I couldn't find any direct reference to when it was drained. Earlier in the thread it said early 1960s. The OS map dated 1966-68 shows it there, but that would have been the publication date, and survey would probably have been a couple of years earlier, and, though it would only have been a rough addition of some changes, I would have expected it to have noted the disappearance of a reservoir!. The 1976 map shows it gone (with images of 6 trees in the middle of where it was), so early to mid 1960s would seem to be about right.
On re-reading this thread recently I recalled some stuff I researched back in the late 90's when I too was looking at the remnants of the old reservoir. The subject resurfaced again last week when I noted boarding being erected around the old site of the scrapyard which occupied the former reservoir site.
Clearly the recession is over and old brownfield sites suitable for housing are back in demand and perhaps Reservoir Mews might be the new must have address in coming years.
My research was in part to investigate how the watercourses in the area had been altered and how some may have been used to feed not just the reservoir but some of the extant local ponds.
Metchley pool being one of them bounded by Cross Farm Road and Bantock Way.
During a visit to the Institute of Hydrology in Wallingford we were treated to a demonstration of a smart piece of software which divided the UK into one metre squares and represented the direction of surface and sub-surface water flow by a simple arrow point using the main eight cardinal compass points. In the few minutes allocated I managed to zoom into my home location and rapidly drew a few of the arrow heads on the palm of my hand for later reference.
The attached picture is a melange of three images taken from the 1914 OS 2nd edition, the 1953 OS with contour lines emboldened and an earlier OS reprint where the "hills" are hachured with Metchley Pool centred. Grateful thanks to the Library of Birmingham and the excellent Library of Scotland online map resource.
https://images.birminghamhistory.co.uk/coppermine/albums/userpics/10449/Metchley_Pool_1.jpg
The purpose being to define the original watercourses that fed the pool and how they flowed from the Harborne ridge southwards to feed the pool which lay within the boundary of Metchley Abbey and was probable dammed along the southern boundary by the monks to act as a storage pond for edible fish.
The pool today flows out into a culvert which crosses beneath Cross Farm Road and is quite audible but then vanishes beneath waste land to the northwest of Woodleigh Avenue and probably runs beneath the today's Harborne Park Road/Metchley Lane junction to join the Bourn Brook near the site of the old reservoir.
What makes this area all the more interesting is that this was the site of an even bigger body of water which called Lake Harrison by University researchers in the mid 1920's. They postulated that a large freshwater lake formed from melt waters along the southern boundary of the ice front during the last Ice Age. The lake running southwards being bounded by the Lickey Hills chain and the Romsley Ridge.
Between the wars the survey teams carried out excavation work in and around the University and Hospital sites gathering erratic material dragged down from the North West by the advancing ice front.
Lake Harrison's levels were maintained by "leakage" between the Lickey Hills and probably formed the Arrow valley to the south.