I'm comparing concentration C of a contaminant in the same spatial region at two time point 2000 and 2010 with sample size of N_2000 = 51 and N_2010 = 26 (not all the samples are from the same location), mean of mu(C)_2000 = 47 and mu(C)_2010 = 27 (determined by block kriging of all point observations using the 'gstat' package of R) and variance of V(C)_2000 = 89 and V(C)_2010 = 68 (kriging variance). To determine if there has been any significant change over the last 10 years, we first need to determine the variance of change in the area:
V(delta C) = V(C)_2000 + V(C)_2010 - V(C)_2000,2010
where, V(delta C) is the variance of change over time; and V(C)_2000,2010 is the covariance between the two temporal samples. Does anyone know how to determine the V(C)_2000,2010 term in the above equation?