Despite that, the river discharges alter along wet and dry seasons and logically it is not possible, but due to the change of climate, we can consider the streamflow had been flown in somehow constant. Based on the table you attached, it seems that there is a mistake because the values are significantly variance, or there is a monotonic trend in your results.

Mohammed Nanekely I read that Mann-Kendall test is not affected by outliers so doesn't that cover the significant variance part? I didn't read anything about the test being affected by data with significant variance. The table shows the data for the month of August (wet season) for 30 consecutive years if that helps. I still can't understand the issue.

Harsh Upadhyay surely MK test is not affected by outliers like the one of value (4118.688) of the 18th year, but the rest of the data lies within the limits (0 to almost 2000). I see your result seems to be a monotonic trend as we expected previously.

Mohammed Nanekely So what do you propose as a solution for the said issue. I have tried standardising the data to reduce the range but that didn't work as the test yielded the same result (Z=0). If the data does have a monotonic trend, why isn't the test reflecting the same? Is there a way to make this time series data identically distributed?

I see either the data have been recorded (gathered) mistakenly, or you can sort out the data into quartiles & deciles, and look what do the results show?

another way you can use is ''The Seasonal Kendall (SK) test'' which is a special case of the Mann Kendall Trend Test for a specific time period as your case of August only.