How Much Water is Enough?
When we finished last month, we had discovered how to find values for potential evapotranspiration (ETo) using AZMET data. This month I want to discuss how we can put that information to practical use. Specifically, I want to answer the question: How much water is enough? Before I continue, however, I want to emphasize that my purpose is not to explain a "by the numbers" system for watering your plants. Rather, I want you to develop a feel for how much water it takes to keep your plants happy and healthy.
The ETo values we learned about last month provide the basis for determining how much water has been removed by evapotranspiration. These numbers show the predicted amount of water in inches that a reference crop (turf grass) would use during a certain period of time. Of course unless your plants happen to be the same as the reference crop, they may not use water at the same rate as the reference, so you must multiply the listed ETo values by a correction factor called a crop coefficient to predict how much your particular plants would have used. A good source of data on crop coefficients has been compiled by the United Nations Food and Agriculture Organization (FAO) and can be found at: http://agen.tamu.edu/wqit/petnet/tools/crop/cropcoe2.html
The crop coefficients shown in the FAO reference even take into account the stage of growth of the crop - initial growth, mid-season growth, and late season growth because plants use water at different rates throughout their life cycles. Table 1 shows a few samples of the kind of date available at the Web site.
Suppose you are growing tomatoes. According to the FAO figures, the crop coefficient for tomatoes at mid-season is 1.2- That means that you must multiply the ETo values obtained from the Bonita AZMET site by 1.2 in order to calculate how much water your tomatoes are using. For example, the ETo values given for the first week of August at Bonita are shown in the second column of Table 2 and the corrected values for tomatoes are shown in the third column.
What these numbers are telling us is that during the first seven days in August 2.11 inches of water would have had to be added to your tomatoes to make up for what was lost due to evapotranspiration. If any rain had fallen during that period, you could subtract that from the total. In the case of the Bonita AZMET site, a total of 0.02 inches of rain fell during the period so only 2.09 inches of water irrigation water would have been required to make up for what was used. To get an idea of how many gallons are represented by 2.09 inches, we have to do a little arithmetic.
One square foot is 12 inches on a side and represents 12 inches X 12 inches = 144 square inches of area. To cover that area with 1 inch of water, requires 144 square inches X 1 inch = 144 cubic inches of water. Since there are 231 cubic inches of water in a gallon, we can calculate that 144 cubic inches of water represents 144 cubic inches/231 cubic inches per gallon = 0.62 gallons of water. In other words, it takes a little over one-half gallon of water to supply 1 inch of water to a square foot of soil.
Since my tomatoes are planted in 4 feet by 4 feet squares, each square covers 16 square feet. Supplying 2.09 inches of water to a square would require 16 square feet X 2.09 inches X 0.62 gallons per square foot or about22 gallons of irrigation water. This of course assumes that all of the water I delivered was available to the tomato plants.
Does this mean that if I had poured 22 gallons of water on each of my tomato squares after a week, without watering my tomatoes would be perfect? Of course not. First, all of the water I supplied would not have reached the thirsty roots of my tomatoes. Second, my tomatoes might have expired of thirst if I had not watered them for a week. And third, the soil in my garden probably has different water holding characteristics than the soil at Bonita, so they might require greater or lesser amounts of water than the ETo values would indicate. What the numbers give me is a place to start. I should apply this much water and watch to see what happens. If the plants wilt, then I need to increase the amount a little. If the soil remains soggy, I can probably decrease the amount a little.
Hopefully this discussion has given you a little insight into how much water is enough. Until next month, happy surfing.
Gary A, Gruenhagen, Master Gardener