Standardized Precipitation-Evapotranspiration Index

The SPEI fulfils the requirements of a drought index since its multi-scalar character enables it to be used by different scientific disciplines to detect, monitor and analyze droughts. Like the sc-PDSI and the SPI, the SPEI can measure drought severity according to its intensity and duration, and can identify the onset and end of drought episodes. The SPEI allows comparison of drought severity through time and space, since it can be calculated over a wide range of climates, as can the SPI. Moreover, Keyantash and Dracup (2002) indicated that drought indices must be statistically robust and easily calculated, and have a clear and comprehensible calculation procedure. All these requirements are met by the SPEI. However, a crucial advantage of the SPEI over other widely used drought indices that consider the effect of PET on drought severity is that its multi-scalar characteristics enable identification of different drought types and impacts in the context of global warming.

SPEI详细的介绍请见Vicente-Serrano SM, et al., 2010。随论文，Vicente-Serrano开发了SPEI Calculator程序，其中PET应用仅包含温度参数的计算方法(Thornthwaite, 1948)，这个程序自由下载，使用说明参考spei_manual_en.pdf。

应对SPEI批量操作需求，我在不掌握SPEI计算过程的基础上，以Matlab调用SPEI Calculator程序实现批量SPEI指数计算，具体情况见附件中SPEI_Cal.m和SPEI_Batch.m，代码运行流畅，调用返回结果正确。

图 1~3分别表示1-、3-、6-、9-、12-month尺度SPEI指数，Orig-12-month SPEI是测试数据附带的测试结果。

图 1

图 2

图 3

如图 3所示，上下两图干湿交替情况基本相同。不过，仔细查看两组数据，我们发现数据之间仍存在微小的差异，比如1901年1月12-month的SPEI在上下两图分别为1.152和1.164104，原因可能在于SPEI Calculator运行环境的差异，但不表示本地运行的前者是错误结果。

SPEI干旱/湿润等级划分(Yu, et al., 2014)

Categories	SPEI values
Extremely dryness	Less than - 2
Severe dryness	-1.99 to -1.5
Moderate dryness	-1.49 to -1.0
Near normal	-1.0 to 1.0
Moderate wetness	1.0 to 1.49
Severe wetness	1.50 to 1.99
Extremely wetness	More than -2

附上测试代码及数据（SPEI.rar）。

附上栅格处理参考代码（SPEIcal.rar）。

References

[1] SPEI.

[2] SPEI Calculator.

[3] Vicente-SerranoSM, Beguería S, López-Moreno JI (2010) A multiscalar drought index sensitive to global warming: the standardized precipitation evapotranspiration index. Journal of Climate, 23, 1696-1718.

[4] Thornthwaite CW (1948) An approach toward a rational classification of climate. Geographical Review, 55-94.

[5] How to run .exe file from matlab.

[6] Yu M, Li Q, Hayes MJ, Svoboda MD, Heim RR (2014) Are droughts becoming more frequent or severe in China based on the standardized precipitation evapotranspiration index: 1951–2010? International Journal of Climatology, 34, 545-558.