1、BRITISH STANDARD BS 7562-3: 1995 Planning, design and installation of irrigation schemes Part 3: Guide to irrigation water requirementsBS7562-3:1995 This British Standard, having been prepared under thedirectionof the EngineeringSector Board,waspublished undertheauthorityof theStandardsBoardand come
2、sinto effect on 15December1995 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference AGE/30 Draft for comment 88/73803 DC ISBN 0 580 25375 9 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technica
3、l Committee AGE/30, Irrigation and drainage equipment, upon which the following bodies were represented: ADAS Agricultural Engineers Association British Agricultural and Garden Machinery Association Ltd. British Turf and Landscape Irrigation Association Health and Safety Executive National Farmers U
4、nion National Rivers Authority Silsoe College, Cranfield Institute of Technology Silsoe Research Institute UK Irrigation Association Well Drillers Association Amendments issued since publication Amd. No. Date CommentsBS7562-3:1995 BSI 10-1999 i Contents Page Committees responsible Inside front cover
5、 Foreword ii 1 Scope 1 2 References 1 3 Definitions 1 4 Design considerations 1 5 Crop water requirements 2 6 Soil water 2 7 Irrigation efficiency 8 8 Choice of irrigation program 8 Figure 1 Typical soil classification triangle 3 Figure 2 Typical range of crop effective rooting depths 4 Figure 3 Typ
6、ical infiltration rate relative to time for different soil types 5 Figure 4 Typical effect of droplet size on infiltration rate 6 Figure 5 Effect of crop cover percentage and crop type of infiltration rate 7 Table 1 Available water holding capacities of soils 3 Table 2 Typical effect of slope on red
7、uction of infiltration rate 5 List of references Inside back coverBS7562-3:1995 ii BSI 10-1999 Foreword This Part of BS7562 has been prepared by Technical Committee AGE/30 and contains recommendations on good practice in the planning, design and installation of irrigation schemes in the UK. It is in
8、tended for the use of engineers and farmers having some knowledge of the subject. It embodies the experience of engineers successfully engaged on the design and construction of irrigation schemes so that other reasonably qualified engineers may use it as a basis for the design of similar irrigation
9、schemes. This Part of BS7562 contains information and represents good practice at the time it was written and, inevitably, technical developments may render parts of it obsolescent in time. It is the responsibility of engineers concerned with the design and construction of schemes to remain conversa
10、nt with developments which have taken place since publication. BS7562 has been prepared in six Parts as follows: Part 1: Glossary of terms; Part 2: Guide for acquisition of site data; Part 3: Irrigation water requirements; Part 4: Guide to water resources; Part 5: Guide for irrigation equipment; Par
11、t 6: Guide for feasibility and implementation procedures. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from le
12、gal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 10, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment t
13、able on the inside front cover.BS7562-3:1995 BSI 10-1999 1 1 Scope This Part of BS7562 gives guidance and information on irrigation water requirements. It covers the subjects of crop water requirements, irrigation water requirements, scheduling and total volumes of water required for the design and
14、operation of irrigation schemes. 2 References 2.1 Normative references This Part of BS7562 incorporates, by reference, provisions from specific editions of other publications. These normative references are cited at the appropriate points in the text and the publications are listed on the inside bac
15、k cover. Subsequent amendment to, or revisions of, any of these publications apply to this Part of BS7562 only when incorporated in it by updating or revision. 2.2 Informative references This Part of BS7562 refers to other publications that provide information or guidance. Editions of these publicat
16、ions current at the time of issue of this standard are listed on the inside back cover, but reference should be made to the latest editions. 3 Definitions For the purposes of this Part of BS7562 the definitions given in BS7562-1:1992 apply together with the following. 3.1 soil water deficit the amou
17、nt of water required to restore the soil to field capacity 3.2 soil water reservoir the water storage available in the soil within the effective rooting depth of the crop NOTEThe soil water reservoir is affected by variation of root depth, growth stage of the crop and crop variety. 3.3 easily availa
18、ble soil water that fraction of the available water holding capacity (AWC) that can be used by the crop without adversely affecting its growth NOTEThis is normally regarded as water available at tensions of2 bar or less. The value of the fraction depends mainly on the soil type but also on the type
19、of crop and its transpiration rate. 3.4 irrigation cycle the time between the start of successive irrigation applications on the same field 4 Design considerations 4.1 General In designing an irrigation system it is essential that the total amount of water to be applied over the growing season is sp
20、ecified as well as how this will vary through the season, particularly as peak demands are likely to be experienced. This Part of BS7562 shows how this essential information can be assessed. This assessment should be made at the outset of the project. Water availability should be checked with the lo
21、cal National Rivers Authority (NRA) 1)office before any detailed planning is commenced. Two important issues should be considered: seasonal and peak irrigation requirements. A seasonal profile of the crop water requirement of each crop should be determined to assess how often, and by how much, it wi
22、ll need to be supplemented with irrigation. This information establishes requirements for licensing and storage purposes. The peak water requirement should also be established as this determines the irrigation system capacity. Rainfall levels are an important factor in the design of irrigation syste
23、ms as UK weather conditions are view varied. The rainfall data obtained need to be site specific due to for example, rain shadow areas and exposed coastal sites. Climate data on a location close to the irrigated area, over a recent20year period, should be obtained. This should provide a reliable est
24、imate of average weather conditions given the considerable annual variation in weather experienced in the UK. Shorter periods will reduce the accuracy. This weather data can then be used to simulate possible irrigation schedules for design purposes. NOTEThe detailed calculations required in these es
25、timates can be processed with the aid of a suitable computer program 2) . 4.2 Rainfall The designer should decide to what extent, if at all, that monthly rainfall figures are taken into account in the irrigation system design. This decision will be based on many factors including the level of risk a
26、t which the site owner wishes to operate, the crop value and response to water and economics of the design. 1) From1April1996 the Environmental Agency will be the appropriate authority to contact. 2) For information on the availability of a suitable computer program, write to Customer Information, B
27、SI, 389 Chiswick High Road, London W4 4AL.BS7562-3:1995 2 BSI 10-1999 Only a proportion of high intensity and heavy rains can enter the soil and be stored in the root zone to be usefully used by the crop. Frequent light rains intercepted by plant foliage with full ground cover are nearly100% effecti
28、ve in this respect. The effective rainfall should be considered in the system design and planning. The agriculturally effective rainfall, R ae(in mm), is given by the equation: R ae= R t (O + P + E) where 5 Crop water requirements 5.1 General Crop water requirements (CWR) are the basis of the design
29、 and operation of an irrigation system. Factors affecting CWR are climate, crop type and stage of growth. CWR can be calculated using the modified Penman approach 1. This approach determines the water requirements for a full green crop cover, freely transpiring. This is known as the reference crop.
30、In the UK, the reference crop is grass grown to a specific height. Other crops are related to the reference crop by means of a crop factor which allows for different stages of crop growth. CWR is given (in mm) by the equation: CWR = k c ETr where Typically CWR varies between400mm and800mm per season
31、, in the UK. This is met in part by rainfall and the shortfall, of roughly50%, is provided by irrigation. 5.2 The modified Penman approach The modified Penman formula for the calculation of ETr is the most accurate of several methods and is the one normally used. Its accuracy, when used for calculat
32、ions under UK climatic conditions, generally falls within 10% and rarely exceeds a variation of 20%. 5.3 Crop factors The crop factor or crop coefficient k c , is the ratio between potential crop evapotranspiration (ET crop ) and the reference crop evapotranspiration (ETr). The factors which should
33、be taken into account when selecting the appropriate k cvalue include the crop characteristics, time of planting or sowing, stages of crop development, and general climatic conditions. It is essential to collect local data on the crop growing season and the rate of crop development. 5.4 Crop evapotr
34、anspiration Factors which can affect the crop evapotranspiration (ET crop ) include aspects of the climate, status of soil water, salinity of soil and the method of irrigation. All of these should be considered at the system planning stage. Typically for design purposes, calculations are done on a m
35、onthly basis. 6 Soil water 6.1 Soil types The soil type determines the amount of water that the soil can hold for crop use and affects the calculated average irrigation cycle. The way water moves into the soil, i.e. the infiltration rate, largely determines the design of the water application system
36、. The relative proportions of sand, silt and clay particles in the soil, together with the amount of organic matter or humus determine the soil texture. The soil texture triangle shows the11 soil classes. Standard soil analysis methods identify the ratio of sand, silt and clay (seeFigure 1). Soils v
37、ary widely in their available water holding capacity (AWC) ranging from low water holding capacity in sandy soils to high water holding capacity in clay loam soils (seeTable 1). The water holding capacity of a soil can be determined approximately by field sampling or more accurately by a simple labo
38、ratory technique. The water theoretically available in a particular soil is not all easily available to the crop and the irrigation system design should take account of this in the calculation of irrigation cycles. R t is the total rainfall (in mm); O is the run-off (in mm); P is the deep percolatio
39、n (in mm); E is evaporation (in mm). k c is the crop factor (see5.3); ETr is the reference crop evapotranspiration (in mm) (see5.2).BS7562-3:1995 BSI 10-1999 3 Table 1 Available water holding capacities of soils 6.2 Crop effective rooting depths The rooting depth of the crop determines the extent of
40、 the soil water reservoir. The rooting depth for the same type of crop varies considerably according to the type of soil. Variation of root depth occurs for additional reasons (which need to be taken into account by the planner): a) each crop and species of crop has its own specific root system (see
41、Figure 2); b) the age of the crop and its stage of development; c) the soil type; d) the type of soil management; e) the amount and placement of soil nutrients; f) the nature of the soil drainage; g) the water supply in the early part of the growing season. NOTEThis figure is reproduced by permissio
42、n of ADAS from reference 2. Figure 1 Typical soil texture classification triangle Soil type Available water holding capacity (AWC) mm/m Sandy 80 Loamy sand 120 Sandy loam 170 Sandy clay loam 170 Sandy clay 170 Clay loam 170 Silty clay loam 160 Clay 180 Silty clay 180 Sandy silt loam 190 Silt loam 22
43、0 Peats 350BS7562-3:1995 4 BSI 10-1999 6.3 Easily available water Easily available water capacity (EAWC) can be determined from published information but in the absence of this, it is common practice to assume that EAWC is approximately50% of AWC. 6.4 Soil water depletion It should be noted that the
44、 proportion of water abstracted by the plant roots varies according to depth and may need to be taken into account. 6.5 Infiltration rate 6.5.1 General An accurate knowledge of the soil infiltration rate is essential for the design of the irrigation system. This ensures that water is applied at the
45、most efficient rate without causing compaction, erosion or damage to the crop or soil. 6.5.2 Determining infiltration rate in the field Infiltration rates can either be assessed from published tables/graphs or from actual measurement in the field using, for example, a ring infiltrometer. Factors aff
46、ecting the infiltration rate of the soil (seeFigure 3), include: a) soil texture; b) soil structure; c) slope of soil (seeTable 2); d) droplet size (seeFigure 4); e) soil water content; f) crop type and crop cover percentage (seeFigure 5). Figure 2 Typical range of crop effective rooting depthsBS756
47、2-3:1995 BSI 10-1999 5 Table 2 Typical effect of slope on reduction of infiltration rate Figure 3 Typical infiltration rate relative to time for different soil types Slope Reduction of infiltation rate % % 35 10 68 20 912 40 1320 60 U 20 75BS7562-3:1995 6 BSI 10-1999 Figure 4 Typical effect of dropl
48、et size on infiltration rateBS7562-3:1995 BSI 10-1999 7 Figure 5 Typical effect of crop cover percentage and crop type on infiltration rateBS7562-3:1995 8 BSI 10-1999 6.6 Bare soil evaporation Evaporation from the soil surface is especially important at the time of sowing the crop. Evaporation from
49、the soil surface, with a crop growing from seed, occurs from the time of planting until the crop achieves100% cover of the soil. However, evaporation from the soil gradually decreases with time as the crop cover increases. 6.7 Ground water contribution Shallow groundwater contribution can affect the water required by the crop. If and when contribution occurs, then the irrigation requirement will be reduced. In most irrigated soils the upward movement of water from the water table can be ignored but in some fen peat a
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