When determining the capacity of a water resevoir, you must also account for an emergency water reserve. In case of a fire that threatens the water reservoir tank or the village it is important to have a system that must take care of the problem. Below is a simple picture on how to set up an emergency water system.

A water reservoir allows us to purchase a pump for the average demand of water. Howstuffworks.com explained how many municaplities use a two pump system. Potable water from a water treatment plant is pumped to village according to the average demand. The pump flow rate is determined by the average demand. For example, let's say the average demand for du Achi is 650 Liters/min. There will be times thruoghout the day when Adu Achi will be using less or more than 650 Liters/min. If the morning from 8-9 am is the peak time of the day, 1000 Liters/min are needed becasue the demand for water is the greatest (people brushing their teeth, showering, cooking breakfast, etc.) then the reservoir will meet the demand since it can flow at 1000 Liters/min. If after the peak, the demand is less than the average, the slower water pump at 650 Liters/min will once again supply the water. Since the demand is less than the average, the excess flow will go towards refilling the water reservoir. This two pump water system lowers the cost of having to buy a very powerful pump that would have to meet the peak demand.

As Paulius stated, we should follow the Virgina Water Standards. 80L/day/person has been the number we have all seen for the minimum amount of water required. 80 Liters is approximately 21 gallons. Below is a link to a letter written by a human rights group to a warden of a prison. The human rights group was worried that the warden was not supplying enough water to the prisoners and the prisoners were at risk for very serious health problems. 2.5-3 liters is the universal standard for the minimum amount of water needed daily, survival. Personal hygiene requires a minimum of 4 ounces; you have to brush your teeth and rinse your mouth. A minimum of 2-6 liters are needed for flushing for saniation purposes. If toliets are not flushed frequently enough, it can be a source of serious disease and is a major health hazard. A minimum of 20 liters is recommended for waste disposal and personal hygiene. A minimum water amount is definitely not something we should say use as our standard, we must be well above. At the same time, we must make sure we are attempting to supply a reasonable about of water. Furthermore, we don't know how much our water borehole contains and how long it wil last taking our demand. In addition, we may be limited by our pump. Since we are looking at alternative power sources, which really limit our pumps maximum output, we may not be able to meet 80 L/day/person. 3.5 liters is just less than 1 gallon. If we supplied 4.5 times the minimum amount of drinking water, each person would have 15.75 liters/4 gallons.
If I drank 2 gallons a day I'd be going to the bathroom every 5 minutes. Overhydration won't occur if the pitaruity gland, kidneys, liver and heart are working properly. However, for a healthy adult, drinking over 2 gallons of water on a daily basis WILL cause overhydration/hyponatremia, because it exceeds the boyd'y ability to lose the water. Overhydration causes an imbalance in electrolytes, specifically a lack of Na(sodium). Your body shuts down, seizures can occur. There is a debate about overhydration. People argue that it's not about too much water; people need to maintain their balance of elctrolytes by consuming salt. Nevertheless, overhydration will result if people are drinking too much water and not maintaining their electrolyte balance. Below is a link to a report done by the US Army.
http://amsa.army.mil/1msmr/2000/v06_n03_article2.htm
Military bases in the US experienced problems with overhyradtion with troops. In the summer, they monitored soldier water intake for varying amounts of exertion and at certain temperatures. The soldiers were in full battle dress. There's an image below which has the information.

In conclusion, the US Army determined a soldier, who is undergoing the highest level of exertion, should never intake more 11 Liters, 12 quarts per day. The data is really valuable, because it was taken during the summer months, which will resemble the temperatures of Adu Achi. From the human rights group letter we established a minimum, and from the US Army report we have a maximum. So if we provided 15.75 liters for drinking, we'd be 4.75 liters higher than the maximum. These 4.75 + liters could go towards cooking. According to the human rights group letter, food like fruits and vegetables provide some of the daily water intake. Cassavas are an important food in the diet of the Adu Achi people. The Cassava Washing Group provided data on the properties and composition of cassavas. We should find out how much water is acquired by the people when they eat the cassavas. Water conservation is going to be important to maintain the life of the borehole.
Determining how the water will be used, will determine the size of the water reservoir. Adu Achi will not distribute water to luxury appliances, but water will be directed to public facilities (taps, showers, toliets, sinks etc.) and some private homes. Here is information about the flow rate and water usage for important items in America.

Item Liter/Min Total Per Use(L)
Kitchen sink 11 33
Shower or tub 19 190
Toilet flush 11 55
Bathroom lavatory sink 7.5 15
Outside hose faucet 19

Water flow restricting valves and shower heads reuce flow and usage by 50%. With 6,000 villagers, not everyone will be able to take shower everyday. Most likely, people will be able to bath every other day or every three days. The total usage above allows someone to shower for 10 minutes. If we follow the recommendations above to convserve rate flow and usage, consider that a person will not shower for 3 days, 32.5 liters must be added to personal use per day. Thus the total use per day is broken down as follows: 25 liters for waste disposal; 32.5 liters for showering; 10 lliters for drinking, 1 liter for oral hygiene, 5 liters for cooking. In total 74.25 liters per day is expected. It is safe to give another 6 liters per person. 80 Liters/day is good estimate and should not be exceeded. Few few homes will have their own sinks and toliets. Most restrooms and bath houses will be public, so if anything 80 Liters/day/person may be reduced to 70 Liters/day/person. 10 Liters/day/person less means 12,000 fewer liters are needed. Sicne we don't know how large the borehole is and how long it will last yet, when we know this data we should redetermine the exact distribution and need per person. Water conservation is very important and the villagers of Adu Achi are not going to have the same usage, or priviledge, for example, as Americans.

There are 6,000 inhabitants in Adu Achi. However, during certain times of the year more people return to the village and since the village will continue to grow in the future, we're looking to supply water for twice as many people. Each person requires 80 L/day. The ideal amount of water supplied everyday would be 960,000 L, approximately 250,000 gallons. That's an enormous amount of water to be storing for one day. According to watertanks.com, a galvanized steel field tank of capacity 150,000 gallons costs $100,000. A rectangular tank 10'x10'X10' holds 7,500 G, 20'X20'X20' tank holds 60,000 G. A cylindrical Tank (diameter X height) 10'X10' holds 6,000 G, 20'X20' holds 47000 G. Therefore, a tank able to hold one day's supply, 250,000 gallons, is going to be very massive and expensive.