William J. Elliot

Chapter 13: Drainage Principles and Surface Drainage

Introductory paragraphs: Variable climatic patterns and soil characteristics combine to cause many areas to exhibit either excess or deficit soil water conditions. Few areas can maintain optimal growing conditions without some additional drainage or irrigation. In fact, drainage and irrigation practices have been used for thousands of years to create or enhance productive lands. Some notable examples of large drainage projects are the polders in Holland and the fens in England, which are low lands reclaimed from the sea. Much of the Corn Belt or upper Midwest in the United States was drained and made productive following the passage of the Swamp Lands Acts of 1849 and 1850. The drainage in the Midwest was instigated by medical professionals, who realized that it reduced malaria even before the mosquito was the known cause. Good soil drainage has long been recognized as essential for permanent irrigated agriculture to alleviate salinity problems in areas such as the western U.S., Egypt, and India. With the current trend of world population increase, additional land may be needed for production of food and fiber. Drainage is the removal of excess water from the soil surface (surface drainage) or from the soil profile (subsurface drainage). In most cases, surface drainage represents the easiest and least expensive drainage method. Excess water can be directed to shallow drains or ditches by using the natural topography of the area to remove the water. Some of the most recognizable surface drains are the ditches that drain roads and highways. Many areas contain a naturally high water table. Although good surface drainage can be used to remove excess surface water, subsurface drainage consisting of a series of drainage ditches and possibly buried perforated pipe may be necessary to lower the water table to a desired position. Poorly drained lands that require subsurface drainage are usually topographically situated so that when drained they may be farmed with little or no erosion hazard. Many soils having poor natural drainage are, when properly drained, rated among the most productive soils in the world.

Chapter 4: Evaporation and Evapotranspiration

Introductory paragraphs: Two phases of the hydrologic cycle of particular interest in agriculture are evaporation and transpiration. About three-fourths of the total precipitation received on land areas of the world returns directly to the atmosphere by evaporation or transpiration. Most of the balance returns to the ocean as surface or subsurface flow. Evaporation is the transfer of liquid surface water into vapor in the atmosphere. The water molecules, both in the air and in the water, are in rapid motion. Evaporation occurs when the number of moving molecules that break from the water surface and escape into the air as vapor is larger than the number that re-enter the water surface from the air and become entrapped in the liquid. Evaporation, which may occur from water surfaces, wet leaf surfaces, or from water on soil particles, is important in water management and conservation.

Chapter 16: Surface Irrigation

Introductory paragraphs: Surface irrigation is an important method of irrigation in most countries with large irrigated areas. In the United States, 44.9% of the irrigation in 2000 was accomplished with surface methods, according to a survey reported by the Irrigation Journal (2001). In the western states, where this percentage is higher, the major water supply for irrigation is surface runoff, usually stored in reservoirs. Since this water must be conveyed for considerable distances, conveyance canals and control structures are key parts of most irrigation systems in arid regions. The hydraulic principles involved in the design of control structures are presented in Chapter 9 and the design of canals in Chapter 6. Groundwater also provides an important source of water for surface irrigation (Chapter 11). Walker and Skogerboe (1987) present a comprehensive review of surface irrigation.

Chapter 1 : Conservation and the Environment

Introductory paragraphs: Soil and water conservation engineering is the application of engineering and biological principles to the solution of soil and water management problems. The conservation of natural resources implies utilization without waste while maintaining a continuous profitable level of crop production and while improving environmental quality. Engineers must develop economical systems that meet these requirements. The engineering problems involved in soil and water conservation may be divided into the following topics: erosion control, drainage, irrigation, flood control, and water resource development and conservation. Although soil erosion takes place even under virgin conditions, the problems to be considered are caused principally by human exploitation of natural resources and the removal of the protective cover of natural vegetation. Urban-rural interface problems are even more serious because of high population density and increased runoff caused by severe changes in land use. Drainage and irrigation involve water and its movement on the land surface or through the soil mass to provide optimum crop growth. To provide water at places and times when it is not naturally available, surface reservoirs or other storage facilities must be developed for irrigation and domestic use. Where available, ground water supplies can be developed and maintained by recharge techniques. Flood control consists of the prevention of overflow on low land and the reduction of flow in streams during and after heavy storms. In water‑short regions, soil water should be conserved by modified tillage and crop management techniques, level terracing, contouring, pitting, reservoirs, water harvesting, and other physical means of retaining precipitation on the land and reducing evaporative losses from the soil surface.

Chapter 11: Water Supply

Introductory paragraphs: Precipitation is the primary source of renewable fresh-water supply for all agricultural, industrial, and domestic uses. Large-scale desalinization of brackish or salty waters may eventually result in reasonable supplies of water for high-value uses in some locations, but desalinization is energy-intensive and expensive. Developed water supplies in the United States use only 4% of precipitation, which is only 13% of the residual precipitation after allowing for all evaporation and transpiration. There is actually ample water for our needs, though it is often not available at the desired time and place. The development of water resources involves storage and conveyance systems to deliver water from the time and place of natural occurrence to the time and place of beneficial use. This chapter emphasizes the development of water resources for agricultural use while briefly discussing other uses and/or benefits.

Chapter 6: Open Channel Flow

Introductory paragraphs: Open channel flow occurs when a free water surface in a channel is at atmospheric pressure. Common examples of open channel flow are rivers, streams, drainage ditches, and irrigation canals. Open channel flow may also occur in pipes if the pipe is not flowing full and the water surface is at atmospheric pressure. Open channel design is common in many applications of soil and water conservation, including drainage and irrigation ditches, grassed waterways, reservoir spillways, and large culverts. In all these applications, the designer must consider channel shape, slope, hydraulic roughness or resistance to flow, and in many cases, channel resistance to erosion. Channels may be earth or concrete lined, vegetated, lined with impervious material such as rubber or fabric, or lined with erosion resistant material such as large rock or high strength geotextile materials. Channels may be left in a natural condition, shaped to achieve a desired capacity, or designed to minimize bed erosion. In some cases, channels may be confined by vertical sides made from materials that are resistant to erosion or sloughing. A properly designed, earth-lined open channel should provide (1) velocity of flow such that neither serious scouring nor sedimentation will result, (2) sufficient capacity to carry the design flow, (3) hydraulic grade at the proper depth for good water management, (4) sideslopes that are stable, and (5) minimum initial and maintenance costs. Additional requirements must be met for carrying irrigation water, such as low seepage loss. Details on open channel flow and design can be found in Chow (1959), French (1985), and Henderson (1966).

Chapter 5: Infiltration and Runoff

Introductory paragraphs: Infiltration and runoff are two important processes in the hydrologic cycle (Figure 1.1). Infiltration begins when precipitation reaches the land surface. Runoff begins when the precipitation rate exceeds the infiltration rate, and retention and surface storage are filled. The relationship between rainfall, infiltration, and runoff is illustrated in Figure 5.1. Infiltration is the main source of water for vegetative growth and crop production, provides input to groundwater recharge, and transports water-soluble compounds, such as fertilizers, manures, herbicides, and other materials, from the land surface into the soil. Some infiltrated water eventually recharges the groundwater. A large fraction of infiltrated water returns to the atmosphere by evapotranspiration. A small fraction of infiltrated water may reappear as surface water and either runoff or infiltrate again.

Chapter 10: Channel Stabilization and Restoration

Introductory paragraphs: A dominant characteristic of most inland cities of the world is a river. To have access to fresh water, many coastal cities were formed near the outlet of a river. Historically, rivers have represented a source of water, transportation route, food supply, and means for waste removal. As cities grew, the river systems became impacted by human activities. These impacts include pollution, sedimentation, channelization, and impoundments. Nearly every major river system is now controlled to such an extent that it can be represented as a series of reservoirs. Tributaries of these large rivers have also been affected by the placement of impoundments to control flow and flooding. The end of the twentieth century saw a shift from analyzing and designing stream systems strictly for hydraulic function to the design of systems with a balance between hydraulic, environmental, and ecosystem functions. The Kissimmee River in Florida is an example of a river that was channeled from 160 km of meandering river to a series of five impoundments connected by a large drainage canal (Brookes and Shields, 1996). The channelization caused a dramatic change in the ecosystems of the region with significant losses in aquatic and waterfowl species. Procedures are now underway to restore the river to its natural condition. Because of the relatively new science of stream restoration and stabilization, few design procedures are available (FISRWG, 1998; Doll et al., 2003).

Chapter 8: Terraces and Vegetated Waterways

Introductory paragraph: Terraces, vegetated or grassed waterways, and a range of open channels may all be part of a watershed drainage system. Terraces and waterways are two designs requiring application of the principles of channel design, and may need special features to minimize soil erosion. Vegetated waterways and terraces are both special conservation structures for erosion and runoff control.

Appendix A: Conversion Constants

Conversions involving feet and acres are based on the international foot (equal to exactly 0.3048 m) adopted by the United States in 1959. Exact conversions are shown in normal typeface. Approximate conversions are italicized. Entries represented by a dash indicate either very large or very small numbers that could easily be derived from other entries in these tables.