As you may know, Umami is the one of most important factor in Japanese green tea for its quality.
When brewing Japanese green tea, we control temperature of water to get best balance of umami(amino acid) and catechin(astringency/bitterness) and caffeine(bitterness).
(Higher temperature (> 80℃ / 176 °F) get more astringency and bitterness in liquid)
How about water to brew Japanese green tea？
What kind water can bring umami taste well in the tea liquid?
Here is the interesting report about Japanese Green Tea & hardness of water - Research report by Mr. Omori Masashi 's group (Mr. Omori is known as Doctor of Tea in Japan)
Conclusion is that besides soft water, Evian water (hard water) are also good for the case according to the report.
Summary of the report
<Evaluation of the Umami intensity of Tea and Water by the Taste Sensing System>
For different types of water used to brew green tea, the taste of the tea infusion was compared. Hard water is generally considered unsuitable for the leaching of green tea, and leached tea using Evian water, which had high hardness, had the strongest umami intensity as detected by taste sensors. Adjusting the hardness of water using CaCl2 had no effect on umami intensity. However, using it in combination with CaCO3 influenced umami intensity in a manner dependent on the concentration of CaCO3. Using the green teas that were leached with Evian water and Crystal Geyser, a sensory evaluation was carried out using a three-point comparison method. Green tea was leached with Evian water was significantly preferred.
Using an artificial hard water, tea that was leached with Evian water was measured in a sensory and taste sensor evaluation. The taste of the two teas was similar.
This summary is quoted from: (PDF)
(Summary is in English, contents are in Japanese)
Umami Information Center Page
What is Umami (Umami in food in general)
Green Tea (Umami-rich Tea)