surface irrigation design

The gated pipe may be connected to the main water supply via a piped distribution network with a riser assembly like the one shown in Figure 13, directly to a canal turnout, or through an open channel to a piped transition. Time is cumulative since the beginning of the irrigation, distance is referenced to the point water enters the field. In short, there is not a universal algorithm for design and evaluation that eliminates the need for good judgement. wire, is placed above the pipe approximately 18 inches below the surface grade. and reuse, 2.4.2 To reach maximum levels of efficiency, the flow per unit width must be as high as possible without causing erosion of the soil. The time and space references shown in Figure 1 are relatively standard. Before moving to the final design computation, the design of the head ditch, mention is made of using the cutback system under variable field conditions. While these systems represent significant percentages in some areas, they will not be discussed in detail in this paper. 5.2.1 Preliminary design Surface Irrigation is a kind of irrigation where gravity works its best. It is usually better to reduce the field length and repeat these calculations. 3c. The application efficiency, Ea, can be computed using Eq. The evaluation methods can be applied if desired, but the design techniques are not generally applicable nor need they be since the irrigation practices tend to be minimally managed. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.6, Figure 53f. Illustration of a typical reuse configuration. The actual prices of drip irrigation … Slip-form concrete lining in the USA. The results, shown in the table below, indicate that one good design is to divide the field into 4 individual subunits or sets of 50 furrows and utilize an inflow of 0.018 m3/min per furrow during the first irrigations. 5.2.2 Detailed design. In these cases, the volume of water on the surface will continue to advance along the border until it reaches the lower end where it will run off or pond in front of the dyke. The maximum depth can be approximated by Eq. Another furrow irrigation option is to capture runoff in a small reservoir at the end of the field and either pump it back to the upper end to be used along with the primary supply or diverted to another field. The usual procedure is to compute the deep percolation ratio and then find the tailwater ratio as 100 - Ea - DPR in percentages. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.2, Figure 53b. the physical and chemical characteristics of the soil, especially the infiltration characteristics, moisture-holding capacities, salinity and internal drainage; iv. 2.4.2 The depth of water at the end of the border, yL, will be: viii. One can find optimal designs and management practices for a multitude of conditions because designs historically requiring days of effort are now made in seconds. The most common piped method of furrow irrigation uses plastic or aluminium gated pipe like that shown in Figure 14. The procedure given here is intended to be conservative and will yield designs capable of performing at somewhat lower application efficiencies than is perhaps possible using the more comprehensive methods. Proper design of surface irrigation systems takes into account the soil type (texture and intake rate), slope, levelness of the field, stream size, and length of run. The maximum unit flow, Qmax, is difficult to assess. For most surface irrigated conditions, rreq should be as close as possible to the difference between the recession time at each point and the associated advance time. Recession begins at that point and continues until the surface is drained. borders On-demand systems should have more flexibility than continuous or rotational water schedules which are often difficult to match to the crop demand. It cannot be used unless this condition is met. 115. iv. The next step in detailed design is to reconcile the flows and times with the total flow and its duration allocated to the field from the water supply. The soil also con- veys and distributes the water over the field. Designs for flow measurement and drop-energy dissipator structures need more attention and construction must be more precise since their hydraulic responses are quite sensitive to their dimensions. Choosing six sets as the basic field subdivision, the number of furrows in the first set is: For the first irrigation, the volume of the runoff reservoir must be: Recalling that for a first irrigation condition, the time of cutoff is 278.5 minutes, the capacity of the pump-back system is therefore: The number of furrows per set for the subsequent sets is: There are 200 furrows in the field. One would not expect the border infiltration equation to more than double furrow infiltration with furrows spaced less than 1 m apart. Equation 48 for the end of advance was written earlier as Eq. This may appear simpler to some and more difficult to others. It may be furrowed or corrugated, have raised beds for the benefit of certain crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it remains a basin. Equation 50 contains two unknowns, tL and r, which are related by Eq. The computation proceeds as follows: Since the application efficiency will vary with Qo several designs should be developed using different values of inflow to identify the design discharge that maximizes Ea. 2.2.4 Uncontrolled flooding. The interested reader can refer to several references in the bibliography for other graphical techniques which extend beyond those given here, but as one does so, it becomes more important to understand the nature of the hydraulic assumptions. There will be substantially more water on the surface of borders than for furrows. In this case, the design is more easily accomplished because of the higher level of experience and data available. The time of advance to the field's half-way point is found by following the same steps as outlined above by substituting 0.5 * L = 100 metres for the length and t.5L for the advance time to this distance. to surface irrigation Compute a revised estimate of r as follows: 6. Landscape Irrigation Design Manual iii ... water surface 200 ft (100 m) above the point where we need it would create a pressure of 86.6 psi (10 bar). There are several unique features of cutback systems that need to be considered at the design stage. From results already available, the required intake opportunity times, rreq, needed to apply a depth of 8 cm (Zreq) were about 389 minutes and 679 minutes for initial and subsequent field conditions, respectively. The Qo verses tL data are plotted in Figure 60. In sprinkler irrigation the water is Landscape Irrigation Design Manual iii ... water surface 200 ft (100 m) above the point where we need it would create a pressure of 86.6 psi (10 bar). Subsurface microirrigation is suitable for almost all Crops and especially for high-value fruit and vegetable Crops, turf and landscapes. Sprinkler irrigation is the process of irrigation by which water is sprayed on the land surface in the form of artificial rain. Conveyance, distribution and management structures, 2.4.2 Conveyance, distribution and management structures. A similar operating scenario prevails for each subsequent pair until the last set is irrigated when some of the tailwater must be either stored until the next irrigation, dumped into a wasteway, used elsewhere or used to finish the irrigation after the primary inflows have been shut-off. The long-term integrity of the detection material is the problem. If the maximum flow is too small to complete the advance, the furrow length must be reduced. Consequently, a wider range of furrow flows needs to be examined along with their performance characteristics. Initial calculations. For borders and basins, open or piped cutlets as illustrated in Figure 11 are generally used. On small fields, the total supply may provide a satisfactory coverage when used to irrigate the whole field simultaneously. Extrapolate also the r and p values in Eq. Surface irrigation systems have two principal sources of inefficiency, deep percolation and surface runoff or tailwater The remedies are competitive. One simplification of border analyses is that the geometry of the flow is simpler because it can be treated as wide, plane flow. However, the depth of infiltration at the end of the field and at the distance L-l metres from the inlet should be checked as Eq. Compute the advance time for a range of inflow rates between Qmax and Qmin, develop a graph of inflow, Qo verses the advance time, tL, and extrapolate the flow that produces an advance time equal to rreq. However, in looking for a root cause, one most often retreats to the fact that infiltration changes a great deal from irrigation to irrigation, from soil to soil, and is neither predictable nor effectively manageable. The resulting application efficiency would be nearly 56 percent. Basin irrigation design is somewhat simpler than either furrow or border design. Schematic drawing of the furrow cutback system proposed by Garton (1966). Generally, the computations for blocked-end borders are best performed with zero-inertia or full hydrodynamic simulation models which are beyond the scope of this paper. ­­Surface irrigation is arguably the least complex form of irrigation. It should probably be in the range of 1.1 to 1.5. From this analysis the amount of water the system should supply through the season can be estimated. vii. Example relationships between inflow rate and advance time, i. Even today it is often more economical to regulate the inflow rather than to collect and pump the runoff back to the head of the field or to another field, tailwater reuse systems are more cost-effective when the water can be added to the flow serving lower fields and thereby saving the cost of pumping. 2.3.2 Wastewater recovery The management of water in the field channels involves flow measurement, sediment and debris removal, divisions, checks, drop-energy dissipators, and water level regulators. There is substantial field evidence that surface irrigation systems can apply water to croplands uniformly and efficiently, but it is the general observation that most such systems operate well below their potential. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.3, Figure 53c. 5.5.1 Design of As an example of this series of calculations, suppose the advance time is wanted for a field with the following data: 3b. To convert … In this configuration, the head ditch is divided into a series of level bays which are differentiated by a small change in elevation. Consequently, some means of emergency surface drainage is good design practice. The frequency and duration of each irrigation needs to be checked and then the headland facilities selected and designed. Five sets would contain 36 furrows; one set, the first, contains 22. Large furrow flows advance over the field rapidly thereby providing the potential for greater application uniformity and less deep percolation, but also greater tailwater losses as the water flows from the field for a longer time. The operation is relatively simple so long as the total field inflow rate can be regulated to compensate for the lower infiltration during later irrigations. the topography of the land with particular emphasis on major slopes, undulations, locations of water delivery and surface drainage outlets; iii. Calculate the number of furrows in each remaining set as: iv. 60 for basins: 3. 2.2.1 Basin irrigation At the end of the border, the application is ZL from above plus yL, or .1034 m. The depth of infiltration at the distance L-1 metres from the inlet is: Z1 = k (td - tL-1)a + fo (td tL-1) (107). First, the friction slope during the advance phase of the flow can be approximated by: in which yo is the depth of flow at the basin inlet in m, x is the distance from inlet to the advancing front in m, and Sf is the friction slope. The task of sizing these headland facilities will be noted in a later section. Figure 57. Since the water supply is presumably controlled by an irrigation department, the design can be substantially hindered if the delivered flows are not as planned. The surface irrigation design process is a procedure matching the most desirable frequency and depth of irrigation and the capacity and availability of the water supply. Select several field layouts that would appear to yield a well organized field system and for each determine the length and width of the basins. of advance and intake opportunity time, 5.4 Conveying water to the field requires similar structures to those found in major canal networks. For the complete system to work well, each must work conjunctively toward the common goal of promoting maximum on-farm production. As described earlier, the inherent limitation of the cutback design is that the advance phase and the wetting phase must have the same duration. The design procedure outlined below is an extension of the approaches already given and consistent with the level of treatment given herein. The cutback flow following the advance phase must be sufficient to keep the furrow stream running along the entire length. 72): Cutback, therefore, substantially improves the efficiency on this field over traditional methods. Although surface irrigation is thousands of years old, the most significant advances have been made within the last decade. Compare the depletion time with the required intake opportunity time. Usually, border irrigation would require a higher discharge than furrow systems, but as a first attempt at the problem, consider the field supply fixed. A surface irrigation event is composed of four phases as illustrated graphically in Figure 1. A conservative estimate of the field runoff per furrow is: vi. It is in fact the same effort with a slightly different aspect. Starting with a flow near the maximum and working downward using the processes already outlined, advance curves for both infiltration conditions and flow directions can be found. = .76] of the distance between curves K* = 1 and K* = 3 yields However, this practice increases the tailwater problem because the flow at the downstream end must be maintained until a sufficient depth has infiltrated. Furrow irrigation flow rates, cutoff times, and field layouts, 5.4.1 Reuse systems have not been widely employed historically because water and energy have been inexpensive. Figure 2. This guide has reduced the role of these hydraulic techniques to the advance phase to allow the User to participate more in the design process. The changes in the lesser-developed and developing countries are less dramatic. = 0.54. the cultural practices employed in the farming region especially where they may prohibit a specific element of the design or operation of the system. The advance time is then estimated as: Note the value using the volume balance numerical method yielded 65 minutes. Figure 52a. At the end of the drainage period, a pond should extend a distance l metre upstream of the dyked end of the border. 6. The number of furrows in this set is therefore: The field must be divided into an integer number of subsets which may require some adjustment of QT, Qo, or Qcb. It will not be possible to alter the number of furrows irrigating per bay of the head ditch, so the inflow to the entire system must be adjusted. Initial design calculations. surface irrigation, design, management, application efficiency, irrigation performance 1. When the maximum non-erosive flow fails to meet the 30 percent rule, it is usually taken as the furrow flow and the rule is ignored. However, the general situation is that fields must be broken into 'sets' and irrigated part by part, i.e. The reuse system design procedure is as follows: ii. 5.4.1 The approximate wetted perimeter for the furrows is found by returning to the flow area, perimeter, and depth relationships. This can be accomplished with a high, but non-erosive, discharge onto the field. If the advance associated with the maximum flow is too long, then either the required application should be increased (at the risk of crop stress) or the field length shortened. For this: The border designs given here assume the advance phase is completed before the inflow is terminated. The selection of system configurations for the project is in fact an integral part of the project planning process. They range from inadequate design and management at the farm level to inadequate operation of the upstream water supply facilities. Unless the border system is extremely well designed and operated, the downstream pond often creates a substantial threat to the crop in the submerged areas and although dyked at their lower ends, most farmers provide a surface drain for excess water. Furrow irrigation flow rates, cutoff times, and field layouts The computer program given at the end of this paper does not include an option or blocked-end borders. Assuming the borders will run in the 200 m direction on the 0.1 percent slope as above, Figure 59 indicates the inflows that will complete the advance in the respective rreq times are 0.036 m3/min/m for initial irrigations and 0.0215 m3/min/m for later ones. The theoretical and practi-cal … Criteria for the selection of the surface irrigation … Table 9 lists a number of irrigation technologies and a figure representing the costs. This can be caused by physical constraints (e.g., steep land slopes, shallow soils, poor water supplies, … In order to express intake as a depth of application, Z must be divided by the unit width. When the water is shut off, it recedes from the upper end to the lower end. Thus, the border slope is usually the best-fit subplane or strip. 2.4. 67 is: Usually the design of basins will involve flows much smaller than indicated in Eq. Subsurface microirrigation is a low-pressure, low-volume irrigation system that uses drip tubes buried below the soil surface. For the first field rreq = 214 minutes. Water is applied to individual borders from small hand-dug checks from the field head ditch. Using this information along with target application depths derived from an analysis of crop water requirements, the detailed design process moves to the selection of flow rates and their duration that maximize application efficiency, tempered however by a continual review of the practical matters involved in farming the field later. Figure 57 imposes this layout on the field. 99: Finally the application efficiencies of the alternative flows and flow directions are found using Eq. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.5, Figure 52e. The results for this example are shown in Figure 59. Head ditch outlets for borders and basins (after Kraatz and Mahajan, FAO, 1975). The units of Z are again m3/m of length/unit width. Furrow systems use outlets which can be directed to each furrow. When the design is shown to be within this constraint on flow, the next computation is to find the furrow advance discharge which just accomplishes an advance in treq minutes. Furrow irrigation configurations (after USDA-SCS, From Figure 52e, the same process yields a Compute or interpolate the inlet discharge required to complete the advance phase in approximately 30 percent of rreq, correcting if necessary for non-erosive stream velocities. The next step in the design process involves collecting and analysing local climatological, soil and cropping patterns to estimate the crop water demands. For the advance phase. As a final thought in this section, something should be stated regarding costs associated with surface irrigation. Equation 71 was given to assist the designer in avoiding this problem, but it is only a guideline. The furrows were placed on 0.5 m intervals across the 100 m direction (and running in the 200 m direction). And since the objective of the design is to completely refill the root zone, either the time of cutoff needs to be extended or the design value of Zreq should be reduced to approximate the depth infiltrated in the least watered areas to ensure this constraint. The first difference is that while the depletion and recession phases are generally neglected in furrow design, both phases must be included for borders. The first of these can be the maximum allowable flow in the furrow, Qmax. 50 and the half-way advance was written as Eq. Surface irrigation is where water is applied and distributed over the soil surface by gravity. Furrows provide better on-farm water management flexibility under many surface irrigation conditions. 49. The major variables involved in surface irrigation system design and operation are: (a) infiltration rate, (b) slope, (c) rate of advance, (d) rate of recessiqa, (e) boundary geometry, (f) surface … If the analysis has not converged then let T1 = T2 and repeat steps 2 through 5. Since the field and furrow geometries have not changed, the value of Qmax = 1.768 m3/min. These calculations will be left to the interested reader. 4 Introduction to irrigation management WaterWise on the Farm WaterWise on the Farm Evaluating your surface irrigation system 5 The available flow from a … These are transparent to the user of the guide, however, and further explanation for those interested can be found in Walker and Skogerboe (1987). 115: Utilizing Figures 53a-f, the advance time as a function of unit flow can be determined as indicated below. the cropping pattern, its water requirements, and special considerations given to assure that the irrigation system is workable within the harvesting and cultivation schedule, germination period and the critical growth periods; v. the marketing conditions in the area as well as the availability and skill of labour, maintenance and replacement services, funding for construction and operation, and energy, fertilizers, seeds, pesticides, etc. The reuse system shown schematically in Figure 55 is intended to capture tailwater from one set and combine it with the supply to a second set. 2.1 Introduction Again Mannings n can be 0.04 for initial irrigations and .1 for later irrigations due to crop cover. Furrow irrigation designs are often needed either for new irrigation schemes or on existing projects where improvements are needed. If they are within about 0.5 minutes or less, the analysis proceeds to step 4. All other inputs to this problem like infiltration coefficients and roughness are assumed to be the same as in section 5.5.3. Quality of water supply is good and hopefully these deliveries will be made as expected so far as rate, duration, and frequency are concerned. Surface irrigation has evolved into an extensive array of configurations which can be broadly classified as: (1) basin irrigation; (2) border irrigation; (3) furrow irrigation; and (4) uncontrolled flooding. The operation of the system should offer enough flexibility to supply water to the crop in variable amounts and schedules that allow the irrigator some scope to manage soil moisture for maximum yields as well as water, labour and energy conservation. The movement of the water over the soil surface is very sensitive to the relative magnitude of the furrow discharge and the cumulative infiltration rates. The interested reader might want to refer to Strelkoff and Katapodes (1977), Strelkoff and Shatanawi (1984), Shatanawi and Strelkoff (1984), and Yitayew and Fangmeier (1984) for some of these reports. A tentative schedule can be produced by comparing the net crop demands with the capability of the water delivery system to supply water according to a variable schedule. Values of k, a, fo and w along with the volume per unit length required to refill the root zone, Zreq, are design input data. To be skilled in design is to completely understand the relationships among the selectable and manageable variables governing surface irrigation, particularly the effects of infiltration and stream size on advance. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.7. for optimal performance The irrigator will introduce the canal water to the first set and collect the surface runoff from it. Precision land levelling is very important to achieving high uniformities and efficiencies. Land consolidation has been carried out in a number of irrigation projects where implementation has included land reform policies and has resulted in field units amenable to furrow irrigation. The advance time can be calculated using the maximum furrow inflow, Qmax. If a better design is sought, the more advanced simulation models will have to be used. Beyond this 'upper limit' some of the following options also evenly divide the field: The second limitation on the design procedure is whether or not the flow will complete the advance phase in a reasonable time, say 24 hours. The head ditch is divided into a series of level bays with spires or other means of diverting water into the furrows. Discharge-advance relationships for the basin example. Thus, some tailwater will be inevitable but should be minimized. Figure 12 shows a system in which siphon tubes are used as a means of serving each furrow. i. Computation of intake opportunity time. 5.4 The difference between an evaluation and a design is that data collected during an evaluation include inflows and outflows, flow geometry, length and slope of the field, soil moisture depletion and advance and recession rates. (1980) also suggest computing a minimum flow, Qmin, based on a value that ensures adequate field spreading. The Qcb/Qo ratio is taken as .43 reflecting the constraint imposed by the later irrigations. The rreq for the first irrigation is 214 minutes and for the subsequent irrigations it is 371 minutes. And, it should be noted that irrigation of the last two sets cannot be accomplished under a cutback regime without reducing the field inflow, QT, or allowing water to spill from the head ditch during the cutback phase on the last set. In order to facilitate efficient surface irrigation, these structures should be easily and cheaply constructed as well as easy to manage and maintain. Or, tailwater can be used to irrigate separate sections of the field or even other fields. For practical purposes, there may not be a depletion phase and recession can be ignored. The perimeter dykes need to be well maintained to eliminate breaching and waste, and must be higher for basins than other surface irrigation methods. iii. For most furrow irrigated conditions, p2 will have a value ranging from 1.3 to 1.5. 56 is 85.7% which is a substantial improvement over the open-end design. systems. Similarly, the irrigation works themselves are better constructed because of the application of high technology equipment. et al., 1971). Because there is no tailwater problem, the maximum unit inflow also maximizes application efficiency. 5.2 The basic design process border design example, 5.5.4 A blocked-end Figure 58. NfQo should be an integer, but should not exceed Qmax. Basins, of course, are generally 'dead' level, i.e. The term “surface irrigation” refers to systems that deliver water to crops using a gravity-fed, overland flow of water.

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