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 Pasture & Range: September 1996 Other Pasture & Range Articles |
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by Hugh Aljoe |
Visits with ranchers in Texas and Oklahoma reveal that rainfall has been widely scattered and highly variable. All agree that the drought is nowhere close to being over and there is a real concern about having enough reserve forage going into the winter. For many producers there is another concern even greater than forage availability and that is water availability.
Even in areas where substantial amounts of rainfall have been received and forages have responded in an almost compensatory manner, many ranchers are worried that the stock ponds will 'dry up' and they will be forced to move the livestock unless they receive some run-off soon. This is especially true if they are rotationally grazing and depend on a stock pond as the primary water source.
Now, it may seem odd that a so-called forage specialist writes about water, but as any forage manager well understands, water is the life blood within any forage system. With abundant water comes the potential for abundant life. With limited water, ... , well you get the picture. Now during this drought, we need to not only assess our forage systems, but we also need to take a good look at our water situation. With that in mind, I have put together some formulas and figures that have some practical applications.
According to the NRC, the average daily requirement of water for an animal unit (AU) will range between 8-15 gallons per day depending on physiological and environmental conditions. Since averages have little meaning during a drought, let's consider the extreme. As a rule of thumb and for the sake of simplicity, I like to assume intake at 1 gallon of water per 1 pound of dry matter consumed. Although we all know most forages contain some water, let us generalize water intake for an AU at 30 gallons per day. Proper terminology when working with stock ponds expresses water volume in acre feet (the amount of water 1 foot in depth under a surface acre).
Take note that...
1 cubic foot = 7.481 gallons
So how much water does an acre foot of water represent relative to livestock needs?
(325,872.4 gal./cu. ft * [30 gal./d./AU x 7 d./wk.])
or
1 acre foot will water 362 AU's for 1 month
(325,872.4 gal./cu. ft * [30 gal./d./AU x 30 d./mo.])
The following are some basic formulas that can be used to determine water volume:
triangle shape pond = 1/2 width x length x avg. depth
circle shape pond = 0.785 x diameter x diameter x avg. depth
(diameter = circumference * 3.14)
Sometimes a good imagination helps in determining an appropriate geometric shape.
For example, let's say we have a stock pond that is relatively circular. We find that by walking around the edge of the water, the pond is about 750 feet in circumference.That means that the diameter is about 240 feet (750 feet / 3.14). Applying this to our formula, we find the volume as follows:
| Volume (circle) | = 0.785 x diameter x diameter x avg. depth |
| = 0.785 x 240' x 240' x 8' = 361,728 cu. ft. = 2,706,087 gal. = ~62 ac. ft. |
If our cattle herd numbers 150 head, then 62 acre feet of water could conceivably sustain the herd for about 600 days [2,706,087 gallons/(150 head x 30 gallons per day)].
Obviously, we have only discussed water quantity. Quality is also a factor, and it is becoming increasingly more important so as water levels drop. Having a source of fresh water is the preferred situation, but it is not always feasible. Limiting the access to ponds by livestock except at designated watering points is a proven means of enhancing and /or maintaining good water quality. If you have questions, contact me or any of the specialists at the Noble Foundation.