Defining Drought … It’s Not Just the Rainfall
Whenever California has the potential for a below normal rainfall season, like now, the D” word starts making its way into the media and everyday conversation. But what exactly constitutes a “drought”. There is no simple answer and it certainly depends on who you ask and where they live.
The American Meteorological Society defines drought as “A period of abnormally dry weather sufficiently long enough to cause a serious hydrological imbalance”. The important takeaway from this broad-brush definition is the use of term “hydrological imbalance” and not rainfall deficit. This is especially true in California where the State’s diverse infrastructure means water falling in the northern half of the state strongly impacts hydrologic imbalances many hundreds of miles away in the south.
And there is no single metric, be it the Drought Monitor, reservoir levels or seasonal rainfall deficits, that is the right tool for everyone. The needs of a water district manager are very different from those of a farmer, a local industry, a ski resort operator or a homeowner.
Broadly, drought is subdivided into four categories: meteorological drought, hydrological drought, agricultural drought, and socioeconomic drought.
Meteorological drought is a measure of the “degree of dryness” resulting from rainfall and snowfall deficits. There is additionally a very important temporal aspect which impacts the other types of drought by its dependence on not only the degree of deficit but also its longevity. These deficits can be measured as the number of days without rain or the percent of an average amounts of precipitation over days, months, years of even decades.
Hydrological drought is a function of the water supply available from rivers, reservoirs and groundwater; and very importantly the infrastructure to distribute that water. The temporal aspect is even more important with hydrological drought as there can be significant time lags between when precipitation occurs, and it impacts surface or subsurface supplies. [This is one of the reasons that meteorologists in California and the West use July 1 to June 30 rainfall season as opposed to the hydrologist’s October 1 to September 30 water year (http://ggweather.posthaven.com/rainfall-season-vs-water-year)]
Agricultural droughts operate on a short time scale as a precipitation deficit during even a short growing season can have significant impacts. These impacts are exacerbated (or mitigated) by crop type, the availability of stored water (i.e., hydrologic drought) plus soil type and moisture.
Socioeconomic drought is the impact on human activities and the related economies and is a function of all of the previous three types of drought as well as metrics like population change and water usage patterns.
A variety of indices and other metrics have been developed to attempt to quantify drought, though one that is good for agricultural drought may not be as adept at capturing socioeconomic impacts.
The bottom line is that drought has many intersecting layers and the effects of any or all of these drought types is dependent upon the user and his location. Drought is complicated!
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876