Mercer Island purchases potable water from Seattle Public Utilities (SPU), delivered via two large-diameter supply lines, then distributed via the City's own water utility infrastructure. Nearly all of the water derives from the Cedar River Watershed and the South Fork Tolt River Watershed. As a member of the Saving Water Partnership Mercer Island participates in various efficiency programs that include rebates for upgrades of irrigation systems and older toilets, as well as educational materials and leak detection kits. See www.savingwater.org. Irrigation is one of the largest consumers of water on the Island, although some waterfront homes have State permits to draw irrigation water directly from Lake Washington. Depending on each summer’s temperature and rainfall, irrigation needs rise or fall accordingly. Healthy soils, mulching, plant selection, and good irrigation practices are an important part of helping to reduce water use across Mercer Island, conserving for the future. Indoor water use is largely driven by the number of people, house size, and the efficiency of water fixtures such as toilets and showerheads.