Drip irrigation is increasingly replacing more traditional irrigation systems in both commercial and home gardens. Drip irrigation has been shown to dramatically slow salt intrusion into soil, decrease water loss through evaporation, decrease water loss from soaking into the ground where roots are not present, and discourage soil-borne pests by depriving them of much of their water supply. Ultimately, however, most growers look to the bottom line to determine whether or not a drip system is appropriate to their needs.
The Cost of Water
Home gardeners groan when they see their summer water bills, which can double and triple during the hottest months of the growing season. However, those bills are a font (pun intended) of useful information in developing a cost-benefit analysis of watering systems, specifically the number of gallons used in the specified time period, and the cost of that water before taxes and other fees are added in. To determine the cost per gallon of water, divide the cost by the number of gallons. For instance, if a household uses 200 gallons a day, at a cost of $1.518, dividing the latter by the former yields a per-gallon cost of $0.0076, or almost four-fifths of a penny per gallon.
Water Use in the Garden
Determining how much household water is going into the garden requires a comparison of water bills over time. Presumably, the extra water used in the hotter months is mostly working to keep plants growing. (If there is evaporation from a pool, that could account for some of the difference, too.) If that number is another 150 gallons a day on average from May through September, then the rough cost of growing a garden is 150 gallons times five months times 30 days per month times $0.0076 equals $168.75.
To check the accuracy of this number, recalculate the water use in the garden from the household water pressure and time spent watering in a month. For example, say that the water from a hose flows at the rate of 45 gallons a minute, and that a hose is used to water the garden for 10 minutes every three days on average from May through September. To determine the water use in the garden, multiply 45 gallons times 10 minutes times five months times 10 days per month times $0.0076, which equals $171 -- close to the number above. Thus, the cost to water the garden in this example comes to roughly $170.
Drip System Costs
Drip system manufacturers make entry-level all-in-one system kits designed to water a specific number of square feet of garden space. This is very helpful in determining how much a drip system is going to cost to install in a household plot. As of late 2010, such a kit for a good-sized garden is typically around $150.
While most parts of a drip system will see at least several years of use before needing to be replaced, in any given year after the first, some cost will be incurred to maintain the system. Making an educated guess that all parts will need to be replaced after about five years of use yields a yearly maintenance cost of about 20 percent, or $30.
Drip System Efficiency
Manufacturers claim that the water savings over sprinklers is about 50 percent. In other words, drip systems are supposed to use only about half of the water of sprinkler or ditch irrigation systems, which would reduce the growing season water bill in the previous example to $85, saving $85.
The Cost-Benefit Ratio of a Drip Irrigation System
In the first year, using the numbers in the examples, the homeowner would spend $150 to save $85, yielding a cost benefit ratio of 0.56. But over time, the benefit improves as the next years' inputs are only $30 to maintain the system. Looking at the cost-benefit ratio over five years yields $270 total cost divided into $425 savings, which equals 1.57, meaning that for every dollar invested into the system, $1.57 will be returned.