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Sensitivity of agricultural drainage systems to changes in climatic inputs
Agricultural Water Management, 21 (1992) 57-66
57
© 1992 Elsevier Science Publishers B.V. All rights reserved. 0378-3774/92/$05.00
Sensitivity of agricultural drainage systems to changes in climatic inputs A.C. Armstrong, R. Arrowsmith and D.A. Castle ADAS Field Drainage Experimental Unit, Anstey Hall, Marls Lane, Trumpington, Cambridge, UK (Accepted 23 January 1992 )
ABSTRACT Armstrong, A.C., Arrowsmith, R. and Castle, D.A., 1992. Sensitivity of agricultural drainage systems to changes in climatic inputs. Agric. Water Manage., 21: 57-66. Drainage design and the performance of existing pipe systems is examined in the light of anthropogenetically induced climate change. It is shown that changes in UK drainage design could be influenced more by structural changes in agricultural production systems demanding higher standards of performance than by small shifts in rainfall totals. Although existing drainage systems might not meet their design criteria, any short-fall will be relatively minor for small increases in design rate. A hydraulic model is used to predict the performance of existing pipe drainage systems in the light of increasingly intense rainfalls. It is shown that existing systems could be stressed hydraulically, increasing the possibility of drainage failure, potential generation of surface runoff, and attendant risk of mole drain collapse.
INTRODUCTION
In the light of the many predictions of changes to the climate, due possibly to anthropogenetic forcing (Houghton et al., 1990; Legget, 1990; UKCCIRG, 1991 ), it is prudent to examine some of the possible consequences of changes in the input parameters to all aspects of the whole agricultural production system. Practical agriculture is a complicated enterprise, in which the growing of plants is only one component, influenced by the demand for agricultural products, the availability of land, equipment and labour, and the management of these resources. This study is concerned with examining the impacts of changes in climatic inputs to one of the management options, that of subsurface drainage. Artificial drainage is often an essential prerequisite for successful agricultural production. For this reason a large proportion of the agricultural land in England aPresent address: British Waterways, Greycaine Rd, Watford WD2 4JR, UK. Correspondence to." A.C. Armstrong, ADAS Field Drainage Experimental Unit, Anstey Hall, Marls Lane, Trumpington, Cambridge, CB2 2LF, UK.
58
A.C, ARMSTRONG ET AL.
& Wales is already drained, particularly in those areas where cereals are grown on heavy clay soils (Robinson and Armstrong, 1988 ). Consequently, any discussion of the performance of agriculture under changed climatic inputs should include the issue of drainage needs. The need for improved drainage to maintain production levels in the face of greater rainfall inputs has already been noted for the Leningrad area of USSR (Iakments and Pitovranov, 1988 ). Issues that need to be addressed include: ( 1 ) the need for drainage under the changed regime, where either more or less intensive drainage may be needed, or the design criteria that are appropriate may change; and (2) the performance of existing drainage systems under altered conditions. Field drainage systems are designed to last at least 25 years, and frequently are found to function after much longer periods. If the time scale for changes in the climate is of the order of decades, then systems that are being installed at the present time may well still be functioning under altered climatic inputs! It is not the intention here to speculate on the magnitude of the changes that may occur, but rather to discuss some of the consequences of given changes. The direction of change in rainfall for the UK is still debated, and it is not the intention to enter this debate (recent reviews are given for example by Rowntree, 1990; Rowntree et al., 1990; and UKCCIRG, 1991 ). Rather, simple (possibly naive) assumptions about the dimensions of change are adopted, and the impacts identified. Such a procedure is adequate to identify the sensitivities of the agricultural system to potential changes. If the system proves to be insensitive to some changes, then no action need be taken. If on the other hand, systems are extremely sensitive, then this will indicate areas where more research on the direction of change is needed. DESIGN OF NEW DRAINAGE SYSTEMS
Procedures for the design of drainage systems require the consideration of rainfall inputs derived from past climatic records. In the UK the design procedures (documented for example by Bailey et al., 1980; MAFF, 1982; Castle et al., 1984 ) use statistics of rainfall during the field capacity period tabulated for 55 agro-climatic areas. Choice of the daily rainfall for drainage design then depends on the type of drainage systems (which determines whether a 5-day or a 1-day rainfall period is used ), and the crop type (which determines the return period). Once the design rainfall rate (q) and the water table height (h) have been established, then it is possible to use one of the many well known equations (see for example the review by Lovell and Youngs, 1984) to indicate the required drain spacing. This involves estimating two significant parameters, the depth of the soil profile (d) below drain depth, the hydraulic conductivity of the soil (K). Usually the total depth of the soil profile is easy to determine,
SENSITIVITY OF AGRICULTURAL DRAINAGE SYSTEMS TO CHANGES IN CLIMATIC INPUTS
59
and the design water table is assumed to be 50 cm below ground depth. The major problem is normally encountered in determining the hydraulic conductivity of the soil, a parameter which is both difficult to measure reliably and highly variable in space. Once the parameters are determined, then the steady-state solution can be adopted in which the discharge through the drains (q) is equal to the design rainfall. The Hooghoudt (1940) equation then gives the estimate for the drain spacing (L) required. The actual soil depth below the drain (d) is replaced by the smaller 'effective' depth (de) to account for flow convergence in the vicinity of the drains. For a uniform soil:
LZ=K/q [8 d e h + 4
h 2]
where L is the drain spacing (m); K is the hydraulic conductivity of the soil ( ( m / d a y ) ; q is the design flux (rainfall rate) ( m / d a y ) ; de is the effective depth of soil below the drain (m); and h is the design water table height at mid-drain spacing (m). Because the effective depth, de, is also a function of the drain spacing, an iterative method is required to solve the equation. Details of the calculation are given by ILRI (1974), and are easily programmed on a computer. Clearly, the drain spacings calculated by this method will change if the input rainfall values also change. The consensus of General Circulation Models for the U K expects an increase in winter rainfall, perhaps of the order of 10%. There is as yet no general consensus as to the changes of summer rainfall (UKCCIRG, 1991 ). Further, a change in winter rainfall amounts could be the result of either an increase in the number of rainfall days without any change in intensity, or an increase in intensity over the same number of days. Nevertheless it is prudent to examine the consequences of a potential increase in winter rainfall intensity, which would lead to an increase in the rainfall rates used to design agricultural drainage systems. The sensitivity of drainage design procedures can be examined by calculating the drain spacing required to meet standards under both existing and possible changes rainfall inputs within the context of a given design situation. That chosen is a moderately permeable soil in the Cambridgeshire area in the East of England, assuming a 5.0 m deep silty soil with a hydraulic conductivity of 0.3 m/day. This situation is perhaps typical of a high productivity situation in an area where the impacts of climatic change might be expected to be high. Nevertheless, the conclusions drawn from this situation can be expected to be applicable over a wide range of humid temperate agricultural systems. Table 1 shows the drain spacings calculated using the Hooghoudt equation for a variety of cropping types, and for a variety of rainfall rates. Current rainfall rates used for drainage design are taken from agro-climatic area 28 (Smith and Trafford, 1976 ). The effect of increasing the design rainfall rate by 5, 10, 15 and 20% are also shown. The different cropping types
60
~X,f'.ARMSTRONG ET AL.
TABLE l Drainage design criteria and calculated drain (m) spacings tbr current climate and tbr rainfall values increased by 5, 10, 15 and 20% Land use
Grass Grass Arable Roots Horticulture Horticulture
design Water table (m above drain)
Return period (y)
0.7 0.5 0.5 0.5 0.5 0.3
l 1 2 5 t0 10
Current design rate ( m m / d )
Rainfall increased by % 0%
6.0 6.0 7.4 9.0 10.0 10.0
21.1 20.5 16.6 16.1 14.5 14.1 12.9 12.5 12.0 11.7 8.0 7.7
+5%
+10%
+ 15%
20%
t9.9 15.6 13.7 12.1 11.3 7.5
19.4 15.2 13.3 11,7 11.0 7.3
19.0 14.8 13.0 11.4 10.7 7.1
impose both different return periods (i.e., different degrees of risk) and different levels of water table control. In addition to the range of crops with a standard design criteria of a water table 50 cm below ground level, results are also given for grass (the least sensitive crop) with the less rigorous design criterion of a water table 30 cm below ground level, and for the most sensitive horticultural crops with the design water table set at 70 cm below ground level. From these results, it is immediately clear that the variation in required drain spacing due to changes in the rainfall inputs, is small compared to the variation of drain spacing resulting from the imposition of more demanding standards of performance required by changes in crop type. It has been widely speculated that an increase in mean temperature will increase the range of crops that can be grown in the UK, particularly with the introduction of high value crops which are temperature limited (e.g., Parry, 1989; Parry et al., 1989 ). These crops are frequently more demanding in their sensitivity to waterlogging than are many of the current field crops. It is thus to be expected that, as high value crops are introduced in response to the changed opportunities, higher standards of performance will be required from drainage systems. WATER-TABLE LEVELS F R O M E X I S T I N G D R A I N A G E SYSTEMS
The degree of water table control afforded by existing systems in the light on increased rainfall inputs can also be examined using the same Hooghoudt drain spacing equation. Re-arranging the equation, the water table height at mid-drain spacing can be solved for any known input rainfall rate and drain spacing. The solution is a simple quadratic equation, which is easily programmed. Only the positive root to the equation is of interest. Table 2 shows the calculated mid-drain water table heights, for the same situation considered in Table I. The drain spacings used are those listed in
SENSITIVITYOF AGRICULTURALDRAINAGESYSTEMSTO CHANGESIN CLIMATICINPUTS
61
TABLE 2 Actual height of the water table (m above drain height) at mid-drain spacing for drainage systems designed for current climate and for rainfall values increased by 5, 10, 15 and 20%
Grass Grass Arable Roots Horticulture Horticulture
design Water table (m above drain)
Return period (y)
0.7 0.5 0.5 0.5 0.5 0.3
1 1 2 5 10 10
Current design rate ( m m / d )
Rainfall increased by % + 5%
+ 10%
+ 15%
20%
6.0 6.0 7.4 9.0 10.0 I0.0
0.734 0.526 0.526 0.526 0.526 0.318
0.767 0.552 0.552 0.552 0.553 0.336
0.800 0.578 0.578 0.578 0.579 0.354
0.833 0.603 0.604 0.604 0.605 0.372
Table 1 for the current climate. The increase in water table height can be seen by comparing the original design height with that predicted. In general, the increases in water table height are small. Only for the least rigorous drainage criteria, that of grass with a design water table 30 cm below ground level, is the increase more than 10 cm for even a 20% increase in rainfall. For the majority of the results, the increase in water table heights is quite small. It is suggested therefore that, although existing drainage systems might not meet their design criteria, any short-fall will be relatively minor for small increases in design rainfall rate. Again, it is suggested that the adoption of more rigorous design standards associated with more sensitive crops will be more important than the effect of changed inputs to existing systems. H Y D R A U L I C F U N C T I O N I N G O F E X I S T I N G D R A I N A G E SYSTEMS
The performance of existing drainage systems under changed climatic inputs is also of concern. For the consideration of hydraulic functioning, the critical parameters are the height of the peak discharge event and the severity of surcharge within the pipe system. The performance of drainage systems can be evaluated by the use of the H Y D I N T hydraulic model which considers the passage of a synthetic input hydrograph (Hodgkinson et al., 1989). This model predicts the drain discharges and the hydraulic head during the passage of a rainfall event through a pipe system. From a knowledge of the pipe hydraulic parameters (diameter, gradient and roughness) the levels of surcharge above the pipe are calculated. The hydraulic head line is calculated for the case of a pipe receiving input at equal rate along its length, with no input at the upslope end (Trafford and
62
A.C. ARMSTRONG ET AL.
Dennis, 1974) derived from the Colebrook-White relationship (Ackers, 1963):
Cq 2 h= ~ - L where h is the hydraulic head (m); q the inflow rate per unit length ( m / m ); and L the total pipe length (m), and the coefficient C is given by:
C=8F/(~egD 5) in which g is the acceleration due to gravity ( m / s / s ) ; D is pipe diameter (m); and F the friction factor. The amount of surcharge over the pipe is given by the difference between the hydraulic head and the pipe height. Surcharge within a drainage system can be of concern where it is likely to intersect critical horizons in the soil. If the pipe drainage systems is used as a collector for mole drainage, then surcharge rising to mole drain depth will restrict the flow of water from the mole drains to the collectors. The submergence of the mole channels may lead to premature collapse, particularly at the most vulnerable point, that of entry to the pipe trench (Harris, 1984). In addition, surcharge reaching the ground surface is potentially dangerous, as this implies that the water pressure in the pipe system is sufficient to generate surface runoff. This phenomenon is observed where pipes are blocked and a 'blow-out' of water at the surface ensues. The HYDINT model has been developed as a tool for examining the effectiveness of existing drainage systems, and in particular to allow the designer to examine the likely durations of surcharge for a given design. It can, however, be equally applied to the issue of climatic change, being used to predict the hydraulic performance of a drainage system as it accepts higher peak flows. For this study, a hydrograph from the Brimstone Farm drainage experiment in Oxfordshire (Cannell et al., 1984 ) was scaled to predict the potential flow through a 200 m long lateral at 40 m spacing. For these calculations a 80 mm corrugated plastic pipe at a depth of 90 cm below ground level at a gradient of 0.48% has been assumed. The critical mole drain depth is 45 cm above pipe depth. The observed hydrograph, with a peak value of 3.89 litres/s has a return period of about 2.5 years. The hydrograph generated by current events is first examined, and then the effects of increasing the height of the peak flow by 5, 10, 15 and 20% are identified. These increased peaks are used to represent the potential effects of an increase in rainfall intensity. The pattern of inputs, and the maximum surcharge for both the current situation and with the peak increased by 15% is shown in figure 1. The time of maximum surcharge occurs at the same time as the peak of the hydrograph. However, the maximum surcharge does not increase beyond a limiting value at which the hydraulic head line intersects the soil surface. At this point, no more surcharge can be accommodated, and surface runoff is generated. For
SENSITIVITYOF AGRICULTURALDRAINAGESYSTEMSTO CHANGESIN CLIMATICINPUTS
63
Input flow M a x surchMoe
Surface 140 runoff 4.0
4.0 3.5
3.5 160
-160
3.0
3.0. -140
oE
2.5
•
2.0
120 100
15
..o •
•
2.5.
~
2.0.
E o
-I00
"15
-~_ - 8 0
60
1.0
| ~
1.0.
Z~ "60
"
40
"40
o.s
o.s.
I
o
o
i;',.I.
5
10
15
20
25
30
35
40
45
-120
50
Time hours
~
f'5
2'0 2's
"20
s'o 3'5 4'0
4'5 5b
Time hours
Fig. 1. Flow and surcharge patterns for current rainfall and for the same event increased by 15%. TABLE3 Changes in height and duration of pipe surcharge for present designs and for increased rainfall intensities Current climate
Peak flow litres/s Maximum surcharge m above pipe Duration of surcharge (h)
3.89 0.65 2.5
Rainfall increased by +5%
+10%
+15%
20%
4.08 0.74 2.5
4.28 0.84 3.0
4.47 0.90 3.5
4.67 0.90 4.0
drainage systems that include mole drains, the critical soil horizon is the roof of the mole drain channel, (45 cm below the surface for this model), and it is clear that this level of surcharge is exceeded for several hours (Table 3). The duration of surcharge increases as the peak drainflow increases, so the period during which the mole drain outlets are submerged also increases. Observations (Castle, 1989) have indicated that the mole drainage system is most vulnerable at the outlet, and it is suggested that this increased duration of surcharge may well decrease mole drain life. Once the limiting situation is reached, where surcharge reaches the surface, then no further increase in drain discharge can be generated. Any increase in peak input rates beyond 15% for this system will generate increasingly large amounts of overland flow, but no further increase in drain flow. Water will then flow across the ground surface, which may lead to a potentially erosive situation. These results indicate that as the peak inflow (i.e. the maximum rainfall intensity) increases, the importance of the constraints imposed by the
64
A.~.. ARMSTRONG ET AL.
capacity of the drainage system will also increase. This suggests that simple predictions cannot be made about the relationships between inputs and outputs, but must also consider the properties of the transmission systems between them. Any significant increase in rainfall intensities will therefore require the intensification of existing pipe drain systems in order to maintain the current drainage status.
CONCLUSIONS
This study has shown that predicted climatic changes can expected to have an effect on the design and performance of agricultural drainage systems. The major driving force will be the potential change in land use, requiring higher standards of performance from drainage systems. If higher value crops are subsequently adopted, then it seems almost certain that higher standards of drainage would be demanded. Although existing drainage designs under existing crops will generally be adequate to accommodate the small increases in rainfall rate, they will not prevent the build up of greater surcharge as the pipe system passes increased flow peaks. It is probable that this short term build up of surcharge would be detrimental to the performance of highly sensitive crops. The importance of changes in land use, which have been identified as the major driving force in determining design criteria are, therefore, likely to reinforced by the problems of short term performance identified as the consequence of an increase in rainfall intensity. Both of these considerations suggest that in designing future drainage systems it seems prudent to consider increasing the design capacity to allow for a potential increase in peak rate, which can be accommodated by using larger pipe diameters than are strictly required at present. Without such measures, the risk of surface runoff is likely to increase, and the life of mole drainage channels is likely to be reduced. It is concluded that changes to the climate that are likely to lead to problems are those associated with the short term behaviour of drainage systems, associated with peak flows and the transient build up of surcharge during storm events. A major concern is thus the better prediction of the rainfall intensities as a consequence of climate change. ACKNOWLEDGMENTS
Financial support for this work from the UK Ministry of Agriculture, Fisheries and Food (MAFF) is gratefully acknowledged.
SENSITIVITY OF AGRICULTURAL DRAINAGE SYSTEMS TO CHANGES IN CLIMATIC INPUTS
65
REFERENCES Ackers, P. (1963) Charts for the hydraulic design of channels and pipes. Hydraulics Research Paper No. 2, Hydraulics Research Station, HMSO, London. Bailey, A.D., Dennis, C.W., Harris, G.L., &Horner, M.W. (1980) Pipe size design for field drainage. R&D Report No. 5, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge. Cannell, R.Q., Goss, M.J., Harris, G.L., Jarvis, M.G., Douglas, J.T., Howse, K.R., & Le Grice, S. (1984) A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil. 1. Background, experiment and site details, drainage systems, measurement of drainflow and summary of results 1978-80. Journal of Agricultural Science, Cambridge: 102, 539-559. Castle, D.A. (1989) Soil and permeable backfill displacement by the mole plough and winged subsoiler and the effect on water entry into the pipe drain. Land and Water Use, (Proceedings of the Eleventh International Congress on Agricultural Engineering, Dublin 1989, Vol. 1), Ed. V.A. Dodd & P.M. Grace, Balkema, Rotterdam. Castle, D.A., McCunnall, J. & Tring, I.M. (1984) Field drainage: principles and practices. Batsford Academic, London. Harris, G.L. (1984) Effect of mole submergence on the life of mole channels. Agricultural Water Management: 8: 361-374. Hodgkinson, R., Arrowsmith, R. & Armstrong, A.C. (1989) HYDINT manual. Unpublished report, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge. Hooghoudt, S.B. (1940) Bijdragen tot de kennis van eenige natuurkundige grooteden van den grond. 7, Algemeene beschouwing van het probleem van de detail ontwatering en de infiltratie door middel van parallel loopende drains, greppels, slooten en canal. Versl. Landbouwkd. Onderz.: 46, 515-707. Houghton, J.T., Jenkins, G.J. & Ephraums, J.J. (editors) (1990) Climate change: the IPCC scientific assessment. Cambridge University Press, Cambridge. Iakments, V.N. & Pitovranov, S.E. (1988) The effects on agriculture in the Leningrad region. p. 639-661 of"The impact of climatic variations on agriculture, Volume 1: assessments in cool temperate and cold regions" ed. M.L. Parry, T.R. Carter & N.T. Konijn, Kluwer, Dordrecht. ILRI (1974) Drainage principles and applications. 4 vols, Publication no 16, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. Leggett, J. (1990) Global warming: the Greenpeace report, Oxford University Press, Oxford. Lovell, C.J. & Youngs, E.G. (1984) A comparison of steady-state land-drainage equations. Agricultural Water Management: 9, 1-21. MAFF (1982) The design of field drainage pipe systems. MAFF Reference book 345, HMSO, London. Parry, M.L., Carter, T.R. & Porter, J.H. (1989) The greenhouse effect and the future of British agriculture. Journal, Royal Agricultural Society of England: 150, 120-132. Robinson, M. & Armstrong, A.C. (1988) The extent of agricultural field drainage in England and Wales 1971-80. Trans. Inst. Br. Geogr. N.S: 13, 19-28. Rowntree, P.R. (1990) Estimates of future climate change over Britain. Part 2: results. Weather: 1990, 79-89. Rowntree, P.R., Callender, B.A. & Cochrane, J. (1990) Modelling climate change and some potential effects on agriculture in the UK. Journal of the Royal Agriculture Society of England: 150, 150-170. Smith, L.P. & Trafford, B.D. ( 1976 ) Climate and drainage. MAFF Technical Bulletin 34, HMSO London.
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A.C. A R M S T R O N G ET AL.
Trafford, B.D. & Dennis, C.W. (1974) The role of the pipe in field drainage. Technical Bulletin 74/15, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge, 25pp. U KCCIRG (United Kingdom Climate Change Impacts Review Group) ( 1991 ). The potential effects of climate change in the United Kingdom. HMSO, London.
57
© 1992 Elsevier Science Publishers B.V. All rights reserved. 0378-3774/92/$05.00
Sensitivity of agricultural drainage systems to changes in climatic inputs A.C. Armstrong, R. Arrowsmith and D.A. Castle ADAS Field Drainage Experimental Unit, Anstey Hall, Marls Lane, Trumpington, Cambridge, UK (Accepted 23 January 1992 )
ABSTRACT Armstrong, A.C., Arrowsmith, R. and Castle, D.A., 1992. Sensitivity of agricultural drainage systems to changes in climatic inputs. Agric. Water Manage., 21: 57-66. Drainage design and the performance of existing pipe systems is examined in the light of anthropogenetically induced climate change. It is shown that changes in UK drainage design could be influenced more by structural changes in agricultural production systems demanding higher standards of performance than by small shifts in rainfall totals. Although existing drainage systems might not meet their design criteria, any short-fall will be relatively minor for small increases in design rate. A hydraulic model is used to predict the performance of existing pipe drainage systems in the light of increasingly intense rainfalls. It is shown that existing systems could be stressed hydraulically, increasing the possibility of drainage failure, potential generation of surface runoff, and attendant risk of mole drain collapse.
INTRODUCTION
In the light of the many predictions of changes to the climate, due possibly to anthropogenetic forcing (Houghton et al., 1990; Legget, 1990; UKCCIRG, 1991 ), it is prudent to examine some of the possible consequences of changes in the input parameters to all aspects of the whole agricultural production system. Practical agriculture is a complicated enterprise, in which the growing of plants is only one component, influenced by the demand for agricultural products, the availability of land, equipment and labour, and the management of these resources. This study is concerned with examining the impacts of changes in climatic inputs to one of the management options, that of subsurface drainage. Artificial drainage is often an essential prerequisite for successful agricultural production. For this reason a large proportion of the agricultural land in England aPresent address: British Waterways, Greycaine Rd, Watford WD2 4JR, UK. Correspondence to." A.C. Armstrong, ADAS Field Drainage Experimental Unit, Anstey Hall, Marls Lane, Trumpington, Cambridge, CB2 2LF, UK.
58
A.C, ARMSTRONG ET AL.
& Wales is already drained, particularly in those areas where cereals are grown on heavy clay soils (Robinson and Armstrong, 1988 ). Consequently, any discussion of the performance of agriculture under changed climatic inputs should include the issue of drainage needs. The need for improved drainage to maintain production levels in the face of greater rainfall inputs has already been noted for the Leningrad area of USSR (Iakments and Pitovranov, 1988 ). Issues that need to be addressed include: ( 1 ) the need for drainage under the changed regime, where either more or less intensive drainage may be needed, or the design criteria that are appropriate may change; and (2) the performance of existing drainage systems under altered conditions. Field drainage systems are designed to last at least 25 years, and frequently are found to function after much longer periods. If the time scale for changes in the climate is of the order of decades, then systems that are being installed at the present time may well still be functioning under altered climatic inputs! It is not the intention here to speculate on the magnitude of the changes that may occur, but rather to discuss some of the consequences of given changes. The direction of change in rainfall for the UK is still debated, and it is not the intention to enter this debate (recent reviews are given for example by Rowntree, 1990; Rowntree et al., 1990; and UKCCIRG, 1991 ). Rather, simple (possibly naive) assumptions about the dimensions of change are adopted, and the impacts identified. Such a procedure is adequate to identify the sensitivities of the agricultural system to potential changes. If the system proves to be insensitive to some changes, then no action need be taken. If on the other hand, systems are extremely sensitive, then this will indicate areas where more research on the direction of change is needed. DESIGN OF NEW DRAINAGE SYSTEMS
Procedures for the design of drainage systems require the consideration of rainfall inputs derived from past climatic records. In the UK the design procedures (documented for example by Bailey et al., 1980; MAFF, 1982; Castle et al., 1984 ) use statistics of rainfall during the field capacity period tabulated for 55 agro-climatic areas. Choice of the daily rainfall for drainage design then depends on the type of drainage systems (which determines whether a 5-day or a 1-day rainfall period is used ), and the crop type (which determines the return period). Once the design rainfall rate (q) and the water table height (h) have been established, then it is possible to use one of the many well known equations (see for example the review by Lovell and Youngs, 1984) to indicate the required drain spacing. This involves estimating two significant parameters, the depth of the soil profile (d) below drain depth, the hydraulic conductivity of the soil (K). Usually the total depth of the soil profile is easy to determine,
SENSITIVITY OF AGRICULTURAL DRAINAGE SYSTEMS TO CHANGES IN CLIMATIC INPUTS
59
and the design water table is assumed to be 50 cm below ground depth. The major problem is normally encountered in determining the hydraulic conductivity of the soil, a parameter which is both difficult to measure reliably and highly variable in space. Once the parameters are determined, then the steady-state solution can be adopted in which the discharge through the drains (q) is equal to the design rainfall. The Hooghoudt (1940) equation then gives the estimate for the drain spacing (L) required. The actual soil depth below the drain (d) is replaced by the smaller 'effective' depth (de) to account for flow convergence in the vicinity of the drains. For a uniform soil:
LZ=K/q [8 d e h + 4
h 2]
where L is the drain spacing (m); K is the hydraulic conductivity of the soil ( ( m / d a y ) ; q is the design flux (rainfall rate) ( m / d a y ) ; de is the effective depth of soil below the drain (m); and h is the design water table height at mid-drain spacing (m). Because the effective depth, de, is also a function of the drain spacing, an iterative method is required to solve the equation. Details of the calculation are given by ILRI (1974), and are easily programmed on a computer. Clearly, the drain spacings calculated by this method will change if the input rainfall values also change. The consensus of General Circulation Models for the U K expects an increase in winter rainfall, perhaps of the order of 10%. There is as yet no general consensus as to the changes of summer rainfall (UKCCIRG, 1991 ). Further, a change in winter rainfall amounts could be the result of either an increase in the number of rainfall days without any change in intensity, or an increase in intensity over the same number of days. Nevertheless it is prudent to examine the consequences of a potential increase in winter rainfall intensity, which would lead to an increase in the rainfall rates used to design agricultural drainage systems. The sensitivity of drainage design procedures can be examined by calculating the drain spacing required to meet standards under both existing and possible changes rainfall inputs within the context of a given design situation. That chosen is a moderately permeable soil in the Cambridgeshire area in the East of England, assuming a 5.0 m deep silty soil with a hydraulic conductivity of 0.3 m/day. This situation is perhaps typical of a high productivity situation in an area where the impacts of climatic change might be expected to be high. Nevertheless, the conclusions drawn from this situation can be expected to be applicable over a wide range of humid temperate agricultural systems. Table 1 shows the drain spacings calculated using the Hooghoudt equation for a variety of cropping types, and for a variety of rainfall rates. Current rainfall rates used for drainage design are taken from agro-climatic area 28 (Smith and Trafford, 1976 ). The effect of increasing the design rainfall rate by 5, 10, 15 and 20% are also shown. The different cropping types
60
~X,f'.ARMSTRONG ET AL.
TABLE l Drainage design criteria and calculated drain (m) spacings tbr current climate and tbr rainfall values increased by 5, 10, 15 and 20% Land use
Grass Grass Arable Roots Horticulture Horticulture
design Water table (m above drain)
Return period (y)
0.7 0.5 0.5 0.5 0.5 0.3
l 1 2 5 t0 10
Current design rate ( m m / d )
Rainfall increased by % 0%
6.0 6.0 7.4 9.0 10.0 10.0
21.1 20.5 16.6 16.1 14.5 14.1 12.9 12.5 12.0 11.7 8.0 7.7
+5%
+10%
+ 15%
20%
t9.9 15.6 13.7 12.1 11.3 7.5
19.4 15.2 13.3 11,7 11.0 7.3
19.0 14.8 13.0 11.4 10.7 7.1
impose both different return periods (i.e., different degrees of risk) and different levels of water table control. In addition to the range of crops with a standard design criteria of a water table 50 cm below ground level, results are also given for grass (the least sensitive crop) with the less rigorous design criterion of a water table 30 cm below ground level, and for the most sensitive horticultural crops with the design water table set at 70 cm below ground level. From these results, it is immediately clear that the variation in required drain spacing due to changes in the rainfall inputs, is small compared to the variation of drain spacing resulting from the imposition of more demanding standards of performance required by changes in crop type. It has been widely speculated that an increase in mean temperature will increase the range of crops that can be grown in the UK, particularly with the introduction of high value crops which are temperature limited (e.g., Parry, 1989; Parry et al., 1989 ). These crops are frequently more demanding in their sensitivity to waterlogging than are many of the current field crops. It is thus to be expected that, as high value crops are introduced in response to the changed opportunities, higher standards of performance will be required from drainage systems. WATER-TABLE LEVELS F R O M E X I S T I N G D R A I N A G E SYSTEMS
The degree of water table control afforded by existing systems in the light on increased rainfall inputs can also be examined using the same Hooghoudt drain spacing equation. Re-arranging the equation, the water table height at mid-drain spacing can be solved for any known input rainfall rate and drain spacing. The solution is a simple quadratic equation, which is easily programmed. Only the positive root to the equation is of interest. Table 2 shows the calculated mid-drain water table heights, for the same situation considered in Table I. The drain spacings used are those listed in
SENSITIVITYOF AGRICULTURALDRAINAGESYSTEMSTO CHANGESIN CLIMATICINPUTS
61
TABLE 2 Actual height of the water table (m above drain height) at mid-drain spacing for drainage systems designed for current climate and for rainfall values increased by 5, 10, 15 and 20%
Grass Grass Arable Roots Horticulture Horticulture
design Water table (m above drain)
Return period (y)
0.7 0.5 0.5 0.5 0.5 0.3
1 1 2 5 10 10
Current design rate ( m m / d )
Rainfall increased by % + 5%
+ 10%
+ 15%
20%
6.0 6.0 7.4 9.0 10.0 I0.0
0.734 0.526 0.526 0.526 0.526 0.318
0.767 0.552 0.552 0.552 0.553 0.336
0.800 0.578 0.578 0.578 0.579 0.354
0.833 0.603 0.604 0.604 0.605 0.372
Table 1 for the current climate. The increase in water table height can be seen by comparing the original design height with that predicted. In general, the increases in water table height are small. Only for the least rigorous drainage criteria, that of grass with a design water table 30 cm below ground level, is the increase more than 10 cm for even a 20% increase in rainfall. For the majority of the results, the increase in water table heights is quite small. It is suggested therefore that, although existing drainage systems might not meet their design criteria, any short-fall will be relatively minor for small increases in design rainfall rate. Again, it is suggested that the adoption of more rigorous design standards associated with more sensitive crops will be more important than the effect of changed inputs to existing systems. H Y D R A U L I C F U N C T I O N I N G O F E X I S T I N G D R A I N A G E SYSTEMS
The performance of existing drainage systems under changed climatic inputs is also of concern. For the consideration of hydraulic functioning, the critical parameters are the height of the peak discharge event and the severity of surcharge within the pipe system. The performance of drainage systems can be evaluated by the use of the H Y D I N T hydraulic model which considers the passage of a synthetic input hydrograph (Hodgkinson et al., 1989). This model predicts the drain discharges and the hydraulic head during the passage of a rainfall event through a pipe system. From a knowledge of the pipe hydraulic parameters (diameter, gradient and roughness) the levels of surcharge above the pipe are calculated. The hydraulic head line is calculated for the case of a pipe receiving input at equal rate along its length, with no input at the upslope end (Trafford and
62
A.C. ARMSTRONG ET AL.
Dennis, 1974) derived from the Colebrook-White relationship (Ackers, 1963):
Cq 2 h= ~ - L where h is the hydraulic head (m); q the inflow rate per unit length ( m / m ); and L the total pipe length (m), and the coefficient C is given by:
C=8F/(~egD 5) in which g is the acceleration due to gravity ( m / s / s ) ; D is pipe diameter (m); and F the friction factor. The amount of surcharge over the pipe is given by the difference between the hydraulic head and the pipe height. Surcharge within a drainage system can be of concern where it is likely to intersect critical horizons in the soil. If the pipe drainage systems is used as a collector for mole drainage, then surcharge rising to mole drain depth will restrict the flow of water from the mole drains to the collectors. The submergence of the mole channels may lead to premature collapse, particularly at the most vulnerable point, that of entry to the pipe trench (Harris, 1984). In addition, surcharge reaching the ground surface is potentially dangerous, as this implies that the water pressure in the pipe system is sufficient to generate surface runoff. This phenomenon is observed where pipes are blocked and a 'blow-out' of water at the surface ensues. The HYDINT model has been developed as a tool for examining the effectiveness of existing drainage systems, and in particular to allow the designer to examine the likely durations of surcharge for a given design. It can, however, be equally applied to the issue of climatic change, being used to predict the hydraulic performance of a drainage system as it accepts higher peak flows. For this study, a hydrograph from the Brimstone Farm drainage experiment in Oxfordshire (Cannell et al., 1984 ) was scaled to predict the potential flow through a 200 m long lateral at 40 m spacing. For these calculations a 80 mm corrugated plastic pipe at a depth of 90 cm below ground level at a gradient of 0.48% has been assumed. The critical mole drain depth is 45 cm above pipe depth. The observed hydrograph, with a peak value of 3.89 litres/s has a return period of about 2.5 years. The hydrograph generated by current events is first examined, and then the effects of increasing the height of the peak flow by 5, 10, 15 and 20% are identified. These increased peaks are used to represent the potential effects of an increase in rainfall intensity. The pattern of inputs, and the maximum surcharge for both the current situation and with the peak increased by 15% is shown in figure 1. The time of maximum surcharge occurs at the same time as the peak of the hydrograph. However, the maximum surcharge does not increase beyond a limiting value at which the hydraulic head line intersects the soil surface. At this point, no more surcharge can be accommodated, and surface runoff is generated. For
SENSITIVITYOF AGRICULTURALDRAINAGESYSTEMSTO CHANGESIN CLIMATICINPUTS
63
Input flow M a x surchMoe
Surface 140 runoff 4.0
4.0 3.5
3.5 160
-160
3.0
3.0. -140
oE
2.5
•
2.0
120 100
15
..o •
•
2.5.
~
2.0.
E o
-I00
"15
-~_ - 8 0
60
1.0
| ~
1.0.
Z~ "60
"
40
"40
o.s
o.s.
I
o
o
i;',.I.
5
10
15
20
25
30
35
40
45
-120
50
Time hours
~
f'5
2'0 2's
"20
s'o 3'5 4'0
4'5 5b
Time hours
Fig. 1. Flow and surcharge patterns for current rainfall and for the same event increased by 15%. TABLE3 Changes in height and duration of pipe surcharge for present designs and for increased rainfall intensities Current climate
Peak flow litres/s Maximum surcharge m above pipe Duration of surcharge (h)
3.89 0.65 2.5
Rainfall increased by +5%
+10%
+15%
20%
4.08 0.74 2.5
4.28 0.84 3.0
4.47 0.90 3.5
4.67 0.90 4.0
drainage systems that include mole drains, the critical soil horizon is the roof of the mole drain channel, (45 cm below the surface for this model), and it is clear that this level of surcharge is exceeded for several hours (Table 3). The duration of surcharge increases as the peak drainflow increases, so the period during which the mole drain outlets are submerged also increases. Observations (Castle, 1989) have indicated that the mole drainage system is most vulnerable at the outlet, and it is suggested that this increased duration of surcharge may well decrease mole drain life. Once the limiting situation is reached, where surcharge reaches the surface, then no further increase in drain discharge can be generated. Any increase in peak input rates beyond 15% for this system will generate increasingly large amounts of overland flow, but no further increase in drain flow. Water will then flow across the ground surface, which may lead to a potentially erosive situation. These results indicate that as the peak inflow (i.e. the maximum rainfall intensity) increases, the importance of the constraints imposed by the
64
A.~.. ARMSTRONG ET AL.
capacity of the drainage system will also increase. This suggests that simple predictions cannot be made about the relationships between inputs and outputs, but must also consider the properties of the transmission systems between them. Any significant increase in rainfall intensities will therefore require the intensification of existing pipe drain systems in order to maintain the current drainage status.
CONCLUSIONS
This study has shown that predicted climatic changes can expected to have an effect on the design and performance of agricultural drainage systems. The major driving force will be the potential change in land use, requiring higher standards of performance from drainage systems. If higher value crops are subsequently adopted, then it seems almost certain that higher standards of drainage would be demanded. Although existing drainage designs under existing crops will generally be adequate to accommodate the small increases in rainfall rate, they will not prevent the build up of greater surcharge as the pipe system passes increased flow peaks. It is probable that this short term build up of surcharge would be detrimental to the performance of highly sensitive crops. The importance of changes in land use, which have been identified as the major driving force in determining design criteria are, therefore, likely to reinforced by the problems of short term performance identified as the consequence of an increase in rainfall intensity. Both of these considerations suggest that in designing future drainage systems it seems prudent to consider increasing the design capacity to allow for a potential increase in peak rate, which can be accommodated by using larger pipe diameters than are strictly required at present. Without such measures, the risk of surface runoff is likely to increase, and the life of mole drainage channels is likely to be reduced. It is concluded that changes to the climate that are likely to lead to problems are those associated with the short term behaviour of drainage systems, associated with peak flows and the transient build up of surcharge during storm events. A major concern is thus the better prediction of the rainfall intensities as a consequence of climate change. ACKNOWLEDGMENTS
Financial support for this work from the UK Ministry of Agriculture, Fisheries and Food (MAFF) is gratefully acknowledged.
SENSITIVITY OF AGRICULTURAL DRAINAGE SYSTEMS TO CHANGES IN CLIMATIC INPUTS
65
REFERENCES Ackers, P. (1963) Charts for the hydraulic design of channels and pipes. Hydraulics Research Paper No. 2, Hydraulics Research Station, HMSO, London. Bailey, A.D., Dennis, C.W., Harris, G.L., &Horner, M.W. (1980) Pipe size design for field drainage. R&D Report No. 5, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge. Cannell, R.Q., Goss, M.J., Harris, G.L., Jarvis, M.G., Douglas, J.T., Howse, K.R., & Le Grice, S. (1984) A study of mole drainage with simplified cultivation for autumn-sown crops on a clay soil. 1. Background, experiment and site details, drainage systems, measurement of drainflow and summary of results 1978-80. Journal of Agricultural Science, Cambridge: 102, 539-559. Castle, D.A. (1989) Soil and permeable backfill displacement by the mole plough and winged subsoiler and the effect on water entry into the pipe drain. Land and Water Use, (Proceedings of the Eleventh International Congress on Agricultural Engineering, Dublin 1989, Vol. 1), Ed. V.A. Dodd & P.M. Grace, Balkema, Rotterdam. Castle, D.A., McCunnall, J. & Tring, I.M. (1984) Field drainage: principles and practices. Batsford Academic, London. Harris, G.L. (1984) Effect of mole submergence on the life of mole channels. Agricultural Water Management: 8: 361-374. Hodgkinson, R., Arrowsmith, R. & Armstrong, A.C. (1989) HYDINT manual. Unpublished report, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge. Hooghoudt, S.B. (1940) Bijdragen tot de kennis van eenige natuurkundige grooteden van den grond. 7, Algemeene beschouwing van het probleem van de detail ontwatering en de infiltratie door middel van parallel loopende drains, greppels, slooten en canal. Versl. Landbouwkd. Onderz.: 46, 515-707. Houghton, J.T., Jenkins, G.J. & Ephraums, J.J. (editors) (1990) Climate change: the IPCC scientific assessment. Cambridge University Press, Cambridge. Iakments, V.N. & Pitovranov, S.E. (1988) The effects on agriculture in the Leningrad region. p. 639-661 of"The impact of climatic variations on agriculture, Volume 1: assessments in cool temperate and cold regions" ed. M.L. Parry, T.R. Carter & N.T. Konijn, Kluwer, Dordrecht. ILRI (1974) Drainage principles and applications. 4 vols, Publication no 16, International Institute for Land Reclamation and Improvement, Wageningen, The Netherlands. Leggett, J. (1990) Global warming: the Greenpeace report, Oxford University Press, Oxford. Lovell, C.J. & Youngs, E.G. (1984) A comparison of steady-state land-drainage equations. Agricultural Water Management: 9, 1-21. MAFF (1982) The design of field drainage pipe systems. MAFF Reference book 345, HMSO, London. Parry, M.L., Carter, T.R. & Porter, J.H. (1989) The greenhouse effect and the future of British agriculture. Journal, Royal Agricultural Society of England: 150, 120-132. Robinson, M. & Armstrong, A.C. (1988) The extent of agricultural field drainage in England and Wales 1971-80. Trans. Inst. Br. Geogr. N.S: 13, 19-28. Rowntree, P.R. (1990) Estimates of future climate change over Britain. Part 2: results. Weather: 1990, 79-89. Rowntree, P.R., Callender, B.A. & Cochrane, J. (1990) Modelling climate change and some potential effects on agriculture in the UK. Journal of the Royal Agriculture Society of England: 150, 150-170. Smith, L.P. & Trafford, B.D. ( 1976 ) Climate and drainage. MAFF Technical Bulletin 34, HMSO London.
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Trafford, B.D. & Dennis, C.W. (1974) The role of the pipe in field drainage. Technical Bulletin 74/15, ADAS Field Drainage Experimental Unit, Trumpington, Cambridge, 25pp. U KCCIRG (United Kingdom Climate Change Impacts Review Group) ( 1991 ). The potential effects of climate change in the United Kingdom. HMSO, London.