Abstract
We analyze trend and persistence in Standardized Precipitation Index (SPI) time series derived from monthly rainfall data at 133 gauging stations in Pernambuco state, Brazil, using a suite of complementary methods to address the spatially explicit tendencies, and persistence. SPI was calculated for 1-, 3-, 6-, and 12-month time scales from 1950 to 2012. We use Mann–Kendall test and Sen’s slope to determine sign and magnitude of the trend, and detrended fluctuation analysis (DFA) method to quantify long-term correlations. For all time scales significant negative trends are obtained in the Sertão (deep inland) region, while significant positive trends are found in the Agreste (intermediate inland), and Zona da Mata (coastal) regions. The values of DFA exponents show different scaling behavior for different time scales. For short-term conditions described by SPI-1 the DFA exponent is close to 0.5 indicating weak persistency and low predictability, while for medium-term conditions (SPI-3 and SPI-6) DFA exponents are greater than 0.5 and increase with time scale indicating stronger persistency and higher predictability. For SPI-12 that describes long-term precipitation patterns, the values of DFA exponents for inland regions are around 1, indicating strong persistency, while in the shoreline the value of the DFA exponent is between 1.0 and 1.5, indicating anti-persistent fractional Brownian motion. These results should be useful for agricultural planning and water resource management in the region.
Highlights
Rainfall variability leads to a deficit or excess of water with respect to normal conditions at a given location
Standardized moisture and crop stress) exhibit values of detrended fluctuation analysis (DFA) exponents close to 0.5, indicating weak persistency and low predictability, while for medium-term conditions (SPI-3 and Standardized Precipitation Index (SPI)-6, which are related with medium-term soil moisture conditions and seasonal to medium-term trends in precipitation) DFA
Exponents are greater than 0.5 and increase with accumulation period, indicating stronger persistency and higher predictability; (iii) for SPI-12 series that describe long-term precipitation patterns which are relevant for streamflows, reservoir levels, and groundwater levels, the values of DFA exponents for Sertão and Agreste are around 1, indicating strong persistency, while in the coastal Zona da Mata region the value of the DFA exponent is found to be between 1.0 and 1.5, indicating anti-persistent fractional Brownian motion; (iv) for both trend and persistence of SPI, we find a significant difference between the regions
Summary
Rainfall variability leads to a deficit or excess of water with respect to normal conditions at a given location. Several indices have been developed to evaluate the severity of dry/wet conditions including the Palmer Drought Severity index (PDSI) [6], the Standardized Runoff Index (SRI) [7], the Standardized Precipitation Index (SPI) [8] and Standardized Precipitation Evapotranspiration Index (SPEI) [9]. The SPI stands out from the point of view that it only requires rainfall data for computations (and no other additional variables or empirical parametrizations), it can be compared among different regions with different climatic conditions [10], and it has already been widely applied to analyze dry/wet conditions in different parts of the world, in many different contexts [11,12,13,14,15].
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