it would helpful to share the link to the original paper as well.

Osman Zakaria Wohor I edited the question. Thank you for your suggestion.

Hong Luan Luan Anyway, I found the paper.

Here is my take: the annual rates of P leaching is measured as (P Kg ha−1 year−1). this expressed in the above formulas as Pgw. So, Pgw= qauntity of P leached to ground water [kg/(ha*a)]. thus the a=annum =yearly=per year=year−1 This is also known as annual P lost in the soil.

see this article for clarity,

Hussain, M.Z., Hamilton, S.K., Robertson, G.P. et al. Phosphorus availability and leaching losses in annual and perennial cropping systems in an upper US Midwest landscape. Sci Rep 11, 20367 (2021). https://doi.org/10.1038/s41598-021-99877-7

Dear Hong Luan Luan ,

As indicated by Osman Zakaria Wohor “a” stands for annual. I see that the literature uses the following ways to describe the units, all mean the same:

kg/(ha*a)

kg/(ha*year)

kg ha−1 a−1

kg ha−1 yr−1

To confirm what is already stated, see page 28 here (see also enclosed file):

http://www.eurobioref.org/images/deliverables/D9%201%203_230713_Quantis_VF.pdf

Where they mentioned in the formula:

Pgw = quantity of P leached to ground water [kg P/(ha.yr)]

So, “a” means yr.

Best regards.

PS. Google Pgw = Pgwl * Fgw and you find numerous studies using this formula.