land drainage: agricultural benefits and environmental impacts

Land Drainage: Agricultural Benefits and Environmental Impacts By J. MORRIS , BSc, MSc*

ABSTRACT

The paper reviews the context and contribution of agricultural land drainage in the UK and discusses the design and performance of land drainage improvement and maintenance schemes. The environmental impacts of land drainage are considered, and how drainage activities can be, and are being, modified to meet environmental criteria. Key words: Agricultural drainage; economics; cost benefit; environmental impact .

INTRODUCTION

Government incentives for land drainage improvements such as river flood alleviation, arterial drainage schemes, and field drainage installations, have been an important component of agricultural support in the UK. More recently, over-supply of agricultural commodities, pressure on public funds, concern with the environmental impacts of intensive agriculture, and competition from non-agricultural land uses, have called into question the desirability (or indeed feasibility) of agricultural systems in the UK which do not protect environmental quality. The environmental impact of land drainage for agriculture has been central to these issues.

AGRICULTURAL DRAINAGE OBJECTIVES

Water is essential for agriculture, but too much of it can limit crop growth and cropping activities. The basic aim of agricultural drainage is to alleviate or prevent the problems caused by excess water in order to facilitate more productive agriculture.

The agricultural drainage problem may be due to coastal or fluvial flooding, and/or the presence of seasonal or permanent high water tables. Poor drainage due to flood inundation often calls for river re-channelling and embanking works. Where poor drainage is due to waterlogging, the pre-occupation is with improving arterial watercourses, installing piped under-drainage, and other measures such as subsoiling, which improve the movement of water in the soil.

This paper was presented for discussion at the Institution's Annual Symposium on Agriculture and the Environment held in York on 21-22 March 1989.

*Senior Lecturer in Agricultural Economics and Management, Silsoe College, Crdnfield Institute of Technology.

J . I W E M , 1989, 3, December

Fig. 1 summarizes the principal benefits of improved drainage for grass and arable enterprises. They can be generalized in terms of improvements in the soil/plant/water growing environment and in the 'trafficability' of soils, which allow more timely access for field activities, together with a reduction in crop damage caused by inundation.

A review of relevant agricultural drainage literature has been given previously'. Much research into crop response to drainage has measured soil wetness in terms of depth to water table, and has related yields to this parameter2x334. Experimental results show a wide difference in the tolerance of crops to the timing and duration of waterlogging or inun- da t i~n ' .~ . Recent UK research has examined the yield effects of waterlogging at different stages of crop development, concluding that water table control to 0.5 m is adequate provided that incur- sions above this level are brief7. With respect to field trafficability and work-day probability, soil strength and load bearing capacity vary inversely with water content and water table height8,".'". Better drainage reduces the risk of untimeliness yield penalties for critical operations such as seed-bed preparation or fe r t i l i~ ing ' ' ,~~J~. In many respects requirements for trafficability and workability may impose stricter conditions than the crop itself. In grassland systems, improved drainage can (a) reduce the risk of surface damage or 'poaching', (b) allow earlier access for grazing stock, (c) allow the timely application of nitrogen fertilizer, and (d) facilitate a move to more intensive and predictable silage making sy~tems'~.

DRAINAGE METHODS

The purpose of drainage design is to control the level of the water table and the risk of flooding according to limits required for agricultural production. River, arterial and field drainage in the agricultural catchment must be considered as a whole.

Field drainage, which remains at the discretion of the private farmer, is mainly a traditional art based on experience of local soils and conditions. Open ditches may adequately serve soils which are free draining; otherwise the tube drains or unlined mole drains may be required, with or without permeable stone fill. The scientific approach to drainage design attempts to control the water table by finding suitable drain spacing and depth for the expected rainfall and hydraulic characteristics of the soil using

55 1

MORRIS ON

ARABLE LAND

- Warmer soils

- More efficient use of Increased yields & quality at

Less variation throughout the

harvest

Less risk of failure

Improved arable productivity

Increased caused by poor drainage crop growth field

disease

nitrogen

- Reduction in damage

- Reduced risk of plant

- Improved soil conditions -Improved timeliness of field operations

GRASSLAND

- Reduced risk of animal disease

Improved species comDosition \

Earlier growth vields of grass

Improved soil se: conditions

' :ended )sons grazing ~~

\? More cuts of grass

Quicker liveweight gain

Greater stock carrying 'capacity

Higher haykilage yields

Improved grassland productivity

\Earlier top dressing II with fertilizer \ Reduced poaching

Fig. 1. Benefits of improved drainage for arable and grassland uses

mathematical equations and modelling. Agronomic trials provide guidelines for preferred water table heights, which for most crops average 500 mm below the surface. Tube drains are often laid at 0.8-1.5 m depending on soil type and field conditions with about 0.15 m freeboard at the outfall. Mole drains are placed at about 0.5-0.6 m depth". With respect to flood risk, practice has provided criteria for arterial designs to give minimum accep- table 'whole year' flood frequencies. This is 1-in-2 to 1-in-5 for most field crops, and 1-in-10 years for high-value horticultural crops. Desired standards for flood risk during the April-October period are more stringenP.

In principle, the criteria and methods for drainage can be used to create and retain wet conditions for natural habitats, if so required.

BENEFITS OF FARM DRAINAGE

For the farm business, the benefit of land drainage may occur in a number of ways17,18,19,20,21.

Drainage may allow the reclamation of land previously unsuited to commercial agriculture. Improved drainage could allow the intens$cufion of existing enterprises, particularly arising from the use of yield-increasing inputs and practices, such as improved forage conservation. Alternatively, better drainage could facilitate land-use change and a

552

switch to new, more profitable enterprises, such as a move from permanent pasture to an arablelley rotation. Furthermore, reduced production costs could result due to improved working conditions, or savings in forage conservation costs due to extended grazing seasons. Where drainage incorporates a significant part of the farm, there may be benefits for activities on other parts of the farm; for example when increased stocking on lowland pastures may release upland areas for arable production whilst maintaining herd size.

Fig. 2 provides a basic framework for agricultural drainage benefit assessment. Improved land drainage offers a degree of flood alleviation and/or an improvement in outfall levels in the arterial drainage system. The extra value of intensifying existing, or moving to new, farming systems, can be expressed in terms of changes in enterprise gross margins and farm-level fixed costs. Methods for appraising land drainage improvements are described e l~ewhere~*~*~ .

VALUE AND PERFORMANCE OF AGRICULTURAL LAND DRAINAGE

During the 1970s there were increased demands for rigour in the design and appraisal of agricultural land-drainage schemes. Schemes were criticized for their somewhat arbitrary and optimistic estimates of

JJWEM, 1989, 3, December.

LAND DRAINAGE: AGRICULTURAL BENEFITS AND ENVIRONMENTAL IMPACTS

Shift in land Avcrage % of total use class benefits (Uha) benefits

.1 LAND DRAINAGE SOLUTION

Degree of Flood Improved Outfall / Alleriation - 1 [- Facility

% of total area

Field Drainage

Yield Response Trafficability Yield Response Cost of Fleld Drainage -Installation and Maintenance

Additional Plant, Labour and

Benefits to Improved Field Drainage

to Flood Alleviation

Increased Productivity of Exlsting Systems or Change to New Systems In Benefit Area

-Reduced Damage to Crops

Grant Aid \Change of Landuse Outside Benefit Area

Market Prices ~ l n c r e a s e d Gross Increased Fixed Margins \ New -/- Costs

Net Returns

and Benefits Reduced Damage ~ NET AGRICULTURAL Intangible Costs, to Property etc. BENEFIT

Fig. 2. Determination of drainage benefits at farm level

benefits and inadequately detailed design and cost estimates. As the UK moved towards self sufficiency and surplus in indigenous foods, appraisals were criticized for not adjusting benefit estimates to reflect either direct or hidden government subsidies to farmers. Furthermore, there was growing concern that, whilst agricultural benefits might be over- valued, environmental impacts received insufficient attention either at the design or analysis stage.

In this context, and with a view to improving the assessment of agricultural drainage benefits, Silsoe College estimated the value to farmers of changes in drainage regimes on 22 water authority and internal drainage board schemes in England, covering over 300 farmers and 10000 haz4. At farm level, the greatest benefits were associated with land-use change to higher value cropping, and the installation of field drains (Tables I to IV). In the case of grassland, benefits were particularly associated with- increases in nitrogen use and changes in grass conservation for winter feed. Key factors influencing farmer uptake of benefits were the extent to which poor drainage restricted desired land use, security of tenure, farm type and size, and recent changes in farm management.

The study also allowed an evaluation of scheme performance. The schemes covered a broad range of sizes and situations, including river embankment and regrading, arterial improvements and pumping. Benefits over a 30-year project life (this required some extrapolation of observed benefits over remaining project life for younger schemes) were set

,

against scheme costs. Fifteen of the 22 schemes appeared to be profitable in financial terms at the Treasury’s 5% discount rate. The poor performers were those with large increases in capital costs beyond those predicted, or limited farmer uptake of benefits.

The best performing schemes were large pumped schemes offering considerable economies in capital , 19”: 1 :i 1 1

188 285 35 18 195

5 243 2 1

150 100 I 0 0

Notes: Benefits = net revenues (extra revenues less extra variable and fixed costs), 1982 prices. Based on 22 schemes covering 300 farmers, 1300 blocks of land on 10OoO ha. Shift: land uses are classified as follows 0 - unused

2 - permanent grass 3 - temporary grass

The shift i s defined as the number of classes moved, e.g. a change from permanent grass to a crop/ley rotation i s a shift of 2.

4 - crop/ley rotation

6 - horticulture 1 - rough pasture 5 - crops

J.IWEM, 1989, 3, December. 553

MORRIS ON

TABLE 11. DISTRIBUTION OF BENEFITS ASSOCIATED WITH DRAINAGE

Average Extra N (kdha) benefits (€/ha)

Post-schcmc Average ‘YO of total

Drained 198

15V 100

% of total % of total benefits area

% of total area

52

49 51

100

100 100

19 1 4

19

100

~

Zero 1-50 51-100 101-150 151-200 200+

15 1 3 9

100

and running costs, where flooding problems had been virtually eliminated and improved arterial networks had removed the drainage constraint. Here, previously wet areas were assimilated into the rest of the farms’ mainly arable cropping sytem.

Without exception, the agriculturally beneficial and worthwhile schemes contained informal or formal farmer drainage organizations which pushed for major improvement works to be done, and provided subsequent follow-up, operation and maintenance.

The evaluation showed that during this period of national expansion in agricultural output, land drainage investments were financially attractive where poor drainage was a real constraint on commercial farming. The ‘reality’ of this constraint was usally voiced by farmers themselves. In other cases, particularly where farms were small and fragmented, and followed traditional methods, benefit uptake and financial performance of schemes were relatively low.

TABLE IV. DISTRIBUTION OF BENEFITS ASSOCI,XTED WITH C H A N C e IN GRASS CONSERVATION METHOD

Change in method

No change More cuts Hay to silage Grazing to hay Grazing to silage Hay to grazing

Average benefits (ffha)

40 51 67 53 60

104

52

% of total Yo of total benefits 1 area

56 I 71 1 1

RIVER MAINTENANCE EVALUATION

Under present circumstances, there is little need or justification for new agricultural drainage schemes. Attention has now turned to the scrutiny of land- drainage maintenance programmes, both in terms of improved programming of physical works25, and the application of management criteria to justify levels of service and prioritize maintenance work’‘. With respect to agricultural criteria, Silsoe College have, in collaboration with Severn-Trent Water, developed a simplified model for evaluating river maintenance expenditure which combines basic data on agro-climate, soil types, topographical and catchment characteristics, field drainage, flood risk and land use, to identify arterial drainage design par- ame te r~~’ (Fig. 3). On the basis of research evidence and field observations, drainage productivity classes have been defined in terms of water table depths during the critical spring period. Water tables between the surface and 300 mm depth indicate a ‘breakdown’ productivity situation, between 300 mm and 500 mm a ‘low’ productivity class, and greater than 500 mm a ‘normal’ productivity situation unimpeded by drainage. The method estimates field water table depth on the basis of freeboard in the adjacent watercourse for specified soil hydraulic conductivity and flood plain topogra- phy.

Fig. 3. An approach to river maintenance for agriculture

Where the water table is controlled by underdrainage, the desired freeboard is determined by drain depth in the field as influenced by soil type, drainage problem, and pipe-run and gradient. Pro- ductivity class is normal provided that freeboard is sufficient to avoid submergence of outfalls. Other- wise productivity classes could be low or break down.

Where sites drain naturally to the river, drainage equations can relate water table height to freeboard (hydraulic head), seepage rate (hydraulic conductivity), land slope, and rainfall. For specified local

554 J-IWEM, 1989, 3, December.


conditions, freeboard is used to estimate water table height and thereby a drainage productivity class.

Surveys in the catchment area can be used to compile scenarios for each land-use class/ productivity level combination. Each is described in terms of financial performance, reflecting the impact of waterlogging and flood risk. These scenarios are then used to show the benefit associated with maintaining a given level of service, the likely 'disbenefit' associated with lower standards, and the justification for maintenance expenditure. The method is currently being refined to allow for variable rainfall conditions, floodplain ditch layouts, and type and seasonality of maintenance operations. The method is also being extended to incorporate the impact of maintenance on environmental quality.

ENVIRONMENTAL IMPACT OF LAND DRAINAGE

Land drainage may have a number of environmental consequences for natural vegetation, wildlife, groundwater levels and quality, and catchment hydrology. Concern for the impact of land drainage on the environment has focused on the permanent loss of wetland sites and the effects of changes in agricultural practices following drainage.

Natural vegetation can be affected in two main ways28, (a) where the expansion af agricultural production displaces natural plant communities, and (b) where lower water tables result in a change in species composition in previously wetland sites. Wildlife resident in or indirectly dependent on wetlands can be depleted by the drainage, change of use and consequent loss of habitat29.m.31v. Depend- ing on local conditions, land drainage may divert water into rivers, thereby reducing the recharge of the aquifer, and may cause a lowering of groundwater levels. This can result in the drying up of ponds, reduced summer stream flows, and peat shrinkage. In addition, changes in land use associated with field and arterial drainage may lead to modifications in the hydrological cycle. Whereas field drainage, by quickly drying out soils, tends to increase the storage capacity and reduce runoff and flood risk during heavy storms, improved arterial drains tend to create earlier peak flows in the main channels and thereby increase flood risk32,33v34. River works, including regular maintenance, can disrupt the hydrological and biological balance of the river, with consequences for fish and other aquatic c r e a t ~ r e s ~ ~ J ~ .

The alteration of the landscape and ecology of a site can have a major impact on its use for recreation and amenity. Anglers, bird watchers, outdoor sports-persons, and those in pursuit of rural tran- quillity may be affected because of land drainage.

J.IWEM, 1989, 3, December

INCORPORATING ENVIRONMENTAL CRITERIA INTO DRAINAGE MANAGEMENT

In response to environmental concern, a number of guidelines were drawn u to reconcile drainage and

conserving or enhancing environmental features, and for prior consultation with the Nature Conser- vancy Council, were embodied in the 1981 Wildlife and Countryside Act. More recently operational guidelines have been compiled for environmentally sensitive rivers and land-drainage manage- ment4"-41342, and for field drainage43. In addition, methodologies for assessing likely environmental impact are being developed and applied44,4s, Gener- alized environmental impact statements were required by MAFF in 1985 for grant-aided land- drainage schemes46, and environment assessments are now required for significant agricultural water- management projects under the European Com- munity Directive 851337. More rigorous assessment of agricultural benefits is also likely to reduce the optimism of many drainage proposal^^^^^^.

A most significant agriculturaYenvironmental initiative has been the designation of 17 environmentally-sensitive areas within which farmers are given financial incentives to farm in accordance with environmental priorities. A number of locations, namely the Somerset Levels, the Suffolk Rivers, the Broadlands, are sites where some form of managed drainage sustains a valued complex of traditional farming, ecology and landscape. Here there are opportunities for continuing or developing drainage management strategies which reconcile agricultural and environmental interests.

In many respects the data, techniques and exper- tise which has developed for designing agricultural drainage are applicable in principle to designing measures to protect or create wetland habitats. For instance, much research effort has examined soil- -plant water relationships for the design of agricultural drainage schemes. The same approach could be used to describe water regimes required for natural wetland species. The sensitivity of natural species to changes in soil water regimes can be assessed, particularly with respect to conditions preferred for agricultural species.

In the same way that engineers have created an environment for commercial agriculture, they can be encouraged to develop engineering and management methods to create or protect the desired water regimes for 'natural' habitats. By way of example, Silsoe College examined the feasibility of wetland creatiodretention in general drained areas4'. Cur- rently the influence of river and arterial drain levels on ditch-side and field soil water conditions, as they influence habitats for natural species, are being examined@. The results have some interesting implications for site management, and point to considerable scope for reconciling farming and

555

environmental needs3'. P 8*39. Responsibilities for

MORRIS ON

wildlife interests within a traditionally-managed agricultural system. In clay valley bottoms in North Wales, river maintenance, in the absence of field drainage, often has limited influence on surrounding water tables. This observation, together with a closer examination of agricultural drainage needs, has recommended less frequent maintenance, with concomitant ecological benefits4’.

These studies suggest that the hydrological, engineering, and agronomic principles and methods which are used to design drainage for agriculture are relevant for the design and management of water regimes in areas where priority is given to conservation, recreation or amenity. Furthermore, selected aspects of cost-benefit methods can be used to assess the value of wetland conservation, not only in terms of agricultural output foregone, but also the value of (non-priced) services flowing from these sites’”.

1.

2.

3.

4.

5 .

1

CONCLUSIONS

Agricultural drainage has made, and will con- tinue to make, a major contribution to UK farming. The efficient operation and maintenance of land- drainage systems will, if anything, increase in importance for many commercial farmers facing diminishing profit margins. Given changing agricultural and environmental priorities, environmental criteria will play a major part in future land-drainage improvement and maintenance activities. Initiatives by environmental groups, farmers, and Government are helping to reconcile different interests, especially in areas where many valued environmental qualities are sustained by a managed, albeit traditional, farming system. The recent policy initiatives on farm diversifi- cation and extensification should provide further scope for matching economic and environmental criteria. In this context, a greater understanding of the inter-relationship between water regime requirements for agricultural and environmental management can only be beneficial. It is an opportunity for environmental and agricultural specialists to work together.

REFERENCES

MORRIS, J., HESS, T. M., RYAN, A. M., AND LEEDS-HARRISON, P . B. Drainage Benefits and Farmer Uptake. Report to SevemTrent Water Authority, 4 volumes, Silsoe College, Silsoe. Bedford. MK45 4DT. UK, 1984.

4. SCAGGS, R. W. Water management model for high watertable soils. Am. Soc. Agric. Engrs, 197.5, paper 7.5/2524.

5. TRAPPORD, B. D. The Evidence in Literature for lncrrawd Yield Due to Field Drainage. Technical Bulletin, Field Drainage Experimental Unit, ADAS, No. 7215, 1972.

6. TRAFFORD, B. D. Soil Water Regime.s - What Is Known, the Work Which Is in Hand and Suggestions for Progress. Technical Bulletin, Field Drainage Expcrimental Unit. ADAS, No 74/13, 1974.

7. CANNELL, R. W., AND BELFORD, R. K. Crop growth after transient water logging: advances in drainage. In: Proc. of ASA E National Drainage Symposium, Chicago, 1982, 16S170.

8. GODWIN, R. J. , AND SPOOR, G. soil factors influencing work days. Agric. Engnr., 1977, 32, 87-90.

9. SPOOR, G., AND GODWIN, R. J . Soil deformation and shccr strength characteristics of some clay soils at different moisture contents. J . Soil. Sci., 1979, 30, 483-498.

10. JARVIS, R. H. The effects of timeliness of soil-engaging operations on crop yield. Agric. Engnr., 1977, 32, 84-86.

11. SMITH, L. P. The effect of weather, drainage efficiency and duration of spring cultivations on barley yields in England. Outlook on Agriculture, 1972, 7 , 7%83.

12. ARMSTRONG, A. C. Field drainage and field work days: results from a national experiment. Agric. Engnr., 1977,32, 93-Y4.

13. WENDTE, L. W., DRABLOS, C. J. W., AND LEMBKE. W. D. The timeliness benefit of subsurface drainage. Am. Soc. Agrk.. Engrs, 1978, 21, 484-488.

14. PARKER, A. Benefits and Economics of Grassland Drainage. Ministry of Agriculture, Fisheries and Food, Land and Water Services Report RD/FE/14, 1983.

Design of Field Drainage Pipe System.r. Reference book 345, HMSO, London, 1982.

Down to Drainage - Arterial Drainage and Agriculture. Advisory Leaflet 736, Ministry of Agriculture, Fisheries and Food, London, 1979.

17. TRAFFORD, B. D. The Langabeare drainage experiment. Agric., 1971, 78, (7), 305-311.

18. HUNTER, J. M., AND TRAFFORD, B. D. An economic appraisal of six years’ results from a drainage experiment on clay land. Exper. Husbandry, 1979, 35, 1-6.

19. BAILEY, A. D. Benefits from drainage of clay soils. Am. Sot. Agric. Engrs, 1979, Paper No. 79-2549.

20. MORRIS, J., AND CALVERT, J . Evaluating returns from drainage. ADAS Quarterly Review, 1976, 20, 151-161.

21. MORRIS, J . , AND Hsss, T. M. Farmer uptake of agricultural land drainage benefits. Environ. and Planning, 1986, 18, 1649-1664.

22. HESS, T. M., AND MORRIS, J . A computer model for agriculture land drainage scheme appraisal. In: Ministry of Agricultural, Fisheries and Food, River Engineers Annual Conference, Cranfield, 1985.

23. MORRIS, J., AND HESS, T. M. Agricultural Rood alleviation benefit assessment: a case study. J . Agric. Economics, 1988, 39, 3, 402412.

24. MORRIS, J., BLACK, D. E, AND HESS, T. M. Drainage Benefits and Farmer Uptake: A Regional Study. Report to Ministry of Agriculture Fisheries and Food, Silsoc College. Bedford, 1987.

25. BIRKS, C. J. Planning river maintenance. In: Ministry of Agriculture, Fisheries and Food, River Engineers Annual Conf.. Cranlield, 1985.

26. FITZSIMONS, J. AND CHATTERTON, J. B. The use Of benefit:coSt techniques in developing objective land drainage maintenance programmes. In: Ministry of Agriculture, Fisheries and Food, River Engineers Annual Conf., Cranfield, 1985.

15. AGRICULTURAL DEVELOPMENT AND ADVISORY SERVICE. The

16. AGRICULTURAL DEVELOPMENT AND ADVtSORY SERVICE. Getting

2. VISSER, J. H. De Landbouwwaterhuishonding in Netherland. Rapp Comm Onderz, 1958, LandbouwWaterhuish. ned., TND No 1. 231.

3. SIEBEN, W. H. Het verband tussen onwatering en opbrengst bij de jonge Zavelgronden in de Noordoostpolder. Van Zee tot Land, 1964, 40, Zwolle, Netherlands: Willink.

27. Hms, T. M., GEDS-HARRISON, P. B., AND MORRIS, J. Evaluation of rivet maintenance in agricultural areas. Proc. 11th int. Congress on Agricultural Engineering, Dublin, 1989, 501-507.

28. HILL, A. R. The cnvironmcntal impact of land drainage. J . Environ. Mangt., 1976, 4, 251-274.

556 J.IWEM, 1989, 3, December.


29. MOUNTFORD, J . 0. AND SHEAIL, J . Effects of drainage on natural vegetation. In: Agriculture and the Environment (D. Jenkins, Ed.), Proc. ITE Symposium, Institute of Terres- trial Ecology, Cambridge, 1984, 98-101.

30. HELLAWELL, J. M. River Regulation and Nature Conser- vation. In: Regulated Rivers Research and Management, Wiley, 1988, 425-443.

31. WII.L.IAMS, G., AND BOWERS, J . K. Land drainage and birds in England and Wales. Royal Society for the Protection of Birh Conservation Review, 1987, 1, 25-30.

32. BAILEY, A. D., AND BREE, T. Effect of improved land drainage on river flood flows. In: FIood Studies Report - Five yean on. Telford, 1981, 131-142.

33. NEWSON, M. D., AND ROBINSON, M. Effects of agricultural drainage on streamflow case studies in Mid-Wales. J. Environ. Mangi., 1983, 17, 33S348.

34. ROBINSON, M. Agricultural drainage can alter catchment flood frequency. Britiyh Hydrofogical Symposium, 1987,6.1-6.10.

35. TOMS, R. The environmental impact of land drainage work. In: Conservution and Land Drainage (Water Space Amenity Commission), 1975, 6 1 5 .

36. MANN, R. H. K. Fish and Fisheries of Regulated Rivers in the UK. Regulated Rivers: Research and Management, Wiley, IYXX, 411-424.

37. MIERS, R . 11. A guide for land drainage engineers on conservation, amenity and recreation. In: Conservation and Land Drainage (Water Space Amenity Commission), 1975, 36554.

38. DRUMMOND, 1. Conservation and land drainage. Wat. Space, 1977, 11, 2>30.

39. WATER SPACE AMENITY COMMISSION. Conservation and Land Drainage Guidelines and Working Party Report, WSAC, London, 1984.

40. NEWBOLD, C. PURSCLOVE, J., AND HOLMES, N. T. H. Nature Conservation and River Engineering, Nature Conservancy Council, Peterborough, 1983.

41. LEWIS, G. AND WILLIAMS, G. Rivers and Wildlife Handbook: a Guide to Practices with Further Conservation of Wildlife in

Rivers. Royal Society for the Protection of Birds and RoyaI Society for Nature Conservation, 1984.

42. MINISTRY OF AGRICULTURE, FISHERIES AND FOOD. Conservation Guidelines for Drainage Authorities, MAFFIDepartment of EnvironmentWelsh Office, 1988.

43. MINISTRY OF AGRICUI~URE. FISHERIFS AND FOOD. Field Drainage and Conservation, MAFF booklct 2522, 19x6.

44. GARDNER, J . L., DEARSLEY, A. F., AND WOOLNOUCH, J . R. The appraisal of environmentally sensitive options for flood alleviation using mathematical modelling. J. Instn. Wat. Environ. Mangt., 1987, 1, (2) . 171-184.

4.5. TKJRNER. K. A N D BROOKS, J . Flood Protection for an Environ- mentally Sensitive Area: An Economic, Ecological Analy- sis. Special Technical Session on Economics of Flood Control and Non-Structural Measures. International Com- mission on Irrigation and Drainage, Dubrovnik, Yugoslavia, 1988.

46. MINISTRY OF AGRICULTURE, FISHERIES AND FOOD. Investment Appraisal of Arterial Drainage Schemes. MAFF Land and Water Services, River Coastal Engineering Group, London, 1985.

47. CONSTRLJC~ON INDUSTRY RESEARCH AND INFORMATION Asso- CIAnoN. The Rerention of Wetlands: Lowland Areas Gener- ally Drained by Gravity. ClRlA project RP318, Silsoe College, Bedford, 1984.

48. YOUNCS, E. G., LEEDS-HARRISON, P. B. AND CHAPMAN, J. M. Modelling water-table movement in flat low-lying lands. Hydrolog. Processes, 1989, (3), (in press).

49. SUTHERLAND, D. Evaluation of Land Drainage Maintenance on Mdlltraeth Marsh, Ynys Mons, Gwynedd. Unpublished MSc thesis, Silsoe College, Bedford, 1988.

SO. MORRIS, J. Evaluating the wetland resource. I . of Environ. Mangi., 1986, 24, 147-156.

A copy of the discussion to this paper, and a copy of the full version of the procccdings of the Symposium, can be obtained from the Institution’s headquartcrs.

J.IWEM, 1989, 3, December. 551

land drainage: agricultural benefits and environmental impacts

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