Your idea doesn't work because:
- Irrigation water is held in dams that don't leak (or shouldn't) so that doesn't lead to groundwater recharge.
- The more that is held in dams the more that is lost to evaporation which is not useful to anybody including the downstream ecology.
- It is used for irrigation where most is lost to evapotranspiration not to groundwater, if your irrigation is soaking down below the root level you are doing it wrong and may be raising the water table and so contributing to salination. This has happened in too many irrigation systems around the world including the Murray-Darling.
- The figure was arrived at to allow sufficient flow in the rivers for environmental, agricultural and domestic purposes downstream, many rivers cease flowing none the less in dry times. If the figure was more it would be favouring those where the rain falls at the expense of those users downstream. And yes higher figures have been suggested by those who would benefit at the expense of others.
You must also take into account that the system must respond to el nino - la nina cycles as well as any seasonal pattern. This is not a reliable annual rainfall nor a reliable seasonal pattern such as annual snow-melt. It's a hard land.