11. Spatiotemporal Variability and Mapping of Groundwater Salinity in Tadla: Geostatistical Approach

Agricultural productivity may be constrained by many factors such as water scarcity, soil degradation, and use of marginal quality water. In Morocco, the main degradation processes occurring for irrigated areas are on-site impact (soil salinization and/or alkalinization) and off-site impacts (pollution of groundwater by salts and nitrates). Since 1995, the Moroccan Public Irrigation Agency has installed and maintained a network of soil and groundwater monitoring stations. For the present study, we selected the soil and water sampling sites on spatial representativeness in the perimeter, the main soil types, and the hydrogeological variants. Soil salinity, alkalinity, and sodicity as well as groundwater salinity, nitrates, and water table level were recorded to determine spatiotemporal variability and dynamics of groundwater salinity. A good understanding of its evolution in space and time will make possible to obtain reliable models for spatiotemporal prediction, estimation of the missing data, cartography, and over the long term for the delineation of risky zones. The spatiotemporal analysis of the groundwater salinity shows the presence of a strong spatial dependence and a weak temporal dependence. The spatiotemporal dependence of the residuals is very weak and primarily consists in random fluctuations. Consequently, a simple model was adopted, containing two components: a spatial component explaining more than 50% of the total variability of groundwater salinity and a temporal component that explains almost 77% of the remaining variability. Overall, this model explains more than 90% of total observed variability. Cartography of the average groundwater salinity was also established by kriging, by computing mean spatial variograms on the basis of per site data. The spatial variogram of the northern area was adjusted by the Gaussian model characterized by a sill of 3 dS²/m² and a range of 12,526 m, while the southern area was adjusted by a Gaussian model with a sill of 0.2 dS²/m² and a range of 9,674 m, with a nugget effect of 0.06 dS²/m².