flows and levels. In general, the District did a competent job relating the predicted environmental responses (including their magnitude and general degree of uncertainty) to the proposed range of withdrawals. The overall strategy of the study and the way it was implemented were appropriate and adequate to address the goals that the District established for the WSIS.
Several critical issues that are beyond the control of the District or were outside the boundaries of the WSIS limit the robustness of the conclusions. These issues include future sealevel rises and increased stormwater runoff and water quality degradation of surface runoff engendered by future population growth and increases in impervious area and pollutant generation associated with urban development. The predicted effects of sea level rise and land use change on water levels and flows in the river are greater in magnitude than the effects of the proposed surface water withdrawals, but they have high uncertainties. The District should acknowledge these limitations in its final report and, using an adaptive management strategy, it should plan to run its models with more recent rainfall and land use records and with emphasis on water quality as well as quantity.
In addition, the workgroups did not appear to consider the possibility of “back-to-back” extreme events (e.g., several extreme droughts separated by only a short period of normal rainfall) in their impact analyses. They also tended to present mean responses to changes in driver variables with little or no consideration of the variance in response. Although mean values are the most likely responses from a statistical perspective, ranges (or variances) of responses also should be considered in analyzing potential environmental impacts of changes in driver variables. Such responses may be less likely than mean values, but they may not have negligible probabilities and could be more detrimental than the mean responses.
Insofar as the MFL regulations limit the withdrawal allowable during low flow periods, the Committee remains concerned whether MFLs will be rigidly enforced in the future. If there is an extended drought in the future, water suppliers might not be able to withdraw water from the river for months or even years on end. It is not obvious that this would be socially acceptable. Finally, now that the WSIS is nearly complete, the District should reexamine the results from their earlier water supply study, which concluded that additional groundwater withdrawals would lead to undesirable impacts on natural vegetation. The Committee recommends that the District compare the levels and nature of impacts associated with withdrawals from the two (surface and groundwater) sources of additional water supply for the region.