Section 36.13 - Safe Yield(1) Safe yield of a water source is calculated as 55% of the drought basin yield plus the reservoir storage volume, if any, for that water source. For the purposes of calculating safe yield:(a) drought basin yield is the annualized Q90 streamflows in a water source based on averaging estimated near natural monthly Q90 streamflows. It is an estimation of the water that would be available in an unimpacted river basin during the probable driest period that is likely to occur; and(b) reservoir storage volume is the modeled volume of water remaining in reservoir storage at the end of a simulated one-year drought comprised of annualized monthly Q90 flows, calculated as available storage minus any registered or permitted withdrawals and any required release volume; and(c) reservoir storage volume is capped at the firm yield of the reservoir.(2) Should registered volumes for a water source, taking into account any determinations made by the Department pursuant to 310 CMR 36.27(4), exceed safe yield, no additional volumes of water will be available through permitting. Where such registered volumes do not exceed safe yield there is no presumption that withdrawals will be permitted. The allocation of water available for permitting within the safe yield will be determined, and may be limited by, application of streamflow criteria, statutory and regulatory requirements, and site-specific considerations pursuant to M.G.L. c. 21G and 310 CMR 36.00.(3) Safe yield, reservoir storage volume(s), total registered volumes and currently permitted volumes for each water source will be published on the Department's website.(4) In a water source where reservoir storage volume is a factor in determining safe yield, reservoir storage volumes are available only to those permittees with legal access to the reservoir(s).(5) For water sources where an estimate of near natural annualized Q90 streamflow is not applicable because the water source is groundwater-driven (the southern portion of South Coastal, Cape Cod, Islands, and portions of Buzzards Bay), safe yield is determined through analysis of water available during the probable driest period through groundwater recharge for each water source. Adopted by Mass Register Issue 1273, eff. 11/7/2014.