Spatial non-stationary relationship between temperature and precipitation variation patterns and drought in the southwestern Caspian Sea

Abstract One of the consequences of temperature and precipitation variation is drought, the continuation of which is accompanied by destructive environmental effects and depletes surface and groundwater resources. In this study, the most significant consequence of variation in temperature and precipitation—namely, drought—is examined and analyzed. To this end, the required data were first obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) at a spatial resolution of 0.25° × 0.25° for a 44-year period (1980 to 2023). Subsequently, the trends in the SPI and RDI indices were determined using the Mann-Kendall trend test, and the Anselin Local Moran’s I (ALMI) statistic was employed to identify the type of their spatial patterns. In addition, ordinary least squares (OLS) regression was used to determine the relationship and harmony of the variables across Guilan Province. The results showed that the RDI index exhibited a negative, decreasing trend in the months of March, May, June, and August. Furthermore, the results of this analysis regarding the mean SPI index indicate a negative trend during the months of May, June, and August; it should also be noted that the intensity of both the RDI and SPI indices increased during the period. Given that average precipitation during the months of May, June, and August follows a decreasing trend, a corresponding decreasing trend in the drought indices has also become significant. In addition to a negative trend in the mean RDI index for the months of March, May, June, and August, a negative trend in the RDI index for all cells during these months was also confirmed. Furthermore, the results obtained from the MK trend test for the SPI index of the cells in August confirm a negative trend, which is consistent with the findings from the same test applied to the mean SPI index for August. In this regard, spatial analysis of the RDI and SPI drought index trend test indicates that the high-high (HH) and low-low (LL) cluster patterns have formed as separate zones in different parts of the province, from the plains to the mountains and from east to west, in March, May, June, and August. Also, a very good spatial relationship was obtained between SPI and precipitation in all months of the year (except October). Furthermore, the spatial relationship between the variables RDI and precipitation, as well as between RDI and SPI, was significant at the 0.05 level during certain months. Overall, it can be stated that non-random behavior governs the spatial patterns of the RDI and SPI indices as well as precipitation, and there is a notable spatial correspondence among them.