In certain cases the observations display a definite trend that needs to be taken into account. This is the case, for example, when one needs to interpolate piezometric heads observed from wells in an aquifer system where a regional gradient is present. To remove the trend from the data it is possible to work with residuals (= measurements - trend) that have a constant mean. However this detrending procedure may be dangerous as it may introduce a bias in the results. For this reason it is important tha the trend be recognized not only from the raw data but also from the physical behavior of the system from which the data come. If the trend cannot be removed from the observations by simple subtraction, the universal kriging approach (2,3) can be used.