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5.2.2. Precipitation The facility receives an average of 17.5 inches of precipitation per year as shown on the isohyetal map of San Joaquin (Plate 7) and Western Regional Climate Center(WRCC). WRCC provided monthly mean precipitation data from 1948 to 2004. For Lodi, California, the mean annual precipitation was 17.57 inches which ranged from an average of 0.03 inches in September to an average of 3.45 inches in January. This data supports the 17.5 inch yearly average obtained from the isohyetal map. 5.2.3. Design Storm The requirements for the design storm for the landfill are identified in the waste discharge requirements (Appendix A). The Foothill Landfill waste management units have been designed based on the 100-year storm event. 5.2.4. Evaporation The mean annual pan evaporation for Lodi measured from 1948 to 2002 was 64.92 inches (Western Regional Climate Center). The mean monthly evaporation ranged between 0.06 inches in December and 10.60 inches in July. These values can be adjusted by multiplying by 0.7 or 0.8 to more closely estimate the evaporation from natural surfaces. 5.2.5. Runon/Runoff Volume/Pattern The natural topography of the Foothill Landfill area controls runon onto the site. In addition, the active fill area is a significant distance from the property boundary, so runon into refuse fill from surrounding properties is minimized. To control runoff, a perimeter ditch surrounding the landfill directs storm water runoff into a permanent sedimentation basin on the east end of the site. The sedimentation basin acts as an infiltration pond. Runon near the active area is diverted to a sedimentation area that is located just southeast of Module 1 (future Module 2). If this sedimentation area overflows, the water flows overland Foothill Sanitary Landfill Page 20 of 62 Public Works/Solid Waste JTD County of San Joaquin—Revised 10/6/05