"decrease within" is a perfectly valid and usable phrase in written English.
You typically use it to describe a decrease in value or amount over a certain period of time. For example, "Fuel consumption has decreased within the last 18 months."
It is appeared in velocity decrease within inter-joint blocks.
With this medication regimen, the hypomanic syndrome will probably decrease within a short period of time.
In the subsoil, soil water retention showed a gradual decrease within each horizon.
Since hydrate dissociation is an endothermic reaction, faster hydrate dissociation causes more rapid temperature decrease within the hydrate zone.
The values of β decreased logarithmically with increasing settlement, showing a marked decrease within the initial settlement range.
The Ministry of Finance gave notice in November 2003 that the proportion of temporary posts should decrease within the public administration, including universities.
Mid hepatic IVC long-axis revealed a correlation between the time after donation and CI ( 20% CI decrease within 5 minutes until 0% after 20 minutes).
We tracked the dissolved Al regularly during the exposure period and observed a rapid decrease within the first hour (Table 2).
Increasing propagule sources heterogeneity and connectivity appeared to decrease within-metacommunity similarity within landscapes.
The average of the two 24HRs was used because it is a commonly used, albeit naive practice to decrease within-person variation in the estimated usual intake.
To decrease within-sample variation possibly introduced by collecting manipulated ants across different days, three individuals that were found manipulated on the same day were used for each sample type.
