Glossary of terms
The 1995 `acres irrigated' data element of this category differs from previous 5-year compilations. The acres irrigated by surface, sprinkler, and micro-irrigation methods will be reported separately in 1995. In previous compilations, the crop acreage was divided into either flood or "other". Public-supplied irrigation deliveries to golf courses, parks, nurseries, turf farms, cemeteries, and other landscape areas are reported as a commercial use.
Surface irrigation systems distribute water across the soil surface by gravity flow, some use ponding or flooding strips of land, others use furrows and small ditches that wet only part of the soil surface during application. The application efficiency of surface irrigation systems may range form 35 to 85 percent (John E. Andrews, USDA-Natural Resource Conservation Service, written commun., 1994).
Sprinkler irrigation systems spray water through the air that falls on the plant and soil surface much like natural rainfall. Sprinkler systems lose water due to evaporation and wind drift during application. The application efficiency of sprinkler irrigation systems range from 60 to 95 percent (John E. Andrews, USDA-Natural Resource Conservation Service, written commun., 1994), although values as low as 15 percent may occur for container nurseries (Smajstria and others, 1988).
Microirrigation irrigation systems, also referred to as low-volume systems, wet only a discrete portion of the soil surface by directing water drops, small streams, mist, or sprays near the ground surrounding individual plants during application. The application efficiency of microirrigation systems range from 60 to 90 percent (John E. Andrews, U.S.D.A.-Natural Resource Conservation Service, written commun., 1994).
Subsurface irrigation systems, also referred to as seepage and subirrigation systems, provide water to crop roots by raising the water level above an established water table height or above a restrictive soil layer. Subsurface systems can use pumps, ditches, and underground conduits to control the height of the water table. Not all of the water used is available for beneficial use due to deep percolation losses, lateral losses to surrounding areas, and unavailable water below the root zone. The application efficiency for subsurface systems can vary widely with different site conditions and management practices. For the purposes of this effort subsurface irrigation values will be totaled with micro-irrigation values
Irrigation efficiencies vary with the type of irrigation system used and with other factors such as soil, crop, and climatic conditions including wind speed, relative humidity, and air temperature. Application efficiency also depends on how well a system is designed and managed (Smajstria and others, 1988). Reports by the U.S. Department of Agriculture (1967), Jensen (1983), and Withers and Vipond (1980) describe many types of irrigation systems.
An areal estimation method uses a representative withdrawal determined at measured locations. The average withdrawal is multiplied by the total number of wells or diversion locations in an area. Alternatively, a representative value for a power use coefficient may be calculated from sample data and divided into the total power consumption. The representative value generally used is the sample mean of the data, it's accuracy as an estimator can be controlled through the sample size (Luckey, 1972).
A commonly used method to estimate water use is to calculate consumptive use using crop coefficients and existing formulae. The irrigation water requirement is computed by subtracting effective precipitation from the theoretical consumptive use. The irrigation water requirement divided by the irrigation efficiency (U.S. Department of Agriculture, 1976) gives an estimate of total irrigation withdrawals. This method requires that information on the crops, acres irrigated, and local climate be collected or estimated, and assumes that the irrigation water supply is adequate for operations.
Another method of estimating irrigation water use is by a local survey. Efficient collection and processing of data will be achieved if the questionnaires are easy for the user to complete. Irrigation questionnaires can usually be distributed by State or local agencies to the selected irrigators, and then returned to the U.S.Geological Survey office for data processing. Questionnaires should include a statement of authority, a deadline, a concise description of the requested data, and a name and phone number that users can contact with questions about the data-request forms. Information may include total irrigated acreage, crop and pasture acreage, type of irrigation system, quantity or flow rate of water used by source, irrigation frequency, number of irrigation wells, depth, capacity, aquifer, total energy usage, power coefficients and system efficiency. Optional data includes: crop water shortages during past year, acres harvested and yields, energy sources, water management practices and improvements, and the agricultural and irrigation sources that are generally contacted when the irrigator needs additional guidance
At golf courses many factors affect the amount of water used including course design, acreage, irrigation systems, soils, and local irrigation practices. Normally, more water per unit of area is applied to the greens and tees than to the fairways. It is best to develop irrigation estimates using course coefficients developed from information for your State. The survey should request the types of grass maintained, acres irrigated of each type, sources of water supply, frequently of irrigation, quantity of water diverted, and the quantity pumped monthly and seasonally during the calendar year. The survey may also request the number of irrigation pumps, total hours pumped, the static and pumping water-levels for wells, and the name and phone number of the person that supplied the information. The data requested should be limited to the objectives of the project and the needs of the cooperator.
Apportioning withdrawal estimates among ground-water, surface-water or reclaimed wastewater sources may be difficult. The amount of time each source is used may be used to divide the total amount. State agencies and county extension agents may be able to provide a seasonal estimate of sources used for irrigation in your State and the amount and sources of reclaimed wastewater. Patterns of use by source for other categories may suggest a reasonable division. Information should include: total irrigated acreage, crops and pasture acreage, irrigation system, discharge or flow rate by source, irrigation frequency, number of wells, depth, aquifer, power consumption and system efficiency.
Field measurements of consumptive use by crops are complex and time consuming and therefore are expensive and thus impractical to measure. Often County Extension Service agents and USDA regional irrigation specialists in your State will know of some measurements of consumptive use. Because consumptive use in some Districts in the irrigation category is larger than the total withdrawals in most other water-use categories, it is important to estimate consumptive use when the water-use data are being compiled. Consumptive-use values estimated by Districts during the 1990 compilation ranged from 21 to 100 percent and averaged 56 percent (Solley and others, 1993).
Consumptive use is generally estimated using empirical formulae. Many of the approaches used to estimate the monthly irrigation water requirements are based on theoretical relationships that represent typical characteristics of water use by crops. Problems are inherent with theoretical methods. Basin estimates may exceed the amount of water used by some crops. The Natural Resource Conservation Service (U.S. Department of Agriculture, 1970) published a report that describes the use of a modified Blaney-Criddle formula that can be used to estimate consumptive use for different crop types. The Blaney-Criddle (1950) method, originally developed in a semiarid region, is used extensively in the United States. Numerous modified versions of the technique have been developed to adapt better to various areas of the country. Many other empirical approaches are used to estimate evapotranspiration, and include the Penman method, Thornthwaite method, Stephens-Stewart method, and pan evaporation methods (Jones and others, 1984). The different methods have been developed to reflect the availability of data.
Canal and ditch seepage is one of the major sources of water loss during the conveying process. In order to extrapolate seepage measurements throughout a large estimation area, the water distribution systems must be adequately characterized. Canal reaches should be classified by soil type and conveyance properties including mean flow, wetted perimeter, channel slope, and geohydrologic setting. Generally, the soil types and conveyance properties can be determined, but the geohydrologic analysis commonly is inadequate. The hydraulic conditions under which canal seepage occurs need to be specifically determined in some randomly selected test reaches and then qualitatively estimated for the remainder of the water distribution system. Several methods used to measure canal seepage are ponding tests, inflow-outflow, and seepage-meter studies (Rohwer and Stout, 1948).
Most conveyance losses are returned to surface-water or ground-water systems. Irrigation water returning to the surface-water system can often be measured using standard methods (Rantz and others, 1982). Irrigation water returning to the ground-water system is often more difficult to quantify. Water-balance studies can be used to estimate actual conveyance losses and return flows (Roush, 1988).
In some parts of the United States the growing season is long enough that double and triple cropping on the same irrigated acreage can occur. For this reason, irrigated acres will be accumulated to reflect the total acreage in areas where multiple-cropping of the irrigated acreage is practiced during the year. Therefore, when any acre of land is cropped twice, it is counted as 2 irrigated acres. This is necessary because additional water must be applied to the second and third crops. Counting acreage in this manner means the "water applied per acre" will be correct. The irrigation method used on the subsequent crops may also vary and should be investigated and documented for your State.
_____, 1966, Determining consumptive use for water planning developments: Los Angeles, Water Resources Center, University of California, 35 p.
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_____, 1988, Colorado irrigation guide (revised): Denver, Colo., Soil Conservation Service, 1 v., various paging
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_____, 1982, Measurement and computation of streamflow--Volume 2, Computation of discharge: U.S. Geological Survey Water Supply Paper 2175, p. 285-631.
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_____, 1967, Soil Conservation Service, National Engineering Handbook, Part 623, Section 15, Irrigation: Chapter 2, Irrigation water requirements (1967); Chapter 3, Planning farm irrigation systems (1967); Chapter 9, Measurement of irrigation water (1973); Chapter 11, Sprinkler irrigation (1983): Washington, D.C., Soil Conservation Service, separate pagination.
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