grondwater = ground water
verhang = fall, drop (van Dale) = level difference

see refs:

USGS SIR 2006-5220: Ground-Water Levels in the Ozark Aquifer along ...Ground-Water Levels in the Ozark Aquifer along the Viburnum Trend, ... Comparing monthly precipitation and daily mean water-levels differences at these two ...
pubs.usgs.gov/sir/2006/5220/sir5220abs.html

SOFIA - Circular 1275: Development of Water-Management System and ...Differences in average ground-water level conditions in northern Palm Beach County during the dry season in October 1970-74 and 1990-94 are similar to the ...
sofia.usgs.gov/publications/circular/1275/gwchange.html

...this is definitely NOT a mere level difference; it is measured in m/km.

Verhang
Het verhang is het relatieve hoogteverschil van een watergang, uitgedrukt in m/km (meters per kilometer). Zo is het (gemiddelde) verhang van de beek de Geul 4,3 m/km, zijnde 250 m (het totale hoogteverschil) : 58 km (de totale lengte).

http://www.waterland.net/index.cfm/site/Nederland Waterland/...

Ref. 1

Discharge and Velocity
The rate at which groundwater moves through the saturated zone depends on the permeability of the rock and the hydraulic gradient. The hydraulic gradient is defined as the difference in elevation divided by the distance between two points on the water table.

Ref. 2

Groundwater movement is dependent on the degree of interconnection of the porous space
(permeability) and the gradient or slope of the water table. These factors vary greatly depending
on the aquifer type. Groundwater in a carbonate aquifer can occasionally move through limestone
caverns as rapidly as surface water (1-3 ft/sec). In sandy aquifers, groundwater can move as
slowly as 3 ft/day or even as slowly as 1 in/day. For those reasons, groundwater cleanup, or
remediation, can only be done by enhanced methods.* These methods are complex and
expensive, making groundwater pollution prevention quite economical. There are obvious health
protection reasons for groundwater pollution prevention, as well.