STANDARDIZED PRECIPITATION INDEX (SPI) IN MONITORING OF MOISTURE CONDITIONS
ON THE TERRITORY OF THE REPUBLIC OF SERBIA
Standardized precipitation index (SPI) belongs to the parameters, indices that characterize the moisture conditions. SPI is relatively simple, but popular and very applicable index of moisture condition. Only data on precipitation amount are used for its calculation. In fact, standardized precipitation index is precipitation amount recorded during some period of time which is represented through the values of random variable that has standardized normal probability distribution. Values of statistical parameters necessary for carrying out of this transformation are obtained by the procession of long-term series of precipitation data. Application of this procedure enables simple classification of some precipitation amount - its quality definition on the basis of appropriate probability. At the same time, intercomparability is obtained of unequal length periods precipitation data as well as precipitation data from the seasons and locations with various precipitation regimes, precisely precipitation distribution.
Operative procedures within monitoring of moisture conditions carried out by the Department of Agrometeorology of the RHMS of Serbia also include calculating of SPI values on the basis of precipitation amounts recorded in previous 30, 60 and 90 days. Besides these calculations that are made in time step of one day, SPI values are also calculated for previous 1, 2, 3, 4, 5, 6, 9, 12 and 24 months after the passing of each calendar month. A part of obtained results is accessible and regularly updated on Internet page of the RHMSS presentation ”Moisture conditions”. They are presented as cartographic surveys and in the form of tabular surveys as per meteorological stations. Instead of numerous SPI values, as more appropriate for using, quality assessment of the moisture conditions as per criteria defined for operative use (explanation is given in attached Table). Criteria are defined only on the basis of SPI probability distribution, precipitation probability distribution in the last instance. They are not defined by cause/effect analysis, that is, index calibration according to the recorded consequences. However, quite a frequent there is a significant correlation between the magnitude and duration of precipitation regime anomaly and detrimental consequences in agriculture, water management, etc. So, such criteria might help in assessing the consequences of unfavorable moisture conditions.
Besides Internet page titled “Moisture conditions”, information on SPI values defined on the basis of precipitation amount in previous 60 days are also found in agrometeorological bulletins, except during winter periods. Bulletins mostly give qualitative moisture assessments as per the above mentioned criteria instead of numerical values, too. In decade agrometeorological surveys and tabular attachments of monthly information there are the assessments of decade moisture conditions for chosen meteorological stations. It is accepted that the SPI value for the last day of the decade represents moisture conditions for the whole decade. In monthly agrometeorological information, the assessment of moisture conditions during the month is given on the basis of mean SPI value obtained from three values that represent monthly decades. Numeric values of standardized precipitation index are used for the preparation of graphical presentation of changes of moisture condition in monthly agricultural agrometeorological information. Mean daily values of SPI for five agricultural areas of Serbia are presented. Everyday values are defined for the groups of stations that are so formed as to represent certain areas.
The whole spectrum of time scales in which SPI can be defined makes possible its wide application: from short-term planning to the research of long-term fluctuations in precipitation regime. Thus, SPI values defined for the periods of one to three months are relatively well correlated with storage of productive moisture in surface soil layers so that they can be used for the assessment of moisture conditions for the growth and development of agricultural crops. SPI values defined for longer time periods indicate to prevailing characteristics of moisture conditions during vegetation period, calendar year etc., and there is often a correlation of these values with the levels of surface and ground waters in observed area.