53:171 Water Resources Engineering
Problem #18
Sequent Peak Method for Evaluating Reservoir Capacity

A reservoir is proposed for a 150 mi2 drainage basin.  Hydrologic and water demand data were gathered for the proposed site for a critical drought period.  The data represent the monthly basin runoff (in inches), the monthly precipitation falling at the reservoir site (in inches), the monthly evaporation from the proposed reservoir (in inches), and the average water demand (in mgd).  After it is built, the average reservoir surface area is assumed to be 5% of the basin drainage area.

Month Runoff Precip Evap Demand
() (in) (in) (in) (mgd)
Oct 1.54 4.10 2.25 125
Nov 1.62 3.75 1.50 127
Dec 1.99 4.20 1.00 143
Jan 2.99 4.00 1.05 137
Feb 2.05 4.60 1.75 150
Mar 2.10 3.50 3.00 145
Apr 3.20 3.20 4.30 153
May 1.50 3.00 5.50 155
Jun 0.35 4.00 5.95 161
Jul 0.20 4.50 5.00 158
Aug 1.15 3.20 4.10 152
Sep 2.85 3.90 3.20 153

For the critical dry period, use the sequent peak method to determine:

  1. What is the reservoir capacity (in acre-feet) required?
  2. When (what month) would the reservoir drawdown begin?
  3. If the same inflows and outflows occurred for a second year, would the reservoir refill?  If so, when (what month)?

Hint: First convert all the inflows and outflows to the same units (acre-feet).  Be sure to account for the (unequal) number of days in each month (where necessary).  Note that with the reservoir built, runoff occurs the less than 100% of the drainage area (the remainder is occupied by the reservoir).

Last changed on 03/11/14 by Gabriele Villarini.