When atmospheric water demand rises because of heat, knowing how much rain has fallen is not enough to judge whether drought exists. The rainfall itself may be insufficient if soil, vegetation, and water surfaces lose more water to evaporation than they receive. This is why the Standardized Precipitation Evapotranspiration Index (SPEI) was developed. It is an extension of the Standardized Precipitation Index (SPI) that incorporates the climatic water balance in its calculation instead of rainfall alone, and therefore reflects the effect of rising temperatures on drought more accurately. The index is based on calculating the monthly difference between precipitation and potential evapotranspiration, that is, D = P − PET. The values of this difference are then aggregated over a chosen time scale ranging from one to forty-eight months, converted into cumulative probabilities according to a suitable probability distribution, and then transformed to the standard normal distribution. Negative values indicate a water deficit and positive values indicate a surplus, and they are interpreted using the same categories used for SPI. Distributions that suit the tails of the distribution, where drought occurs, are usually employed, the best known being the three-parameter log-logistic distribution (Genlog), the Pearson Type III distribution (PE3), and the Generalized Extreme Value distribution (GEV). In the study under discussion, potential evapotranspiration was calculated using the Penman–Monteith equation with the Hargreaves–Samani modification, a method that was also used to compensate for missing evaporation data at some stations, particularly Diwaniyah and Samawah. The index was then calculated on a twelve-month scale for five stations in the Middle Euphrates region, namely Hillah, Karbala, Najaf, Diwaniyah, and Samawah, based on a reference period from 1991 to 2021. Goodness-of-fit tests showed that the Genlog distribution was the most suitable for Hillah, Karbala, Diwaniyah, and Samawah, while the GEV distribution was the most suitable for Najaf. The effect of evaporation is clearly visible in the number of drought months during the past period. According to SPEI, about 77 months were recorded in Hillah, 70 months in Samawah, 61 months in Najaf, 60 months in Karbala, and 59 months in Diwaniyah. The difference in Samawah is striking, as the number of months reached 70 according to SPEI compared with only 44 according to SPI. This means that a large part of its drought is due not to low rainfall alone, but to high temperatures and increased evaporative losses. Historical drought episodes were also longer according to SPEI than according to SPI at most stations. In Najaf, the episode lasted about three and a half years between 1998 and 2002 according to SPEI, compared with about one year and four months according to SPI, and in Hillah about three years and three months between 1999 and 2002, compared with about two years. The highest historical intensity was recorded in Diwaniyah at about (‑1.97) between 1998 and 2000. This period, the late 1990s and early 2000s, clearly stands out as the most severe drought era in the series studied. When the index is applied to the projected climate series for the period 2021–2060, it becomes clear that SPEI reveals more severe drought than SPI. Under the moderate RCP4.5 scenario, the study projects that Diwaniyah will experience an episode between 2050 and 2052 with an intensity of about (‑2.97), which falls within the exceptional drought category, whereas the most severe SPI episodes at the same station do not exceed about (‑1.6). The longest projected episode in Samawah extends about two and a half years between 2050 and 2053, while Najaf experiences its most severe episode between 2048 and 2050, Karbala between 2056 and 2058, and Hillah between 2040 and 2041. According to the detailed figures, Hillah and Diwaniyah remain the longest in duration and Karbala the shortest. Thus the risk is concentrated in the middle of the century, when the effect of high temperatures compounds the scarcity of rainfall. The index is not without limitations. It requires temperature data and additional data to estimate evapotranspiration, and the quality of its results is affected by the estimation method used, which here is partly based on compensating for missing data. The choice of probability distribution also affects the final values, and future results remain tied to the uncertainty of climate models. It is therefore preferable to read SPEI together with SPI, since the difference between them shows the extent to which heat contributes to worsening drought. Thus, SPEI adds to the picture of drought a dimension that a rainfall-only index cannot capture, and makes its results closer to the reality of a hot, arid region such as central Iraq, where evaporation swallows a large share of every drop of rain. It is therefore an indispensable tool for anyone planning water management under a climate that grows hotter year after year. Keywords: Drought, SPEI, SPI, potential evapotranspiration, Middle Euphrates, climate change, RCP4.5 scenario. Source: Zahraa Hussein Mohammed Aziz (2024). Projections of Climate Changes in Hydrological Regime of the Middle Mesopotamian Plain. Master’s thesis in Civil Engineering (Water Resources), Department of Civil Engineering, College of Engineering, University of Babylon, Iraq.