FIGURE 4–19 Furrow irrigation of lettuce. (Source: USDA) irrigation is commonly used in orchards (Figure 4–18). Furrow irrigation is a version of flood irrigation in which the surface flow is limited to channels between ridges. Vegetables may be irrigated in this way (Figure 4–19). 4.8.9 SOURCE OF WATER For home gardening, municipal water is the most frequent source of water for irrigation. The advantage of this source is that it is ready for use, provided the user has a means of connecting to it, eliminating the initial cost of providing an irrigation system. The source is reliable, since water is needed each day for a variety of uses in the home; the city ensures a continued supply. The disadvantages of this source of water are its high cost, the possibility of rationing in certain areas at times when plants need water the most, and the sensitivity of certain plants to municipal water treatments (chlorine and fluoride). For a large landscape, a well may be an alternative source of water. Wells are practical where the water table is at a readily accessible level. Sinking a well is expensive, and the quality of water is affected by soil and rock materials and may yield hard water (containing rock mineral deposits). Underground water may be contaminated by soil surface pollutants. Three types of wells may be used for irrigation. Driven wells are relatively inexpensive and easy to install. The unit is small and thus unobtrusive in the landscape. However, it yields a limited amount of water and is prone to blockage from debris over time. Driven wells are practical in areas where the water table is high. A well point must be found, which is perhaps the most difficult aspect of the whole process. Once found, shafts are driven into the soil using a sledgehammer. Shafts may each be about 5 feet long (1.5 meters) and are driven one after another, connecting the sections as one proceeds. Dug wells take up more space but also yield more water. If the site is properly selected, a dug well produces good-quality water year-round. A pump is needed to lift the water for use; dug wells may be lined with rock or concrete. The third type of well, a drilled well, is rather expensive and not a common option for homeowners. It is installed by using large drilling equipment similar to that used for prospecting oil. Driven Well A method of drawing groundwater by driving shafts into the water table. 4.8.10 WATER QUALITY Water quality for irrigation is not only critical to the success of growing plants but it also affects the design of irrigation systems. The issue is less critical if municipal water is the water source for irrigation. Water quality problems may be physical, biological, and chemical. The physical problems relate to the presence of mineral particles—sand, silt, and clay—in the water source. These sediments can clog pipes and sprinkler nozzles. Chemical problems pertain to the presence of dissolved salts like calcium, magnesium, 4.8 Soil Water 135
icarbonate, and iron. These chemicals can precipitate out of the water, causing clogging of pipes and nozzles as in the case of physical pollutants, as well as corrosion of metal parts if water pH is low. Also, chemicals like iron and manganese can stain walls and sidewalks. Untreated water can carry a variety of microbes and algae. These problems are associated with water wells and surface water sources of irrigation water. Polluted irrigation water is more problematic for drip irrigation systems than sprinker systems because of the tiny orifices of the emitters and tubes used in microirrigation. Some filtration is required for such pollution-sensitive systems. 4.8.11 EVAPOTRANSPIRATION AND ITS EFFECT ON IRRIGATION Moisture in the root zone of the soil is lost through plants by the process of transpiration through plant surfaces. Soil moisture is also lost from the soil when water vapor is lost by evaporation from the soil surface. These two processes produce a combined effect, called evapotranspiration, that is responsible for most of the water removal from the soil. The efficiency of irrigation is dependent on evapotranspiration. To minimize this factor, water must be applied at the right time of day by the most water-efficient method and protected from rapid loss. Water applied to bare soil is rapidly lost by evaporation from the soil surface. Mulching, where practical, may be applied to reduce this moisture loss. Irrigation in the early morning allows water to seep into the soil and thereby reduces the loss that occurs at high noon. Methods of irrigation vary in water use efficiency; drip irrigation provides water in the most efficient manner. 4.8.12 BEST TIME OF DAY TO IRRIGATE The critical considerations in watering plants in the landscape or garden are to deliver adequate moisture to the root zone, reduce waste, and avoid the persistence of a microclimate conducive to disease organisms. Water should be delivered at a rate at which it can infiltrate the soil and for a long enough time to achieve deep wetting of the root zone. Such watering can happen any time of day. In terms of waste, certain times of day, especially around noon, provide the greatest opportunity for water loss from the soil and other surfaces due to evapotranspiration (see next section). Contrary to popular belief, plant leaves will not be scaled when they are watered at high noon. Rather, moisture loss is very high at this time of day. The other important consideration is avoiding the persistence of high humidity and high moisture, which predisposes plants to diseases. Watering plants such that there will be time for leaves to dry before nightfall reduces disease incidence. Most landscapes are watered in the early part of the day for the reasons given. Precipitation Rate The amount of rainfall received by an area within one hour. 4.8.13 PRECIPITATION RATE It is important that an automatic lawn irrigation system be carefully designed for efficient and effective high water use. The topography of the area, soil water-holding capacity, and plant needs must be considered in the design. One factor of paramount importance is the precipitation rate, the measure of the amount of rainfall received by the area within one hour. A high precipitation rate is considered to occur when more than 1 inch (2.5 centimeters) of rain falls within an hour; a low rate is less than 1 inch per hour. An area of predominantly sandy soil has a high soil infiltration rate and thus requires a high precipitation rate to satisfy plants’ water needs. Using this information, one can select a system that will not be overworked (because of use for an excessively long period at one time). Sprinkler heads used in subirrigation systems may be divided into three groups according to precipitation rates. Low-precipitation-rate sprinklers are used where the soil can tolerate a large amount of water in a short period. Moderate-precipitation-rate sprinklers are run for a longer time and can have variable output. They are located in areas that are average in infiltration rate. It should be mentioned that although a homeowner 136 Chapter 4 Plant Growth Environment
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