EDO Standardized Precipitation Index, 48-month accumulation period (SPI-48), blended and interpolated (version 1.2.0)

The 48-month Standardized Precipitation Index (SPI-48) is a meteorological drought indicator to monitor precipitation anomalies over 48-month accumulation periods and is a proxy indicator for long-term impacts, for example, reduced reservoir and groundwater recharge. This indicator has a temporal resolution of 1 month and is derived from daily rainfall observations at SYNOP (Surface Synoptic Observations) stations from the MARS database (https://ec.europa.eu/jrc/en/mars) interpolated at 0.25 decimal degrees, and blended with Global Precipitation Climatology Centre (GPCC - http://gpcc.dwd.de/) monthly total precipitation at 1 decimal degree. Interpolation at 0.25 decimal degree resolution and blending with GPCC data are according to Barnes iterative correction objective analysis (Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive Barnes objective map analysis scheme for use with satellite and conventional data. J. Climate Appl. Meteor.,22, 1487–1503.). The SPI represents precipitation anomalies at a given location and time, as the deviation from the long-term mean in a standard normal distribution, with more negative values representing a more severe drought. SPI is calculated by comparing the observed total precipitation amounts for an accumulation period of a number of months, indicated by a number following SPI-48, with the long-term “climatological” rainfall distribution for that same period of the year, derived from historical rainfall records in a specific reference period. A given accumulation period corresponds to different potential impacts of a drought, with shorter accumulation periods generally representing fast-changing aspects of the water system such as surface soil moisture and flow in small rivers, and longer accumulation periods representing slower changing aspects of the water system such as groundwater and baseflow in larger rivers. However, the exact relationship between accumulation period and impacts depends on the natural environment (e.g. geology, soils) and human interference (e.g. irrigation). Details on the calculation of the SPI and blending and interpolation procedures are given in the indicator factsheet.48个月尺度标准化降水指数（SPI-48，Standardized Precipitation Index）是一类气象干旱指标，用于监测48个月累积周期内的降水异常情况，可作为长期干旱影响的替代指标，例如水库水位下降与地下水补给减少。 该指标的时间分辨率为1个月，其数据源自MARS数据库（https://ec.europa.eu/jrc/en/mars）中地面天气观测（SYNOP, Surface Synoptic Observations）站的日降雨观测资料，经0.25经纬度分辨率插值后，与全球降水气候中心（GPCC, Global Precipitation Climatology Centre，http://gpcc.dwd.de/）发布的1经纬度分辨率月总降水量数据融合。其中0.25经纬度分辨率插值与GPCC数据融合流程遵循Barnes迭代校正客观分析方法（Koch, S. E., M. DesJardins, and P. J. Kocin, 1983: An interactive Barnes objective map analysis scheme for use with satellite and conventional data. J. Climate Appl. Meteor.,22, 1487–1503.）。 标准化降水指数反映特定地点与时段的降水异常情况，即某时段累积降水量与长期平均水平的偏差，并通过标准正态分布完成归一化处理；数值越负，代表干旱程度越严重。SPI的计算逻辑为：将指定月数累积期（SPI-48后缀数字即代表累积月数）的实测总降水量，与源自特定参考时段历史降雨记录的同期长期气候学降雨分布进行对比。 不同累积时长对应干旱的不同潜在影响：较短的累积时长通常反映水系统快速变化的特征，例如表层土壤湿度与小型河流径流；较长的累积时长则对应水系统缓慢变化的特征，例如地下水与大型河流的基流。不过，累积时长与干旱影响之间的确切关联取决于自然环境（如地质、土壤）与人类活动干预（如灌溉）。 关于SPI的计算方法，以及数据融合与插值流程的详细信息，请参见该指标的情况说明书。