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**Essay Questions**

Click here to access the Equation Sheet.

25. (3 pts) (modflow) Explain the difference between a **stress period** and a **time step** in MODFLOW.

26. (4 pts) (modflow) Describe how an **iterative solver** works. Explain how each of the following

parameters affect the solution process:

a) Maximum number of iterations

b) Acceleration parameter

c) Head change criterion for convergence

27. (3 pts) (stochastic) Explain the difference between the **random sampling** and the **latin hypercube** methods associated with the parameter randomization options for stochastic (Monte Carlo) simulations.

28. (3 pts) (bc) What is the objective of **regional to local model conversion**? Why not just start out with a local-scale model?

29. (3 pts) (modflow) The following diagram represents a section of a **river** within a MODFLOW cell:

a. What is the general equation for conductance (in terms of K, A, and L.)?

b. Show how the variables in the diagram would be used to compute the river conductance. In other words, insert the variables into the proper location in the equation you entered for part (a).

c. Compute the value of the river conductance (in ft^2/day) for the reach. Assume that L=300 ft, M = 6 ft, W = 40 ft, and K = 10^-2 cm/s.

30. (4 pts) (head-darcy) An artesian aquifer is overlain by three soil layers and a small pond as shown below.

a. (3 pts) What is the **equivalent** vertical hydraulic conductivity for the system? [cm/s]

b. (1 pt) What is the **head loss** through the three layers? [ft]

c. (2 pts) What is the **Darcian velocity**/ of flow through the three layers? ft/s]

d. (1 pts) **How much water** flows into the pond from the aquifer per square foot of area in one month's time (30 days)? [ft^3]

Show your work and label your answers.

31. (3 pts) (sustain) Groundwater systems are in a state of equilibrium prior to groundwater development. As groundwater resources are developed via pumping, the water withdrawn by the wells comes from **three sources**. Describe each of the three sources.

32. (3 pts) (sustain) Define the term **groundwater sustainability**.