irrigation drainage and river engineering.pdf

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30Contents Part A: Irrigation and Drainage30.1 Irrigation fundamental concepts 30.1.1 Introduction 30.1.2 Soil moisture 30.1.3 Crop water requirements 30.1.4 Irrigation efficiency 30.1.5 Effective rainfall 30.1.6 Salinity and leaching requirement Irrigation methods 30.2.1 Introduction 30.2.2 Surface irrigation 30.2.3 Sprinkler irrigation 30.2.4 Trickle irrigation 30.2.5 Sub-irrigation 30.2.6 Irrigation canal design Drainage of agricultural land 30.3.1 Introduction 30.3.2 Sub-surface drainage of irrigated land 30.3.3 Drainable surplus 30.3.4 Drainage of lands subject to excess rainfall 30.3.5 Drain spacing 30.3.6 Drain flow 30.3.7 Drainage layouts 30.3.8 Drainage of heavy soils 30.3.9 Bedding systems 30.3.10 Surface drainage for irrigated land 30/3 30/3 30/3 30/3 30/4 30/5 30/5 30/6 30/6 30/6 30/7 30/9 30/9 30/9 30/9 30/9 30/10 30/10 30/10 30/11 30/11 30/11 30/12 30/12 30/12

Irrigation, Drainage and River EngineeringW Pemberton BSc, FICEHead of Irrigation and Drainage Department Sir Murdoch MacDonald and Partners Head of River Engineering Department Sir Murdoch MacDonald and Partners30.4.2 Rivers as natural drains 30.4.3 Economic issues 30.5 Hydrology 30.5.1 Introduction 30.5.2 Measurement 30.5.3 Statistics 30.5.4 Flood flow calculation methods 30.5.5 Hydrographs 30.5.6 Curve number method 30.5.7 The Flood studies report Channel regime 30.6.1 Regime flow 30.6.2 Regime formulae 30.6.3 Practical applications Sediment transport 30.7.1 Basic concepts 30.7.2 Sediment transport estimates 30.7.3 Sediment transport equations 30.7.4 Stable channel design Channel design 30.8.1 Channel flow formulae 30.8.2 Channel stability 30.8.3 Other considerations Channel improvements 30.9.1 Channel clearance 30.9.2 Realignment 30.9.3 Revetments and lining

C E Rickard BSc, CEng, MICE, MIWEM

30/12 30/13 30/13 30/13 30/13 30/13 30/13 30/14 30/14 30/14 30/14 30/14 30/14 30/15 30/15 30/15 30/15 30/15 30/16 30/16 30/16 30/16 30/17 30/17 30/17 30/17 30/17 30/19 30/19 30/20

30.2

30.6

30.7

30.3

30.8

30.9

Part B: Land Drainage and River Engineering30.4 Land drainage and flood alleviation 30.4.1 Objectives of land drainage 30/12 30/12

30.10 Embankments 30.10.1 Introduction 30.10.2 Design

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30.10.3 Stability 30.10.4 Construction 30.10.5 Rood walls 30.11 Detention basins, washlands and catchwater drains 30.11.1 Detention basins 30.11.2 Washlands 30.11.3 Catchwater drains 30.12 Structures 30.12.1 Introduction 30.12.2 Retaining walls 30.12.3 Bridges 30.12.4 Weirs

30/20 30/20 30/21 30/21 30/21 30/21 30/21 30/22 30/22 30/22 30/22 30/22

30.12.5 Gated control structures 30.12.6 Tidal outfalls 30.13 Pumping 30.13.1 Single or multiple pumps 30.13.2 Motive power 30.13.3 Pumps 30.13.4 Control 30.13.5 Pump station building 30.13.6 Other types of pumping installation References

30/23 30/24 30/24 30/24 30/24 30/24 30/25 30/25 30/26 30/26

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PART A: IRRIGATION AND DRAINAGE 30.1 Irrigation - fundamental concepts

Table 30.1Moisture content (percentage by weight) Soil type Permanent wilting point 4 8 18 30 Available moisture 4 7 10 15

30.1.1 Introduction Irrigation is desirable where natural rainfall does not meet the plant water requirements for all or part of the year. Irrigation is essential for agriculture in the desert but even in areas such as northern Europe it can improve the yield of crops normally grown under rainfall conditions only. 30.1.2 Soil moisture The soil can be considered a moisture reservoir. Soils can be classified under the International Soil Science Association (ISSA) system as follows: Fraction Coarse sand Fine sand Silt Clay Particle size (mm) 2-0.2 0.2-0.02 0.02-0.002 < 0.002 Field capacity Coarse sand Fine sand Silt Clay 8 15 28 45

With knowledge of the crop rooting depth, the available soil moisture and the crop water requirements, it is possible to select a suitable irrigation interval (time between irrigations). Not all water in the root zone is readily available to the crop. It is normal to allow the crop to deplete only 50% of the available moisture before irrigating again. More detailed guidelines are given by the Food and Agricultural Organization.1 30.1.3 Crop water requirements Crop water requirements are defined as the depth of water required to meet the water loss through evapotranspiration CETcrop) of a crop. The effect of climate on crop water requirements is given by the reference crop evapotranspiration CET0) which is defined as the rate of evapotranspiration from an extensive surface of green grass of uniform height (8 to 15cm): ET^-k^ET, (30.1)

Water is held by the soil in the soil pores. The amount of water held can be defined as follows: (1) Saturation: the state of complete soil wetness when no further water may be added to the soil. (2) Field capacity (FC): the condition reached after water has drained from the soil by gravity. (3) Permanent wilting point (PWP): the condition reached after plants have extracted all the moisture they can from the soil. (4) Available water: defined as (FC-PWP), the amount of water held by the soil that plants can use. Plants respond to how tightly the water is held by the soil which is defined as soil moisture tension. Generally, it is assumed that the soil moisture tension at field capacity is 0.3 bar pressure. Soil moisture tension at PWP is assumed to be 15 bar. Typical moisture contents for various soils are shown in Table 30.1.

where kc is the crop coefficient which varies with crop, growth stage, growing period and prevailing weather conditions The most reliable method of estimating T0 is generally considered to be the PENMAN method. This method is best described by Doorenbos and Pruitt1 which also gives details of crop coefficients for a wide range of crops. Values of Tcrop are normally calculated for 10-day periods. A typical crop coefficient curve is shown in Figure 30.1. A simpler method was proposed by Blaney and Criddle2 in

Planting date

Approx. 10% ground cover

Crop coefficient K

Crop Initial development

70-80% ground cover

Mid-season

Late

Figure 30.1 Example of crop coefficient curve. (After J. Doorenbos and W. O. Pruitt (1977) Crop water requirements. Food and Agriculture Organization Irrigation and Drainage Paper No. 24.)

Maturity Harvest

Table 30.2 Monthly percentage of annual daytime hours (p) for different latitudesLatitude North

South40 42 44 46 48 50 52 54 56 58 60

Jan. JuL 6.76 6.63 6.49 6.34 6.17 5.98 5.77 5.55 5.30 5.01 4.67

Feb. Aug. 6.72 6.65 6.58 6.50 6.41 6.30 6.19 6.08 5.95 5.81 5.65

Mar. Sep. 8.33 8.31 8.30 8.29 8.27 8.24 8.21 8.18 8.15 8.12 8.08

Apr. Oct. 8.95 9.00 9.06 9.12 9.18 9.24 9.29 9.36 9.45 9.55 9.65

May Nov. 10.02 10.14 10.26 10.39 10.53 10.68 10.85 11.03 11.22 11.46 11.74

Jun. Dec.10.08 10.22 10.38 10.54 10.71 10.91 11.13 11.38 11.67 12.00 12.39

JuL Jan. 10.22 10.35 10.49 10.64 10.80 10.99 11.20 11.43 11.69 11.98 12.31

Aug. Feb. 9.54 9.62 9.70 9.79 9.8910.00 10.12 10.26 10.40 10.55 10.70

Sep. Mar. 8.29 8.40 8.41 8.42 8.44 8.46 8.49 8.51 8.53 8.55 8.57

Oct. Apr. 7.75 7.69 7.63 7.57 7.51 7.45 7.39 7.30 7.21 7.10 6.98

Nov. May 6.72 6.62 6.49 6.36 6.23 6.10 5.93 5.74 5.54 5.04 4.31

Dec. Jun. 7.52 6.37 6.21 6.04 5.86 5.65 5.43 5.18 4.89 4.56 4.22

Note: Southern latitudes apply 6-month difference as shown.

which the monthly crop water requirements Tcrop (in millimetres) are found by multiplying the mean monthly temperature Tm (0C) by the monthly percentage of annual daytime hours p and a monthly crop coefficient k\ETcrop = (OA6Tm + Z)kp

divided into three parts: (1) field application; (2) field canal; and (3) distribution efficiency. 30.1.4.1 Field application efficiency (Ea)

Table 30.2 shows the monthly percentage of p for different latitudes. A sample calculation of water requirements for maize planted mid May near Saskatoon (latitude 520N) is shown in Table 30.3. A simple calculation of gross irrigation requirements (/gross) can be made as follows:

Ea is dependent on soil type and type of irrigation system used. Typical values are given in Table 30.4.

Table 30.4Irrigation method Application practices a, % water application efficiency Soil texture heavy light Sprinkler Daytime application, moderately strong wind Night application Poorly levelled and shaped - Well levelled and shaped Poorly graded and sized Well graded and sized

i^-(fT^,-W EI

(30.2)

where Tcrop is the crop water requirements, /?e is the effective rainfall and Ea is the field application efficiency

Table 30.3Mean monthly temp. 0 Water % annual Crop requiredaytime coefficient ments (mm) (V (P)

Trickle Basin

60 70 80 60 75 55 65

60 70 80 45 60 40 50

Period Days

(Q

Furrow Border

30.1.4.2 Field canal efficiency 15/5-31/5 1/6- 3/6 4/6-30/6 1/7- 8/7 9/7-31/7 1/8-17/8 18/8-31/8 1/9-16/9 17 3 27 8 23 17 14 16

(Ef)

11.6 19.7 19.3 21.38.7

5.95 1.11 10.02 2.89 8.31 5.55 4.57 4.53

0.35 0.35 0.96 1.05 1.14 1.14 1.02 0.75

27.8 6.6 164.1 51.2 159.9 112.6 83.0 40.8 646.0

Ef is dependent on type of field channel used and area served. Blocks larger than 20 ha Blocks up to 20 ha Unlined canals Lined or piped Unlined canals Lined or piped 0.90 0.95 0.80 0.90

30.1.4 Irrigation efficiency It is necessary to account for losses of water incurred during conveyance and application to the field. Efficiencies can be

30.1.4.3 Distribution efficiency (Ed) Distribution efficiency (Ed) is dependent on area se