“…Recently, several methods have been developed to determine the NRS for a specific area, including spatial-temporal variability analysis [18,23,[26][27][28][29][30], geostatistical [31][32][33], stratified sampling [34,35], and bootstrap methods [20,[36][37][38][39]. The spatial-temporal variability analysis usually uses some statistics, such as relative difference or correlation coefficient, to determine the NRS and the representative sites [18,23,26,27,30,40].…”