These maps show the extent of drought in the United states using different indicators of moisture. The different maps are more fully explained below. These maps are updated weekly, on Monday nights.
What The Drought Maps Mean
Palmer Drought Index — The Palmer Drought Index has become one of the best known and most used methods of estimating soil moisture since it was developed in the 1960s. The index users zero to indicate normal soil moisture, negative numbers for dry conditions, and positive numbers for wet ones. The index is calculated using a combination of soil moisture and temperature, so it can be normalized to any region of the country, during any season. A slow moving index, it does not accurately reflect short term changes.
Precipitation Needed to End Drought — How much rainfall will it take over the period of a month to end current drought condition (if they exist). This figure refers more to hydrological drought, where ground water supplies are in need of recharging, than it does to agricultural drought, where growing conditions can be greatly improved with simply a return to normal rainfall patterns. While eventually normal rainfall in an area will end a drought, the precipitation needed to end drought takes normal rainfall into account, and perhaps counter-intuitively, more rainfall will be required in a normally rainy climate to end a drought than would be required in a drier climate
Soil Moisture Anomalies — This indicator measures the difference between what the expected soil moisture would be, given historical observations, and the actual moisture in the root zone of the soil.
Crop Moisture Index — This measurement of soil moisture was developed to more accurately reflect the effects of recent precipitation (or the lack of it) than does the Palmer Drought Index. For example, a heavy rainfall may improve agricultural conditions in the short term, causing a decrease in the soil moisture anomaly, but not be much help in improving long-term drought conditions. As such, its effect may not appear in the PDSI.
Field Soil Capacity — This measure is a percentage ratio of the amount of moisture in the upper and lower levels of the soil to the amount of moisture the soil could absorb. Because even dry soil can become saturated immediately following a storm, this measurement is typically made 48 hours after the last rain event.