Credits: Biovision-Infonet
The required storage capacity of a water reservoir depends on:
- The daily required volumes and quality of water measured in litres.
- The length of the dry seasons during which these volumes and quality of water are required.
Example on water demand for a homestead:
While the number of days in a dry season can be estimated fairly easy, such as 180 days without rain in a semi-arid region, the volume of water required for each of the 180 days can be calculated using the following guidelines on daily requirements of water for a rural homestead:
| Water users | Daily requirements Litres | Number of days without rains | Required volume for a dry season Litres |
| 1 person | 15 | 180 | 2,700 |
| 1 grade cow | 50 | 180 | 9,000 |
| 1 local cow | 20 | 180 | 3,600 |
| 1 goat | 5 | 180 | 900 |
| 1 sheep | 5 | 180 | 900 |
| 1 hen | 0.3 | 180 | 54 |
| 3.3 mm on 4048 m2 (1 acre) with drip irrigation | 1,336 | 60 days x 1,336 | 80,150 |
| 5 mm on 4048 m2 (1 acre) with furrow irrigation | 2,024 | 60 days x 2024 | 121,440 |
| 4.3 mm on 4048 m2 (1 acre) with sprinkler irrigation | 1,741 | 60 days x 1741 | 104,460 |
(RELMA 2001, by I.V.Sijali)
A homestead with 6 persons, 4 local cows, 20 goats and sheep (shoats) and 20 hens who wants to irrigate 2023 m2 (1/4 acre) with drip irrigation requires the following volume of water for a 180 days dry season without any rains:
| Litres | |
| Clean water from roof for domestic use: 6 persons x 2,700 L | 16,200 |
| Unclean water from a water hole in a riverbed: 4 cows x 3,600 L | 14,400 |
| Unclean water from a water hole in a riverbed: 20 shoats x 900 L | 18,000 |
| Unclean water from a ground tank or a pond: 20 hens x 54 L | 1,080 |
| Unclean water from a ground tank or a pond for irrigating 1/4 acre: 80,150 x 1/4 | 20,038 |
| Total storage requirement | 69,718 |
| Add 20% loss due to evaporation and seepage | 13,944 |
| Total storage requirement for a 180 day long dry period | 83,662 |
This example shows that a rural homestead in a dry area could use 3 types of water sources:
1) A roof catchment tank with a storage capacity of at least 16,200 + 20% loss = 19,440 litres for fresh clean water for domestic use.
2) A well in a riverbed or a pond that can supply 14,400 + 18,000 + 1,080 + 20% loss = 40,176 litres of unclean and, perhaps, saline water for the livestock.