Assessing the Impact of Drought and Upstream Dam Construction on Agriculture in Arid and Semi-Arid Regions: A Case Study of the Middle Draa Valley, Morocco

doi:10.21203/rs.3.rs-5218228/v1

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Assessing the Impact of Drought and Upstream Dam Construction on Agriculture in Arid and Semi-Arid Regions: A Case Study of the Middle Draa Valley, Morocco

https://doi.org/10.21203/rs.3.rs-5218228/v1

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This study assesses the impact of drought and upstream dam construction on agriculture in arid and semi-arid regions with the Middle Draa Valley (MDV) case study in Morocco. This Valley, nestled in the arid landscapes of Morocco, has a rich tapestry of history, culture, and natural beauty. However, its oases are facing a water shortage crisis due to the effects of climate change and human pressure on water resources for irrigation. We analyzed the Standardized Precipitation Evapotranspiration Index (SPEI) and the Normalized Difference Vegetation Index (NDVI) to monitor vegetation dynamics in this valley. The SPEI shows a declining trend, indicating increasing water scarcity, which has led to the overexploitation of groundwater and significant vegetation degradation. The NDVI remained relatively stable until 2015 but has since declined, signaling ecosystem deterioration. The correlation between SPEI and NDVI in the oases suggests that vegetation depends not only on precipitation and evapotranspiration but also on other factors such as insufficient releases from upstream dams and the use of groundwater for irrigation. This is confirmed by analyses of the correlation between dam releases and NDVI, as well as data on groundwater table levels. This situation poses an urgent warning for decision-makers to consider which water management measures and agricultural practices to adopt. The most practical solution to avoid further degradation of the oases in the short term is to conserve the remaining groundwater resources exclusively for irrigation date palms and to develop strict long-term water policies.

Oases

Dam

NDVI

SPEI

Climate change

Water resources

Climate change represents one of the most pressing challenges of our time, with devastating and widespread consequences across the globe (Calvin et al., 2023). Scientific evidence unequivocally indicates that human activities, particularly the emission of greenhouse gases resulting from fossil fuel combustion and deforestation, are the primary drivers of this phenomenon (Calvin et al., 2023; Gillingham & Stock, 2018). The effects of climate change manifest in rising global average temperatures (Calvin et al., 2023; Karmaoui et al., 2019), more frequent and intense extreme weather events (Weilnhammer et al., 2021), melting glaciers and ice caps, and sea-level rise (Shahgedanova, 2021). These changes have profound repercussions on ecosystems, biodiversity, food security, human health, and economic stability (Fankhauser & S.J. Tol, 2005; Luo et al., 2020; McMichael et al., 2006).

The Middle Draa Valley (MDV), located in southeastern Morocco, is among the areas heavily impacted by the effects of climate change, leading to the progressive degradation of date palm oases (Moumane et al., 2022). This region, once renowned for its lush landscapes and thriving oases, faces growing challenges due to increasing temperatures, decreasing precipitation, and more frequent and intense drought episodes (Karmaoui et al., 2019). In addition to the challenges posed by climate change, the MDV also suffers from dwindling groundwater resources due to overexploitation (Johannsen et al., 2016). Over the years, the growing demand for water for agricultural irrigation, drinking water supply, and other human needs has placed unprecedented pressure on the region's underground aquifers, confirmed by the increase in motor pump numbers (Ait Lamqadem et al., 2018). As a result, the oases and the communities that depend on them for their livelihoods are facing growing challenges in terms of food security, health, and well-being that need to be studied using an international approach to assess the impact of climate conditions and human pressure on the vegetation of the area.

Many approaches have been used in recent research to study the impacts of climate change on vegetation in many areas of the world by using many recognized indices. Among the most used indices, we find the normalized difference vegetation index (NDVI) and the Standardized Precipitation Evapotranspiration Index (SPEI). NDVI, proposed for the first time by Rouse et al. (1974), gives information about the density and health of vegetation (Taiwo et al., 2023). The approach of SPEI was first proposed by Vicente-Serrano et al. (2011) as an improved drought index, particularly suitable for studies of the effect of global warming on drought severity by integrating both precipitation and potential evapotranspiration (Byakatonda et al., 2020; Vicente-Serrano et al., 2011). The combination of these two indices makes it possible to monitor the effects of drought on variation of vegetation on a spatial and temporal scale.

This approach was used in many studies to assess the impacts of climate change on ecosystems in many arid and semiarid areas like the case of the MDV region. For example, in Iran, NDVI and SPEI are used, in the Khuzestan Province, to identify areas at high risk of drought, and to determine when drought occurs (Nejadrekabi et al., 2022). In Northern Xinjiang China, Luo and al analyzed the dynamic characteristics of drought and vegetation cover, evaluating their interactions from 1998 to 2015 by using SPEI and NDVI (Luo et al., 2020). Results indicate that both NDVI and SPEI exhibited fluctuating and increasing trends, leading to a slight improvement in vegetation status. In Morocco, in the sub-basin of Lakhdar of Wadi Oum Er Rabia basin, a study was conducted to examine LULC from 2000 to 2020, focusing on how vegetation cover affects land erosion (Ait El Haj et al., 2023). The study comes out with a conclusion indicating that there is a certain correlation between NDVI and the Standardized Precipitation Index (SPI), more particularly with the annual and average variation SPI12. The particularity of SPI compared to SPEI is that the SPI is an index that only uses rainfall data to measure drought, ignoring other factors like evaporation and temperature that also affect the water status of an area (Ait El Haj et al., 2023). Therefore, the advantage of the SPEI is that it is very appropriate for areas with high temperatures, like our study area, because it accounts for the effects of evapotranspiration in its calculations.

Otherwise, in arid and semi-arid areas, the vegetation depends not only on precipitation and evapotranspiration but also on other factors such as dam releases and the use of groundwater. So to effectively study changes on vegetation in arid and semi-arid areas, it's essential to take a comprehensive approach that considers all the factors influencing these changes. The correlation between NDVI on one hand, and SPEI, dam releases, and groundwater on the other, reveals the impact of each factor relative to the others on vegetation. This will give the field of influence of every factor on spatial and temporal scales. So this study aims to assess comprehensively and accurately the dynamics of MDV ecosystems in response to climatic variations, dam releases, and the use of groundwater. The SPEI allows us to quantify drought or moisture conditions in the region and analyze their temporal evolution. Dam releases facilitate the assessment of their long-term impacts on vegetation. Additionally, groundwater table levels indicate their influence on vegetation. Meanwhile, NDVI offers insights into vegetation health and density.

2.1. Study Area

The Middle Draa Valley (MDV), located in the southeastern part of Morocco, which is a region of significant geographical, cultural, and historical importance, was used as a case study (Fig. 1). The MDV is a lush and fertile oasis that stretches for about 200 kilometers along the Draa River, from Agdez (Long= -6.45 °, Lat = 30.69 °) to Mhamid Elghazlane (Long=-5.72 °, Lat = 29.82 °), making it the longest oasis in Morocco. Along the MDV, we find six oases: Mezquita, Tinzoline, Ternata, Fezouata, Ktaoua, and Mhamid. The valley is limited by the Atlas Mountains in the north and the Anti-Atlas Mountains in the south. The oasis's land size is around 26,000 hectares, whereas this valley spans more than 15,000 km2 (Karmaoui, 2019). The main crops are palm trees, cereals, alfalfa, subsistence vegetables, and livestock. The cultivation of date palms is particularly important, with the region being known for producing high-quality dates. In this arid region, more than 70% of the local population depends on the agricultural sector (Karmaoui, 2019). The irrigation is ensured by precipitation, the release of water from the Mansour Eddahbi dam upstream, and the utilization of groundwater resources (Heidecke, s. d.). 97% percent of the water demand in MDV is attributed to irrigation purposes (Klose, 2013). According to data provided by the Draa Oued Noun Hydraulic Basin Agency from 1989 to 2015, the average annual precipitation in Zagora City, located in the middle of the MDV, is approximately 70mm. The maximum temperature in Zagora is on average 30°C over the year (from 18°C in January to 44°C in July). The minimum temperature in Zagora is on average 12°C over the year (from 0°C in January to 20°C in July and June). This Valley faces many environmental challenges, including water scarcity and the encroachment of desertification. Sustainable water management practices are crucial to maintaining the health of the oasis and supporting the local agricultural communities.

2.2. Precipitation and temperature data

Data used are precipitation and temperature from 1995 to 2020 sourced from the ERA5 dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the calculation of SPEI curves. Specifically, the precipitation data utilized in this study has undergone rigorous validation through previous research endeavors, affirming its reliability and accuracy (Khettouch et al., 2023). These validations have demonstrated the robust performance of ERA5 precipitation data, thus providing a solid foundation for our subsequent analyses and conclusions. Then, we used precipitation and temperature data from NASA's Prediction Of Worldwide Energy Resources (POWER) project from 2020 to 2022 to complement our dataset.

2.3. piezometer data

Piezometer data between 2010 and 2021 was acquired from the Draa Oued Noun Hydraulic Basin Agency. The location of the piezometers is indicated on the map of the study area in Fig. 1. These data serve as valuable indicators of aquifer behavior, aiding in the assessment of water availability. In this study, piezometer data was leveraged to assess the impact of groundwater availability on the dynamics of vegetation in the MDV. Integrating these datasets aims to elucidate the intricate relationship between groundwater resources and vegetation dynamics within this region. Through meticulous analysis, this study explores how fluctuations in groundwater levels influence the density and health of vegetation, indicated by NDVI, across this valley. By quantifying the temporal patterns of groundwater availability and correlating them with vegetation dynamics, this research endeavors to provide valuable insights into the vulnerability of ecosystems to hydrological changes and to enhance our understanding of the complex interactions between hydrological processes and ecosystem functioning in arid environments like our case study.

2.4. Data of Mansour Dahbi releases

Mansour Eddahbi Dam releases data between 1995 and 2021 was also acquired from the Draa Oued Noun Hydraulic Basin Agency. As this study endeavors to evaluate the degradation of crops within the MDV, the inclusion of data on releases from the Mansour Eddahbi dam appears essential to know if there is an existing correlation between the NDVI and releases from the dam. While the correlation between NDVI, an indicator of vegetation health, and dam releases may offer valuable insights into the relationship between water availability and crop conditions, it is essential to incorporate the actual release data from the dam to comprehensively assess the irrigation dynamics and their impacts on crop degradation.

Figure 2 shows that the releases from Mansour Eddahbi dam are notably irregular, with significant differences from one year to another, and a general negative trend over the years.

2.5. Remote Sensing Data

Normalized Difference Vegetation Index (NDVI) dataset was obtained from google earth engine. The NDVI, derived from Landsat 5™, Landsat 7 ETM+, and Landsat 8 OLI/TIRS imagery with a resolution of 30m, serves as a crucial indicator of vegetation health and density over time. Landsat 5™ provides satellite images between 1995 and 2000, Landsat 7 ETM + provides satellite images from 2000 to 2013, and Landsat 8 OLI/TIRS provides satellite images from 2013 to 2023.Two distinct methods to determine the NDVI for each oasis were used. Initially, we analyzed the spatial and temporal evolution of NDVI across the entire surface area of each oasis. This comprehensive approach allowed for a thorough examination of NDVI trends over time, encompassing variations across the entire expanse of each oasis. Additionally, we utilized a second method, focusing on delineating the evolution of NDVI within specific 1 km² segments within each oasis, spanning the period from 1995 to 2023. By zooming in on these smaller spatial units, we aimed to capture localized nuances and variations in NDVI dynamics, providing a more detailed understanding of how vegetation health and density evolved within discrete areas of each oasis over the specified timeframe. Employing these complementary methods enabled a comprehensive assessment of NDVI trends at both the macro and micro levels within each oasis, facilitating a nuanced understanding of vegetation dynamics across these ecologically significant landscapes. Overall, the processing of Landsat images was conducted on the Google Earth Engine platform. Through the analysis of temporal and spatial changes in NDVI using Landsat's historical imagery, we gained valuable insights into seasonal and spatial patterns, phenological changes, and the overall dynamics of ecosystems. This time series data allows us to assess the impact of environmental factors on vegetation, such as climate variations or land-use and land-cover changes.

2.6. NDVI Time Series Construction

We used the dataset of Landsat images of the surface reflectance between 1995 and 2023 to calculate the NDVI time series for all the periods between January and December. The surface reflectance images provide processed images with high resolution. The NDVI values were derived by utilizing the red and near-infrared bands from the satellite imagery. For each pixel, we employed the following formula to calculate NDVI:

NDVI = (NIR - Red) / (NIR + Red) (Rouse et al., 1974)

where NIR represents the near-infrared band and Red denotes the red band.

The classification density of vegetation relies heavily on the Normalized Difference Vegetation Index (NDVI), a widely used metric derived from remote sensing data. NDVI values provide insights into the density and health of vegetation cover within a given area. The density of vegetation is classified as follows. when NDVI values fall below 0, it typically indicates the presence of water bodies. In the range between 0 and 0.15, NDVI suggests low-density vegetation, often corresponding to sparse or stressed vegetation. Moving into the range between 0.15 and 0.3, NDVI signifies moderate-density vegetation, indicating healthier and more extensive vegetation cover. Beyond an NDVI of 0.3, the classification denotes high-density vegetation, indicating lush and vigorous vegetation, often indicative of dense forests or highly productive agricultural areas.

2.7. SPEI Index

The Standardized Precipitation Evapotranspiration Index (SPEI) stands out as a sophisticated drought assessment tool, blending precipitation and temperature data to offer a more nuanced perspective compared to the Standardized Precipitation Index (SPI) solely reliant on precipitation metrics (Ayugi et al., 2020). By factoring in the atmospheric evaporative demand, dictated by temperature fluctuations, the SPEI provides a holistic insight into drought dynamics, capturing the impact of heightened temperatures on evapotranspiration rates. The SPEI built in the R Program language version 4.3.2 was used to compute the SPEI.

Comparable to the original SPI, a negative value indicates dry conditions, whilst a positive value depicts wet conditions (Gozzo et al., 2019). These values for the SPEI define the characteristics of drought or wet conditions in terms of severity, intensity, and duration of occurrence. In this study, the threshold of SPEI ≤ 0 was inferred to signify dry events whereas SPEI ≥ 0 represented wet events over the study domain. The classification of SPEI used in many recent studies that examined the spatiotemporal evolution of drought in arid and semi-arid areas is presented in Table 1 (Ayugi et al., 2020; Luo et al., 2020; Polong et al., 2019).

Table 1

Drought severity classifications
SPEI	Drought severity classes
< – 2	Extreme drought
– 1.99 to – 1.50	Severe drought
– 1.49 to – 1.00	Moderate drought
– 1.00 to 1.00	Near Normal
1.00 to 1.49	Moderately wet
1.50 to 1.99	Severely wet
> – 2	Extreme wet

This study defined the frequency of dry/wet events over the study domain as given in Equations (2);

$$\:Fs=\:\frac{{n}_{s}}{{N}_{s}}\times\:100\%\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:$$

where n_s is the number of months of drought events (SPEI < 0), and N_s is the total of the months for the study period.

2.8. Mann–Kendall Test

The study employed the Mann-Kendall (MK) test to detect the significance of the SPEI-12 analyses obtained from the processing of precipitation and temperature data using the R program. This is a climate index that measures drought or moisture conditions over 12 months, taking into account both precipitation and evapotranspiration. The non-parametric feature of the MK test allows us to confirm the existence of a trend in any dataset against the null hypothesis of no trend. Additionally, it does not require the sample to conform to any specific probability distribution since it works well even with insufficient or abnormal values. The significance of the trend was tested at the 5% significance level. A Z-score exceeding the magnitude of critical values at the 5% significance level denotes a monotonic trend of drought/flood events. The S value denotes variance, which is used to calculate the significance of the trend. Numerous hydro-climatic studies across various domains have employed MK tools for trend analysis (Ayugi et al., 2020; Luo et al., 2020). Moreover, the Sen Slope estimator was used to detect the magnitude of the trends. The details of the Mann-Kendall Test determination are defined by Luo et al. (2020). The determination of the Mann-Kendall Test is performed using R language version 4.3.2.

2.9. Season of palm trees

Date palm is one of the oldest fruit crops grown in the arid regions of the Arabian Peninsula, North Africa, and the Middle East where dates play a major food source and income source for local populations and play significant roles in the economy, society, and environment in these areas (Chao & Krueger, 2007). In terms of area, Morocco holds the seventh position globally, ranks eleventh in the number of date palms, and twelfth in date production (Sedra, 2015). Therefore, it is interesting to study the effects of climate change and human pressure on water resources on this tree of primary importance for oasis populations such as the case of MDV. The aim is to investigate the evolution of greenness during the season when date palms predominate over other crops in the MDV between 1995 and 2023, using NDVI. To select the most appropriate season characterized by the dominance of palm trees as the primary agricultural activity in the oases, we analyzed the NDVI curves across all seasons: winter, spring, summer, and autumn. After a thorough examination, we opted for the summer season, as it more accurately represents the prevalence of palm trees. This decision was further supported by on-site visits. Upon visiting the oases and conducting field investigations, we observed that during winter which begins from January to March, the primary agricultural activities in the Draa oases include palm cultivation, alfalfa cultivation, and seasonal vegetable production, with wheat farming being dominant. Spring, which begins from April to June, witnesses the presence of palm trees, alfalfa, and vegetables, again with wheat farming taking precedence. It is also the harvesting season. Summer, which begins from July to September, sees palm tree cultivation as the predominant agricultural activity, alongside alfalfa, with palm trees prevailing. In autumn, the agricultural season commences anew. From our observations, we deduced that the NDVI's evolution during the summer season correlates closely with the density and health of palm trees.

2.10. Correlation Analysis

2.10.1. Correlation between vegetation density and health (NDVI) with drought (SPEI)

The examination of the correlation between NDVI and SPEI entails overlying the temporal progression graphs of both indices. This analytical approach juxtaposes the fluctuating patterns of vegetation health and density, as indicated by NDVI, with the concurrent climatic conditions, as represented by SPEI. By superimposing these graphs, we can discern potential relationships between vegetation dynamics and water availability, facilitating a deeper understanding of ecosystem responses to changing environmental factors. The correlation analysis between NDVI and SPEI involves investigating their relationship across periods of both negative and positive SPEI values. This examination seeks to elucidate whether fluctuations in SPEI, denoting drought or surplus water conditions, correspond with changes in NDVI, reflecting shifts in vegetation health and density. A positive correlation suggests that as SPEI values turn negative, indicating drier conditions, NDVI decreases, signaling a reduction in vegetation greenness and density. Conversely, during periods of positive SPEI values, a positive correlation indicates that NDVI increases, reflecting improved vegetation health due to favorable water availability. However, if these correlations do not manifest, it suggests that factors beyond precipitation and evapotranspiration influence vegetation dynamics, emphasizing the multifaceted nature of ecosystem response to environmental stimuli.

2.10.2. Correlation between vegetation density and health (NDVI) with Dam water availability

The correlation study between NDVI and Dam releases involves the application of the Pearson correlation coefficient. This statistical measure evaluates the linear relationship between two continuous variables, in this case, NDVI values representing vegetation health and the volume of water released from the dam. By employing the Pearson correlation coefficient, we can quantitatively assess the strength and direction of the association between these variables. This analysis provides valuable insights into how variations in dam releases may impact vegetation health within the study area, facilitating a more comprehensive understanding of the relationship between water resource management and agricultural outcomes in the MDV.

Figure 3 sums up the methodology used in this study and explained in the last paragraph :

3.1. The temporal evolution of vegetation of Oases

The interannual variations in NDVI for the Draa Oases are shown in Fig. 4. It indicates that the average NDVI in 1995 changed from one oasis to another. For Mhamid oasis, the average NDVI is 0.094; for Ktaoua, it is 0.09; for Fezouata, it is 0.119; for Ternata, it is 0.18; for Tinzoline, it is 0.181, and for Mezquita, it is 0.143. The NDVI values in the oases of Tinzoline, Mezquita, and Ternata, located in the northern part of the Draa Valley, are notably higher compared to those in the southern oases, such as Fezouata, Ktaoua, and Mhamid. These results show that the more we head toward the south the NDVI becomes lower. The NDVI is closely linked to vegetation density, and it is observed that vegetation density decreases from north to south in the Draa Oases. To analyze the variation of the NDVI of the oases, we can categorize the NDVI variation curves into three main stages: from 1995 to 2003, from 2003 to 2015, and from 2015 to 2024. However, the Mezquita oasis follows a different pattern, with stages from 1995 to 2003, from 2003 to 2019, and from 2019 to 2023.

From 1995 to 2003, the average NDVI fluctuated with a general decrease across all oases, suggesting a general decline in vegetation density during this period, with the degree varying from one oasis to another, except for the Mezquita oasis which maintained a generally stable NDVI. For M'hamid Oasis, the results presented in Fig. 4(a) revealed a decline in the average NDVI from 0.0947 to 0.0586, indicating a decrease at a rate of 0.0045 per year. Similarly, for Ktaoua Oasis, the results from Fig. 4(b) showed a reduction in the average NDVI from 0.0905 to 0.0552, with a rate of decrease at 0.0044 per year. In the case of Fezouata Oasis, as presented in Fig. 4(c), there was a decrease in the average NDVI from 0.119 to 0.0738, corresponding to a rate of decline of 0.0056 per year. Ternata oasis, illustrated in Fig. 4(d), indicated a decrease in the average NDVI from 0.1808 to 0.0732, with a rate of decrease at 0.0134 per year. Similarly, for Tinzoline oasis, shown in Fig. 4(e), there was a decline in the average NDVI from 0.1817 to 0.0879, indicating a decrease at a rate of 0.0117 per year. Between 1995 and 2003 as it is presented in Fig. 4(f), the NDVI of the Mezquita oasis exhibited notable fluctuations, indicating dynamic changes in vegetation cover over the specified period. These fluctuations ranged from a maximum value of 0.17683 to a minimum of 0.12275, with a negative trend at a rate of 0.0026 per year. The analysis indicated that the rate of decrease in Ternata and Tinzoline oases was significantly higher compared to other oases. The results suggest a notable variation in NDVI dynamics across the MDV from 1995 to 2003. Overall, there was a general decrease in average NDVI across all oases during this period, indicative of a decline in vegetation density. However, the extent of this decline varied among different oases. Notably, M'hamid, Ktaoua, Fezouata, Ternata, and Tinzoline oases all experienced significant decreases in NDVI, with Ternata and Tinzoline showing the highest rates of decline. Conversely, the Mezquita oases displayed fluctuating NDVI values during this period, maintaining relatively stable vegetation cover overall. The analysis highlights Ternata and Tinzoline oases as experiencing particularly pronounced decreases in NDVI compared to other oases, suggesting potentially heightened environmental stress in these areas.

From 2003 to 2015, there was a general increase in NDVI across all oases of the MDV albeit with varying rates of change from one oasis to another, except Mezquita Oasis which continued to increase until 2019. For M'hamid Oasis, depicted in Fig. 4(a), there was an increase in the average NDVI from 0.0586 to 0.1242, representing a rate of 0.0054 per year. Similarly, for Ktaoua Oasis, as shown in Fig. 4(b), there was an increase in the average NDVI from 0.0552 to 0.1226, indicating a rate of 0.0056 per year. Likewise, for Fezouata Oasis, illustrated in Fig. 4(c), there was an increase in the average NDVI from 0.0738 to 0.1147, corresponding to a rate of 0.0034 per year. The analysis of Ternata Oasis, presented in Fig. 4(d), showed an increase in the average NDVI from 0.0732 to 0.1902, with a rate of increase at 0.0097 per year. Similarly, for Tinzoline Oasis, as demonstrated in Fig. 4(e), there was an increase in the average NDVI from 0.0879 to 0.2004, indicating a rate of 0.0093 per year. Finally, for the Mezquita oasis, as indicated in Fig. 4(f), there was an increase in the average NDVI between 2003 and 2019, rising from 0.1227 to 0.2196, corresponding to a rate of 0.006 per year. Those results show that M'hamid and Ktaoua oases exhibited similar rates of NDVI increase, while Fezuata showed a slightly slower rate of increase. Conversely, Ternata and Tinzoline displayed the highest rates of NDVI increase among all oases. The Mezquita oasis showed a steady increase in NDVI over the entire period, albeit with a lower rate of increase compared to Ternata and Tinzoline oases. These results suggest spatial and temporal variations in vegetation dynamics across the MDV, influenced by local environmental conditions and potential management practices.

From 2015 to 2023, the average NDVI decreased across Mhamid, Ktaoua, Fezouata, Ternata, and Tinzoline oases, suggesting a general decline in vegetation density during this period, with the degree varying from one oasis to another, except for the Mezquita Oasis, which began to decrease in 2019. For M'hamid Oasis, depicted in Fig. 4 (a), there was a decrease in the average NDVI from 0.1242 to 0.0694, representing a rate of 0.0068 per year. Similarly, for Ktaoua Oasis, as shown in Fig. 4(b), there was a decrease in the average NDVI from 0.1226 to 0.0772, indicating a rate of 0.0056 per year. Likewise, for Fezouata Oasis, illustrated in Fig. 4(c), there was a decrease in the average NDVI from 0.1147 to 0.0682, corresponding to a rate of 0.0058 per year. The analysis of Ternata Oasis, presented in Fig. 4(d), showed a decrease in the average NDVI from 0.1902 to 0.0808, with a rate of decrease at 0.0136 per year. Similarly, for Tinzoline Oasis, as demonstrated in Fig. 4(e), there was a decrease in the average NDVI from 0.2004 to 0.0892, indicating a rate of 0.0138 per year. Between 2019 and 2023, there was a decrease in the average NDVI of Mezquita oasis from 0.2196 to 0.1047, corresponding to a rate of 0.0287 per year. The results suggest a concerning trend of decreasing vegetation density across multiple oases in the MDV from 2015 to 2023. This period witnessed a general decline in average NDVI across M'hamid, Ktaoua, Fezouata, Ternata, and Tinzoline oases, indicating a reduction in overall vegetation density and health. The rates of decrease varied among the different oases, with Ternata and Tinzoline experiencing particularly steep declines. Notably, the Mezquiita oases, which initially maintained stable NDVI values, also began to decrease in 2019, indicating a shift towards declining vegetation density in this area as well. Specific analysis of M'hamid, Ktaoua, Fezouata, Ternata, and Tinzoline oases showed consistent decreases in NDVI over the period, with varying rates of decline. These findings highlight the vulnerability of vegetation in the MDV to environmental pressures during the studied timeframe. A common trait observed among the NDVI curves of various oases is the convergence of NDVI values across all seasons towards the same minimum in the year 2023. Despite the seasonal fluctuations in vegetation growth and health, the NDVI values of different oases tend to align, reaching their lowest point in synchronization during that particular year.

Between 1995 and 2023, the NDVI consistently registers higher values during winter and spring compared to the other seasons. The consistent pattern of higher NDVI values during winter and spring suggests that these seasons likely correspond to the cultivation stage in the MDV, a hypothesis supported by on-site visits. The cultivation stage typically involves healthier vegetation due to reduced water stress and favorable weather conditions during these seasons. The analysis of NDVI curves from the MDV oases suggests a consistent pattern indicating that the NDVI values for the summer season are consistently the weakest among all curves.

The analysis of NDVI evolution in the MDV's oases reveals that, for the majority of the time, the vegetation density can be classified as low, especially in the Fezouata, Ktaoua, and Mhamid oases. This conclusion is based on the observation that the NDVI values frequently remain below 0.15, which is indicative of sparse or minimal vegetation cover. In contrast, during certain periods, particularly between 2010 and 2020, the vegetation density in the Mezquita, Tinzoline, and Ternata oases can be classified as moderate density. This classification arises from the NDVI values exceeding 0.15 during this time frame, indicating an increase in vegetation activity and greenness.

3.2. The Spatial and Temporal Distributions of Palm Trees

To evaluate the evolution of density and health of palm trees, our methodology entails a comprehensive analysis of NDVI trends during the summer season over the years from 1995 to 2023. By focusing on the summer months, when palm trees dominate the crops of the MDV, we aim to capture the most representative data regarding their general condition. This approach allows us to track changes in vegetation density and health over time, providing valuable information on long-term trends and fluctuations that may impact palm trees. Through this systematic assessment of vegetation dynamics, we can effectively gauge the health and density of palm trees and identify any potential factors influencing their evolution.

The temporal trend of the NDVI during the summer season closely mirrors the trajectory of the average NDVI over the same period. This parallel evolution suggests a strong correlation between the two variables, indicating that the seasonal changes in vegetation greenness, as captured by the NDVI, align closely with the broader patterns observed in the average NDVI. The trend of the NDVI in the oases of Ternata and Tinzoline indicates a declining tendency, whereas Mezquita oases generally exhibit a stable trend between 1995 and 2023. The NDVI values in the southern oases of Mhamid, Ktaoua, and Fezouata generally demonstrate a trend of stabilization between 1995 and 2023, albeit with consistently lower values compared to their northern counterparts. The curves depicting the NDVI converge to a general, similar value in the years 2004 and 2023.

The observation of a significant decrease in the Normalized Difference Vegetation Index (NDVI) after 2015 for the oases of Tinzoline, Ternata, Fezouata, Ktaoua, and Mhamid, as shown in Fig. 5, and after 2019 for the Mezquita oasis, is an unprecedented phenomenon according to the processed dataset. This downward trend in NDVI indicates a marked decline in vegetation density and health in these oases over the past years. Conversely, the NDVI values observed in the southern oases of Mhamid, Ktaoua, and Fezouata consistently present lower levels compared to their northern counterparts such as Ternata, Mezkita, and Tinzoline. This disparity in NDVI suggests notable differences in palm density and health between these regions.

These findings are particularly alarming as they reflect the devastating effects of climate change and the ongoing degradation of the environment in these regions. Such a decrease in NDVI can have serious consequences for oases ecosystems, compromising biodiversity, agricultural productivity, and the livelihoods of local populations who depend on these oases for their food, water, and well-being. These findings underscore the urgent need to adopt effective adaptation and mitigation measures to preserve these fragile ecosystems and ensure their long-term sustainability.

3.3. The Temporal evolution of Drought of oases

From the analysis of the results of the Standardized Precipitation Evapotranspiration Index (SPEI) conducted using the R program, irregularities in precipitation patterns in the Draa region are observed, with a negative influence of temperatures. This combination of climatic factors has contributed to a predominantly dry climate in the region over the years. The collected data reveal significant variations in precipitation patterns, with periods of prolonged drought interspersed with a few wet years. According to statistics, drought years accounted for approximately two-thirds of the total duration between 1995 and 2022, with a frequency ranging between 65% and 67%: 65% for the Mesquita, Ternata, and Tinzoline, Ktaoua and Fezouata oases, and 67% for the Mhamid oasis as indicated in Table 2. These figures highlight the predominance of drought periods in the Draa Valley region over the past decades.

Applying the Mann-Kendall (M–K) test and Sen’s slope estimation, the annual SPEIs of oases decreased at a linear rate between − 0.0267 and − 0.0448 yr − 1. The lower the SPEI, the heavier the drought occurrence; thus, a decreasing trend of SPEI indicates an increasing drought occurrence in all oases of the MDV. The z-value measures the strength and direction of the trend observed in the SPEI data. Negative values indicate a decreasing trend, while positive values indicate an increasing trend. In this case, all the z-values are negative for all oases (between − 2.272 and − 1.718), indicating significant decreasing trends in SPEI across all regions (Table 3). As p-value represents the probability of observing the test statistic (z) as extreme as the one calculated if the null hypothesis (no trend) were true. A small p-value, typically less than the chosen significance level (alpha), as seen in Mezquita, Ternata, and Ktaoua oases (0.04186, 0.04186, and 0.02309 respectively), indicates strong evidence against the null hypothesis. In the case of Tinzoline, Fezouata, and Mhamid regions, where the p-value is greater than the typical significance level of 0.05 (0.05532, 0.08565, and 0.0722 respectively), it suggests a decreasing trend according to other parameters such as Z, S, and tau, although not statistically significant at the 5% level.

Table 2

Frequency of drought in the Draa Oases
	Mezquita	Tinzoline	Ternata	Fezouata	Ktaoua	Mhamid
Fs	65%	65%	65%	65%	65%	67%
linear decrease rate	-0.0366	-0.0325	-0.0358	-0.0295	-0.0448	-0.0267

Table 3

Results of SPEI Mann–Kendall Test of Draa Oases
	Mezquita	Tinzoline	Ternata	Fezouata	Ktaoua	Mhamid
z	-2.0349	-1.9164	-2.0349	-1.7188	-2.272	-1.7978
n	28	28	28	28	28	28
p-value	0.04186	0.05532	0.04186	0.08565	0.02309	0.0722
S	-104	-98	104	-88	116	-92
varS	2562	2562	2562	2562	2562	2562
tau	-0.2751323	-0.2592593	-0.2751323	-0.2328042	-0.3068783	-0.2433862
Alpha	0.05	0.05	0.05	0.05	0.05	0.05
Significance	Significant decreasing	No significant decreasing	Significant decreasing	No significant decreasing	Significant decreasing	No significant decreasing

Table 4

Classification of drought
	Mezquita	Tinzoline	Ternata	Fezouata	Ktaoua	Mhamid
	Number of months
Extreme drought	9	6	7	9	15	8
Severe drought	26	25	20	19	17	25
Moderate drought	46	53	52	49	46	54
Near Normal	221	229	236	233	235	222
Moderately wet	20	10	8	15	5	11
Severely wet	3	2	2	0	0	5
Extreme wet	0	0	0	0	0	0

3.4. Temporal evolution of water table level in MDV oases

As noted in the methodology, groundwater is among the critical factors ensuring water demand in the MDV. Recognizing its central role, we study the evolution of groundwater levels to seek to understand its fluctuations and assess its impact on vegetation dynamics. Understanding the interaction between groundwater availability and vegetation health is paramount, as it provides crucial information about the broader dynamics of the MDV’s ecosystem. Through this integrated approach, we aim to elucidate the complex relationship between groundwater resources and vegetation dynamics, thereby contributing to a more complete understanding of the ecological dynamics within this vital ecosystem.

So, our analysis revealed a consistent decline in water table levels across all oases in MDV from 2010 to 2021, (the Ktaoua Oases was excluded because of insufficient data). This decline signals a concerning pattern of diminishing groundwater resources throughout the region, posing significant challenges for both agricultural sustainability and ecosystem health. The decline in the water table level in the Draa Oases record declines between 13 m and 3 m as shown in Fig. 8. This decrease in the water table in the last decade indicates the overexploitation of underground water resources in the MDV which has harmful impacts on the agricultural situation in the region, and that is indicated by the spatial and temporal distribution of vegetation presented by the evolution NDVI. The analysis of the evolution curves of groundwater levels in the MDV reveals a worrying trend: that of a constant and irreversible decrease. This observation suggests that the region's water tables do not benefit from adequate recharge to compensate for withdrawals and losses linked to human exploitation and unfavorable environmental conditions

3.5. Correlation analysis between vegetation density and health with drought

The annual average SPEI and NDVI from 1995 to 2022 are shown in Fig. 7. From 1995 to 2022, an intriguing correspondence emerged in the evolution of NDVI and SPEI across the Fezouata, Ktaoua, and Mhamid oases. However, this relationship exhibits varying degrees of variation from one period to another. For instance, during the period from 1997 to 2002, characterized by negative SPEI values indicating drought conditions, a discernible decrease in NDVI is observed, reflecting the adverse impact of water scarcity on vegetation health. Conversely, between 2008 and 2009, when positive SPEI values signify a wetter period, a notable increase in NDVI is evident, highlighting the positive influence of ample water availability on vegetation growth. These fluctuations underscore the sensitivity of oases ecosystems to changes in precipitation and evapotranspiration, with drought conditions inhibiting vegetation growth and wetter periods facilitating its resurgence. Despite observing a certain similarity in the variation of NDVI and SPEI, a notable exception arises between 2010 and 2018. During this period, despite experiencing the succession of many years of drought, it is intriguing to note that NDVI consistently maintains values above the average. During that period, the density and health of vegetation depended on factors beyond precipitation and evapotranspiration, such as the release of dams upstream of the MDV and the use of underground water resources for irrigation.

In the oases of Tinzoline and Ternata, there is a notable dependency in the evolution of NDVI and SPEI during the period spanning from 1999 to 2009. This correlation indicates a synchronized relationship between vegetation health and climatic conditions, where fluctuations in NDVI closely correspond to changes in SPEI values. However, divergent patterns emerge during the periods from 1995 to 1999 and from 2009 to 2018, where this dependency decreases. During these intervals, the evolution of NDVI appears to diverge from the trajectory of SPEI, suggesting that other factors beyond precipitation and evapotranspiration may exert stronger influences on vegetation dynamics.

The Mezquita oasis presents an intriguing ecological phenomenon where the NDVI demonstrates a distinctive fluctuation pattern, seemingly disconnected from the SPEI values. Despite the conventional understanding that NDVI tends to mirror relative variations in precipitation and evapotranspiration in the oases of Fezouata, Ktaoua, and Mhamid, the Mezquita oasis exhibits a divergent behavior, hinting at underlying complexities in their ecosystem dynamics in this oasis. This independent oscillation suggests the presence of additional factors influencing vegetation health and density namely the use of groundwater and Mansour Eddahbi dam releases for irrigation.

3.6. Correlation analysis between vegetation density and health with dam water availability

The examination of the correlation between NDVI and dam releases exposes varying degrees of association across different oases within the MDV as illustrated by Fig. 9. In Tinzoline, Ternata, and Fezouata oases, a significant positive correlation is evident between these parameters, with correlation coefficients ranging from 0.6 to 0.66. This correlation underscores a significant relationship between vegetation health and the volume of water released from the dam in these regions. Conversely, a low correlation is observed for the oases of Ktaoua and Mhamid, with correlation coefficients of 0.49 and 0.36, respectively. While still indicative of a connection between NDVI and dam releases, the strength of this relationship is slightly lower compared to the previous oases. However, the case of Mezquita oases presents a stark departure from these trends, with a correlation coefficient of only 0.08. This indicates an almost negligible correlation between NDVI and dam releases in Mezquita, highlighting unique environmental dynamics or management practices that differ significantly from other areas within the MDV.

Droughts are widely acknowledged for their substantial impacts on the social economy, environment, and humans compared to other natural disasters (Gebremeskel Haile et al., 2019). The results indicate an increase in droughts throughout the Draa oases between 1995 and 2022 due to reduced precipitation and increased evaporation (Karmaoui, 2015; Ouysse et al., 2012). The frequency of droughts in the Draa region is more than 65% indicating that the majority of the time this region records low precipitation. As discussed, the MDV is located in the Southeast of Morocco, and because of the dry conditions, the majority of land in the MDV is arid. The findings of this investigation also revealed that less than 7% can be classified as moderately wet in all oases, as shown in Table 4. This suggests that the region predominantly experiences low levels of precipitation and high temperatures, leading to prolonged periods of dryness. This dryness affects the main economic activity, agriculture, in the MDV, especially during the last decade, confirming the predictions of local research (Johannsen et al., 2016).

The SPEI of the MDV showed a decreasing trend from 1995 to 2022, indicating an aggravation in drought frequency. Moreover, the results of the M–K test show that the SPEI decreased significantly after 1995. This phenomenon arises from the effects of climate change, characterized by rising temperatures and diminishing precipitation patterns in the region of MDV as confirmed and predicted by recent local studies (Karmaoui et al., 2019; Ouysse et al., 2012). Analyzing the difference in SPEI patterns between the different oases shows that the climate in all regions follows the same evolution (decrease trend). Consequently, the SPEI is a useful indicator of climate change effects on the NDVI dynamics (Luo et al., 2020). Remote datasets were used in this analysis to compute changes in the NDVI, which showed a slight decreasing trend (0.001/a) for the average NDVI from 1995 to 2023 in Fezouata, Ternata, and Tinzoline oases, indicating a decrease in density and health of vegetation, which is consistent with the results of previous studies (Johannsen et al., 2016; Moumane et al., 2022). For Mhamid and Ktaoua oases in the extreme south of the Draa Valley, the finding indicates that there is a general stabilization of NDVI at low levels compared with other oases (between 0.12421 and 0.0505). This finding is attributed to the considerable distance of these oases from the Mansour Eddahbi Dam, preventing them from benefiting from its low flow. In contrast, the nearby northern oases downstream of the dam (Mezquita, Tinzoline, and Ternata) exhibit significantly higher NDVI values (between 0.0734 and 0.2196) due to the dam's releases. The correlation between NDVI and dam releases confirms the dam's influence on vegetation health and density. The case of Mezquita illustrates the different patterns compared to all other MDV’s oases where the NDVI shows a general increasing trend between 1995 and 2019 indicating an increase in agricultural production because of overexploitation of groundwater as illustrated by a decrease in the level of the water table (more than 8m between 2010 and 2021) and low flow of the dam upstream. The results of the study indicate that the common factor among all oases of the MDV is the collapse in both the density and health of vegetation over the last decade especially after 2015 and after 2019 for Mezquita Oasis. The collapse is due to the succession of years of drought recently, indicated by the SPEI curves, the beginning of the depletion of the non-renewable groundwater reserve, as presented in Fig. 8, and insufficiency of the reserve of dams upstream of MDV to ensure the demand for increased water needs for irrigation (Johannsen et al., 2016).

For the correlations between vegetation indices (NDVI) and climate indices (SPEI), the results indicate that the correlation between the NDVI and climate factors presented a certain dependence variation, revealing that certain climate extreme indices had stronger impacts on vegetation in the Draa oases, particularly for the Fezouata, Ktaoua, and Mhamid oases. However, a notable exception arises during the period between 2010 and 2018 in said oases. Despite several successive years of drought, it is intriguing to observe that the NDVI consistently maintains values above average. This phenomenon can be attributed to the intensification of agricultural activity, depicted by NDVI evolution curves in Fig. 7, particularly through the overexploitation of groundwater resources for irrigation by increasing groundwater pumping (Ait Lamqadem et al., 2018; Karmaoui et al., 2016), resulting in a notable decline in the water table level in the region, as depicted in Fig. 8 and predicted by last studies (Johannsen et al., 2016). Otherwise, the finding indicates also that NDVI of Tinzoline and Ternata oases depend not only on precipitation and evapotranspiration (SPEI) but also on other parameters namely releases of dams upstream and the use of groundwater. During the majority of the period from 1995 to 2023, notably between 2009 and 2018, an observed disparity in the evolution of NDVI and SPEI emerged, with NDVI indicating high vegetation density during periods of extensive drought in said oases. This situation is due to the intense agricultural activity in regions by using groundwater and Mansour Eddahbi Dam releases for irrigation. On the other hand, in Mezquita oases located just upstream of Mansour Eddahbi dam, NDVI presents an evolution independent of that of the SPEI indicating that vegetation in said oasis depends on the low flow of the dam upstream and the use of groundwater for irrigation.

Furthermore, the correlation between the health and density of vegetation with the release of the dam indicates a relatively strong correlation with the vegetation of the oases of Tinzoline, Ternata, and Fezouata (R greater than 0.6). The relatively strong correlation coefficients, exceeding 0.6, indicate an association between the volume of water released from the dam and the vegetation dynamics in these areas. This implies that changes in water releases have a notable impact on the health and density of vegetation within these oases. Otherwise, the southern oases, Ktaoua and Mhamid, have a relatively positive low correlation compared to the previous oases of the north (R equal 0.49 and 0.36 respectively). The oases in the northern part of the MDV benefit more from dam releases compared to those in the southern regions. This advantage is due to their proximity to the Mansour Dahbi Dam, which ensures a more consistent and reliable supply of water for irrigation, and that is justified also by NDVI which is higher in the north oases than those of the south. The particular case of the Mezquita oasis, which comes just downstream of the Mansour Eddahbi dam, presents neither a correlation with the releases of the dam nor with SPEI in the majority of the period studied. This is due to a flow rate that comes from the dam during a large period of the year and the use of groundwater for irrigation according to the investigation with the population of the region.

For the evolution of the density and health of date palms, the study targets the summer season, the season of predominance of date palms as the main agricultural activity in MDV as it was confirmed by site visits. So, the evolution of the NDVI of summer over the years reflects more the evolution of the health and density of date palms. The results of the analysis illustrate that the trajectory of the NDVI of the summer season presents almost the same evolutionary characteristics as the evolution of the average NDVI with values lower than the average. This difference between the summer and average NDVI is due to LULC in the other seasons of the year where we have, as was noted during the site visits, in addition to date palm, and other crops such as alfalfa, vegetables, and wheat. It is for this reason that the greenness is high in other seasons compared to the summer. Concerning its evolutions over the years, we noticed the same patterns as the average. This reveals that date palms are influenced like other crops by climate change and pressure on water resources. As date palm can grow in very hot and dry climates it requires abundant underground water near the surface or access to irrigation (Chao & Krueger, 2007). The beginning of exhaustion of groundwater resources in the last few years, as predicted by (Johannsen et al., 2016) and illustrated by Fig. 8, and the shortage of surface water resources, indicated by the SPEI, justify the collapse of date palm texture in the last decade. Additionally, with the same current drought conditions, MDV date palms will continue to experience increasing and unprecedented collapse in the coming years.

This paper examines the dynamic characteristics of drought and vegetation cover in the MDV from 1995 to 2023 and assesses the interactions between the two, along with the influence of dam releases and groundwater on vegetation. On an inter-annual scale, SPEI showed a fluctuating and decreasing trend over all oases whereas NDVI presented fluctuating and different trends from one oasis to another. The NDVI of the oases of Fezouata, Ternata, and Tinzoline showed average downward trends from 1995 to 2023, indicating the degradation of crops, while those of Ktaoua and Mhamid showed general stability trends during this period, except after 2015 when we noticed a notable decrease. Otherwise, The NDVI of Mezquita Oasis showed a general upward trend until 2019 when NDVI began to decrease exponentially indicating the collapse of crops in the area. In addition to the downward trend of the SPEI, the evolution of dam releases upstream and the water table level in the valley also show a downward trend, worsening the water situation and consequently leading to vegetation degradation.

The correlation between NDVI and SPEI of all oases indicates that vegetation health and density depend on precipitation, the Mansour Eddahbi Dam releases, and groundwater. This finding is confirmed by decreasing water table levels and the correlation between dam releases and NDVI, particularly for the Tinzoline, Ternata, Fezouata, Ktaoua, and Mhamid oases. The northern oases benefit more from the dam releases than those in the south. The case of Mezquita Oasis, located just downstream of the dam, is particular. The said oasis benefits more from the continued low flow of the dam as the investigation with the local population confirms it. But in the last decade, the study finds that there is a collapse of vegetation, indicating the degradation of crops, in all the oases of MDV which is attributed to the decrease of the volume of dam releases and depletion of groundwater.

The research results offer a vital theoretical foundation for the planning, management, and development of regional vegetation conservation, agriculture, forestry, and animal husbandry in Morocco. So as a recommendation, this ecosystem and water situation in the MDV is a warning to decision-makers of an imminent environmental, economic, social, and cultural crisis in this valley in the coming years due to its importance at all levels. The impact of ecosystem degradation has had and will continue to have negative effects, as agriculture, tourism, culture, and community are directly linked to the green beauty of the valley. There is only one option to be taken today, otherwise we will lose all possibilities of some mitigation of this situation in this valley: it is to take action. The only solution is to determine the priority of economic activity in the region and to take environmental disasters seriously as a menace to life. The only solution that appears the most practical to avoid further degradation of the oases in the short term is to conserve the rest of the underground water resources only for the irrigation of the rest date palms to maintain a certain continuity of life. The solution requires political will that proposes another economic activity in the region, other than agriculture, which allows the population to continue to settle down and live, reduces pressure on limited water resources in the region for irrigation, and advances the reflection on the strict policies of water to be adopted in the oases of Morocco in the long term.

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Funding Declaration

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Author Contribution

A.M. wrote the main manuscript. J.A., M.A., and M.D.H. reviewed and provided revisions to the manuscript. All authors read and approved the final version of the manuscript.

Acknowledgement

We extend our sincere gratitude to the Hydraulic Basin Agency of Draa Oued Noun in the name of Mr Aziz Aouijil, Head of the Water Resources Monitoring, Evaluation, and Planning Department, for their invaluable support in providing the necessary data to carry out this study. Their collaboration and provision of comprehensive data were crucial to the successful completion of our research. We greatly appreciate their commitment to supporting scientific inquiry and advancing our understanding.

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No competing interests reported.

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Assessing the Impact of Drought and Upstream Dam Construction on Agriculture in Arid and Semi-Arid Regions: A Case Study of the Middle Draa Valley, Morocco

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Abstract

Figures

1. Introduction

2. Material and methods

2.1. Study Area

2.2. Precipitation and temperature data

2.3. piezometer data

2.4. Data of Mansour Dahbi releases

2.5. Remote Sensing Data

2.6. NDVI Time Series Construction

2.7. SPEI Index

2.8. Mann–Kendall Test

2.9. Season of palm trees

2.10. Correlation Analysis

2.10.1. Correlation between vegetation density and health (NDVI) with drought (SPEI)

2.10.2. Correlation between vegetation density and health (NDVI) with Dam water availability

3. Results

3.1. The temporal evolution of vegetation of Oases

3.2. The Spatial and Temporal Distributions of Palm Trees

3.3. The Temporal evolution of Drought of oases

3.4. Temporal evolution of water table level in MDV oases

3.5. Correlation analysis between vegetation density and health with drought

3.6. Correlation analysis between vegetation density and health with dam water availability

4. Discussions

5. Conclusion

Declarations

Funding Declaration

Author Contribution

Acknowledgement

References

Additional Declarations

Status:

Version 1