Let V denote rainfall volume and W denote runoff volume (both in mm). According to the article "Runoff Quality Analysis of Urban Catchments with Analytical Probability Models" (J. of Water Resource Planning and Management, 2006: 4-14), the runoff volume will be 0 if Vv, and will be k(V-va) if V>v. Here v is the volume of depres- sion storage (a constant), and k (also a constant) is the runoff coefficient. The cited article proposes an exponen- tial distribution with parameter A for V.

a. Obtain an expression for the cdf of W. [Note: W is nei- ther purely continuous nor purely discrete; instead it has a "mixed" distribution with a discrete component at 0 and is continuous for values w > 0.]

b. What is the pdf of W for w> 0? Use this to obtain an expression for the expected value of runoff volume.