Droughts

Floods and droughts seem to be two adverse extremes, so after exploring floods let’s turn to another type of natural disasters, droughts. It seems not hard to understand what the word “drought” means. We can easily get a picture in mind of dry earth with multiple deep cracks, withered crops and so on. IPCC Glossary makes a definition of drought: “A period of abnormally dry weather long enough to cause a serious hydrological imbalance.” However, IPCC SREX indicates that there are numerous definitions about drought which bring some difficulties to the research of drought. Heim Jr., 2002 describes droughts as four main categories: 

• meteorological (climatological) drought is an atmospheric condition with none or less than average precipitation
• agricultural drought makes a severe damage to crops although the deep layer of soil may contain adequate moisture
• hydrological drought refers to a persistent period of time of reducing water supply(surface or subsurface, e.g.: streamflow, lakes, reservoir and groundwater)
• socioeconomic drought relates to imbalance between demand and supply of some economic products affected by other three types of droughts

      

        Figure1 Contraction/Desiccation cracks in dry earth       Figure2 A livestock carcass in Marsabitm in northern Kenya,         (Sonoran desert, Mexico)                                                  which has suffered prolonged drought

Drought indices

Although there are some difficulties to explore the change of droughts, because of no consensus on the definition of drought and lack of observation data of soil moisture, there are some drought indices created to help us learn more about droughts.

Standard Precipitation Index (SPI) which is one of broadly used drought indices and solely based on standardized precipitation. The SPI consists of a transformation of long-term historic precipitation records to normal distribution with zero mean and standard deviation of unit (McKee et al., 1993). The SPI is appropriate for quantification of most categories of droughts, because of the strong relationship between itself and different elements (e.g. stream flow, ground water level, soil moisture content) on different time scale (Lloyd-Hughes and Saunders, 2002). Hayes et al., 1999 indicated advantages and disadvantages of the SPI. Three advantages: 1. Simplicity, based solely on rainfall; 2. Versatile, calculated on multiple timescales; 3. Normal distribution, consistent frequencies of severe and extreme drought classifications at any timescales and locations. Disadvantages: 1. depend on the quality of precipitation data; 2. limited data coverage; 3. timeliness of the preliminary data; 4. be misleading in regions with some seasonal rainfall regime; and other limitations.

Table1 Drought classification by SPI value and corresponding event probabilities

Consecutive Dry Days (CDD) is another widely used index which also only based on rainfalls, referring to maximum consecutive days with limited precipitation during a period of time below a certain threshold, typically 1 mm per day (Frich et al., 2002). The advantages of this indicator is that it can be used for a whole year or any seasons which you would like to choose (can see the example in Figure3).

Figure3 Projected annual and seasonal changes in dryness assessed for 2046-2065

(annual time scale) and 2081-2100(annual time scale and two seasons)

Palmer Drought Severity Index (PDSI) is different with two above indices because it considers not only precipitation, but also evapotranspiration. Dai, 2011 compared four forms of PDSI and  found them having a high correlation with soil moisture observation data in the US and Eurasia, also with yearly streamflow and land water storage respectively in most part of objective regions. It is a common index with some good properties, however there are also some limitations. It should be noted that PDSI is appropriate for central US, which means its less compatible property across different areas and over time (IPCC SREX). Therefore, when PDSI is applied for a certain region, it should be calibrated for local situation.

Here, I just make a introduction of these drought indices which are always seen in drought assessment articles. There are many other indicators, such as Precipitation Potential Evaporation Anomaly (PPEA) and Standardized Precipitation Evapotranspiration Index (SPEI). These indices have their own shortcomings and focus on just one or some aspects of droughts, therefore it is good choice to combine local properties to choose indices which can suit the objective region best. However, even though the projection is still not the completely right answer, because the change of environment.