The phrase "a downward gradient" is correct and usable in written English.
It can be used in contexts related to physics, mathematics, or geography to describe a slope or decline in elevation or value.
Example: "The road has a downward gradient, making it easier for cyclists to gain speed."
Alternatives: "a declining slope" or "a negative gradient."
At the bottom of the cliff, the trail takes on a gentler downward gradient through an open canyon.
With some exceptions e.g., large apartment complexes in downtown Chicago people do not reside in the downtown areas, and there is a steady downward gradient in population density per unit area (and more open land and single-family residences) as one moves from the inner city toward the open country.
A downward vertical gradient of roughly 0.01 down to 200 m b.g.l. was observed and reproduced by calibrated models.
A downward hydraulic gradient also occurs in summer, but the increased pumping throughout the model domain induces horizontal flow from the deeper part of the aquifer, including down gradient areas to the southwest.
The limited pond data available from Char Para indicate a more pronounced difference of 2 m between the pond and groundwater (downward gradient of 0.2) in June 2009 and no significant gradient later in August through October.
Also, the increased contribution of shallow water reaching shallower supply wells as a result of enhanced downward gradients during the summer pumping season might substantially exceed the deep storage of any wellbore flow that occurs during winter idle periods.
In effect, an overall management strategy of generally decreased pumping in areas of downward gradients and generally increased pumping in areas of upward gradients could help optimize wellbore flow to reduce arsenic concentrations while having little effect on total withdrawals.
During the year, reversals in vertical head gradient yield reversals in arsenic temporal trend, and downward gradients in the dry season correlate with increases in arsenic.
