DOI 10.1515/pesd-2015-0032 PESD, VOL. 9, no. 2, 2015

THE USE OF STANDARDIZED INDICATORS (SPI AND SPEI) IN PREDICTING DROUGHTS OVER THE REPUBLIC OF MOLDOVA TERRITORY

Nedealcov M.1, Răileanu V.1, Sicirc;rbu R.1, Cojocari R.1

Key words: Standardized Precipitation Index (SPI), The Standardized Precipitation Evapotranspiration Index (SPEI), temporal estimation, spatial analysis.

Abstract The drought events frequent manifestation over the Republic of Moldova territory, in the context of climate change requires a scientific monitoring adjusted to international researchers. In recent years, internationally, the estimation of this phenomenon occurs through standardized indexes. The most used of these, being the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Since there is no a unified definition of drought, the World Meteorological Organization proposes to calculate the indexes, through developed calculation software. Thus, based on multi-annual data (1980-2014) a regional spatio-temporal estimation concerning drought in the Republic of Moldova was performed, thereby realizing the regional investigations framing in the international ones.

Introduction

So far, neither around the world nor in Republic of Moldova, there is no a universally accepted terminology regarding the definition of drought, this is the explanation at the section of using wide range of indices to estimate this phenomenon. In this context, the World Meteorological Organization proposes the calculation of Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) – as the basic indicators in order to quantify the intensity, duration and spatial extent of drought. Thus, the SPI index proposed by McKee et al. [2] and SPEI index elaborated by Serrano, Begueria and Moreno [3] were the basis for drought monitoring in recent decades over the Republic of Moldova territory.

1 Institute of Ecology and Geography ASM

150 Nedealcov M., Răileanu V., Sicirc;rbu R., CojocarI R.

Initial material and research methods

In the recent study have been used monthly data series concerning precipitation and air temperature recorded at 16 stations and 11 precipitation stations of State Hydrometeorological Service of the Republic of Moldova for a period of 35 years (1980-2014).

Soil moisture conditions 'respond' to precipitation anomalies on a relatively short time scale. Groundwater, river flow rate and the accumulations in reservoirs, reflects longer-term precipitation anomalies. For these reasons, McKee et al. [2] calculated SPI for different time intervals (3, 6, 12, 24 and 48 months). The software that allows automatic calculation of the index is available on the World Meteorological Organization web page [6]. SPI is a simple index based on the probability of precipitation, and for its calculation are required only data regarding monthly precipitation for a period of at least 30 years. The precipitations are normalized, using a probability distribution, so that the SPI values are in fact, seen as the standard deviations from the median. Positive SPI values characterize wet periods and negative ones - dry periods. SPI distribution for the whole period is normal; the average is equal to zero and the standard deviation - with the unit. The main disadvantage of this index is that it uses only precipitation, without taking into account the thermal regime and evapotranspiration.

Table1. The quantification of the drought distribution probability, according to SPI

The SPEI is calculated based on the amount of data that characterize atmospheric precipitation, temperature and latitude of the place, which allows to take into account the potential evapotranspiration. SPEI is based on the water balance, and it can be compared to the Palmer Drought Severity Index (PDSI). SPEI It is based on the original procedure of calculation of index and uses the same SPI available time scales. The SPEI calculation is based on monthly difference between precipitation and potential evapotranspiration, which is a simple methodology of water balance and can be calculated, as well, at different time scales. Therefore, for SPEI calculation, is used a comprehensive set of data characterizing atmospheric precipitation, temperature and the potential evapotranspiration. In this context, it was created special software to automatically

The use of standardized indicators (SPI and SPEI) in predicting droughts 151

calculate the SPEI, for a wide range of time scales. The software is available for free on the Spanish National Research Council web page [1, 5].

The quantification and the probability drought distribution (the example of SPI) are presented in Table 1.

Analysis of the results

Therefore, at regional level for the first time, automatic calculation of above mentioned indexes has took place, which has helped to essential improve researches in this area. Thus, for the last decades (1981-2014), it was carried out a scientific monitoring of droughts: duration, intensity and the area of manifestation in the Republic of Moldova.

Table 2. Data from the SP

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使用标准化指标(SPI和SPEI)来预测摩尔多瓦共和国的干旱情况

Nedealcov M、Răileanu V 、Sicirc;rbu R、Cojocari R

关键词:标准化降水指数(SPI)，标准化降水蒸散指数(SPEI)，时间估计，空间分析。

摘要:在全球气候变化的背景下，干旱灾害频繁地发生在摩尔多瓦共和国境内，所以需要国际上的研究人员对它进行监测。近年来，国际上一般采用标准化降水指标对干旱情况来进行评估。其中最常用的是标准化的降水指数(SPI)和标准化的降水蒸散指数(SPEI)。由于没有对干旱的统一定义，世界气象组织建议使用计算软件来计算这些指数。通过对多年数据(1980-2014)的分析来对摩尔多瓦共和国的干旱情况进行区域时空估计，从而完善了该区域国际研究体系。

介绍：迄今为止，无论是在世界上还是在摩尔多瓦共和国，关于干旱的定义都没有一个被普遍接受的标准，这是采用泛用的指数来预测这一现象的原因。在这个背景下，世界气象组织提出了用标准化降水指数(SPI)和标准化降水蒸散指数(SPEI)作为基本指标，来衡量干旱的强度、持续时间和空间上的分布范围。所以，由McKee等人提出的SPI指数，由Serrano, Begueria 和Moreno 所提出的SPEI 指数，是近几十年来摩尔多瓦共和国的干旱监测的基础。

1.原始材料和研究方法

在最近的研究中，摩尔多瓦共和国16个气象站站点和11个国家水文气象服务站记录了过去35年（1980-2014）的降水和气温。土壤水分条件能反映短期的降水的异常。地下水、河流流量和水库蓄水量的变化则能反映较长期的降水的异常。出于这些考虑，McKee等人根据不同的时间间隔计算了SPI（3、6、12、24和48个月）。自动计算这些数值的软件可以在世界气象组织网站中找到。SPI是一种基于降水概率的简单指数，它的计算需要至少30年的月降水数据。通过概率分布使降雨量数据标准化，所以SPI值实际上是与中值的标准差。正的SPI值具有湿周期和负周期的特征。整个周期内的SPI分布是正常的，它的平均值等于0和标准差（有单位）。该指数的主要缺点是它只使用降水数据，而不考虑热机制和蒸散量的影响。

表1.根据SPI值计算干旱分布概率

根据大气降水、温度和纬度的数据计算出SPEI的数值，进而考虑了潜在的蒸散量的影响。SPEI基于水平衡原理，它可以与帕尔默干旱严重指数(PDSI)作比较。它基于原始的索引计算方法，并使用和SPI相同的时间尺度。该方法的计算是基于月降水与潜在蒸散量之间的差额。这是一种简单的水平衡方法，可以在不同的时间尺度上计算。

因此，就SPEI的计算而言，它使用了一组综合的数据来描述大气降水、温度和潜在的蒸散量的关系。在这种背景下，一种特殊的软件被开发出来，它可以用来自动计算不同的时间范围内的SPEI。该软件可在西班牙国家研究委员会网页中免费使用。

表1给出了定量与干旱概率分布(SPI的例子)。

2.分析结果

因此，在该区域上，首次用软件对上述指标进行了计算，这对该领域的研究具有重要的促进作用。进而，我们对过去的几十年里(1981-2014)摩尔多瓦共和国的干旱持续时间和强度以及干旱的具体表现进行了科学的检验。

表2.来自SPI-SPEI数据库的数据