio of drawdowns at two distances. Since other terms are constant, Step 3: Given data. - at . - Need at . Step 4: Relation of drawdowns.
Step 5: Simplification. Since , For large , the logarithmic ratio depends weakly on . Assuming , approximate ratio: Let’s assume (typical for confined aquifers). Step 6: Compute drawdown. If increases further, ratio stabilizes around 0.79. Taking refined average: Final Answer:
