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Dale Tate's Garden Site
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Dedicated to the Senior Citizen's Community Gardens sponsored by the City of El Paso's Parks and Recreation Department located at bout 600 yards down an unimproved road toward the Franklin Mountain from the intersection at 9175 Stahala and Gabriel streets in El Paso Texas.
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Southwest Desert Gardening in Texas, New Mexico, Arizona, Colorado,
California, Nevada, and Utah.|
Using Water in The Desert |
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Dale Tate Let’s look at what this means .32 inches = 2.66 feet X 50 = 133 sq ft of garden. A one inch rain on 133 sq ft is 133 / 12 = 11.084 cu ft of water. There are 7.48 gallons of water per cu ft so that would be 11.084 X 7.48 = 82.63904 gallons of water. These people are only using 10 gallons per day so 10 / 82.63904 = .121 inch of water. It takes about 80 days to grow a crop of sweet corn such as we have had for our corn roast so multiply the 80 X .121 and you come up with 9.68 inches of water used to grow the crop. The Irrigation Principals and Practices 4th edition at http://ag.arizona.edu/pubs/garden/mg/vegetable/irrigating.html In its TOTAL CONSUMPTIVE USE AND DAILY PEAK USE OF WATER for the Intermountain Desert and Western high Plains indicate that sweet corn can be grown with .24 to .35 inch per day, approximately twice of the .121 inch per day used by the Chapin Experiment found at: http://www.hydrosource.com/web_clp/971101/chap2.htm I have heard that our local irrigation district allow a farmer 26 inches of water to grow corn so it obvious someone need to know more about how to use water including the Members of the Weldon Yerby Community Garden Society. Water conservation is an evolving technology so exactly how much water can be conserved is unknown. Water is precious, especially in the desert, as in El Paso County so allow me to inform you of some water math. Rain fall has been measured in inches since (I don’t know when), therefore irrigation requirement is also measured in inches. One inch of free water from nature means something like this. One cubit foot of water is 7.48 gallons and that much you have received with a one inch rain on each 12 sq. feet of your garden space. Most of our garden sites are about 5.5 X 42.5 feet or 238 sq. ft. so one would divide 238/12 and get 19.834 cubit feet that would fall on your garden with a one inch rain. That means you have just received (19.834 X 7.48) about 150 gallons of free water. We get this about 7 to 9 times a year. Any additional water we use cost someone some money, in our case it is the City of El Paso Parks and Recreation Department. We are growing gardens for fun, well so what? Boys and Girls play soccer on city parks for fun and the parks also use water. A short paper, as I hope this will be, cannot begin to tell you how much to water, but maybe I can tell you how not to waste water. The T-tape or otherwise known as drip tape with a 10 pound pressure regulator that we are using in the garden dispenses 40 gallons per hour for each 100 feet. A conscious water user must first know how much water is being used. Plants use water as moisture as damp soil, not free water as in wet soil that can be squeezed out. Now keep this in mind, if one placed a two inch deep pan out in the sun for June, July, and August and put an amount of water in the pan each morning equivalent to one inch of rainfall, you would never fill the pan because the evaporation rate from and open pan exceeds the one inch per day. When I see wet soil in our garden, I am well aware that the gardener does not know much about water and is wasting a valuable resource by evaporation. They could use a two or three inches of organic mulch on top of the drip tape, or do as some do and place the drip tape three or four inches under the top of the soil. Some agriculturists call this dust mulching or subsurface drip irrigation (SDI), which allows little or no water to be lost from evaporation. I understand the City is to exempt (SDI) systems from the coming water restrictions. The root growing depth of most of our vegetable plants is in the 12 to 24 inch range. In our garden you are indeed fortunate to have 24 inches of soil. Soil is like a sponge. When you saturate a sponge with water and the lay in on a table, you will see that water will drain out of it for a few moments and then stops. The water retained in the sponge, we can called sponge capacity and that’s all the water that can be used, that water on the table is wasted. Our garden soil have what is called “field capacity”, that being the amount of water the soil will hold against the force of gravity. Water applied above this amount drains into the subsoil and is wasted just like the water draining out of the sponge onto the table. You can check the depth of your soil with a posthole digger. After one knows the depth of their soil, one way to check the depth of water penetration with what is known as a soil probe which is only a ¼ inch steel rod with a handle on in for pushing into the soil. This is important to know in order to avoid watering the sub-soil and wasting water. You are also wasting plant food or fertilizers. Most plant foods are water soluble so what you are doing when you over water are dissolving the plant nutrients in water and caring it down below the root zone. Soils are mineral particles ranging in size from very small (clay) to the very large (course sand). The larger soil particles create less field capacity for holding water. It also means that the water used will penetrate deeper in the soil, however it does not mean that you can produce a crop for less water, what it does mean is that you will use less water but water more often. Hard squabble farmers call this a droughty field. That is what we have, however every gardener site is different depending on the amount of organic matter they have added to it, but generally one can estimate 1 foot of soil will hold from 1 to 1.4 inches of water so any time you see someone applying more than ¾ inch of water to 9 inches of top soil, you may know that person is abusing a privilege. Plants need moisture, soil with nutrients, and air to thrive. Sometimes plant die because of saturated soil which excludes air. Few people are astute enough to recognize the amount of water you are wasting by watering the sub-soil but even a half-wit can see water flooding out of your garden patch and know this is wasting water. This is caused by people using automatic timer set for too often or for too long or by people manually turning their water on and leaving the garden without remembering to turn the water off. Personally, I use an automatic metering valve that can be turns on for an amount of water in gallons and then turns off. My rational for using this device is that if I’m not looking at my garden and know how much water to use, then I should not be using water anywhere. Probably shouldn’t be even left along all night to care for myself. We do get some rain – recently we received about one and ½ inch of rain at our gardens, but those automatic timers are still watering. I cut some water off, and really I would just as soon the water for that patch is never turned back on because to owner don’t deserve to have a garden. I do the same when I see water running manually with the owner no where in site. Table 1: Determination of soil moisture content: How Soil Feels and Looks
Now for those who would like to know more, you can start here. Farmers who grow plants for profit have a very scientific method of determining when and how much to water. Land grant universities have developed a scale called PET (potential evaporation transpiration) index for each location or environment within their state. This index is the water need of a four inch high mature cool season grass to grow successfully. This conclusion is drawn from the historical record of the past thirty years of climate data including temperature, wind speeds, and humidity. They also know that each species of plants has a different moisture requirement. They call this a crop coefficient (cK). This cK is normally a percentage of the PET and this percentage is most times less than 100 percent of PET. Another factor used is the stage of plant growth of these crops and this is also a percentage of the cK. Now to use this information they need soil sensors placed in predetermined manner in their fields. Now to use all this information, they must have computers connected to the sensors to turn the irrigations systems on and off. There are fewer and fewer small family farms because it takes a large operation to be profitable. It’s called the economy of scale. Evapotranspiration
(ET) is a measurement
of the total amount of water needed to grow plants and crops. This term comes
from the words evaporation (i.e., evaporation of water from the soil) and
transpiration (i.e., transpiration of water by plants). Different plants
have different water requirements, so they have different ET rates. The following table is historic PET for El Paso so there is no way that one need to use more water on our gardens than the 66.05 inches per year minus the amount of rain fall we receive.
How much water is this? Let’s compute for a total year assuming we have a 238 square foot garden and that you are going to water for maximum production using the total 66.05 inches of water / 12 is 5.5 feet X the 238 sq ft = 1309 cubit feet X 7.48 gallons per cubit feet equals 9,791.32 gallons of water used on one garden site in a year. That’s equivalent to approximately 178 fifty-five gallon barrels of water for each garden site. Our objective is save the amount of water received as rain and at least 35 % of the 9,791.32 gallons desired. To do this we would minimize the amount of water lost to evaporation and to leaching. There is also a thing called deficit irrigation, a fact is that some vegetable crops need water stress at various times for the highest quality produce. Cantaloupes, Jalapeno Papers, and Tomatoes come to mind. The Cantaloupes and Tomatoes will not get sweet without some stress and Jalapeno Peppers will not get hot. Garden plants seem to acclimate to an environment so it is quite conceivable that one could be quite successful in saving another 20 to 25% by practicing deficit irrigation. Most desert acclimated landscape plants must also be stressed for water to reach their most natural of beautiful state, yet most subsurface drip system installed by the green industries do not allow for this. These so called xeriscape plants will also acclimate to excess water when provided, but often not into the beautiful plant desired. That makes one wonder the point of xeriscapeing to begin with. Water moves down thru the soil by gravity. Water moves upward and sideways by capillary action. Subsurface drip irrigation (SDI) is the latest, and possibly the most sophisticated and most efficient method available for irrigating agricultural crops, landscape and turf. As often demonstrated, SDI is capable of providing the greatest yields and water use efficiency (WUE) of any irrigation method now in use in the World. Actually some of us may be using more water than (PET) requirement for El Paso as some seem to be using water all the year without regard to the season. Approximately 40 percent of the garden sites at the Weldon Yerby Senior Citizen's Community Gardens have already been retrofitted with the drip irrigation equipment installed and there are two gardens that have subsurface drip irrigation (SDI). On average, I think we senior citizens are ahead of the rest of the city in water conservation measures taken. Our Garden Society will promote SDI on all the sites by education and example. March 29, 2003 |
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