Is there a downwash equation?
What you probably are looking for is the Biot-Savart law.

This is originally describing the magnetic field generated by an electric current; however, it was soon found that it equally well describes the induced speeds caused by a vortex. The vortex is the bound vortex of the wing plus the free vortices trailing behind it. Integration over their length will give the induced speed at a given point. This speed is proportional to the inverse of the distance to the vortex squared. The vortex strength depends on air speed and span loading; the proportionality being with the inverse of flight speed, aircraft mass and the inverse of wingspan squared.

There is also a dependency of downwash speed with the inverse of air density at the altitude where the glider flies, but this will be canceled out when the effect at ground level is calculated. What is left is an inverse dependency on the local air density at ground level.

Note that you will only get the induced speed, not the pressure from the Biot-Savart law. In order to calculate the pressure increase, you need to add the equation for the pressure rise of the downwash interfering with the ground. This pressure is proportional to the downwash speed squared and air density, so now all density effects are canceled out and you will be left with those factors for the pressure rise from an aircraft flying overhead:


the square of the induced downwash speed


which depends in turn on:


the inverse of height squared
aircraft mass
the inverse of wingspan squared
the inverse of flight speed


Note also that this does not consider viscous effects.

In order to know the limit of detectability, you also need to know the sensitivity of your measuring equipment.