Data from GRACE satellites and radar interferometry (InSAR) can be used to remotely assess groundwater storage change. It can be compared with more widely used groundwater management tools such as aquifer water budgets. We have a paper on the subject on its way, study site is Central Mexico.

What are some approaches to measure and manage a nation's water supply

Nation's water supply is depends upon the water demand for various areas like agricultural, domestic, livestock, industry etc. 

First you have to find out the demand of each criteria. Then total availability of water (ground + Surface). accordingly you will come to know the water demand -supply ration for small area to large area (Nation).

Find out the amount of water used to produce a unit of a material (finished goods) whether industrial or any other, for a country & compare this with those which are pioneers in the field.From this it will be possible to find out where the wastage occurs.

@ Pascal:  I am eager to read your paper. 

@ Abhijit: Is data about the demand of each criteria publicly accessible for India?

@Jacob Yu: The data which is required for demand is accessible in India. According to the standard requirements for each criteria demand will be calculated? 

Similarly supply also can calculate.

You need to work out the national water balance. This is non-trivial as you need to determine surface and ground water storage capacities, soil water content, groundwater flows, surface water flows, evaporation/transpiration, water usage (urban, rural, industrial, agriculture), rainfall-run-off, groundwater recharge rates and so forth. You are dealing with an open system. Groundwater is hard and demand fluctuates.

I agree with most of the answers mentioned above by some experts. While their responses are correct but to me is just part of the answer! In many situations especially in developing economies, the amount of un-accounted-for-water (UAfW) surpasses half of the produced quantity. As such it is wasteful to use high technology to look for new sources whether ground or surface without controlling the UAfW! Secondly and maybe  as important is the issue of pollution control. The amount of waste (liquid or solid) that reaches our sources of water is unimaginable. That reduces the amount that is available to consumers. It is therefore paramount to work in an integrated manor or holistically so that all tenets of water are obeyed! I believe we can add more water to the 'table' by using the strategies mentioned above. Looking for new sources of water should be the last option.

As it is the multiple-factor task including climate change, existence, requirements and distribution of water resources to solve such problem it is only possible to create model or to adapt already known models for the conditions which will allow to design scenarios and to solve them. In the basin of the Aral Sea such model ( ASBmm) is used. It allows even to optimize water distribution in the conditions of various scenarios of development of the states, including climate change modeling ( cawater-info.net)

To manage a Nation's water supply it is critical to have a sound, comprehensive monitoring network.  You cannot manage what you do not measure.  There has to be a continuous committed investment in assessing the surface and groundwater resources as well as the quality of those water resources.  To omit monitoring is to introduce uncertainty in planning and design.   The creation and maintenance of a long-term quality-assured water resources database is essential to water resources management, conservation, and future strategic planning.

@ D. Phil Turnipseed

Is the database your are referring to this one?

http://help.waterdata.usgs.gov/ 

@ Jacob Yu

Jacob, Thanks for your interest in the USGS.  I was not referring to any database in my note, just the critical need of monitoring in water resources management.  It is paramount in the design and planning of efficient and sustainable water resources management. The USGS does maintain a long-term quality assured data base that is free for download, but I've found that water data in other countries around the globe are not always free for download.  The URL you provided is one link to the USGS database for water resources information.   For a quick and easier access to surface water data you might try this:  http://waterwatch.usgs.gov/  

Again, thanks for your note.

Dear Jacob,

Your question is a multi-disciplinary one. To start with,I suggest you review the conceptual frameworks available from sustainability perspectives (e.g. DPSIR) so that  you can analyze what are the main driving forces, their impcats and how they can be responded. There are lots of articles published during recent years on the subject.

In addition to considering subjects pertinent to water governance, there are some methodological frameworks which suggested new methods in how to look at water balance at the large scale (e.g. Water Accounting) rather than the traditional hydrological ones. The advantage is that such methods link water resources to environment and economic considerations which has become a hot spot in scientific community nowadays. I hope you find the answer helpful.   

For sustainable water management one has to make water balance. For example water input terms  like rainfall (Average on long term basis) , surface water available and safe exploitation of ground water resources. Regarding out put terms it depends upon the area in general agricultural require more water that too depends upon cropping pattern followed and irrigation practices adopted in the area. Other major consumer are industrial as well as domestic water requirement. Industrial requirement again depends upon the the kind of industry. Domestic requirement depend mainly upon population it varies from 100-200 litre per day or even more. So depending upon the agro climatic condition the water budgeting can be done by adopting thumb rules.

In addition to what already stated in the replies above by many other colleagues, I would like to refer to “Delft Hydraulic Model of 1979. Before managing any water resources systems, it is necessary to understand the interaction and complications of its three major subsystems (physical, institutional, and infrastructural), controlled input, boundary conditions, desired and undesired outputs. 

I'd like to add that when looking at measuring and managing a nation's water supply, you will almost certainly run into problems when rivers are shared across national and even internal state boundaries. To add on to Saseendran S Anapalli, I therefore strongly suggest adding in experts who are familiar with the political side of international water sharing issues. One prominent example here is Farakka Barrage on the Ganges, located on the border between India and Bangladesh.