Geology 127 Hydrogeology Assignment 2

Directions: Show all your work, including any necessary conversions. Label all axis and graphs. Use appropriate number of significant figures. Use short, concise sentences to explain your answer where appropriate.

• A forested watershed has an area of 97 ha. A peak stream flow of 1.9 cubic feet per second (cfs) at the outlet was measured following a 24 hour rainstorm with an average intensity of 1 inch per hour.
  1. Calculate the total amount of rain that fell during the storm. (4pts)
  2. Calculate the runoff coefficient using the rational equation. (4pts)
  3. What assumption must be made to use the rational equation? (4pts)
  4. Estimate the range of peak stream flows you would expect from a similar storm if the watershed had been developed with single family homes (use Table 2.3 in Fetter). (4 pts)

• The following values were recorded during a double-ring infiltrometer test:

1. Plot the cumulative infiltration (cm) as a function of time (sec) (3pts).
2. Plot the infiltration rate (cm/hr) as a function of time (sec) (3pts).
3. A rainstorm with an intensity of 2.0 in/hr lasts for 10 minutes. When does

Hortonian overland flow begin? (4pts)

1. How much water from the above storm is infiltrated? (6pts)
2. How much water from the above storm runs off as infiltration excess? (6pts)

• The following graph shows discharge for the Cosumnes River at Michigan Bar, CA. The watershed area above the gauging station is approximately 536 square miles.
  1. How long do the surface water effects from the storm last. (2pts)
  2. Draw the approximate baseflow component of the hydrograph. (3pts)
  3. Estimate the increase in baseflow (GW) from this storm in cfs. (3pts)

• Derive an expression for porosity in terms of bulk density and solid density using the equation for porosity (n=V_v/V_t), bulk density (ρ_b=m_s/V_t), and solid density

(ρ_s=m_s/V_s)where V_v is the volume of the voids, V_t is the total volume, and m_s is the mass of the solids. (4pts)