Smith Mountain Lake Level Predictive Model Accuracy Improvement Water Management Committee May 14,

Basic Question 1.The current predictive model uses historical streamflow Historical streamflow includes both historical rainfall runoff and historical groundwater flow However, summer groundwater flow is determined by winter/spring recharge Would using the actual groundwater flow based on the previous winter/spring recharge improve prediction accuracy? 2.Testing this hypothesis requires knowing how accurately lake levels can be predicted using real data 2

Water Balance Model ∆Vol = (Inflow +Rain – Evap – Discharge - GW) ∆T Inflow = Roanoke + Blackwater + Pigg + Shoreline Rain = Rain on lake surface (Rocky Mount & Bedford average) Evap = evaporation from lake surface Discharge = Leesville dam discharge (USACE) GW = groundwater loss from lake bed ∆T = time period (typically hour, day or month) Lake level changes determined from lake volume changes Convert Leesville and SML actual lake levels to SML adjusted lake level 3

Calculating Adjusted SML Lake Height Use AEP 2006 survey for SML and Leesville lake areas and volumes vs. lake heights Assume a linear dependence of surface area on height A(h) = A(h ref ) + (a ref )* (h – h ref ); SML: A(h ref ) =A S = acres, h ref = 795 and a ref = a S = Leesville: A(h ref )= A L = 3260 acres, h ref = 613 and a ref = a L = Changes in lake volume are a quadratic equation in h (see notes) Assume a simple transfer of Leesville excess volume to SML 4

Typical Model Values (cfs) ParameterWinterSummer Inflow Rain90120 Evaporation20120 Discharge Groundwater363 Inflow and discharge dominate changes in lake volumes Groundwater losses are a secondary loss Rainfall and evaporation largely cancel one another Withdrawals for water usage are considered negligible, cfs, and are implicitly included in the GW loss term 5

6

7

Roanoke gage is corrected for Salem/Roanoke water withdrawals There is a clear seasonal trend in streamflow ratios Inflow Extrapolation 9

Same analysis downstream from Leesville Seasonal trend is present, but reversed Inflow Extrapolation 10

Aug Sep Apr May Jun

DateModel Diff (inches) Model Diff (cfs) Inflow (cfs) Discharge (cfs) Rainfall (cfs) Month Prediction Differences vs Inflow, Rain and Discharge Rates

GW = 50 cfs Scaled area = 1350 mi 2

Conclusions The results to date indicate the current water balance model is not sufficiently accurate for testing the groundwater hypothesis There are four possible reasons for this result 1.The water balance model does not have all the correct terms 2.The conversion of actual lake levels to SML adjusted heights is incorrect 3.The scaling methodology for the 3 USGS gages does not correctly handle the more rural downstream areas around SML and Leesville 4.The regions around SML and Leesville need to be considered as a separate hydrographic region Reasons 3 and 4 are considered the most likely explanations, but pursuing the 4 th possible reason may be the most productive. More accurate inflow data are needed in order to test the original groundwater hypothesis