Why has drought become increasingly focused and broad-spectrum since the beginning of the 21st century? Has its environmental and socio-economic identification and information been focused, especially in the context of climate change? Has climate change been identified in the occurrence of drought at different spatio-temporal scales and using different approaches? What are the possible outcomes? Is the purpose of studying and analyzing drought trends in different regions of the world determined by the topography and varying warming rates?

Since the start of the 21st century, increasing focus has been put on drought and its wide range of environmental and socioeconomic effects, particularly in the context of climate change. The identification of changes in drought occurrence has been done at different spatiotemporal scales and using different approaches, with results that may not be fully comparable. This study aims to analyse drought trends in the northwestern region of Italy, encompassing the Piedmont and Aosta Valley regions, characterized by diverse topography and warming rates. The analysis is carried out over the last 60 years using the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at 3- and 12-month timescales and deriving drought events at the local and regional spatial scales. By leveraging on a continuous and spatially coherent precipitation and temperature dataset, we explore the temporal and spatial variability of drought conditions and compare results obtained with different approaches. Our results reveal widespread drying trends in the region, with temperature playing a crucial role. The SPEI indicates more extensive and steeper negative trends than the SPI due to temperature increases. However, the onset and cessation of drought events are predominantly driven by precipitation anomalies, while temperature plays a key role in longer-term drought conditions. Both the SPI and SPEI consistently identify local and regional drought events. In the 1990–2020 period, drought event severity, duration, and intensity generally increased compared to during the 1960–1990 period, even though this increase is less significant than the one shown by the SPI and SPEI. Nevertheless, the spatial scale of the analysis plays a significant role in interpreting these trends. Localdrought characteristics are more influenced by temperature increases in the SPEI, whereas regional droughts are more affected by precipitation patterns, as seen in the SPI, with more frequent short-term droughts aggregating into longerterm deficits. Drying trends are more pronounced in lower, less rugged areas, while alpine regions show fewer drought trends. Interestingly, drought characteristics and trends are found to be more correlated with terrain ruggedness than with mean elevation. In fact, a clear drying trend is not found at a region-wide level but is instead found when considering homogeneous areas defined by terrain ruggedness. Furthermore, changes in the number of drought episodes and in their severity, duration, and intensity are found to be correlated with terrain ruggedness at all timescales. These findings emphasize the need for high-resolution, region-specific studies to better understand how droughts evolve in complex terrains like the northwestern Italian Alps. Future research should investigate whether similar outcomes are found in other regions and what the potential causes are as this is instrumental for evaluating how these trends may continue to evolve under projected climate change scenarios.Drought is considered to be one of the main natural disasters, with widespread effects affecting large portions of the world’s population (Wallemacq et al., 2015) and causing severe financial losses (García-León et al., 2021) and ecosystem impacts (Crausbay et al., 2020). Drought also has both short- and long-term effects on water availability (IDMP, 2022), which are relevant when considering the global increase in water demand in the past and the predicted challenges in meeting that demand in the future (UNESCO, 2018; Wada et al., 2016; Burek et al., 2016). These drought-related phenomena are also likely to become more impactful as droughts are predicted to become more severe and frequent under climate change conditions (Dai, 2011, 2013; Trenberth et al., 2014; Ward et al., 2020; Pörtner et al., 2022). Understanding if and how changes will occur on a local scale is thus necessary in order to develop adequate adaptation responses. Several studies on meteorological-drought trends exist at the global and continental scales (e.g. Ault, 2020; VicenteSerrano et al., 2022; Ayugi et al., 2022). Many studies have also been carried out in northern Italy – often in the context of the wider Mediterranean or Alpine region – analysing either precipitation series (Bordi and Sutera, 2002; Brunetti et al., 2002; Hoerling et al., 2012; Haslinger and Blöschl, 2017; Pavan et al., 2019) or precipitation and temperature series (Hanel et al., 2018; Falzoi et al., 2019; Arpa Piemonte and Regione Piemonte, 2020; Baronetti et al., 2020; Vogel et al., 2021). Overall, these studies have found an increase in meteorological-drought occurrence in northwestern Italy, particularly after the 1970s, even in the cases in which recent drought events have not been found to be exceptional when compared to historical records (Haslinger and Blöschl, 2017; Hanel et al., 2018). Despite some agreement about the changes in precipitation, the seasonality reported by the studies differs significantly, with precipitation decreases found either in the winter (Brunetti et al., 2002; Hoerling et al., 2012) or summer season (Haslinger et al., 2012; Hanel et al., 2018; Pavan et al., 2019). Among these studies, those also considering temperature values consistently showed rising temperatures – and, thus, a rise in evaporative demand – to be a main factor in drought increases. Besides drought trends in wider areas, interest in regional expressions of climate change has also been growing. One of the most investigated regional phenomena is the enhancement of warming rates with elevation or elevation-dependent warming, explored on the basis of both surface measurement (e.g. Mountain Research Initiative EDW Working Group, 2015) and of climate models (e.g. Palazzi et al., 2019). In general, despite conflicting results regarding the presence of an elevation effect on warming rates and the lack of adequate climate data for mountainous regions, a consensus on enhanced warming rates at higher altitudes emerges (Rangwala and Miller, 2012; Pepin et al., 2022). The change in orographic precipitation gradients, i.e. the elevation-dependent precipitation change, has also been widely investigated, with less consensus on the results. A comprehensive metaanalysis of both in situ studies of precipitation data from mountainous regions (including the Alps) and global gridded databases from the early 1950s to the late 2010s reported a relative decrease in precipitation compared to in lowlands, although without high confidence (Pepin et al., 2022). Furthermore, analyses such as that of Giorgi et al. (2016) have shown the importance of the spatial resolution in understanding processes in topographically complex regions, reporting that increases in summer precipitation in higher-elevation areas of the Alpine range could only be detected by dense observation networks and described by high-resolution regional climate models. Several studies on meteorological droughts are based on evaluating drought indices, such as the SPI (Standardized Precipitation Index) and the SPEI (Standardized Precipitation Evapotranspiration Index), giving a statistical interpretation of the temporal variability of meteorological data. Besides the temporal variability of wetting and drying events, droughts are further characterized by the duration and intensity of these events, which are relevant and independent indicators of drought characteristics. Less investigated is the spatial variability in drought characteristics and the change in indices obtained by averaging over different areas (Haslinger et al., 2012) and possibly leading to different outcomes in terms of drought magnitude and trends. The spatial dimension is relevant not only when larger or smaller regions experience the adverse effects of droughts but also when droughts occur on complex terrains, ranging from lowlands to hills and mountains. In this case, drought characteristics may differ among terrains, and the frequency of occurrence of drought periods of a certain magnitude may be diversified. Elevation is thus a main factor to be considered to describe this complexity but may not be the only one. Given these considerations, in this study, we aim to tackle the following research questions:1. Are there temporal trends in drought indices such as the SPI and SPEI and how do these trends translate into changes in the characteristics of drought events, e.g. duration, severity, and intensity? 2. Is there a relationship between drought trends and the topographical characteristics of a landscape? If so, is elevation the topographical variable most correlated to these trends? 3. Do these conclusions change if drought events are defined at different spatial scales? To investigate these questions, an area such as the western Po River basin is particularly suitable. The region is part of the European Alps that divides the Mediterranean and continental Europe, with opposite projected changes in precipitation and different responses to climate oscillations. Also, the region comprises wide plains, hilly areas, and high mountains, with possible effects of elevation gradients and topography on drought characteristics and trends. Despite the presence, as detailed above, of studies on drought in the chosen region, these lack either the needed spatial resolution or the focus on different choices for drought characterization and on possible effects of terrain characteristics on drought conditions. is reported (although with some differences in the degree of correlation). Altogether, these results indicate that, on shorter timescales, droughts in higher-mean-elevation areas tend to be more clustered. Even so, visual inspection of the spatial distribution of local-drought characteristics for the SPI-3 and SPEI-3 (see Fig. 4a–h) shows a high spatial variability of characteristics. However, the higher mean elevation points of the mountainous part of the domain do show quite uniform drought characteristics that are consistent with the observed correlations. It can therefore be stated that, despite some significant effects of the mean elevation on the characteristics of drought periods, heterogeneous local orography and meteorological conditions play a key role. When considering terrain ruggedness, the resulting correlation values are generally less significant than for mean elevation at the 3- month scale but more significant at the 12-month scale. The SPI-12 and SPEI-12 run characteristics display no correlation with longitude or latitude (see Fig. 4i–r) and no correlation with mean elevation in terms of the number, severity, and duration of runs. The only statistically significant correlation found is with the SPI-12’s DIL, with higher mean elevation areas reporting less intense events, consistently with the results obtained for the SPI-3. Conversely, indices at the 12-month scale have significant correlations with terrain ruggedness for the number of runs and their DSL and DDL, with rugged terrain reporting less numerous, less severe, and shorter droughts. 4.2.2 Temporal analysis of local-drought characteristics Trend analysis based on the obtained local-drought characteristics reports only a few cells (always less than 3 % of the domain) showing significant changes for drought duration, severity, and intensity (DDL, DSL, and DIL) (not shown here). In comparison, the SPEI-3 shows a far greater number of cells, slightly more than 10 % of the total area, with significant increasing trends for DSL and DIL, distributed almost exclusively along the Alpine chain, particularly near the southern border. The yearly average change, in terms of the percentage of the relative DSLandDIL for the cell, ranges from 1 % to 11 % and 0.01 % to 1 % for severity and intensity, respectively. Despite the overall lack of significant trends, clear differences can be found between the characteristics of drought runs that started before and after 1990. The SPI-3 and SPEI3 display, on average, an increase in the number of droughts (more markedly in the case of the SPEI-3) and in their DIL. Opposite results are found in terms of DSL and DDL, with the SPI-3 indicating a shift towards less severe and shorter droughts and vice-versa for the SPEI-3. Significantly, this difference seems to be caused mainly by cells located in the flat part of the region, where the SPEI-3 indicates a shift towards greater DSL and DDL (not shown here). The rest of the region shows similar results for the two indices. Figure 4. Spatial distribution of drought run characteristics at 3- and 12-month timescales. The SPI-3’s number of runs (a), average severity of local drought DSL (b), average length of local drought DDL (c), and average intensity of local drought DIL (d). The SPEI-3’s number of runs (e), DSL (f), DDL (g), and DIL (h). The SPI-12’s number of runs (i), average severity of local drought DSL (l), average length of local drought DDL (m), and average intensity of local drought DIL (n). The SPEI-12’s number of runs (o), DSL (p), DDL (q), and DIL (r). The Alpine chain, especially in the north, shows a shift towards a higher number of less severe, shorter, and less intense droughts. The SPI-12 and SPEI-12, on the other hand, report agreeing results and show, on average, a change towards a lower number of more severe, longer, and more intense droughts across the domain. The only exception is the Alpine chain, where, for a small but continuous area, a change towards less numerous, less severe, shorter, and less intense droughts is found. These relative changes are highly correlated to the mean cell elevation and, even more so, to the ruggedness of the area (as confirmed by Table 2). For example, at the 3-month scale, the flat part of the region sees a change towards less numerous, more severe, longer, and more intense droughts, while the Alpine chain shows an opposite change. Changes in the SPEI-12 run characteristics also display a similar correlation for DSL, DDL, and DIL but the opposite in terms of the number of droughts. Therefore, it seems that the SPEI-12 droughts got more numerous, more severe, longer, and more intense in the lowlands, and, although not quite as strongly, the opposite has happened in the Alpine chain. The SPI-12 does show an increase in the number, severity, and duration of droughts in the lowlands and a decrease in the mountains but no correlation for DIL. Changes in local-drought characteristics, as opposed to average values, report higher correlations with terrain ruggedness than with mean elevation. Overall, correlation values are also higher than those found for average local-drought characteristics, and visual inspection of the spatial distribution (not shown here) does show quite a homogeneous distribution of drought characteristic changes between the mountains (especially on the windward side, i.e. the one facing the Po Plain) and the plains and hills. The only outliers are the Aosta Valley in the northwest and another valley close to the western border, with changes that are often similar to those in the lowlands. Still, most of the changes found by comparing the two periods are not found to be significant according to the twosample t test and, thus, do not denote a change in the probability distribution of local-drought characteristics. The cells with significant changes (reported in Fig. 5) are mostly distributed between two areas: changes towards more severe (according to the SPI-12, SPEI-3, and SPEI-12), longer (according to both indices at the 3-month scale), and more intense (according to both indices at the 12-month scale) droughts are reported for the eastern-most part of the domain; changes towards less severe and shorter droughts are reported mostly in the northern part of the Alpine chain for the SPI and SPEI at the 3-month scale, while almost no significant shifts towards less intense local droughts are found. 4.3 Region-wide drought event analysis This section shows the results obtained from the analysis of region-wide drought events (see Sect. 4.3). Similarly to the previous sections, both the characteristics of drought events and their change over time are discussed. 4.3.1 Region-wide drought event characteristics Region-wide drought events are calculated from the SPI and SPEI series at 3- and 12-month scales. As the most interesting example, Fig. 6 shows the result for the SPEI-12. The analysis displays similar results between the two indices at the same timescale, with all main events being identified by both the SPI and SPEI and with high agreement between the extent of the area in drought conditions over time. The analysis at the 3-month scale reports about 60 events (see Fig. A2), while the analysis at the 12-month scale reports less than 20 events (for the SPEI-12, see Fig. 6; for the SPI12, see Fig. A2). Region-wide drought events at the longer timescale are more severe and longer than those at the shorter timescale, as expected, but intensity and area values are similar. Regarding relative differences between the drought characteristics of the SPI and SPEI at both timescales, DSE is similar between the two indices, DDE is higher for the SPEI, and both DIE and DAE are higher for the SPI. On the other hand, when considering the mean highest area affected by drought conditions in every single event, both indices report similar results at both timescales. Overall, this indicates that the same deficit tends to affect a slightly wider area with a higher intensity but for less time when only precipitation is considered, while it tends to affect the same overall area with less intensity and for a longer time when both precipitation and temperature are considered. Region-wide drought event analysis on the SPI-12 and SPEI-12 was also useful in indicating the main drought events that happened in the region in the last 60 years. Of these, the last one, starting in the winter of 2021 and still ongoing at the end of the available data time series, was identified as perhaps the most extreme in the series. In particular, the wide area affected by drought during this event and its severity, second only to the longest 2001–2008 event, mark it as an exceptional drought for the region. The intensity value is also the highest of all detected events, but this may not be significant given that this last event had not yet ended at the time the analysis was done. Certainly, the fact that its severity is higher than the severity of the 2001–2002 event as detected by the SPEI-12 also adds to how exceptional this last event is. 4.3.2 Temporal analysis of region-wide drought event characteristics Trend analysis reports no significant results for the drought characteristics of region-wide drought events. Figure 5. Cells with significant changes between the mean drought characteristics for the 1958–1990 and 1990–2023 periods according to the two-sample t test. (a–d) Mean drought severity (DSL) change for the SPI-3 (a), SPEI-3 (b), SPI-12 (c), and SPEI-12 (d). (e–h) Mean drought duration (DDL) change for the SPI-3 (e), SPEI-3 (f), SPI-12 (g), and SPEI-12 (h). Red and blue arrows indicate a worsening and bettering of drought conditions (i.e. higher or lower severity and longer or shorter duration). Comparing the values before and after 1990 does show results that are consistent with those found for local droughts (Table 3): drought events have become more severe, longer, and more intense at both timescales. Also, similarly to drought runs, the number of drought events has increased at the shorter 3-month timescale while having decreased at the longer 12-month scale. Another difference is in the DAE, which has decreased at the 3-month timescale and has increased at the 12-month timescale. Overall, this seems to indicate that, on a regionwide level, drought conditions worsened between the periods 1960–1990 and 1990–2020, with short-term deficits becoming more common over slightly smaller areas, leading to more generalized deficits over wider areas at the longer timescales. Despite many of the described changes not being significant according to the two-sample t test, DSE and DDE for the SPI-12 do show a statistically significant shift in the mean before and after 1990. Changes in DSE and DDE for the SPEI-12 also show p values close to the 5 % level, although these do not fall below the 5 % threshold. This seems to confirm that the shift towards worse region-wide drought conditions (higher severity and longer duration)Figure 6. Region-wide drought event analysis conducted on the SPEI-12. (a) Time series of percentage of cells experiencing drought conditions (only the portion below the −1 threshold, upper part of the diagram) and minimum index value in the domain (lower part of the diagram). Each event is highlighted in yellow and labelled. (b) Drought event characteristics: drought intensity (DIE), mean drought area (DAE), and drought severity (DSE). is more evident at longer timescales and that this shift is mainly caused by a change in precipitation patterns. Despite the apparent importance of precipitation, the only significant trend in terms of the percentage of the domain experiencing drought conditions (index lower than −1) over time is found for the SPEI12, with a slope coefficient of 2.92 × 10−4 yr−1 . 5 Discussion and conclusion In this study, 60 years of precipitation and temperature data are analysed in order to characterize changes in drought conditions in the Piedmont and Aosta Valley areas. In Sect. 1, three questions were posed. The first question asked whether there are temporal trends in drought indices such as the SPI and SPEI and how these trends translate into changes in the characteristics of drought events (in terms of duration, severity, and intensity). Evidence of widespread drying trends in the region is found through the trend analysis of the SPI and SPEI series. Temperature plays a key role in defining these drying trends as the SPEI reports negative trends for wider areas and with greater slope coefficients than the SPI. This is to be expected given the clear trends in temperature due to climate change and is consistent with other studies conducted in the area (see, for example, Falzoi et al., 2019; Baronetti et al., 2020). Still, the areas showing the more severe drying trends do not coincide with the areas showing the highest warming rates, indicating that changes in droughts are governed by the interplay between temperature and precipitation. When moving from drought indices to drought event identification, it is interesting to note that the start and end of single drought periods seem to be mainly determined by precipitation anomalies, which is in contrast to the importance of temperature in determining long-term conditions. Despite the worsening of drought conditions related to precipitation and temperature being clear, the effects on the characteristics of individual drought events are weaker. Some evidence of an increase in the severity, duration, and intensity of drought periods after 1990 is found, although this is often not statistically significant. A tendency for drought periods at the 3-month timescale to become more numerous and for drought periods at the 12-month timescale to become less numerous is observed at both a local and regional scale. Thus, while the percentage of time under drought conditions has become greater at both timescales, it seems that a larger amount of shortterm deficits aggregate into long-term deficits with higher duration. In addition to this, a significant positive trend in the percentage of the area under drought conditions according to the SPEI-12 is detected. Overall, however, changes in local-drought characteristics between the two halves of the analysed series are seldom significant, making it difficult to assess whether the increase in severity, duration, and intensity of drought periods is actually part of a general tendency, which would be coherent with the detected worsening drought conditions.The higher resolution of the analysed data, compared to previous studies, makes it possible to show quite heterogeneous results with regard to the presence of drying and/or wetting trends, as well as drought characteristics, in different portions of the region. As a possible explanation of this result, our analysis studies relations between terrain characteristics and drought characteristics, finding several significant correlations. This type of analysis is in common with a growing body of literature focused on the elevation effects on drought characteristics, with studies conducted in the Qinghai–Tibet plateau (Feng et al., 2020), the Lorestan Province in Iran (Hosseini et al., 2020), the Indus River basin (Dubey et al., 2023), and the Canary Islands (Carrillo et al., 2023). These studies, using mean elevation as a topographic variable, find different results with regards to the distribution of drought trends at high and low elevations, and, as such, no general claim about the tendency of different elevation areas to show drying and/or wetting trends can be made. The second question in the introductory section asked whether there is a relationship between drought trends and the topographical characteristics of the landscape and, if so, whether elevation is the topographical variable most correlated to these trends. Terrain characteristics and mean elevation show a significant influence on the observed trends and changes in drought characteristics, with drying trends being more severe the as the area becomes lower and less rugged. In fact, when the mountainous parts and the flat part of the domain are considered separately, the first shows no significant drought trends, while the second reports significant drying trends for both the SPI and SPEI at multiple timescales. This is particularly true for the flat areas of the region, where trends are stronger than in low-elevation rugged (hilly) areas. In the case of drought period characteristics, decreases in severity, duration, and intensity are mostly found in the Alpine range, while increases are mostly found in the smoother and lower-lying areas. Overall, drought characteristics and changes in time seem to be better correlated to the terrain ruggedness than to elevation alone. Thus, our finding of more severe drying trends and worsening drought characteristics in the lower-altitude part of the region proves the importance of considering topographic effects in areas with highly diverse terrain. More importantly, our study shows that mean elevation, although certainly a variable to be considered, should not be the only topographic variable taken into account. The third question in the introductory section asked whether the outcomes depend on the chosen spatial scale of the analysis. Interestingly, the local-drought analysis and the region-wide analysis result in some differences. Changes in the characteristics of local-drought periods are affected by temperature increases as drought periods obtained from the SPEI series show more pronounced increases in severity, duration, and intensity than those obtained from the SPI series. Contrarily to this, drought events at a region-wide scale show more marked shifts in severity and duration for the SPI than for the SPEI, denoting a more significant influence of regional precipitation patterns than of temperature on droughts at a regional scale. It would be of interest to understand if this is valid for the region of interest, northwestern Italy, or if a similar result could be valid in other areas of the world and, more generally, what the causes could be from a meteorological and climatic point of view. The same consideration can be made for the other results of our analyses. Although strong correlations between drought trends and the mean elevation and ruggedness of the terrain are found, the attribution of these results to physical phenomena is not straightforward. The presented methodology does not focus on this aspect, and, given the complexity of the involved phenomena, attribution is outside the scope of our study. However, our finding of different meteorological conditions between the Alpine chain and the surrounding Po Plain is consistent with other studies concerning the presence of an increase in alpine summer convective precipitation not in common with the surrounding areas (Giorgi et al., 2016; Grose et al., 2019). In this study, we restrict our focus to near-past and current conditions and do not consider predictions of future conditions (although strong drying trends in some portions of the study area are detected). Further research is needed to study how the local and regional drought characteristics of areas at different elevations and with different reliefs may evolve under climate change. Still, the results presented in this paper can be useful not only for the Piedmont and Aosta Valley regions, where they could be the input for analyses of soil moisture and hydrological droughts, but also for other areas and for drought research in general, showing the need to conduct drought studies at different spatiotemporal scales and underscoring the importance of considering areas with distinct topographical features, as well as giving an indication of which areas are more likely to face dryer conditions.Droughts of the future are likely to be more frequent, severe, and longer lasting than they have been in recent decades, but drought risks will be lower if greenhouse gas emissions are cut aggressively. This review presents a synopsis of the tools required for understanding the statistics, physics, and dynamics of drought and its causes in a historical context. Although these tools have been applied most extensively in the United States, Europe, and the Amazon region, they have not been as widely used in other drought-prone regions throughout the rest of the world, presenting opportunities for future research. Water resource managers, early career scientists, and veteran drought researchers will likely see opportunities to improve our understanding of drought.