10.2 USBR terminology
10.3 Financial and economic considerations
10.4 Land classes and subclasses of the USBR system
10.5 the USBR mapping symbols
10.6 USBR land classification specifications
Sophisticated methods of land classification for irrigated agriculture were first evolved by the United States Bureau of Reclamation in the 1920s and 1930s. Many of the principles underlying the USBR system have been incorporated in the FAO Framework for Land Evaluation and in this bulletin. The purpose of this Chapter is to summarize the main features of the USBR system and highlight where this differs from the FAO system.
The USBR classification system for irrigated land is carried out in the context of a project plan and with respect to the land uses defined under the project plan, which may be broadly or narrowly defined. There is no formal system for defining land utilization types as in the FAO Framework; instead these are implied in the plan formulation.
The USBR classification system incorporates broad economic considerations from the start, as is also recommended in this publication. This is important because irrigation projects generally involve costly inputs and improvements such as engineering works, irrigation and drainage networks, land clearing and levelling, and others.
The USBR Reclamation Manual (1951) and subsequent Reclamation Instructions lists the following principles of the USBR classification system:
i. Prediction: The classification should reflect future conditions as they will exist after the project is implemented. This recognizes that changes will occur in relationships between soils, water and crops as a result of irrigation and land improvements and that the classifier should use the classes to indicate whether these changes are likely to be favourable or unfavourable.
ii. Economic correlation: This assumes that a unique relationship can be established during a classification, between physical conditions of the land such as soils, topography and drainage and an economic measure of the class ranges. The measure used is payment capacity, i.e. the residual available to defray the cost of water after all other costs have been met by the farmers.
iii. Permanent and changeable factors: The classifier must distinguish between permanent factors, such as soil texture, soil depth, macrorelief, etc., and changeable factors, such as salinity, ESP, pH, microrelief, nutrient status, water table levels, etc. Thus the survey and classification are directed to determining which inputs and improvements to changeable factors are cost effective.
iv. Arability-irrigability: Land which is physically and economically capable of providing a farmer with an adequate standard of living, should water be available for irrigation, is first classified. Such land is called 'arable' (connoting a different meaning of the word to that in common usage). Arable lands constitute areas that warrant consideration for inclusion in a plan of development. Lands which are selected for inclusion in the plan of development are called 'irrigable' lands. This dual-stage procedure is copied in this publication in the successive classification of 'provisionally-irrigable' and 'irrigable' land.
Precise meanings are attached to the use of the following terms in the USBR classification system:
Arable land is land which, in adequately sized units and properly provided with the essential improvements of levelling, drainage, irrigation facilities, and the like, would have a productive capacity, under sustained irrigation, sufficient to:
i. meet all production expenses, including irrigation operation and maintenance costs, and provide a reasonable return on the farm investment;
ii. provide a reasonable repayment contribution toward the cost of project facilities;
iii. provide a satisfactory standard of living for the farm family.
Irrigable land is land initially classified as arable (potentially irrigable) that is subsequently found to be economically justified (benefits exceed costs) under a specific plan of development which includes the water and other facilities necessary for sustained irrigation.
Productive land is the maximum hectarage of irrigable land subject to cropping; a measure that provides a basis for the determination of water requirements, canal capacities and payment capacities. For conditions in Western USA the productive area is about 3 to 6% less than the irrigable area because of non-productive land uses such as farm roads, farm laterals and drains, irrigation structures, fences, buildings, and feed lots.
Full irrigation service land is irrigable land which will receive its full water supply from one source.
Supplementary irrigation service land is irrigable land now receiving, or to receive, an additional or regulated supply of water through new facilities.
Gross classification area is the area mapped and classified in a given survey.
Land class is a category of land that produces a standard payment capacity in financial rather than economic terms. This is the definition used in practice in the USA. Arrangements for on-farm development costs and project repayment vary substantially among developing nations and, in contrast to the USA, are often not firmly established prior to project investigations. Therefore, since the ultimate classification of irrigable lands under an economically justified plan of development would be the same for the classes retained in the plan, whether the initial classification is based on farm financial analysis (farmer's repayment ability) or an economic analysis (irrigation benefits), it could be time-saving and appropriate for international situations to employ a uniform economic evaluation approach to land classification studies at the outset of investigations. Farm financial and project repayment considerations can be evaluated and arranged as may be necessary after an economically justified project plan has been formulated. This approach would simply reverse the order of accommodating the two principal concerns of irrigation suitability investigations, 'financial viability' and 'economic justification', from that employed by the US Bureau of Reclamation. In the end, essentially the same classification of 'irrigable' land should result from either approach.
Land subclass is a category within the class identifying a deficiency or deficiencies, and is indicated by a letter (i.e. s for soil, t for topography, and d for drainage).
Informative appraisals are investigations of selected physical factors to provide needed information for the planning, development and operation of irrigation projects. The investigations may be of present land use, productivity, existing land development, farm water requirements, etc. or informative appraisals of drainability and topography.
Great importance is given in the USBR classification system to farm budget studies and the concept of payment capacity in the determination of arable lands. The land of lowest quality that can be classified as 'arable' is specified as early in the investigations as possible. In the USA the minimum is prescribed by a law which states that irrigable lands shall be classified with respect to their power under a proper agricultural programme to support a farm family and pay water charges (Reclamation Law, 1924). Accordingly, the minimal quality lands have been defined as those capable of supporting a farm family and paying at least the annual operation, maintenance and replacement costs expected to prevail when the project comes into operation. Hence, in the initial arability classification land classes are an expression of relative differences in payment capacity - a financial rather than an economic measure.
The selection of lands for irrigation is phased into two parts:
- the selection of arable land on the basis of farm production financial considerations;
- selection of the irrigable area on the basis of the economics of the project plan, wherein irrigation benefits determined by economic evaluation equal or exceed project irrigation costs.
Satisfactory farm production finance is clearly essential for sustained irrigated agriculture. The application of economics to the project as a whole facilitates the principles of optimum design or scale, and net benefit maximization. In the case of multipurpose projects, the project plan may be formulated to allocate water to irrigation, hydroelectric power, municipal and industrial water supplies, and other purposes. The irrigable area is thus selected in relation to the water allocated to irrigation and to the size and location of the distribution and drainage systems.
The application of project plan formulation criteria generally leads to successive reductions of 'arable' land from the plan of development. Typical adjustments include:
i. the elimination of uneconomic increments such as those that are too costly to serve, drain or provide with distribution works;
ii. the adjustment of land areas to the available water supply and the elimination of tracts located above the water delivery elevations or elevations that can be irrigated economically by lift;
iii. exclusion of isolated segments, odd-shaped tracts, severed areas and public rights-of-way;
iv. elimination of areas unable to meet minimal criteria for economic returns under the plan.
The selection of lands is thus fundamentally guided by criteria requiring the selected lands to:
a. be included in a plan having a favourable benefit/cost relationship;
b. have sufficient amortization capacity to pay assigned construction charges;
c. have ability to meet anticipated annual operation, maintenance and replacement charges.
Based on farm budget studies and a knowledge of the estimated annual operation, maintenance and replacement (OM & R) charges for water to the farm, an estimate can be made of the maximum amount of money that should be spent in developing land for irrigation (permissible land development costs). For example, assume that a farm budget showed a payment capacity of $20 per acre for the best Class 1 land in the project area. Also assume that the annual OM & R charges are estimated to be $7.50. The latter charge is used as the minimal payment capacity for any land to be included in the irrigation project. The difference in payment capacity between the best land and the minimal quality land ($20.00 - $7.50 = $12.50) is the permissible land development cost. At 6% interest the $12.50 difference in payment capacity would be equal to the annual interest on approximately $208. Thus, if the productivity of unlevel terrain once levelled would equal that of the Class 1 land, up to $208 per acre could be expended for land development work before the land would no longer be regarded as financially suited for irrigation development. Such lands requiring development costs in excess of average Class 1 development costs, but within the additional $208 per acre limit would be downgraded to a lower arable class in relation to development cost ranges determined from the payment classes illustrated below. Any lands may be included in the arable category as long as the resulting payment capacity equals or exceeds $7.50 in this particular example. Class ranges might be set at unequal or equal ranges of payment capacity e.g.
$20.00 - $15.84
$15.83 - $11.67
$11.66 - $ 7.50
Six land classes based on production economics are normally recognized. Brief descriptions are as follows:
Class 1 - Arable: Lands that are highly suitable for irrigated farming, being capable of sustained and relatively high yield of climatically adapted crops at reasonable cost. These lands have a relatively high payment capacity.
Class 2 - Arable: Lands that have a moderate suitability for irrigated farming. These are either adaptable to a narrower range of crops, more expensive to develop for irrigation, or less productive than Class 1. Potentially these lands have intermediate payment capacity.
Class 3 - Arable: Lands that have a marginal suitability for irrigated farming. They are less suitable than Class 2 lands and usually have either a serious single deficiency or a combination of several moderate deficiencies in soil, topography, or drainage properties. Although greater risk may be involved in farming these lands than those of Class 1 and 2, under proper management they are expected to have adequate payment capacity.
Class 4 - Special use lands: Lands which in the USA are only suited to certain special uses (e.g. rice, pasture, or fruit) are classified 1, 2 or 3 (to reflect relative payment capacity) along with the appropriate letter designating the land use (crop).
Class 5 - Non-arable: This land is temporarily considered as non-arable because of some specific deficiency such as excessive salinity, questionable drainage, flooding, or other deficiency which requires further studies to resolve. The deficiency or deficiencies are of such a nature and magnitude that special agronomic, economic, or engineering studies are required to resolve the costs or effect on the land. Class 5 designation is tentative and should be changed to either Class 6 or an arable classification during formulation of the recommended plan of development.
Class 6 - Non-arable: Land that is non-arable under the existing or project economic conditions associated with the proposed project development. Generally, Class 6 comprises steep, rough, broken, rocky, or badly eroded lands, or lands with inadequate drainage, or other deficiencies. In some instances lands considered to be Class 6 in one area may be arable in another area because of different economic conditions. In addition to various physical-type deficiencies that result in a non-arable classification, lands initially classified as arable (potentially irrigable) on the basis of payment capacity (farm financial analysis) may be found non-arable if subsequent economic analysis (benefit analysis) indicates that benefits from such lands are less than their costs in a plan of development. Thus, the lower arable class(es) of lands would be considered non-arable and, of course, non-irrigable for economic reasons.
Subclasses are indicated by lower case letters that indicate the reason for the land being downgraded to a lower class. Thus, Class 1 land does not have subclasses, but other classes may be appended with the letters 's', 't', and 'd', singly or in combination to show whether the deficiency is in 'soils', 'topography' or 'farm drainage'. The basic subclasses of the land classes are s, t, d, st, sd, td and std.
A comparison of the FAO Framework and the USBR classification described above, is given in Appendix 1, Figure A.1.
Typically, the mapping symbols employed by the USBR take the form given in Figure 5.
The class and subclass symbols on the top of the line have already been described. The other symbols may be used as required but the rules for their use are set up for each individual classification study; however, the rules must be consistently obeyed throughout any one study.
Land use codes, such as: C - irrigated cultivated, L - unirrigated cultivated, P - irrigated permanent grassland, W - wasteland, etc. can be used, or a symbol may be used to specify a crop.
Productivity and land development codes: Productivity connotes the interaction of the economic factors of productive capacity and costs of production, but excludes land development costs. Thus, in the example given, Class 2 relative productivity and Class 2 land development cost (symbol 22 in the denominator) results in an overall land class 3 in the numerator.
Farm water requirement code: The symbols A, B and C may indicate whether the soil, topographic and drainage conditions, land use, method of irrigation, etc. from informative appraisal, are low, medium or high, respectively, in farm water requirement relative to the average for the surrounding area.
Land drainability code: This normally relates to conditions below a depth of 5 ft (150 cms): X - good drainability, Y - restricted drainability, and Z - poor or negligible drainability.
Figure 5 Example of standard mapping symbol used by USBR
Additional symbols may be added as in the example to indicate special conditions where data are required for farm unit planning and land development. They can be further qualified with subscript numerals to indicate a range in character; e.g. k1, k2, k3 might indicate ranges of depth to gravel.
Seven examples of land class specifications prepared by the USBR are given in FAO Soils Bulletin No. 42.
Land class specifications express the anticipated influence of various mappable physical factors on the projected productivity level, cost of production and cost of land development. Parameters established within land classes for the pertinent soil, topographic, or drainage factors should represent approximately the same range of influence in evaluating irrigation suitability. Thus the range of slope or amount of levelling selected for Class 1 land should represent about the same influence on suitability for irrigation as the range of soil depth or farm drainage requirement permitted in this class.
In the USA, correlation of the study area with nearby irrigated farms is nearly always possible and provides the best means of relating various physical parameters to measured production levels if soil and other environmental and management conditions are similar.
Relative yield levels are first established and physical specifications thought to be correlated with these yield levels are developed. Farm budgets are used to evaluate the relative impact of each selected physical characteristic on net income and as a basis for calculating the maximum permissible development cost for each of the arable land classes, as earlier described.
Farm budgets are usually first developed for the best soil, topographic and drainage conditions on the proposed project area, and the relative yield is taken to be 100%. Costs used for land development on this land include a nominal amount for ditches, diversion structures, farm drains, and smoothing; these costs are budgeted for all land classes. All other development cost estimates should reflect costs above the amount needed for the best quality land. As discussed in Section 10.3, the lower limit is also established. This varies among projects because OM & R charges vary due to differences in the lengths of project distribution and drainage systems, required lifts, power, seasonal water demand, and for other reasons.
A preliminary estimate of water costs is normally essential in establishing a lower cut-off point for arable land in monetary terms before deciding the lowest acceptable productivity level for the least productive soil. For example, the lower limit of arability in one project may be only half of the productivity required in another project due to lower water costs. Different physical specifications of Class 3 lands are then developed. Thus lower quality soil and rougher topography can be profitably used in areas having low cost water or where high value crops are to be grown.
Figure 4a Map of the Southern Conveyor Project, Cyprus, showing the "provisionally-irrigable" land in the study area - Carte du Projet de collecteur dans la partie méridionale de Chypre, indiquant les terres "conditionnellement irrigables" dans la zone étudiée - Mapa del Proyecto del Colector Sur, en Chipre, con indicación de las tierras "potencialmente regables" en la zona del estudio
Figure 4b The same map showing the "irrigable" land - Même carte indiquant les terres "irrigables - El mismo mapa con indicación de las tierras "regables"