DO YOU HAVE ENOUGH WATER FOR YOUR STOCK

Doing a quick water budget will give you more confidence on how your water situation will hold up this summer.

This article was written by Greg Bekker for Agriculture Vicotria’s Sheep Notes newsletter in November 2024.

Rainfall and runoff into dams and rainwater tanks has been in short supply this year. Doing a quick water budget will give you more confidence on how your water situation will hold up this summer. Carting water for domestic use may be a given but to do the same for stock becomes expensive and can be mentally and physically draining when you do not know how long you will have to do it. Most producers do financial, feed and fodder budgets but quite often overlook budgeting for water.

To do a water budget, list the dams by paddock and calculate the water available in each one. Remember the last 300mm of water in the dam may be unusable as animals may get stuck or it may be of very poor quality. The following are examples using figures from the beef cattle and sheep drought feeding and management books:

• Beef cattle drought feeding stock
• Sheep drought feeding book

How much do you need?

For a sheep farm that has 1000 ewes and 25 rams and 500 lambs to finish before Christmas the water requirement for animals would be:

• 1000 ewes x 6l/head = 6000 litres per day 900,000 L
• 25 rams x 10l/head = 250 litres 37,500 L
• 500 lambs x 4l/head = 2000 litres per day 140,000 L

Total water needed for sheep to drink 1,077,500 L

If you have a self-replacing flock, you would need to add these animals. This example budget is until the end of March. When will your dams fill up?

For a beef farm with 100 cows and calves and 3 bulls.

• 100 cows x 100l/head = 10,000 litres per day (150 days till end of March) 1,500,000 L
• 100 calves x 4l/head = 400 litres per day (150 days till end of March) 60,000 L
• 3 bulls x 100l/head = 300 litres per day (150 days till end of March) 45,000 L

Total water needed for cattle to drink 1,605,000 L

These calculations can also be done for each mob and how much water they will need.

How much have you got?

The next thing to do is determine how much water is available for stock in each of the paddocks and how long that will last. Generally, paddocks run out of feed and not water but with the low levels of water in dams this may not be the case this summer.

Tables 1 and 2 give the estimated volumes of small rectangular (Table 1) and gully dams (Table 2) measured at the water level.

Evaporation can be one of the biggest losses from farm dams, especially small or shallow dams. A small farm dam 3m deep or less would lose around 60% to evaporation over summer. Regular monitoring or prior knowledge of a dam’s capacity to hold water is necessary to accurately estimate how long your dam water will last.

Example for a gully dam: 20m x 40m x 3m dam less 60% in evaporation

= 0.48 ML (Table 2) x 60% = 288,00 litres (0.28 ML) lost

= 0.48 – 0.28 = 0.2 ML available at end of summer.

Once you have completed this water budget you will be in a better position to look at options for consolidating and improving stock and domestic water resources for your farming business.

Table 1. Volume of water in small rectangular dams

Volume of water in a small rectangular dam (megalitres) – Batter slope 3:1

Water dimensions (metres)	Depth of water (metres)
Length	Width	0.5	1.0	1.5	2.0	2.5	3.0
5.0	5.0	0.01
5.0	10.0	0.02
10.0	10.0	0.04	0.05	0.06
10.0	15.0	0.06	0.09	0.10
10.0	20.0	0.08	0.12	0.14
15.0	15.0	0.09	0.15	0.18	0.19	0.19
15.0	20.0	0.13	0.21	0.25	0.28	0.28
15.0	25.0	0.16	0.27	0.33	0.37	0.38
15.0	30.0	0.19	0.33	0.41	0.46	0.47
20.0	20.0	0.17	0.29	0.37	0.42	0.44	0.44
20.0	25.0	0.22	0.38	0.49	0.56	0.59	0.61
20.0	35.0	0.31	0.55	0.72	0.84	0.91	0.94
20.0	40.0	0.36	0.63	0.84	0.98	1.06	1.10
10.0	10.0	0.04	0.05	0.06
10.0	15.0	0.06	0.09	0.10
10.0	20.0	0.08	0.12	0.14

Table 2. Volume of water in small gully dams

Volume of water in a small gully dam (megalitres) – Batter slope 3:1
Water dimensions (metres)	Depth of water (metres)
Length	Width	0.5	1.0	1.5	2.0	2.5	3.0
5.0	5.0	0.01
5.0	10.0	0.01
10.0	10.0	0.01	0.02	0.03
10.0	15.0	0.02	0.03	0.05
10.0	20.0	0.02	0.04	0.06
15.0	15.0	0.02	0.05	0.07	0.09	0.11
15.0	20.0	0.03	0.06	0.09	0.12	0.15
15.0	25.0	0.04	0.08	0.11	0.15	0.19
15.0	30.0	0.05	0.09	0.14	0.18	0.23
20.0	20.0	0.04	0.08	0.12	0.16	0.20	0.24
20.0	25.0	0.05	0.10	0.15	0.20	0.25	0.30
20.0	30.0	0.06	0.12	0.18	0.24	0.30	0.36
20.0	35.0	0.07	0.14	0.21	0.28	0.35	0.42
20.0	40.0	0.08	0.16	0.24	0.32	0.40	0.48
5.0	5.0	0.01
5.0	10.0	0.01