On the morning of the first of April 2016 a momentous change will take place that will affect the income tax calculation of nearly every single worker in Scotland.

From that date on wards almost half of what most people pay will be decided by MSPs at Holyrood.

The change was included in the 2012 Scotland Act, alongside a host of other new powers for Scotland, including on other taxes, borrowing and even drink-driving limits.

Earlier this month in his introduction to the second annual report on its implementation, the Liberal Democrat Scottish Secretary Alistair Carmichael set out his stance on the magnitude of the Act.

"Some would have people in Scotland believe that (come the independence referendum) when they make that choice, it is all or nothing; independence or the status quo.

"The Scotland Act illustrates why this is a false choice. Devolution is not a promise on the horizon - it is happening now."

But if the Scotland Act suffered a profile problem before, the pro-Union parties' recent proposals for further devolution appear to have cast it even further into the shadows.

So is the Act, hailed when passed by then Scottish Secretary Michael Moore as the biggest transfer of fiscal powers to Scotland for 300 years, the forgotten victim in the battle for independence?

Or is it largely an irrelevance, a series of measures that have, no matter the outcome in September, been overtaken by events?

Income tax is arguably the biggest change included in the Act. But there is a strong chance that the move could pass by unnoticed by workers.

Under the changes the Westminster Government will reduce the amount of income tax that it charges by 10p in the pound.

That money, or part of it, will be replaced by a separate Scottish income tax, which will be voted on and set by the Scottish Parliament.

If MSPs want to keep the Scottish Government's budget stable they will have to set the new rate at 10p in the pound - a rate that would mean that the ordinary worker will see no difference in their take-home pay.

However, if MSPs want to offer tax cuts or increase spending they do have the leeway to vary the new rate, by an unlimited amount in either direction.

The original idea behind the change was not necessarily to affect Scots pay packets. It was to make politicians more responsible - forcing them to raise at least part of the money that they spend.

Other taxes will see major changes, too, with the Scottish Parliament given powers over stamp duty and landfill tax.

For his part Scottish Finance Secretary John Swinney has outlined plans to scrap stamp duty in favour of what he says will be a "fairer" tax, when he gets control of the power in April next year.

The other big fiscal change is on borrowing powers.

The Treasury has in recent years given the Scottish Government the power to borrow money to invest in long-term projects. But that amount is to rise to a £2.2 billion.

Alongside the financial powers Scottish ministers also have plans to tighten controls on airguns and crackdown on drink-driving on Scotland's roads, both measures which could have wide ranging effects.

Questions remain over how all this will work in practice.

Officials have to designate, for instance, whether someone is or is not a Scottish taxpayer.

Under the Act a worker is a Scottish income taxpayer if they have a main place of residence in Scotland or spend at least the same number of days in Scotland as in another part of the UK.

The exception to this will be Scottish politicians.

Any parliamentarian with a Scottish constituency, be they an MP, MSP or MEP for any part of a tax year, will automatically be counted as a Scottish taxpayer.

After all, you could not have a separate Scottish income tax rate voted on by Scottish politicians but not paid by them, could you? Other complications abound.

Earlier this year a Holyrood committee called for greater transparency if MSPs were to be able to scrutinise Customs' collection of the new Scottish rate of income tax. It also recommended that Scottish ministers should publish migration patterns, to assist MSPs to predict whether setting a different Scottish income tax rate would encourage or discourage worker to declare themselves Scottish taxpayers.

Others raise broader questions. Some unionist commentators have protested at the sense of having different income tax levels and speed limits on either side of the border - arguing that the move could simply further emphasise a sense of separateness between Scotland and the rest of the UK.

A little noticed report by the Centre for Public Policy for Regions (CPPR) think tank has warned that a long-held Scottish Government pledge never to allocate more than 5% of its budget to repaying building project costs could be breached with the new borrowing powers - depending on how you calculate the figures.

Critics claim that this could affect the SNP's plans for "shovel ready" projects while the Scottish Government insists that its commitments are "affordable".

To the SNP the Scotland Act is a missed opportunity. The party campaigned for the UK Government to extend the powers even further. While the legislation was going through Westminster the Scottish Government demanded a further six measures, including control over corporation tax, the Crown Estate, alcohol and tobacco excise duties and broadcasting.

The party has claimed that the income tax powers are a "pig in a poke" that could leave Scots worse off.

It is also clear that, should there be a Yes vote in September, then the Scotland Act will matter little, ultimately overtaken by independence.

In recent months the pro-Union parties have also outlined their own proposals for further powers for Scotland that would seem to supercede some of the powers in the Scotland Act, including some of those on tax, but not others.

The Scotland Act was first announced by the then newly elected Tory-Liberal Democrat Coalition Government way back in 2010.

At that stage there was no certainty that there would ever be an independence referendum, let alone a vote just four short years later.

But no matter what the result in September, the full impact of the Scotland Act's measures remain to be seen and potentially cannot be ignored.

2012 Scotland Act: Key measures

INCOME TAX

From 2016 almost half of the income tax paid by most Scots will be set by MSPs at Holyrood. The move will see the basic, higher and additional rates of UK income tax be reduced by 10p in the pound. Unless the Scottish Government wants to see its budget cut, MSPs will have to vote to set a new Scottish rate at 10p in the pound. But they could also vote to set the level higher or lower, with no limits either way.

Full devolution of stamp duty and landfill tax

The Scottish Government has taken forward legislation to replace these taxes in Scotland with the Land and Buildings Transaction Tax and Scottish Landfill Tax. It is also taking forward legislation to establish Revenue Scotland as the tax administration responsible for the collection of the new taxes.

New borrowing powers

MSPs will be able to borrow up to £2.2bn. A limited version of the power is already in place, designed to enable the Scottish Government to fund £100m of pre-payments for the Forth Road Crossing.

The power to create new devolved taxes

In 2012 the Act gave MSPs the ability to introduce new devolved taxes in Scotland. However, this power is not absolute and any new taxes can only be created in agreement with Westminster.

Powers over airguns

The measure followed the death of two-year-old Andrew Morton, shot in the head with an airgun in Glasgow in 2005. There are an estimated 500,000 air weapons in Scotland. Scottish ministers have set out measures to ensure anyone who owns an airgun will need a licence and a legitimate reason to hold the weapon.

Drink driving limits

l Ministers plan to use the new power to cut the drink-drive limit from 80mg per 100ml of blood, saying this will make roads safer.