[Therion] Typical Head of Water between entry and exit of sumped passages

[Footleg]

I am working on a master survey of the Kingsdale System in Yorkshire,
UK. There is a substantial submerged portion to the system. Over a
rough distance of 1500m of sumped passage, what difference in water
levels could be expected between the upstream and downstream ends of
the sump?

The diving data just records water depth, so it computes both ends of
the sump as having identical water levels. We know in flood conditions
the upstream sump level backs up considerably. But assuming the diving
is done in conditions of low flow, what typical difference might be
reasonable to expect in the altitude of the resurgence and of the
upstream end of the sump (determined by survey data from an entrance)?
My data shows a significant difference, but I do not know how much of
that difference might be expected and how much is due to errors in the
dry passage survey and entrance altitudes?

Footleg