From: Michelle Richman

MEMORANDUM
To: Brian Patton

cc: Rick Raymondi

From: Michelle Richman

Date: April 15, 2011

Re: South Side Return Flows through 2010 as Requested by Clive Strong
Background

This memo refers to the accompanying files containing hydrographs for fourteen south side return flows that fall within the Twin Falls Irrigation District. Please also refer to the accompanying map. Each return flow file includes a hydrograph displaying flow in daily average cubic feet per second, generally for the years 2005 to 2010. It is important to note that the graphed flow includes precipitation runoff. Some of return flows are perennial while others are seasonal. For the perennial return flows, the estimated base flow has been identified on the hydrograph, and the calculated discharge includes this base flow component.

Seven of the fourteen return flow sites have been monitored by IDWR since approximately 2005, and these include A10, Cedar Draw, East Perrine Coulee, I Coulee, Mud Creek, S Coulee, and the Twin Falls Coulee. Five of the fourteen sites were monitored by the Agricultural Research Service until May of 2010. At that time, the site monitoring and subsequent data processing was taken over by IDWR. These five sites include Deep Creek, N Coulee, Q Coulee, S2 Coulee, and Rock Creek at Poleline. Data for the Perrine Coulee are supplied to IDWR by the Twin Falls Canal Company. Data (daily cfs values) for the Lateral 10 Power Plant (L10P) are supplied to IDWR by the power plant.

Two of the fourteen sites return to the Salmon Falls Creek: L10P and the N Coulee. The remaining twelve return flow sites spill into the Snake River.

Methods and Assumptions

Some sites have contracted rectangular sharp crested weirs, and the discharge is computed using the standard equations published by the Bureau of Reclamation. Other sites have no measurement device, and flow is calculated using a rating curve developed from field discharge and stage measurements. For these sites that use rated sections to compute flow, continued additional discharge measurements are needed to validate the initial rating curves.

All of the data processing, quality assurance, and rating curve development have been conducted using Microsoft Excel. The rating curves could potentially be improved through the use of Aquarius rating curve development software, which IDWR has primarily used for its spring monitoring efforts.


Comments
Historical USGS data is available for a couple of the sites, and this can be supplied by IDWR upon request. These sites include Cedar Draw, East Perrine Coulee, Mud Creek, and Rock Creek at Poleline. IDWR can also make available upon request the sum of all measured south side return flows by month and by year.