Analysis of the effect of climatic variability and increasing water demand on the Saïss aquifer, Morocco

. Climate variability has a significant impact on the water table of the Saïss plain, which relies heavily on groundwater for agricultural irrigation and drinking water supply. The study of the spatio-temporal evolution of the groundwater level requires a global and integrated approach that considers both natural processes and human activities. Satellite imagery has provided information on land use and land cover that influences groundwater recharge. The GIS has facilitated the integration of different datasets, allowing spatial analysis, mapping and identification of vulnerable areas to depletion. Geostatistical techniques are used to analyze collected data, identify trends and quantify the relationship between groundwater levels and various influencing factors. The results of this study have shown that the depth of the water table of the Saïss plain between 2000 and 2020 has generally decreased by several metres in the NW and the centre zone, but has improved by a few metres or remained constant in the other areas. This information is valuable for developing sustainable water management plans that take into account both environmental and societal needs.


Introduction
Variations in precipitation patterns, increased frequency of droughts and changes in temperature have all affected the groundwater reserves of the Saïss plain, which are heavily used for agricultural irrigation and drinking water supply.Indeed, the Saïss plain has undergone significant agricultural and industrial development, leading to an increase in the water demand for irrigation, industrial and domestic use.This has led to a decrease in piezometric levels, reducing the amount of water available for irrigation and other uses [1].
Monitoring changes in land use over a given period at regional level is one of the main requirements for analyzing climate change.Land use influences hydrological processes in various ways, including transpiration or interception, infiltration, evapotranspiration and flow, which are classified as loss processes in a watershed's water balance [2,3].
Vegetation plays an important role in the flow of water.But this role varies depending on the seasonal cycle of vegetation, the state of vegetation (density and height) and the saturation state of soils before the rain [4].It has been confirmed that the change in land use has a significant impact on climate through various pathways that modulate surface temperature and precipitation.Several research are based on vegetation index and surface temperature for soil moisture determination and vegetation water stress such as the work of [5][6][7].
On the other hand, measurements of groundwater levels from observation wells are a source of accurate information for sustainable use and planning of an aquifer; however, these series are often available with irregular time intervals and not always continuous in space.Therefore, accurate modelling of groundwater levels in unsampled locations is required to make an accurate database.For a proper management and an optimal exploitation and rational use of water, it's necessary to understand and study the behaviour of aquifer systems [8].This analysis requires a regionalized map of the piezometric level as a basic tool.It is generally serves as a reference for hydrogeological and environmental studies.It enables the comprehension of the morphology, geometry and hydrodynamics of the aquifer [9,10].
In order to ensure a good management of these water resources, it is necessary to have complete and accurate knowledge of the state of the region's aquifer systems.For this purpose, we conducted a geostatistical analysis of groundwater level data.In this study, several steps were taken: Mapping land cover and land use from remote sensing data, analysis of hydro-climatic parameters and spatio-temporal assessment of groundwater depth based on piezometric measurements from 2005 to 2020.

Study Area
Saïss plain is located in the North of Morocco with an area of approximately 2260 km², 95 km long and 30 km wide, between the coordinates Lambert: 460 < X < 553 km and 335 < Y < 385 km.The basin of Fez -Meknes is an important part of the Sebou watershed, and houses two of its largest cities, as well as several of its centers.It forms the central part of the Southern Rifan groove that spreads between two large structures, namely the Rifain area, and the Middle Atlasic Causse (Fig. 1).

Fig. 1. The geographical location of the Saïss plain
From a demographic perspective, 2014 Census data shows that the overall population is 2335228, compared to 1568504 in 1994 (Table 1), with an overall growth rate of 1.99% [11].

Methodology
To assess the impact of climate change on groundwater resources, groundwater level data from 41 observation wells for the Saïss plain and monthly average precipitation data were used for the analysis.These data on piezometric levels, temperatures and precipitation were obtained from the Sebou Hydraulic Basin Agency (ABHS).The approach used in this work is based on the following steps: -First, an exploratory analysis of the spatial data was performed, followed by a data processing and quality control step.This allows the distribution and spatial correlation of the variables to be examined, the normality of the measured datasets to be verified, and the overall trends of these datasets to be identified.
-Analysis of trends in hydro-climatic changes, because the identification of the climatic context of the Saïss plain is of paramount importance, given its direct influence on the hydrological and hydrogeological cycles of the study area.
-Land use mapping using satellite images: Landsat images were used to map land use and vegetation cover.The analysis concerns two scenes for the years 1988 and 2019.These images are acquired from the Global Land Cover Facility (GLCF) and the United State Geological Survey (USGS).The analysis of land use changes is carried out using a supervised classification by the maximum likelihood algorithm.
-Changes in groundwater levels in the plain were analysed by creating a groundwater level map using geostatistical methods: Several spatial interpolation methods were tested and compared: deterministic methods (Inverse Distance Weighting (IDW), Radial Basic Function Array (RBF), and Polynomial Local Interpolation (PLI)), single-variable probabilistic methods (ordinary krigeage (OK), universal krigeage (UK) and (EBK)) and multivariate probabilistic methods (ordinary cokrigeage (OCK) and universal cokrigeage (UCK)) for spatial interpolation of the groundwater level in the Saïss plain.All comparisons between models are evaluated based on indicators of statistical precision provided by cross-validation [10].
-Temporal variability of groundwater resources: The hydrodynamics of the Saïss aquifer depends on several natural and anthropogenic factors.The water mainly used for irrigation and drink, sometimes excessively, exposes this aquifer to overexploitation which threatens their sustainability [1].

Climate change trends
An analysis of the trends in hydro-climatic data shows that the region's climate has experienced a long-term drought since the early 1980s.Figure 2 shows the evolution of interannual rainfall at the Fez-airport station.The inter-annual distribution of rainfall recorded several successive dry months (25 deficit years versus 19 surplus years) (Fig. 3).These droughts have had a negative impact on agriculture and surface and groundwater resources.The Saïss plain has a dry and warm Mediterranean climate.

Changes in springs flows
Some 20 springs are currently being measured in the Saïss plain.This reduced number is due to the fact that some springs have dried up (Ain Chegag, Ain Kharrouba, Ain Boukhnafer, Bergama Sghira, etc.) [1].Seasonal fluctuations and general downward trend in spring flows are important (Fig. 4).The total spring flow measured in 2020 is about 4 m 3 /s, compared to 7 m 3 /s measured in 2008.The drought and the exploitation of groundwater in the Saïss aquifer are responsible for the decrease in the overall spring flows (about 40% decrease).This decrease can be explained by the rainfall deficit that has persisted for several years.

Land Use Mapping
Changes in land use can lead to modify in hydrological processes, both quantitatively and qualitatively [12][13][14][15].For the study of the land use variation in the Saïss plain, 2 Landsat images were processed (1988,2019).Five land use classes have been identified: arable land, water surface, rangeland, urban area and arboriculture/irrigated (Fig. 5).
The results show the extension of the surface of arboriculture/irrigated (including olive trees).The water resources used for irrigation are essentially of underground origin (pumping).One of the consequences of the extension of arboriculture is that rangeland and arable land have decreased by 80% and 6% respectively between 1988 and 2019.

Spatial variability of groundwater resources
The groundwater level time series cover 39 wells in 2005 and 45 wells in 2020.After applying and comparing the different methods of spatial interpolation, namely deterministic and probabilistic methods, ordinary kriging is more accurate than the other methods.
Figure 6 shows the regionalized maps of the depth of the water table of the Saïss plain estimated by the optimal method ordinary kriging (OK) for the years 2005 and 2020.The analysis of this maps shows that groundwater flow is generally from the SW to the NE.In addition, these maps provide global information on the flow rate.It is high in the aquifer's NE and moderate in the SW.The average hydraulic gradient is 5‰.

Temporal variability of groundwater resources
The piezometric evolution is based on the analysis of the data of the Saïss plain piezometric network, developed by the services of the ABHS.For the comparison of water table depth between 2005 and 2020, we have produced a map of groundwater level differences between 2005 and 2020 (Fig. 7).The analysis of the variation in the depth of the aquifer has shown that, in general the state of the aquifer has dropped in the NW and in the middle, on the other hand it has improved or remained constant in the other zones.
In the deficit zone, piezometers are mainly located in the irrigated area with surface water.For example, at piezometer 237/15, the depth of the water table experienced significant fluctuations with an upward trend of 15 m.There is an almost general decline in the piezometric levels of the water table due, on the one hand, to the reduction in natural inputs (rainfall deficit) and, on the other hand, to the increase in agricultural pumping.

Conclusion
This study shows the contribution of the geostatistics in the water depth levels spatialization, and the assessment of the state of groundwater resources.Analysis of land use maps for the period 1988-2019 showed that the region has experienced significant variation over the past 30 years.There is an increase in the surface of arboriculture/irrigated and urban areas to the detriment of arable land and rangeland.The depth of the Saïss aquifer between 2005 and 2020 shows a general drop of several meters in the NW and in the middle of the study area, however it improved by a few meters or remained constant in the other areas.To meet the challenge of conserving water resources, sustainable water management practices are crucial.This includes the promotion of water-efficient agricultural techniques, the implementation of water conservation measures, investment in alternative water sources such as treated wastewater and the development of adaptation strategies to climate variability, such as better irrigation programming and the use of drought-resistant crops.In addition, regulations and monitoring systems must be in place to ensure responsible use of water and to prevent overexploitation of groundwater resources.

Fig. 7 .
Fig. 7. Map of the piezometers with the difference in groundwater level between 2005 and 2020.

Table 1 .
Population trends in the Saïss plain.