have experienced or are bracing for local, regional, or statewide water shortages. US population, and in particular California population has constantly increased during the last decades, so using water wisely is crucial in order to provide enough water also for the future generations. During the 20th century, water diverted south through the California Aqueduct was economically essential to Los Angeles. But fisheries, wildlife and water quality in the bay and delta paid a heavy price. Water is becoming an increasingly important resource throughout California and the United States. The largest single use of potable water in California is water used to irrigate for agriculture. The largest remaining segment of water use is that of public water supplies. Air and atmosphere Buildings in the United States contribute 38.9% of the nation's total carbon dioxide emissions, including 20.8% from the residential sector and 18.0% from the commercial sector (2008). On average, the energy use for typical buildings is assumed to consist of 67% electricity and 33% natural gas The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4 °F (1–3 °C) warmer than its surroundings. In the evening, the difference can be as high as 22 °F (12 °C). Heat islands can increase summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality. One study estimates that the heat island effect is responsible for 5–10% of peak electricity demand for cooling buildings in cities. HVAC systems now will be required to use MERV 13 filtration, up from MERV 8, (2019 CalGreen, effective January 1, 2020) Materials and waste Approximately 170 million tons of building-related C&D materials were generated in the U.S. during 2003. This is a 25% increase in generation from the 1996 estimate of 136 million tons ( which was 25% to 40% of the national solid waste stream). Provisions The residential mandatory measures are provided in chapter 4 and the non-residential ones in chapter 5 of CALGreen Code. About the residential mandatory measures, the Code provides measures like storm water drainage and retention systems thought to prevent flooding of adjacent properties and prevent pollution from storm water runoff by retaining soil on-site or by providing filtering to restrict sedimentation from reaching storm water drainage systems and receiving streams or rivers. To comply, retention basin has to be sized and shown on the site plan, and water