Results from processing VASClimO datasets were used for studying general precipitation characteristics and analyzing trends in drought conditions for the period 1951–2000. It was possible to take full advantage of the E-OBS dataset for analyzing precipitation characteristics and drought conditions for the longest period with data availability covering the years 1950 throughout 2014.
4.1. Precipitation Characteristics
In reference to VASClimO data, annual precipitation totals in Lebanon for individual grid cells ranged from 315 mm in the semi-arid area of Hermel in North-Beqaa (the inland region) to 1222 mm in North-Lebanon. Yearly average precipitation (September through June) varied from 610 mm in central inland Lebanon (Bmahray) to 780 mm in North Lebanon (Kfarhay). In addition, variability in monthly precipitation patterns was observed among the different sites. Average monthly precipitations values were highest in January recording 174 mm in North Lebanon (Kfarhay), 158 mm in South Lebanon (Bazourieh), 139 mm in central Lebanon (Bmahray) and 138 mm in the semi-arid region of inland North Beqaa (Mrah el Abed). Overall, the peak month for precipitation was January.
As for the E-OBS data, values of annual precipitation totals varied from approximately 100 mm in Bmahray to 955 in Bazourieh. Yearly average precipitation varied from 519 mm in Bazourieh to 573 mm in Kfarhay. Average monthly precipitations were highest in December, recording approximately 80 mm in Kfarhay and 82 mm in Bmahray. The year 2014 recorded the lowest precipitation amount in Bmahray in the past 50 years.
On average, 60% of total precipitation in the two datasets occurred between December and February. Intra-annual precipitation changes in both datasets showed increasing precipitation starting September-October and decreasing precipitation starting February.
Overall, the magnitude of the values for individual grid varied considerably depending on the chosen gridded dataset. In general, the annual precipitation totals for the period 1951–2000 were higher in the VASClimO dataset than those in the E-OBS dataset except for the years 1957, 1991, 1997, and 2000 (Figure 2
). However, the variations in precipitation totals at the local level are considerably higher as the values of the grid cells only represent spatially strongly smoothed records. Analyses based on station data described annual precipitation totals of more than 1000 mm in the North, more than 1200 mm in Mount-Lebanon, around 930 mm in the South, and 705 mm in the Beqaa (inland) [15
]. This makes the employed VASClimO dataset a more representative dataset of local precipitation conditions.
4.2. SPI Trends
SPI values were calculated using both datasets on different timescales, namely at 12, 6 and 3 months bases. The use of VASClimO data shows a decreasing trend of SPI-12 for all selected regions, thus indicating a higher tendency of increased drought occurrence throughout the country (Figure 3
). A more careful observation of the results showed that severe (−2.00 ≤ SPI < −1.50) to extreme (SPI < −2.00) drought conditions occurred over the four studied locations at an average of 10- to 15-year-time intervals.
Furthermore, it was observed that moderate drought conditions (−1.50 ≤ SPI < −1.00) occurred at an average of 5-year time intervals.
When comparing drought conditions between two sub-periods, namely 1951–1975 and 1976–2000, it was found that the latter was characterized by a threefold decrease in the frequency of moderate drought conditions and a twofold increase in the frequency of severe drought conditions. In addition, the second period was characterized by both, an extremely wet condition in the year 1994 and an extremely dry condition in the year 1999.
Similarly, the use of E-OBS dataset showed a decreasing trend of SPI-12 for all selected regions, also indicating a higher tendency of increased drought occurrence across the country (Figure 4
). A more careful observation of the results showed an extreme drought condition (−2.00 > SPI) in the year 1958. Severe (−2.00 ≤ SPI < −1.50) to extreme (SPI < −2.00) drought conditions occurred over the four studied locations at an average of 16-year-time intervals. Further, it was observed that moderate drought conditions (−1.50 ≤ SPI < −1.00) occurred at an average of 5- to 6-year time intervals. The year 2014 was classified as the driest period in the past 15 years.
When comparing drought conditions between the same two sub-periods, namely 1951–1975 and 1976–2000 using the E-OBS dataset, no significant changes in the frequency of moderate and severe drought conditions were observed. However, when considering the two sub-periods, namely 1950–1982 and 1983–2014, it was found that the latter was characterized by a 60% increase in the frequency of moderate drought conditions, mainly in the years 2001 throughout 2014.
Averaged SPI-6 values were also calculated from the four center points using both datasets. In general, SPI-6 is commonly used, along with SPI-3 for monitoring agricultural drought conditions [25
]. The fall-winter interval, also named the winter half-year period, includes the 6-month period extending from October till March, while the winter-spring interval includes the 6-month period extending from December till May. In this study, SPI-6 showed a decreasing trend for the four locations when using the VASClimO dataset (Figure 5
). In addition, SPI-3 was calculated for the different locations showing also decreasing trends in SPI values for fall, winter, and spring seasons (Figure 5
). The year 2000 was the driest during the period 1951–2000. In general, an increasing drought frequency was observed to be more prominent during the winter season.
When using the E-OBS dataset, SPI-6 (winter-spring) showed also a decreasing trend (Figure 6
). In addition, SPI-3 was calculated for the different locations, also showing decreasing trends in SPI values mainly for the winter and spring seasons (Figure 6
). More specifically, the winter of 2014, characterized by extreme drought conditions, was the driest in the past 56 years. The winter-spring period of 2014, characterized by severe drought conditions, was the driest in the past 50 years. In general, an increasing drought frequency was observed to be more prominent during the winter-spring period and during the spring season.
A comparison between the yearly SPI values, as derived from the two datasets, was also conducted to obtain an overview of possible agreements and/or disagreements in drought and wet conditions (Figure 7
). In general, there were periods where there was considerable disagreement between the two datasets in identifying wet or dry conditions. This applies, e.g., to the year 1963 where VASClimO indicated near normal to slightly wet conditions, whereas E-OBS indicated moderate drought conditions. Further, there were periods where there were disagreement cases in the classification of wet or dry conditions. This applies, e.g., to 1958 where VASClimO indicated severe dry conditions, whereas E-OBS indicated extreme dry conditions. In addition, it applies, e.g., to 1994 where VASClimO indicated extremely wet conditions, whereas E-OBS indicated very wet conditions. These differences disappear in some regions, while being particularly pronounced in others especially with the presence of significant differences among the regions. For instance, the E-OBS dataset indicated an SPI-12 value of 0.7 in Mrah El Abed (North Beqaa) in 1994, whereas Bazourieh (South Lebanon) recorded a value of 2.25. In another case involving the E-OBS dataset for the year 1963, Bazourieh recorded an SPI-12 value of 0.58, whereas the remaining regions recorded an SPI-12 close to −1.7. This suggests potential inaccuracies in the gridded datasets, which could be attributed to differences in the groups of stations composing the basis of the grids, and therefore, their identified precipitation trends [37
]. Simultaneously, there were periods where there was considerable agreement between the two datasets in the classes of wet or dry conditions. This applies, e.g., to 1995 where both datasets indicated moderate drought conditions.
Overall, comparing the annual SPI trends of the two gridded products showed slightly opposite trend directions. This was mainly due to (1) an exceptionally low rainfall total in 1958 in E-OBS not reflected in VASClimO; and (2) rainfall totals in the years 1991 and 1997 in E-OBS that were exceptionally higher than rainfall totals in VASClimO. In turn, this resulted from a combination of factors, namely the underlying station data and the precipitation estimation methodology. Therefore, it is not possible to assign superiority/inferiority qualifications to the two datasets. Yet, it would be necessary to evaluate the E-OBS dataset, among others, in the future, against long-term station data in the country. Accordingly, it would be highly desirable to increase the number of measurement stations and make available all existing meteorological datasets in the country.
Overall, an increasing drought frequency potentially contributes to higher agricultural, economic, and environmental damages. Results from another study [28
] showed that the length of the fire season was negatively correlated with mean annual precipitation (r
= −0.836 at P
< 0.01). It was also shown by other studies conducted in the Mediterranean ecosystem [38
] that fire probability and number of fire events increased in seasons that were warmer and drier than average. In addition, other studies [24
] showed that large-scale forest fires were associated with high long drought periods, among others.
With the lack of continuous and reliable historical data of rain gauge stations from Lebanon, the use of gridded datasets for deriving SPI has potential use in Lebanon for monitoring drought conditions and possibly their impact on different sectors such as forestry and agriculture. Yet, the resolution of the employed grid cells is too coarse to represent the precipitation characteristics that might arise from topographic variability. The use of other gridded datasets such as E-OBS, which is a European daily high-resolution gridded dataset of surface temperature and precipitation [31
], might have also other limitations. However, the VASClimO and E-OBS datasets have their individual pros and cons. In reference to [30
], VASClimO is specifically designed for trend analysis purposes, while both datasets have the same limitations in comparison to station data, interpolation error, and underestimation of extremes [37
As shown by [37
] in a study targeting a neighboring region, both datasets correlate well with few exceptions for specific years. Furthermore, trend lines of SPI showed slight increases in drought conditions for specific regions.
Overall, VASClimO in the four regions agree with [37
] in identifying the year 1999 as one of the driest years in the record. That year was indicated as one of the warmest years possibly due to the strongest El-Nino event in the 20th century [37
]. As per the E-OBS dataset, the year 1999 was the second driest after the year 1958.
Finally, it would be of value to identify the role of temperature increase in drought conditions. This could not be assessed with the sole use of SPI. However, with availability of additional data, other indices such as the standardized precipitation evapotranspiration index (SPEI) can be used to account for possible effects of temperature variability and temperature extremes [10