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rAcwuru.ror ane small amounL of weep percoiauon �rairuaii-r,1l Lnrougn pervious areas of <br /> the site as the onsite portion of the total annual recharge (call it RI). <br /> b) For average year conditions (monthly time steps), estimate the amount (or percentage) of <br /> building site runoff entering the basin that is expected to drain via percolation, as <br /> opposed to being discharged to the regional storm drain system. If the basin is designed <br /> for full retention/percolation of normal year rainfall-runoff, with off-site discharge only <br /> during peak storms, it may be possible to consider all building site runoff as available for <br /> percolation. <br /> c) Estimate the direct rainfall volume (acre-feet) into the basin as part of the water available <br /> for percolation. Since it appears the retention basin capacity is planned to be shared with <br /> another development site (Buildings A&B), include only the portion of direct rainfall <br /> approximating the percentage of the basin allocated for the project. <br /> d) Combine the estimated monthly recharge volumes from (b) and (c), and subtract the <br /> average amount of water lost to evapotranspiration (ET) from the surface of the retention <br /> basin (monthly). For the a shared retention facility, adjust (i.e., reduce) the ET losses <br /> consistent with the percentage of the basin allocated for the project. <br /> e) The sum of(b) + (c) - (d) would give an estimate of the net monthly off-site retention <br /> basin recharge volumes, and the annual total (call it R2). <br /> f) The sum of Ri + 2 wo give—tfie eshmatecT anrivaIiau Il rec�arge,7�,- r usein t e�- <br /> above mass balance equation. <br /> A-2 <br />