Investigating climate change over 1957–2016 in an arid environment with three drought indexes

Droughts are typical of the climate in Iran and have become more frequent over the last few decades. Iran’s agriculture, water resources, and ecosystems are all vulnerable to drought. In this study, the monthly time series of precipitation, temperature, and potential evapotranspiration were obtained from the CRU TS 4.01 gridded dataset to calculate the Standardized Precipitation Index (SPI), the de Martonne Aridity Index (DMAI), and the Standardized Precipitation-Evapotranspiration Index (SPEI) during two 30-year periods (1957–1986 and 1987–2016). At the national scale, the annual SPI, DMAI, and SPEI have decreased by 0.02, 0.11, and 0.09 unit decade⁻¹, respectively, since the beginning of the twentieth century. Over 1987–2016, these rates increased to 0.29, 0.70, and 0.36 unit decade⁻¹. Over the period 1957–1986, 89.5% of the total area was classified by the DMAI as hyper-arid, arid, or semi-arid, increasing to 94.8% over 1987–2016. Over the period 1987–2016, around 42.1, 57.9, and 48.8% of the grid cells revealed significant (P < 0.1) decreasing trends in the annual SPI, DMAI, and SPEI, respectively (27.1, 45.5, and 35.6% at the 95% confidence level). Winter has made the largest contribution to the temporal variations of all indexes at the annual scale. Spatiotemporal variations of the SPEI were greater than the two other indexes since it considers the non-stationarity in potential evapotranspiration. It proved to be an appropriate climatic index for investigating the spatiotemporal variations of agricultural drought, as it considers the source and demand terms of the water balance equation. Analysis of the frequency of sever to moderate droughts during the two 30-year periods confirms that country’s climate has significantly changed, especially in central, western, southwestern, southern, and eastern regions.