Development of tools to estimate conveyance losses in the Truckee River, USA David McGraw & Greg Pohll & Rina Schumer & Margaret Shanafield Abstract A method to estimate stream conveyance losses, including stream bank seepage and evapotranspi- ration from riparian areas, was developed for the Truckee River in California and Nevada (USA). Aquifer diffusivity is the primary variable required to compute aquifer head and seepage. Head and seepage conceptual models developed in previous studies were calibrated using stream stage and aquifer head at five sites along the Truckee River. The equations for head require numerical convolution to solve. It was found that head is insensitive to diffusivity and a single value for diffusivity provides good results at all study sites. Also, because releases from storage are usually prescribed as a step increase in flow for a given period of time followed by a step decrease in flow, an analytic solution for volume of seepage can be used in place of numerical convolution. Under typical operating conditions, seepage volume represents a small fraction of the total reservoir releases. Uncertainty in seepage estimates can be reduced with increased accuracy in hydraulic conductivity. This work demonstrates a valuable technique to estimate conveyance losses under natural and managed stream flow. Keywords Groundwater/surface-water relations . Groundwater management . Conveyance losses . USA Introduction The purpose of this study is to develop methods to calculate incremental conveyance losses in the Truckee River system due to upstream reservoir releases that can be incorporated into the numerical water-accounting system mandated under a new River operating agreement. It is well-known that flood waves can be modified considerably by bank storage (Pinder and Sauer 1971) and effects of frequent changes in Truckee River stage from releases at upstream reservoirs must be quantified. Conveyance losses include seepage to groundwater and associated evapotranspiration (ET). Although total con- veyance losses will be assessed as a component of this project, these losses need to be calculated based on incremental flows as they are released into the Truckee River. The effect of river stage on groundwater level, discharge, and bank storage is analogous to that of changing water level in a well penetrating a confined aquifer on head, discharge and aquifer storage (Neuman 1979, 1981). Here we are interested in the effect of an increase in streamflow on seepage and head in the aquifer. Although there are non-linear feedbacks between stream stage and head in an adjacent aquifer, resulting in use of iterative models for computation of seepage, small increases in stream stage relative to the aquifer thickness and the slope of the water table allow a linear approx- imation of the problem (Moench and Barlow 2000). Hydraulic head in the aquifer with distance from the stream can be obtained by solving the groundwater flow equation K x @ 2 h @x 2 þ K z @ 2 h @z 2 ¼ S s @h @t þ q 0 ð1Þ where h is hydraulic head, K x and K z are horizontal and vertical hydraulic conductivity, respectively, S s is specific storage, t is time, and q′ is an additional source term (e.g., recharge or evapotranspiration) subject to the following initial and boundary conditions: 1. Water level in the stream is initially at the same elevation as the water level everywhere in the aquifer and aquitard hx; z; t ¼ 0 ð Þ¼ 0. 2. An aquifer of infinite extent @h @x x ¼1; z; t ð Þ¼ 0. 3. Immediate release of water from aquifer storage h(x =0, z, t) = C. 4. The stream flows in a straight line and forms a fully- penetrating vertical boundary to the aquifer. 5. The aquifer is homogenous and of uniform thickness. Received: 2 March 2010 / Accepted: 14 December 2010 * Springer-Verlag 2011 D. McGraw ()) : G. Pohll : R. Schumer : M. Shanafield Desert Research Institute, Division of Hydrologic Sciences, 2215 Raggio Parkway, Reno, NV 89512, USA e-mail: David.McGraw@dri.edu Tel.: +1-775-6737382 Fax: +1-775-6737363 Hydrogeology Journal DOI 10.1007/s10040-010-0701-2