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9-1 <br /> <br /> <br /> <br /> <br /> <br /> <br />9. DROUGHT <br /> <br /> <br /> <br /> <br />9.1 GENERAL BACKGROUND <br />Drought is a significant decrease in water supply relative to what is needed to meet typical demand in each <br />location. It is a normal phase in the cycle of Mediterranean climates such as that of San Mateo County, originating <br />from a deficiency of precipitation over an extended period, usually a season or more. This leads to a water <br />shortage for some activity, group, or environmental sector. Drought is generally defined based on four ways of <br />measuring it (National Drought Mitigation Center, 2021): <br /> Meteorological drought—Based on precipitation deficit compared to normal. Anomalies of precipitation <br />may last from several months to several decades. How long they last depend on interactions between the <br />atmosphere and the oceans, soil moisture and land surface processes, topography, internal dynamics, and <br />the accumulated influence of global weather systems. <br /> Agricultural drought—Based on agricultural impacts due to reduced precipitation and water supply <br />(e.g., crop loss, herd culling, etc.) <br /> Hydrological drought—Based on measurements of stream flows, groundwater, and reservoir levels <br />relative to normal conditions <br /> Socioeconomic drought—Based on direct and indirect socio-economic impacts on society and the <br />economy. Socioeconomic drought occurs when the demand for an economic good exceeds supply as a <br />result of a weather-related shortfall in water supply. If a community has stored enough water to meet its <br />needs in the event of a shortage of rainfall, then it may not experience socioeconomic drought even <br />though its geographic area experiences meteorological drought. <br /> <br />9.1.1 Monitoring Drought <br />National Oceanic and Atmospheric Administration Drought Indices <br />The National Oceanic and Atmospheric Administration has developed several indices to measure drought impacts <br />and severity and to map their extent and locations: <br /> The Crop Moisture Index measures weekly short-term drought to quantify drought impacts on agriculture <br />during the growing season. <br /> The Palmer Z Index measures monthly short-term drought. <br /> The Palmer Drought Severity Index measures the duration and intensity of long-term drought-inducing <br />circulation patterns. Long-term drought is cumulative, so the intensity of drought during a given month is <br />dependent on the current weather patterns plus the cumulative patterns of previous months. Weather <br />patterns can change quickly from a long-term drought pattern to a long-term wet pattern, and the Palmer <br />Drought Index can respond fairly rapidly.