Marginal Export Cost Estimates of Water Bank Transfer
Don Wilson

February 1998

Marginal Export Cost (MEC) has been defined as the extra water needed to carry a unit of water across the Delta to the pumping plants for export while maintaining a constant salinity at a given location. MEC varies widely and it is highly dependent on location and antecedent conditions. Incremental export increases may increase salinity in some areas while decreasing salinity in others. Modeling studies using DSM2 and ANNs can be used to study the complex interrelationship of flows and salinity in the Delta.

The Continuous Impulse Marginal Export Cost (CIMEC) method was used to study carriage water under historic conditions. Two separate investigations were performed. The first investigation looked at the effects of decreasing exports by 500 cfs and then recalculating the SAC flow needed to maintain salinity at a constant level. The second experiment attempted to quantify the MEC associated with the pumping required by the 1991 Drought Water Bank.

Both attempts used ANNs which used CVP, DXCposition, SAC, SJR, and SWP as inputs and trained with DSM2 salinity output. The time period studied for the first investigation used historic data for the five year time span starting Jan 1989 to Nov 1994. The second study used 1991 historic data.

Jersey Pt. and Contra Costa Canal were chosen as the two study locations because they represent two interior Delta locations which have salinity standards which often control Delta operations and where the salinity/outflow relationship is relatively complicated.

Effects of a 500 cfs reduction in SWP pumping.

Jersey Pt. salinity, historic flows and DXC gate position, were used to obtain a baseline case by using the CIMEC SAC flow estimation methodology to calculate SAC flow for Jan 1989 through Dec 1994.

The historic SWP pumping data was then modified by reducing the SWP exports by 500 cfs. SAC flow was then recalculated so that salinity at Jersey Pt remained at historic levels. Figure 1 shows how the 500 cfs reduction affected the calculated SAC flow values. Monthly carriage water was calculated using the following equation:

C.W.=([delta]Exports-[delta]Calculated SAC flow)


C.W.=((Hist. SWP pumping - 500cfs) - (Hist. SWP pumping)) -
((Calc SAC with exports reduced by 500cfs) -
(Calc SAC for historic conditions))

Figure 2 shows monthly calculated carriage water at Jersey Pt. expressed in cfs and also as a percentage of export reduction. Carriage water percentage can either be zero, negative or positive. A zero value implies that there is a one to one correspondence between incremental increases in pumping and the incremental increase in SAC flow needed to maintain salinity levels at a given station. A negative percentage implies that [delta]SAC flow is less than [delta]Exports, while a positive carriage water percentage implies that [delta]SAC flow is greater than [delta]Exports when SAC flow is adjusted to keep salinity constant.

The monthly percent carriage water at Jersey Pt. is generally positive but showed some variation which may be attributed to the varying flows and DXC position.

The total average carriage water value for the entire period was defined as

Avg C.W. = [sum](monthly calculated C.W.) / [sum]([delta]exports)

and was found to be about 9%.

This experiment was repeated for the same time period using salinity at Contra Costa Canal (CCC). Figure 3 shows how the

500 cfs reduction affects the estimated SAC flow values when SAC flow is calculated using historic salinity at CCC. Monthly carriage water values also were calculated and are shown in Figure 4.

When CCC salinity is assumed to be controlling, the monthly carriage water ratio varies from -90% to 60%. The monthly percent carriage water with CCC controlling is much more volatile than the carriage water value observed when Jersey Pt salinity is used. The total average carriage water for the entire period with CCC controlling was 15%.

Estimate of carriage water for 1991 Drought Water Bank (DWB) Pumping

The second part of this experiment was to estimate the MEC associated with the 1991 Drought Water Bank (DWB) pumping. MEC was estimated once for Jersey Pt. EC controlling and also for CCC EC controlling.

Jersey Pt. historical salinity, historic rim inflows and exports and DXC position were used to calculate SAC flow using the CIMEC method described earlier in this report. This calculated SAC flow was used as the baseline case.

The 1991 historic exports for SWP were then modified by subtracting the SWP exports attributed to the 1991 Drought water Bank from the historic SWP export data and SAC flow was recalculated. Monthly carriage water values were calculated and the results are shown in Figure 5. The plot shows the monthly pumping made for the 1991 Drought Water Bank (DWB), the carriage water attributible to the 1991 DWB pumping, and the ratio of (C.W. for DWB pumping)/(avg DWB pumping).

The total average carriage water for the 1991 Drought Water Bank Pumping period with Jersey Pt. controlling came out to be 8.9%

The above process was repeated using Contra Costa Canal historic salinity. Contra Costa Canal historic salinity, historic rimflows and pumping, and DXC position were used to calculate SAC flow using the CIMEC method. Then as before the 1991 DWB exports were subtracted from SWP pumping and the new reduced SWP values were used to recalculate SAC flow. Carriage water was calculated on a monthly basis and the results are shown in Figure 6. Figure 5 and Figure 6 show how calculated carriage water estimates can vary depending on controlling location and changing monthly conditions.

The total average carriage water for 1991 DWB pumping with CCC controlling was calculated to be 14%.

These studies show how existing models can be used to further examine the relationships between salinity at a given location, rimflows, and gate operations. These preliminary results lead us to believe that as we continue to gain an understanding of these complex flow/salinity/gate operation relationships, opportunities to further optimize Delta operations will present themselves.

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