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Water Supply Systems
Dean E. Eisenhauer, Derrel L. Martin, Derek M. Heeren, Glenn J. Hoffman
Pages 171-184 (doi: 10.13031/ISM.2021.9) in Irrigation Systems Management. ,
Abstract. See https://www.asabe.org/ISM for a PDF file of this entire textbook at no cost.
9.1 Introduction
When you consider installing an irrigation system, there are several water supply questions that must be answered. First, where is a suitable supply of water? More specifically, will the water come from a reservoir, a water course, or a well? Second, will water rights need to be addressed? The third question is about the quality of the water: is the water saline or will it be reclaimed water? Such questions are addressed in this chapter.
Although the most plentiful substance on the earth’s surface, water is frequently not available in sufficient quantity when and where it is needed. To overcome these deficiencies, water resources are frequently stored and then conveyed from the time and place of natural occurrence to the time and place of beneficial use. The demand for reliable water supplies continues to increase as world population grows and becomes more affluent. Depending on the location and climate, a large portion of water withdrawals are used for irrigation (Figure 9.1). Where the water supply is inadequate, competition arises between agricultural and urban water users and other users. In many locations, agricultural interests developed the water supply initially, thereby acquiring rights to the water through prior use. As urban water use increases, municipalities frequently can afford to pay higher water costs and can achieve a greater economic return per unit of water than agriculture. One solution to this dilemma is the purchase of agricultural land by municipalities to acquire the rights to the water. Another alternative is to seek rights to water through the legislative and judicial systems. Yet another potentially more attractive alternative is for the nonagricultural water users to pay for improved irrigation delivery or management systems with the water from reduced irrigation consumption going to the municipalities and agriculture being paid for the water (see also section 5.10). While this chapter focuses on water quantity for irrigation, the water quality of a water supply system should also be assessed for any potential negative impacts on the crop and soil (Chapter 7 and Suarez, 2012).
When considering the development of a water supply, water is categorized as surface water or groundwater. Surface water originates from precipitation on the landscape moving downslope to streams and rivers. A portion of the water in streams and rivers is from overland runoff. The balance is baseflow, streamflow that comes from groundwater. When flows are always ample to satisfy water demands, surface waters can be withdrawn directly from the natural water course. The flow of many water courses, however, fluctuates too widely over time to satisfy water demands. For many rivers, peak water demands occur at times of minimal flow. This situation requires the construction of reservoirs to store high flows to be released later for beneficial uses. Reservoirs are normally created by constructing a dam across a stream or river. In special situations reservoir sites are located off-stream. Surface storage may range in size from huge multipurpose reservoirs to small ponds.
Most precipitation that infiltrates and deep percolates beyond the plant root zone eventually reaches the groundwater table, called groundwater recharge. Groundwater is water beneath the earth’s surface that occurs in saturated materials. A zone of saturation in a substratum capable of yielding enough water to satisfy a particular demand is referred to as an aquifer. A major challenge facing water managers is ensuring that withdrawals of groundwater plus baseflow requirements do not exceed recharge. In many areas where groundwater is the major water supply, withdrawals exceed replenishment, and a sustainable water supply is in jeopardy.
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| Figure 9.1. Irrigation withdrawals by state for 2015 (top) and irrigation withdrawals over time (bottom). (Both illustrations from Dieter et al., 2018.) |
9.2 Water Rights and Laws
From a legal perspective in the U.S., water may be classified as diffused surface water, water in well-defined surface channels, water in well-defined aquifers, and underground percolating water. Diffused surface water is precipitation spread across the landscape. Diffused surface water and underground percolating water, because of their diverse nature, are normally regulated by common or civil law. In most states diffused surface water is considered the property of the landowner, who may use it without regard to the water supply of others. In many states, particularly those in the east, the law of diffused surface water has addressed who is responsible for the damage caused by diffused surface water. In some western states, diffused surface water is treated the same as water in well-defined channels.
9.2.1 Surface Water
In the U.S., the right to use surface water in natural watercourses is governed by two different doctrines: riparian and prior appropriation. In different states these doctrines are recognized either separately or in combination. In the future, adjudicated water rights based on highest-value use will become increasingly important.
The riparian doctrine (National Agricultural Law Center, 2020) recognizes the right of an owner of riparian land to make reasonable use of the stream’s flow on the riparian land. Riparian land is contiguous to the stream or other body of surface water from which water is withdrawn. The right-of-land ownership includes the right of access to and use of the water. This right is not lost even if water is not used. Reasonable use of water generally implies that the landowner may use all the water needed for drinking, for household purposes, and for watering livestock. Where large herds of livestock are watered or where irrigation is practiced, the riparian owner is not permitted to exhaust the stream flow. Owners may only use their equitable share of the flow. This doctrine is used in many eastern states.
The doctrine of prior appropriation is based upon the priority of development and use. The first person to develop and put water to beneficial use has the right of continued use. The right of appropriation is generally acquired by filing a claim in accordance with state laws. If the use is beneficial, the appropriator has the right to all water required at the given time and place. This doctrine assumes that it is better to let individuals, prior in time, to take all the water rather than distribute inadequate amounts among all water users. Appropriated water rights are not limited to riparian land and may be lost by nonuse. This doctrine is recognized in most western states, although in some states it is in combination with the riparian doctrine. It is difficult to make generalizations because state laws on water rights differ on specific details and many change with time.
Today, almost all riparian states have moved towards allocating water through a permitting system. Using the same “reasonable use” criteria as common law, the states first determine whether a new use is reasonable. The permitting system allows the state to plan for and maximize water usage in the future. In many states, agricultural uses are exempt from permit requirements.
Some states, such as California and Oklahoma, have developed hybrid allocation systems. Hybrid systems combine aspects of both the riparian and the appropriation systems.
9.2.2 Groundwater
Most states in the U.S. have a different allocation system for groundwater than for surface water. Groundwater allocation systems often differentiate between on-tract and off-tract uses. On-tract use is where water is used on the tract where the well is located. Off-tract use is where water is transferred to another location.
Under the absolute dominion rule (National Agricultural Law Center, 2020 and Driscoll, 1986), a landowner may use as much groundwater as possible. The impact of the groundwater use on neighboring users is not taken into account. Although some states follow this doctrine with allowances for remedies for willful injury, most states have rejected this doctrine as malicious withdraws of water.
The correlative rights doctrine distributes water on an equitable basis among landowners and allows off-tract uses, although these are subordinate to on-tract uses. With the Correlative Rights Doctrine the landowners overlying the same aquifer are limited to a reasonable share of the aquifer supply.
Some of the western U.S. states apply the doctrine of prior appropriation (similar to its application to surface water), which gives the earlier water users priority over later users. The water use amount is limited to beneficial uses.
Another legal approach is to apply the rule of reasonable use. As the name implies the landowners have the right to use the groundwater beneath their land as long as it is deemed to be a reasonable and beneficial use.
In the U.S. these legal approaches, along with others, for groundwater use are applied on a state-to-state basis. For example, the State of Nebraska uses a unique blend of the Rule of Reasonable Use and the Correlative Rights Doctrine along with statutory preferences for use (Aiken, 1980).
9.3 Aquifers
Geologically, the loose and discontinuous layers of decayed rock debris overlying solid bedrock are termed regolith. Soil, where chemical and physical weathering are the most active, is the uppermost part of the regolith. The regolith is a potential storage medium for water. Above bedrock, which is essentially impermeable to water, the rock is fractured and frequently consists of gravels, sands, and soil particles. As illustrated in Figure 9.2 water can be contained in the pores (interstices) of soil, sand, gravel, and rock (Meinzer, 1923). The substrata containing interstitial water is divided into the unsaturated zone and the zone of saturation. Groundwater that can be successfully extracted for a water supply only occurs in the saturated zone. The boundary between the unsaturated and saturated zones is called the water table. The water table may be at or above the soil surface as in swamps, wetlands, and near lakes and continuously flowing streams.
Permeability or hydraulic conductivity is a measure of the ability of an aquifer to transmit water. The total porosity and permeability of an aquifer depends upon the size and shape of the pores. Table 9.1 provides approximate values for total porosity and relative permeability. The specific yield of an aquifer, the portion of the stored water that can be withdrawn for a water supply, is also given in Table 9.1. Except for clay, porosity is generally a good indicator of the amount that can be withdrawn from an aquifer. Clay, although high in porosity, has a low permeability that limits water flow (Table 9.1). Usually, sand, gravel, and fractured rock are good water-bearing deposits that can be developed as a water supply. Table 9.2 summarizes the ranges of hydraulic conductivity for various aquifer materials.
In some geologic formations, groundwater may be confined under pressure between two impervious layers. This “confined” aquifer may create what is termed an artisan condition. For artesian flow to be possible there must be a pervious stratum that is continuous from a region upslope where water can percolate into the aquifer to a downslope region where the aquifer is confined between upper and lower impervious layers. When a well is installed through the upper impervious layer into the confined aquifer, water will rise up the well to a level depending on the hydrostatic pressure on the aquifer at the well location. If the pressure is high enough, water will flow out of the well under this artesian condition. The more normal condition for both confined and unconfined aquifers is that groundwater must be pumped from the well. Figure 9.3 illustrates the geological conditions that foster these various sources of groundwater. For a more detailed presentation of groundwater, the reader is referred to Freeze and Cherry (1979), Todd (1980), and Sterrett (2007).
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Figure 9.2. Several types of interstices found in substrata that can store groundwater: well-sorted sedimentary deposit having high porosity (top left), poorly-sorted sedimentary deposit having low porosity (top right), rock rendered porous by fracturing (bottom left), and rock rendered porous by dissolution (bottom right). (Image courtesy of Barkmann et al., 2020.) |
| Table 9.1. Approximate characteristics of groundwater aquifers (adapted from Schwab et al., 1992). | |||
| AquiferMaterial | Total Porosity(%) | Specific Yield(%) | RelativePermeability |
Dense limestone | 5 | 2 | 1 |
Dense shale | 5 | 2 | 1 |
Sandstone | 15 | 8 | 700 |
Gravel | 25 | 22 | 5,000 |
Sand | 35 | 25 | 800 |
Clay | 45 | 3 | 1 |
| Table 9.2. Range of hydraulic conductivity values for various types of aquifers (adapted from Driscoll, 1986). | ||
Aquifer Formationor Material | Range of Hydraulic Conductivity(ft/d) | |
| Low | High | |
Fine to coarse gravel | 101 | 104 |
Fine to coarse sand | 10-3 | 102 |
Silt and loess | 10-4 | 100 |
Glacial till | 10-8 | 100 |
Karst limestone | 10-2 | 102 |
Shale | 10-9 | 10-5 |
Sandstone, well cemented, unjointed | 10-6 | 10-4 |
Sandstone, friable | 10-4 | 10-1 |
Unfractured igneous and metamorphic rocks | 10-9 | 10-6 |
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| Figure 9.3. Cross section of geologic formations illustrating sources of groundwater for water supply. (Modification of image supplied courtesy of Barkmann et al., 2020.) |
9.4 Groundwater Supplies
Wells, by far, are the most common source of groundwater. Wells are holes drilled downward from the soil surface into an aquifer. Tube wells are drilled by machine to groundwater that is generally less than a few hundred feet and are typically simpler in design than deep wells. Deep wells, discussed in detail later, can be thousands of feet deep to reach deep aquifers. Where the water table is relatively shallow, wells can be dug by hand if not prohibited by regulations. Also, springs or dugout reservoirs can be sources of groundwater for a water supply in some situations. In areas where the water table is only a few feet below the soil surface, dugout reservoirs or open pits can provide access to groundwater. Occasionally, these water sources can be developed into a suitable water supply, if allowed by national and local regulations.
9.4.1 Shallow Wells
Globally, many rivers and streams have flood plains that are irrigated. In many instances, the water table is only a few feet below the level of the water course. In these areas, shallow wells are dug by hand or machine into the groundwater. These wells are typically less than 50 feet deep and less than 10 feet in diameter. Stone, brick, or other materials may be used to stabilize the walls.
Since power requirements for pumping water are a function of total dynamic head and flow rate (Chapter 8), low-flow wells in shallow aquifers have a much lower power requirement than for wells with high pumping lifts and wells with high flow rates. For small-scale irrigation systems using water from a shallow aquifer, a greater variety of options are available for lifting the water, including treadle pumps, pedal pumps, and water wheels (Figure 9.4). Along with small fuel-powered pumps, small electric pumps powered by a solar panel (solar pumps) are used more often for pumping systems with low power requirements than high-power pumping systems (Figure 9.5).
(a)  (b) (c)   | |
| Figure 9.4. Systems for accessing water from shallow aquifers: (a) treadle pump (photo courtesy of iDE, International Development Enterprises); (b) pedal pump (photo courtesy of Maya Pedal Guatemala), and (c) a water wheel in Rajasthan, India (photo courtesy of Carl Anders). | |
 | Figure 9.5. Water delivery system for groundwater from the shallow alluvial aquifer of the Shashe River, Zimbabwe. A solar pump is used to deliver water from the concrete-lined storage pond to a nearby field, and the net provides shade for fish. (Photo courtesy of Annelieke Duker, IHE Delft Institute for Water Education.) |
9.4.2 Tube or Cased Wells
When the water table is relatively shallow, on the order of a few hundred feet, tube wells are used frequently. Tube wells consist of a casing with screen or perforations near the bottom of the well. The casing, normally made of steel, PVC plastic, concrete, or fiberglass, is installed during or after the drilling process to stabilize the hole and allow water, but not subterranean particles, to move into the well. The lower portion of the casing is perforated, slotted, or screened with openings sized to minimize the entry of subterranean particles from the aquifer medium into the well. Refer to section 9.4.4 for a discussion on screen type and size of openings in the screen or casing.
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Figure 9.6. Well constructed in a sand and gravel formation. A casing and screen are always used. A gravel pack is optional. |
9.4.3 Deep Wells and Well Hydraulics
For deep wells, the casing and screen diameter can range from a few inches to a few feet and can range in depth from less than 50 feet to more than several thousand feet. A cross section of a well installed in homogeneous material overlying an impervious rock formation is shown in Figure 9.6. Under static conditions when the well is not being pumped, the water level in the well will rise to the static water table position (Figure 9.7). When pumping begins, the water level in the well is lowered and water from the surrounding material flows into the well. The water table around the well is lowered to the general form of an inverted cone. The vertical distance from the static water table to the water level at the well is known as the drawdown. If pumping continues at a constant rate, the shape of the water table surrounding a well will become nearly stable. The horizontal distance from the well to where the water table is not noticeably lowered by drawdown is known as the radius of influence.
There is a definite relationship between drawdown and discharge from a well. Typical relationships are shown in Figure 9.8. For thick aquifers or artesian formations, the relationship is nearly a straight line. As the aquifer becomes thinner, less discharge occurs for the same drawdown as in a thick aquifer.
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| Figure 9.7. Well hydraulics including static water level and drawdown. |
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| Figure 9.8. Typical relationships between drawdown and discharge of wells. |
Drawdown (s) is the difference between static water level (SWL) and the pumping water level in the well (PWL) and is calculated as:
s = PWL – SWL (9.1)
When a well functions like the straight line in Figure 9.8, the specific capacity, SC, is constant and is calculated as:
(9.2)
where Q is discharge in gallons per minute (gpm) and s is drawdown in feet. Specific capacity is a useful term when predicting drawdown in a well for a given discharge because:
(9.3)
See Example 9.1 for application of Equations 9.1, 9.2, and 9.3.
9.4.4 Well Construction
In constructing and developing a successful well, several features require careful consideration. These features include the method of drilling, well alignment, depth of well, casing material, casing perforations, gravel packing, well development, and well testing. Driscoll (1986) presents significant detail on well design and specifications, as well as well drilling methods. The specifications for wells and the certification of well drillers is often regulated by states in the U.S. Also, it is not uncommon for states to regulate the spacing of wells to prevent interference with neighboring wells.
Most irrigation wells are drilled with cable or rotary tools. With cable tools, a heavy bit is repeatedly dropped onto material at the bottom of the well. Crushed material is removed periodically with a bailer. Wells up to 5,000 feet deep have been drilled with a cable tool. The most common method of drilling, however, is by rotary tools. A bit is rotated by a drilling pipe and a mud slurry is pumped through the pipe to the bit to carry cuttings up the outside of the pipe to the soil surface.
An irrigation well should normally penetrate the water-bearing formations as deeply as possible. The deeper well will usually provide a greater yield of water per foot of drawdown of the water table. It is imperative that the well is vertical for the installation and replacement of pumps. Particularly for wells that have a gravel pack, misalignment because of gravel wedging somewhere below the surface causes the casing to be pushed out of alignment. With proper drilling, well development, and maintenance, a well should last several decades.
Casing materials for wells include steel, wrought iron, concrete, plastic, and fiberglass. Many states have specifications for casing depending on the type of well. Well casings are perforated, slotted, or screened near the bottom of the well to facilitate entering groundwater. Properly-sized perforations prevent subterranean particles from flowing into the well with the water.
Wells drilled in unconsolidated material with rotary drills are usually gravel packed. The selection of the right gravel material is crucial to prevent particles from moving into the well. The gravel needs to be large enough to permit sufficient water flow but with small size pore spaces to prevent solids from moving. The gravel packing material cannot be more than five times the average size of the substrata material if the well is to be stabilized. With these size restrictions, many times a specially manufactured well screen is used rather than the gravel pack. Screens are constructed of brass, bronze, galvanized steel, stainless steel, plastic, or fiberglass to resist corrosion.
Immediately after a well is constructed, it is normally “developed.” The purpose of well development is to make the well sand-free and maximize the flow of water from the aquifer. To prevent pump damage, materials like clay, drilling mud, silt, and sand are removed from the vicinity of the well casing that is screened. There are several methods to develop a well. They are all designed to loosen fine particles so they can be pumped from the well before the permanent pump is installed. The pump used during well development is designed specially to be tolerant of fine particles. The most common method to develop a well is surging. The pump is turned on and then off to allow water to surge back into the well thereby drawing fine materials into the well to be removed by pumping during a repeat of the surge cycle. Another surging technique uses a surge block. The surge block is a tool fastened to the end of the drill. As the drill is moved up and down, it produces a pumping action to draw fine particles into the well.
After the well is developed, a temporary pump is installed for a pumping test. During the pumping test the flow rate (discharge) and drawdown are measured simultaneously. This information is required to select the proper size of pump. Sterrett (2007) provides a practical reference for planning and installation of water wells.
9.5 Surface Water Supplies
To the irrigator, there is great value and need for a dependable water supply that is flexible with respect to the frequency of available water, the rate of water delivery, and how long the water is available. These expectations for a surface water supply are more easily accomplished by pressurized delivery systems than by open channels. Nevertheless, the predominant means of delivering irrigation supplies from irrigation projects is by open channel. Pressurized delivery systems include pipelines which may vary from being underground and permanent to portable, temporary pipe on the soil surface.
9.5.1 Open Canals
Conveyance canals or ditches are frequently used to deliver water from surface storage or wells. A system of open canals often distributes water great distances from its source to the field. Figure 9.9 shows a large open canal and smaller lateral canals.
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Figure 9.9. Canal delivery system in western Nebraska, which delivers water from Seminoe Dam (inset). |
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| Figure 9.10. On-farm ditch providing water to small fields near Delhi, India (top), lateral canal with a weir to provide sufficient head (water surface elevation) for siphon tubes in Nebraska. (Bottom photo courtesy of Steve Melvin, Nebraska Extension.) |
Losses of water by seepage from canals can be a major concern. Water seeping out the bottom of the canal is especially high in earthen canals at the beginning of an irrigation season when soil intake rates are high. Figure 5.8 gives approximations of canal seepage losses depending upon soil texture for unlined canals. Proper soil compaction at optimum moisture content can almost eliminate seepage in some soils.
Irrigation canals are sometimes lined to minimize seepage losses. In addition to reducing seepage, canals are lined to ensure against interrupted operation resulting from channel failure; to provide a more efficient cross section by increasing sideslopes, by reducing the roughness coefficient, by eliminating vegetative growth, and by reducing maintenance. Canals can be lined with a variety of materials. The most common lining material by far is concrete, but other materials include brick, rock masonry, asphalt, soil cement, rubber, colloid clay, and plastic. Concrete meets all the requirements for a lining better than any other material. Its principal disadvantages are high initial cost and possible damage from soil swelling and shrinking, soil chemicals, and freezing and thawing. Concrete can be applied in a variety of ways but continuous pouring with slip-form equipment is the most common.
The purpose of irrigation delivery systems is to provide water to the field in a timely and reliable manner (Figure 9.10). To improve reliability and increase flexibility frequently requires some type of automation of the delivery system. Water is delivered by one of three possible scheduling techniques: demand, arranged, and rotation. A “demand” schedule allows for complete flexibility on the frequency, rate, and duration of water delivery. A common example is a municipal water system; the user can open the faucet at any time (flexibility in frequency), receive a low or high flow rate (flexibility in flow rate), and take the water as long as desired (flexibility in duration). An “arranged” schedule requires the user to request the rate and duration of a water delivery in advance. The advance notice required to receive and to turn the water off is typically one to two days. Arranged schedules often require that the water be turned on or off at a specific time of the day. In a “rotation” schedule, all flow entering a small canal is delivered to only one field. The length of time water is delivered to a field depends upon its size. After delivering water for the prescribed period to one field the flow is shifted to the next.
In addition to the reliability that water is delivered when and as promised, there are two other aspects. One aspect is that the flow remain at the prescribed rate; the second is that flows and water levels in the canal are controlled so that canal structures and soil banks are not damaged.
Many water delivery systems are now automated and there are many types of automated systems. Methods of automatic control differ based upon the control of flow rate or water level in the canal, the control based on measures at the upper end or the lower reaches of the delivery system, and the control being local or remote. More information on developing surface water supply systems for irrigation, including small earth dams, is presented in Huffman et al. (Huffman et al., 2013).
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| Figure 9.11. Installation of a buried pipeline for irrigation water delivery. |
9.5.2 Pressurized Delivery Systems
Pipelines are used extensively to deliver water, especially when the capacity required is low enough for standard pipe sizes or the advantage of a closed delivery system outweigh those of a canal system (Figure 9.11). There are pipeline delivery systems where the pipe is 10 feet or more in diameter. Some of the advantages of buried pipelines include: few problems from damage caused by animals; no vegetative problems; land over the pipeline can be utilized; buried pipelines do not obstruct cross traffic; pipelines do not have to follow elevation contour lines; lower maintenance costs; less hardware required for controlling flows; and less threat of drownings.
Disadvantages of pipelines compared to canals include: initial cost may be higher than canals; and pipelines may plug from sediment or debris more easily.
Pipelines for water delivery systems are increasing in popularity. The conversion is especially rapid in expanding urban areas. Some irrigation districts use monolithic (cast-in-place) concrete pipe for low-pressure conditions. Reinforced concrete pipe are being used uphill and downhill from a supply canal. The uphill pipelines are supplied by pumps while the downhill laterals are normally gravity fed.
In some locations, the downhill laterals have sufficient slope and length to develop the pressure required to operate sprinkler systems without booster pumps. Many pipelines operate with a pressure head of 2 feet or less and lead directly to surface irrigation systems or booster pumps to provide the head for sprinkler or microirrigation systems.
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| Figure 9.12. Illustration of stream depletion, when a pumping well intercepts groundwater that would have flowed into the stream as baseflow (modified from Winter et al., 1998). |
9.6 Surface Water-Groundwater Interaction
It is recognized that surface water and groundwater are connected, to the point that they have been referred to as a single resource (Winter et al., 1998). For example, many streams are gaining streams (gaining water from the adjacent aquifer), while some streams are losing streams. Water resources managers need to account for these interactions when planning at a watershed or basin scale. A specific application for irrigation is the impact that a groundwater well can have on a nearby stream (Figure 9.12), which is called stream depletion (Barlow and Leake, 2012). In this case, the well is pumping water stored in the aquifer which would have flowed to the stream; however, in some cases, a portion of the pumped water can actually come from the stream water, depending on the pumping rate, the length of time of pumping, and how close the well is to the stream.
9.7 Reclaimed Water Supplies
Reclaimed water includes raw and treated sewage water from industries and municipalities (wastewater), runoff from the low end of surface irrigated fields (generally called tail water), and water from subsurface drainage systems. Concerns from tail water are fertilizers, pesticides, and suspended soil particles. From drainage water, the concerns are nutrients, chemicals, and salts. The highest concerns, however, are use of sewage water. The concerns focus on the potential risks of disease from bacteria, virus, and pathogens (Pachepsky et al., 2011). Apart from being a health risk, it is also an environmental issue. If sewage water used for irrigation enters surface waters, such as lakes and rivers, it can contaminate these waters and harm ecosystems. Soils do filter a large amount of pollutants from wastewater. Studies indicate up to 90% of pollutants can be removed but the filtered water may still contain bacteria and viruses.
Developing countries report much higher levels of pathogens in irrigation waters than developed countries (Thurston-Enriquez et al., 2002). For example, wastewater irrigation provides a quarter of all vegetables produced in Pakistan. Globally, an area the size of Germany is primarily irrigated with human sewage water. This translates into a health risk for all people who consume the foods grown on these irrigated lands.
Although standards for the use of reclaimed wastewater exist for food crops eaten raw in the United States, using reclaimed wastewater to irrigate food crops is seldom practiced. In developing countries, raw or partially treated wastewater is often used to grow food crops. Throughout the world wastewater use has become significant and this has encouraged many countries to develop regulations to control water quality to reduce health and environmental risks.
Wastewater use will become more and more attractive for irrigation, given the current and future problems of water scarcity for irrigation. The amount of collected and treated wastewater is sure to increase significantly with population growth, rapid urbanization, and improvement in sanitation service. More information on using reclaimed water for irrigation can be found in Waller and Yitayew (2016).
9.8 Summary
Water is supplied for irrigation from both surface and groundwater. The right to use water for irrigation varies among states and countries, and irrigators should check on which laws apply in their area. Groundwater is extracted by wells that vary from a few feet to thousands of feet. The complexity of the well design depends upon its depth. Surface waters are conveyed to fields by a series of open canals or buried pipelines. Reclaimed water will become a larger source of irrigation water in the future as the demand for fresh water increases.
Questions
1. Prepare a table that summarizes the attributes of the two common doctrines of surface water rights with respect to:
a. How a water right is acquired,
b. Quantity of water that can be used,
c. Types of water use allowed,
d. How or if a water right can be lost, and
e. Where water acquired through these two doctrines may be used.
2. Prepare a diagram of the regolith. Show the vadose zone, phreatic zone, crop root zone, capillary fringe, and the water table.
3. What method of drilling wells is most common in your area?
4. What discharge rates for irrigation wells are typical in your area? What discharge rates are common for domestic wells?
5. If unlined canals can have significant losses of water because of seepage, why are earthen dams effective?
6. In the city nearest to where you live, is water delivered by canal or pipeline? Are farms near you irrigated from canal or pipeline systems or are on-site wells used?
7. A new irrigation well was drilled to a depth of 300 feet. The static water level was 80 feet. The well was test pumped at 1200 gpm and the pumping water level was 140 feet. The planned irrigation system will have a flow rate of 900 gpm. Determine the expected drawdown and pumping water level at 900 gpm.
References
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