In getting started, it is imperative for the farmer and/or surveyor to familiarize themselves with the proposed field or land area they wish to cultivate. The more they know about their land the better their chances will be of selecting the proper irrigation system. Some of the physical characteristics and economic issues that they will need to take into consideration are:
Physical Characteristics
Economic Setting of the Land Area
A surveyor/farmer should also come equipped with topographical and aerial maps. These maps locate everything on a farm which may effect the proposed crop site. Accompanying the maps, the surveyor will bring soil surveys or soil maps. These surveys give educated guesses on how crops will perform in particular areas within a proposed site. To see what a sample of what a soil survey looks like click here. The soil survey was taken from Madison County, Georgia. The survey of the key is below:
As an editor's note, I did attempt to put both the survey and the key inside its own html file, but there was not enough memory to do so.
It should come as no surprise that the soil makeup in the state of Georgia is clay based, more specficially, red clay based. Clay soil is a dense soil where its water permeability rate is very small (i.e. somewhere around .02 inches/hour). In other words, it takes water a long time to seep through clay based soil. On the other hand, there are many other places in the world, much less our own country, where sandy soils exist, such as desertous areas or the midwest region of the United States. In places where sandy soils thrive, the permeability rate is much higher than clay areas (i.e. around 2 to 6 inches/hour).
Another factor surveyors need to take into consideration when building an irrigation system is making sure that the water source is plentiful enough to apply the proper amount of water and that the system distributes the water evenly for maximum potential crop yield. Should a surveyor skip over this step or assess in haste, this decision could have prolonged adverse affects on the field. Dead patches of crops will form due to insufficient amount of water. Disease to crops could occur because of bacteria and fungi growth due to overwatering in particular land areas. Uneven growth of crops could occur because the uneven water distribution. In addition to uneven growth, lack of proper water distribution could also lead to fertilizer damage of the crops.
One way farmers combat these problems, after a thorough investigation of the proposed field, is what is known as the Uniformity Coefficient. The Uniformity Coefficient (or Cu as it is called in the farming community) is defined as "the measuring the amount (as a percent) of water caught in cans positioned at constant intervals within a sprinkled area" (Zimmerman, p. 157). This coefficient, which has been in practice since the 1950s, uses three criterion to determine its value:
The Uniformity Coefficient equation looks like this:
where Cu = Uniformity Coefficient, x = deviation of individual observations, m = mean value of all individual observations, and n = number of observations. A standard irrigation design calls for an 85% Uniformity Coefficient reading. An adequateCu will also depend upon the kind of crop. For example, the Cu will need to be higher, the shallower the root system of a crop, or the shallower the first layer of soil (also known as the top soil).
One other factor that farmers should take into consideration is what is known as evapotranspiration (EVT), or the evaporation of water from soils and the transpiration (or evaporation) of water from plants. This is a consideration that cannot be quantified accurately over night. Farmers have been known to keep fields fallow, or unused, until they have enough EVT data to determine the amount of land they are willing to develop. This process is especially important in very arid or dry regions where there is generally "more land than water with which to irrigate" (Zimmerman, p. 32). As expected, the EVT will be higher in dryer areas than in marsh or swamp regions. The evapotranspiration factor may also determine whether a farmer picks one type of irrigation system over another.
It is also important for a farmer to consider mixed farming development for several reasons. It helps make the farmer depression proof by not "putting his/her eggs all in one basket" on one type of crop. Diversification not only gives the farmer a better chance of surviving economically but it may be necessary for the farmer to diversify his/her crops based up on soil type and change in topography from one field to the next. This may be one reason why a farmer might use more than one circular pivoting sprinkler system within the same square area of land (as suggested in the figures above).
Finally, a system layout design will need to take the following things into consideration: topography, location of the water source (i.e. within, adjacent or imported from somewhere else), and the shape of the field. Whatever type of irrigation system a farmer chooses, it is important that the irrigation field units are as uniformly shaped as possible, preferable rectanglular or square shaped. Irregularly shaped areas should be cut into as many regular shaped units as feasible with the irregular remains concentrated into a minimum of odd-shaped units (Zimmerman, p. 101).
Once all of the preliminary investigations have been completed and all of the physical and economical issues have been taken into considerations, it is time for the farmer to develop the irrigation system. There are several irrigation system designs in which the farmer can choose. Some of the more popular designs include:
For this project we will focus solely on sprinkler irrigation and leave the other types of irrigation systems as investigations for another time. So, why might a farmer choose sprinkler irrigation over the others? Author Larry G. James lays out several reasons in his book, "Farm Irrigation System Design". Sprinkle systems spread water efficiently and is "adaptable to many sois and terrains. It can be successfully used to irrigate:
Sprinkler systems are more versitle than other systems. In addition to supplying water to crops, sprinkler systems can also be used for: frost protection, fertilizer and chemical applications, crop and soil cooling, and germinating seeds (James, p. 69).
Sprinkler Irrigation has many designs to choose from. They include but are not limited to the following: continuous-move, set-move, and solid-set designs. Continuous-move designs works under the assumption that the irrigation system is free to move at all times while water is applied to the field. A popular continuous-move system for commercial use is the center-pivot design, which we will discuss later. The set-move design applies water while the system is stationary, but can be moved to other areas of the field afterwards.The solid-set design uses enough sprinklers such that, in theory, it could irrigate entire blocks of land simultaneously within a cultivated field. Solid-set systems range from being portable (although once in place it will not move until the harvest season is over) to permanent (where sprinklers are permanently set in a field and their water source comes from underground piping). One type of solid-set system is trickle irrigation, where water is emitted in smaller quantities (i.e. in spray/mist form) over a longer period of time.
For the set-move and the solid-set systems, there are three main configurations that farmers use to space out their sprinkler heads. The first is an equilateral triangle spacing (as seen in the picture below). This type of spacing is best suited for solid-set systems. The next two are square (as seen below) and rectanglar spacing. These designs are best suited for set-move systems, mainly because the piping, (or lateral as it is called) in which the sprinkler heads are connected to, is easier to move than piping connected in a trianglular pattern.
Rectangular spacing is generally used over square spacing when a field is known for its elevated wind velocities. This tends to keep the water application uniform. In general, wind as well as the EVT rating will play a role in how far or close the sprinkler heads are spaced from each other.
For continuous-move systems, there are two main designs: the center-pivot design and the linear-move design. The linear-move design usually involves one long lateral pipe with attached sprinkler heads evenly spaced which basically moves from one end of the field to the other. The center-pivot design has the lateral pipe situated in the middle of the field and rotates in a circle around a fixed "pivot structure" (James, p. 187). Like the linear-move system, the lateral on the center-pivot contains the sprinklers that irrigate the crop (as seen in the picture below).
The lateral is kept in a straight line by a computerize alignment system that starts and stops the movement of the lateral. Center-pivots are best suited to "coarse textured soils with high infiltration capacities" and are especially useful where light, frequent irrigations are required. The disadvantage is that most crops are divided into square blocks. In a square shaped field, only 79% of the field gets irrigated. However, farmers sometimes add what is known as a "corner catcher" to the main lateral that "folds out as the corner is approached and folds back toward the main lateral as the machine moves out of the corner toward the near side of the field" (James, p. 187).
Now, let's take a look at the particulars involved with Residential Irrigation System Design.
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