Essentially, estimation of a regionalized variable is to calculate the variability of the parameter. Two common functions, covariance and variogram, which are based on different hypotheses, are calculated for the variographic analysis of a variable using statistical methods. The variogram is however, calculated based on a less restrictive hypothesis and hence is applicable for most of the hydrogeological parameters. E represents the expected value. The variogram is also calculated in different directions by taking the pairs in that particular direction only.
N d is the number of pairs for a particular lag and direction.
An experimental variogram is usually an irregular curve due to large value of lag and tolerance. The transmissivity variogram often shows a nugget effect whereas the water level does not. A variogram being a square function, is always positive starting from the origin. It attains a maximum value or sill after a certain distance, or lag, which is called the range. However, sometimes due to high variability, a sudden rise appears very near to the origin.