Groundwater level spatial variability distribution and risk assessment in a sparsely gauged basin using the Spartan covariance function and auxiliary information

Περίληψη

Knowledge of the spatial variability of the water table level in aquifers that are scarcely monitored provides information to understand the aquifer behaviour at different locations of the basin. This information becomes more important in basins that are under the threat of over-pumping where the water table level has fallen significantly. The spatial distribution feedback gives the potential to identify vulnerable locations. The spatial variability of the water table level in this work is based on hydraulic heads measured during the wet period of the hydrological year 2007-2008, in Mires basin of the Messara valley in Crete, Greece. Three different approaches are used to estimate the spatial variability of the water table level in the basin. All of them are based on Kriging methodology. The first is the classical Ordinary Kriging approach, the second involves information from a secondary variable in terms of Residual Kriging and the third calculates the probability to lie below a certain groundwater level limit that could cause significant problems in groundwater resources availability. The latter is achieved by means of Indicator Kriging. A recently developed non-linear normalization method is used to transform both data and residuals closer to normal distribution for improved prediction results. In addition the recently developed Spartan variogram model is applied to determine the spatial dependence of the measurements. The latter proves to be the optimal model, compared to a series of models tested, that provide in combination with the Kriging methodologies the most accurate cross validation estimations. Groundwater level and probability maps are developed providing the opportunity to assess the spatial variability of the groundwater level in the basin and the risk that certain locations have in terms of a safe groundwater level limit that has been set for the sustainability of the groundwater resources of the basin.