Estimating storage requirement

Estimating storage requirement:
  • Dry period demand method – In this approach one simply estimates the longest average time period without any rainfall for your particular geographic area. This will typically coincide with the dry season which in the India, generally runs from January to May. Your local meteorological office can be consulted to get such estimates. hence, if your household daily demand is 100 litres and the dry season runs on average for 120 days, then the size of your storage should be 12,000 litres.
  • Simple method – In this method, the average annual water consumption is estimated for the household, based on the number of occupants. The average duration of the longest rainless period is also assumed in terms of number of days. This rainless duration period is in turn expressed as a ratio (of the duration of a year) and multiplied by the annual consumption to estimate the volume of water that will be required for this period. A worked example:
  • Consumption per person per day, C = 40 litres
  • Number of people per household, n = 5
  • Longest average dry period = 25 days
  • Annual consumption (litres) = C x n x 365 – Annual consumption 40 x 5 x 365 = 73,000 litres (16,058 gallons) Storage requirement, T = 73,000 x 25 / 365 =5,000 liters
  • Graphical Method – Using the graphical method, one only needs to know the number of persons in the household and the approximately roof area. The graph in below can be used to determine the recommended size of the storage. The graph shows a plot (dashed line) for tank size Selection for a roof area of approximately 225 m2 and a household size of 6 persons. The plot suggests that a 3,000 gallon (13,638 litre) storage tank is recommended.
Storage Structures collection efficiency

  • Clean water from the roof can be stored in Underground sumps or Rain barrels for further use after the suggested filtration methods.
  • Calculation of storage/recharge volume based on the average rainfall and collection efficiency: The storage or Recharge structure capacity can be calculated by the amount of runoff from your roof taking into consideration the losses due to evaporation, leakages etc…
  • The optimum storage = Roof Area in Sq Mx Runoff Coefficient(0.9)x Average Rainfall in mm, Runoff co-efficient is the percentage of water from the roof that comes down in the rainwater pipes which is taken as 90% for RCC roofs, as about 10% will be lost by evaporations, leakages etc… Average design rainfall of Bangalore can be taken as 32mm (970mm/30days).
  • Eg: If the roof area of a building is 112 Sq M, then the storage or recharge structure capacity will be 112 x 0.9 x 32=3225 Liters.
  • We shall first calculate the maximum amount of rainfall that can be harvested from the RCC rooftop Following details are available:
  • Area of the catchment 112 Sq.m.
  • (C) Runoff coefficient = 0.9 (R) Average annual rainfall 970 mm (also call it as height of rainfall 0.97meters)(R)=0.97, Annual rain water potential=(A)Roof area (in Sq.m)X(C)Coefficient runoff from roof)X(R) Annual rainfall (in mm) Annual water harvesting potential from 112 Sq.m. Roof = A x C x R= 112 x 0.9 x 0.97= 97.7 cu.m. (97776 liters)
  • Rain water volume Calculation
  • Area of =112 sq.m. Height of rainfall=0.97m=970mm or 39 inches Volume of rainfall=Area of plot x height of rainfall Over the plot 112 sq.m. x 0.97 m =105.38 cu.m.(105,380liters Assuming that only 60% of the total rainfall is effectively harvested. Volume of water harvested =105,380 Liters x 0.6=63288 Liters This volume I about thrice the annual drinking water requirement of a 5-member’s family. The average drinking water requirement per person maximum 10 liters a day.

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