Documentation question/ error in the InVest Seasonal Water Yield Model


When I was reading the documentation for theory of the Seasonal Water Yield Model, there seems to be something wrong in the formula of the Quick Flow QF.

To calculate the Quick Flow, the formula involves the function of E1, where E1 is the exponential integral function.

But in the documentation, the calculation of E1 is written as: 𝐸1(𝑡) =∫︀𝑒^(−𝑡)/𝑡 dt, with the limitation of (1,∞).

However, after I checked with the source code of the model and found that the integral function E1 maybe should be written as: 3
which means that the formula of the E1 in the documentation is written wrong.

Can somebody answer my doubt?
Thanks a lot!