REVIEW OF THE HYDROMODIFICATION EXEMPTION FOR FIVE RIVERS IN SAN DIEGO COUNTY, CALIFORNIA VICTOR M. PONCE AND LUIS PARRA INTRODUCTION The purpose of this project is to perform a hydrologic study to confirm or deny the renewal of the current hydromodification exemption for the five reaches mentioned in Table 1. These reaches have been exempted by the Hydromodification Management Plan of San Diego County, herein referred to as the HMP document (San Diego County, 2011). Table 1. Summary of exempt river reaches in San Diego County River Downstream Limit Upstream Limit Otay River Outfall to San Diego Bay Lower Otay Reservoir Dam San Diego River Outfall to Pacific Ocean Confluence with San Vicente Creek San Dieguito River Outfall to Pacific Ocean Lake Hodges Dam San Luis Rey River Outfall to Pacific Ocean Upstream river limit of basin plan subwatershed 903.1 upstream of Bonsall near Interstate 15 Sweetwater River Outfall to San Diego Bay Sweetwater Reservoir Dam The hydromodification exemption has been based on the perception that existing large reservoirs upstream of the exempt reaches reduce the river discharge to a larger extent than potential increases which may be attributed to land development downstream. In accordance with the HMP document, these reaches have a contributing area greater than 100 sq mi and 100-yr peak flow rates in excess of 20,000 cfs. The perception to exempt is based on a consensus opinion, based on the experience of the Technical Advisory Committee (TAC), and not obtained through a specific study of discharge contributions in the subwatersheds in question. On May 8, 2013, the San Diego Regional Water Quality Control Board (SDRWQCB) issued a new permit Order R9-2013-0001 (SDRWQCB, 2013). The Board is now requiring that the continuance of the exemption for the said river reaches must be justified by a hydrologic study. Accordingly, the purpose of this study is to perform a hydrologic analysis of the watersheds listed in Table 1 to determine the river reaches for which the continuance of the exemption is justified. SDSU METHODOLOGY The study will use measured discharge on those rivers, and information about reservoir levels to simulate what would be the hypothetical FDC that would exist if the reservoirs were not there. This curve be compared with a FDC considering the reservoir and accounting for fully development conditions downstream, to determine if in the range of analysis an increase of the discharges may cause hydromodification conditions detrimental to the original scenario. An alternate result could also show that hydromodification correction of the negative hydromodification effects of the dam can be expected by the increase in runoff due to land use. As hydromodification is a complex phenomenon established in a large scale range, two expected situation can occur: (1) the combined effect of the dam and potential development could be closer to the hypothetical and natural FDC than simply including hydromodification control for an area already modified by a dam, or (2) the combined effect of the dam and potential development could improve the situation in a portion of the range of analysis, but be detrimental in another range of the range of analysis in which case an exemption to hydromodification is not recommended. TRWE METHODOLOGY To enforce the water balance results carried out by SDSU, TRWE will also do a general SWMM analysis of the watershed to numerically determine the influence of the potential development in the exempt reaches. A similar water balance will be performed, but using precipitation data, percentages of impervious areas, and soil types to try to generate a realistic FDC for each creek. The analysis will also be two-fold: a hypothetical no-reservoir scenario will be compared to a post-development scenario with or without the exemption in place for the percentage of the watershed that is affected by the exemption. The reader should be aware that not all areas in the watershed downstream of the reservoir will be affected, but only those areas draining directly to the exempt reaches (areas draining to the exempt reaches through erodible natural channels or creeks contributing to those reaches will not be included in the exemption analysis). EXPECTED RESULTS If both TRWE and SDSU methodology suggest that the exemption is no detrimental to the conditions of the reach (and even beneficial to counteract the changes the reservoirs have produced) then there is strong scientific evidence to justify the exemption, and the overall document will support the continuation of the exemption. If either TRWE or SDSU (or both) methodologies suggest that the exemption is detrimental to some of the five (5) reaches currently exempt, then a recommendation of the removal of such exemption in this particular river will be presented.