DITCH: a model to simulate field conditions in response to ditch levels managed for environmental...

Agriculture, Ecosystems and Environment 77 (2000) 179–192

DITCH: a model to simulate field conditions in response to ditch levelsmanaged for environmental aims

Adrian Armstrong∗ADAS, Gleadthorpe Research Centre, Meden Vale, Mansfield, Notts, NG20 9PF, UK

Received 23 December 1998; received in revised form 9 March 1999; accepted 1 June 1999

Abstract

Wetland areas are frequently managed by manipulating the water levels in the surrounding ditches, with the aim of restoringor enhancing the wetness of the site, especially for the ecological requirements of some bird populations. The extent to whichwater tables in the centre of fields can be controlled by this action can be simulated through the use of a model, DrainInteraction with Channel Hydrology (DITCH), based on drainage theory. DITCH successfully predicted the pattern of watertable behaviour on two wetlands sites, in the Norfolk Broads and Somerset Levels areas in the UK. The model can also be usedto predict the strength of the soil surface and the extent of surface flooding. When used to examine the effects of alternativeditch management regimes within the two test areas, the model shows that the effects of ditch management options are noteasily converted into impacts in the centre of the fields. If the effects are not sufficient, then hydrological manipulation of sitesto achieve or improve wetland status may require more active intervention. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:Wetlands; Water tables; Environmentally sensitive areas; Soil surface strength; Flooding; Hydrology

1. Introduction

Public awareness and concern for the maintenanceand preservation of wetlands (Maltby, 1986), has leadto positive action to manage wetland resources. In thepast, many wetland areas have been drained, with theaim of increasing agricultural productivity. In an at-tempt to restore wetland status, or to improve the wet-ness of those wetlands that have survived, the samestructures that were used to drain the land are nowbeing used to control the wetness, primarily throughthe control of ditch water levels. These manipulationsare intended to improve the ecological status of the

∗ Corresponding author. Tel.:+44-1623-844-331; fax:+44-1623-844-472.E-mail address:[email protected] (A. Armstrong).

area, as to improve the landscape value. Populations ofbirds, which are often taken as critical environmentalindicator, require soft, wet soils, for feeding; and par-tially flooded areas for breeding. (Tickner and Evans,1991).

Consequently, governments have introducedagri-environment schemes, in which they encourage(often through payments) the management of agri-cultural areas in environmentally sensitive ways. Onesuch scheme in the UK is the Environmentally Sen-sitive Areas (ESA) scheme, in which the Ministry ofAgriculture Fisheries and Food enters into agreementswith farmers to adopt practices which seek to con-serve and enhance areas of high landscape or historicvalue which are vulnerable to changes in agriculturalpractices. Where the ESAs encompass wetland ar-eas, the agreements may prescribe the water levels in

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180 A. Armstrong / Agriculture, Ecosystems and Environment 77 (2000) 179–192

the ditches, and the land owners have to meet thesetargets in order to qualify for the relevant payments.

Within such schemes, the practical issue of deter-mining the correct management options (as definedin the terms of the ESA agreements), particularly theditch water levels that are appropriate, may be subjectto debate. As a tool for understanding some of thepossible outcomes of adopting specific ditch manage-ment regimes within wetlands ESAs, a model, calledDITCH (Drain Intraction with Channel Hydrology)has been developed to examine the consequences ofvarious ditch management regimes (Armstrong, 1993;Armstrong et al., 1993). This paper describes themodel, its development and its validation against datasets collected from two sites in the UK, in the BroadsESA and in the Somerset Levels and Moors ESA. Theresults have been used to indicate the effectiveness ofthe ESA prescriptions in meeting their ecological andlandscape objectives.

2. Model development

To simulate the soil water regime within a field, amodel is required to calculate the sequence of waterlevels in the field in response to the meteorologicalinputs and the imposed boundary (ditch) conditions.The theory of water movement to drains, derived froman understanding of the physics of soil water flow pro-cesses, is based on Darcy’s Law for saturated flow,leading to the development of a theory of drainage sys-tems that can be used for this study (e.g., Van Schilf-gaarde, 1974; Smedema and Rycroft, 1983; Ritzema,1994).

The DITCH model is based on the calculation ofthe water balance in the field, using the fluxes of watermoving through the soil to the peripheral ditches, toestimate the changing position of the water table inthe field. The water elevation in the field on dayt isMt is, thus, given by:

Mt = Mt−1 + (R − ET − Qd)

f(1)

whereR is the rainfall, ET is evapotranspiration,Qdis the discharge through the drainage systems andf isthe relevant porosity. For geometrically regular situa-tions such as a uniform soil drained by parallel ditches,the drainage can be calculated from one of the well-

known drainage equations. For example, for soils ofhydraulic conductivity,K, drained by parallel ditchespenetrating to an impermeable layer at the base of thesoil profile at spacingL, a simple equation such asthe Donnan drainage equation (Ritzema, 1994) can beused to calculate the drainage fluxes:

Qd = 4K(M2t − D2

t )

L2(2)

whereDt is the level in the ditches at timet. The Don-nan equation considers horizontal flow only, but canbe used if the impermeable layer is a small distancebetween below the drains or the ditch bottom. Wherethe ditches do not penetrate to the base of the soil(as is usually the case), then vertical flow componentscan no longer be ignored, and the Hooghoudt drainageequation (described for example by Ritzema, 1994)should be used to replace the simple drainage Eq. (2).The flux between ditch and field,Qd can be in eitherdirection, and therefore includes both drainage (Qd ispositive) and recharge (Qd is negative), so represent-ing both the winter and summer phases of operation.If the level in the ditches is input into the model asan externally constrained set of values, then the waterbalance can then be solved directly.

Although drainage theory gives, strictly, onlysteady-state solutions to the fluxes, the fluxes calcu-lated by the drainage equations can be considered tobe correct if considered over a succession of steadystates during which the position of the boundaries canbe considered to be fixed. This is achieved by use ofa small model time step, which in DITCH is 1 hour.

The DITCH model, thus, parallels the model de-scribed by Youngs et al. (1989), although differing inits choice of .drainage equations, and being simplerin that it considers only the mid-field water table. Afirst approximation to the two-dimensional form ofthe water table can be made by scaling the potentialfunction E (x, y) for rectangular fields described byYoungs et al. (1989 equation 10), between the twofixed points of the ditch level and the field centre wa-ter table. Other models such as DRAINMOD (Skaggs,1982) or SWATRE (Belmans et al., 1983) could poten-tially be adapted for the same function. However, theflexibility afforded by a purpose-built computer codeis preferred, as it requires only data relevant to the is-sue in hand. In particular, the DITCH model does notconsider the unsaturated zone above the water table,

A. Armstrong / Agriculture, Ecosystems and Environment 77 (2000) 179–192 181

which is acknowledged to be a simplification, justifiedby the difficulty of acquiring suitable data for the char-acterisation of the soil hydraulic functions required fora full consideration of the unsaturated phase.

Initially, the DITCH model was described as a the-oretical model (Armstrong, 1993) to demonstrate thatthe degree to which the ditch and field systems wereinterlinked depended both on the frequency of theditches, and the hydraulic conductivity of the soil.In highly permeable soils, the soil water table in thecentre of the field could be tied quite closely to theditch level, but in low permeability clay soils the de-gree to which water could be moved from the ditchto the field centre was extremely limited, and the fieldcould dry out in response to summer evapotranspira-tion despite there being high levels in the surroundingditches.

2.1. Non uniform soil parameters: hydraulicconductivity decreasing with depth

The assumption of a vertically homogenous soil inEq. (2) is not always realistic, as it is frequently ob-served that the hydraulic conductivity of soil decreaseswith depth. Solutions for the flux through drains insoils in which the hydraulic conductivity varies con-tinuously as a function of depth (or height above thedrain) are given by Youngs (1965). If the variationof hydraulic conductivity with height above the pro-file base,K(z) shows an exponential increase from thebase (equivalent to an exponential decrease from thesurface), written as:

K(z) = K0eβz (3)

whereK0 is the hydraulic conductivity at the base ofthe saturated soil (i.e., atz= 0, with z increasing up-wards), andβ is a constant. For non-empty ditches theanalysis of Youngs then gives the drainage equation:

q = 2K0[eβHm − eβHw − βHm + βHw]

β2D2(4)

whereHw is the height of water in the ditches, andHm is the maximum water table height at mid-drainspacing, which gives estimates of the drainage fluxwhich can then be easily included in Eq. (1).

2.2. Layered soils

A second situation that requires to be modelledis the situation where a soil consists of two layers,for which the analysis of drainage fluxes is given byin (Wesseling, 1973 p. 19–31, and Ritzema, 1994,p. 272–277). For a two-layered soil with the drainor ditch in the lower layer, the drainage flux isgiven by:

Hd

q= D1

K1+ L2

8(K1D1 + K2D2)+ L

πK1ln

aD0

u(5)

in which the depth of the layers areD and have con-ductivity K, with subscripts 1 and 2 for the upper andlower layers, respectively,a is a shape factor, andHdis the head difference between the water table and theditch, the force driving the water movement. Again,the direction of the overall flux, which is determinedby the sign of the head difference driving the flow,Hd, can be either positive or negative, depending onthe circumstances, so predicting both drainage andrecharge.

3. Field study sites

The model has been used to study the effects ofditch management regimes in two of the ESA in UK:the Broads ESA and the Somerset Levels and MoorsESA (MAFF, 1989). In these areas, selected sites havebeen monitored, and the results used to validate theresults of the model. Once validated, the model hasthen been used to examine the issues raised by varyingthe ditch regimes within these areas.

3.1. Halvergate, the Broads ESA

The Halvergate marshes are part of a wide expanseof flood plain, traditionally summer grazing pasture,lying in the centre of the Broads ESA, a series oflow-lying river valleys, marshes and fens, in Norfolkand North Suffolk in Eastern England was chosen asthe site for detailed monitoring studies. Since 1989,water levels in six fields and the adjacent ditches havebeen monitored (Armstrong and Rose, 1998b). Fourfields were subject to a water management schemefor the retention of spring and summer water levels(ESA Tier 2) and of these two were subject to the


additional raised water levels in the ditches (ESA Tier3) from January 1994. Reference fields (Fields 5 and6) outside this area, had the normal managed waterlevels (ESA Tier 1) which were kept low in winter fordrainage although allowed to rise in summer for stockwatering. In all fields, water tables and ditch levelswere monitored by continuous recording water levelmeters (Talman, 1980; 1983) supplemented by a openauger holes, ‘dipwells’ (Armstrong, 1983) read on a3–4 week cycle. Monitoring of fields 2, 3 and 5, wasdiscontinued in the summer of 1992, but maintainedon the remaining three fields.

The soils of the area are Gleysols, alluvial claysof the Newchurch series (Clayden and Hollis, 1984).They are well structured in the topsoil, where they arerich in organic matter, but rapidly become structurelesswith depth, and at 2 m depth are ‘buttery’ and anaero-bic. Measured hydraulic conductivity of the soil (mea-sured in situ using the single auger hole technique) re-flects this structural development, and varies betweenvalues in excess of 100 m/day close to the surface, tovalues less than 0.01 m/day at depths below 1 m, andbecoming effectively impermeable at 2 m. The datawere fitted to the exponential function Eq. (3) usinglinear regression techniques.

3.2. Southlake Moor, the Somerset Levels and MoorsESA

The Somerset Levels and Moors ESA forms thelargest remaining lowland wet grassland or grazingmarsh system in England and is consequently of out-standing environmental interest (MAFF, 1991a, b).Within it, Southlake Moor is a self-contained unit con-trolled by an inlet and an outlet sluice. Here, wa-ter levels have been monitored in four fields withinSouthlake Moor and one reference field outside themoor since June 1989 using similar instrumentation asfor Halvergate. Until summer 1991 the dipwells wereshallow (no more than 0.50 m deep), but were replacedby deep (1.50 m) auger holes from then on. Gaugeboards, recording the water levels in the ditches, atboth the inlet and the outlet were also recorded.

The soils are Histosols of the Midelney series (Av-ery, 1955), having a shallow clay cap (approximately0.40 m deep on Southlake Moor) of river alluvium,overlying a peat substrate at least 2 m deep. The claycap arises from practice of ‘warping’, which involves

letting the area flood with the waters from the adjacentriver.

Prior to the establishment of the ESA, the ditchlevels for Southlake Moor were maintained at 3.50 mabove ordnance datum (AOD) – mean sea level) frommid-November to the end of March, and at 3.65 mAOD from April to mid-November. Land levels arebetween 3.7 and 4.0 m AOD. On this site, a raisedwater level regime was initiated at the beginning ofDecember 1988. The water levels were maintained at3.65 m at outlet sluice throughout the year, except forMarch, when the levels were 3.50 m to facilitate ditchcleaning operations.

4. Hydrological modelling

The DITCH model was implemented for both sites,using the observed values of the soil parameters andfield data on land and water levels. For each, the modelwas validated by a comparison with the observed watertables in the centre of the field.

4.1. Halvergate

The DITCH model was applied to the conditions inthe Broads area using the depth-dependent drainageEq. (4). The meteorological data and the observeddaily ditch levels for the relevant fields were used tomodel the water tables in the centre of each of the sixmonitored fields (Fig. 1). Visually, the results showexcellent agreement between the model and the ob-servations. A statistical evaluation of the whole set ofmodel results, which thus included the ability of themodel to represent the variation both between fieldsand within fields, gave a correlation coefficient be-tween the modelled and observed water tables of 0.69,which was considered to be an excellent confirmationof the model. The model efficiency criterion (Loagueand Green, 1991), for the same data set was 0.44,which again was considered to indicate an acceptablelevel of model performance.

4.2. Southlake

Application of the model to Southlake required theuse of the two layer drainage equation 5, even though


Fig. 1. Comparison of observed (*) and modelled (––––) water tables for the six monitored fields, in the Halvergate area of the BroadsESA, Eastern UK, 1990–1994. AOD indicates Above Ordnance Datum – mean sea level.

the dipwell data had suggested a continuity betweenthe two layers in the profile. Direct measurement ofthe hydraulic conductivity of the peat subsoil, usingthe single auger hole technique indicated a subsoilconductivity of the order of 1–2 m/day.

The observed gauge board heights at the outletsluice were used to define the input boundary condi-tions. The modelled water tables were compared tothe observed data (Fig. 2) for the recording period.Visual inspection of the model results showed thatthey were in general very close to observations. Inparticular, the model simulated the lowest depth towhich the water table fell in the middle of the summer

in the last three years of the study. The same compar-ison was not possible for the first three years, duringwhich the field data did not record deeper water ta-bles because of the shallow nature of the recordinglocations. The biggest divergence was that the modeldid not accurately reproduce the depths of floodingcaused by the very high ditch levels.

The statistical comparison of the results was madeusing the same criterion as for the Halvergate site,namely the simple correlation coefficient (0.63) andthe model efficiency (0.38). Although not quite asgood as the results for Halvergate, these were still con-sidered to indicate an adequate model performance.


Fig. 2. Comparison of observed (*) and modelled water tables, (solid line) for four of the monitored fields at Southlake Moor in theSomerset levels and Moors ESA, SW England, 1988–1994.

5. Estimation of soil surface strength

As the ability of birds to probe the soil for food isa major factor in the choice of sites for breeding, theestimation of soil surface strength is required to iden-tify site suitability for bird feeding needs (Tickner andEvans, 1991). For many soils, there is a relatively welldefined relationship between soil strength and watercontent. As a first approximation, soil water contentat the surface is also correlated with the water tabledepth from the surface, so a relationship between soilstrength and water table offers the possibility of mod-elling the suitability of the soil for feeding by birds.This relationship cannot, however, be established froma priori principles, and must be calibrated from fieldobservation.

The penetration resistance of the soil is a measureof the difficulty that a bird might be expected to havein feeding. For work specifically related to bird beakpenetration, a special penetrometer has been devised(Green, 1986) which records the force required to pusha narrow cylinder into the soil a distance of 100 mm,and so mimics the behaviour of a bird beak. At both

sites, the soil surface strength was recorded when-ever the auger holes were read using this penetrome-ter. On each occasion eight replicate measurements ofsoil strength were taken at three locations within eachfield.

The field data showed very different behaviours forthe two sites. At Halvergate, the soil strength mea-surements (Fig. 3), showed no significant correlationbetween water table depth and soil strength for any ofthe six fields. Nearly all the observations are for valuesgreater than a 6 kg force required to penetrate the sur-face, and this value was maintained even when the soilwas flooded. This value is greater than the estimatedmaximum force of 3 kg required for penetration of thesoil by Snipe (Gallinago Gallinago) (Green, 1988). Itis considered that this high surface penetration resis-tance is the consequence of the grazing managementregime of the marshes, which are subject to short peri-ods of high intensity stocking. The consequence is theformation of a hard surface mat of vegetation and com-pacted soil. It is suggested that this capping behaviouris one of the characteristics of clay marshes, such asthe Halvergate marshes, compared to peat marshes.


Fig. 3. Relationships between soil strength and water table depth for each of the monitored fields in the Halvergate area of the Broads ESA.

The results also serve to emphasise the impact of andagricultural management practice affecting the valueof land for ecological purposes.

These results contrast with those obtained at South-lake Moor, (Fig. 4) where a clear correlation betweenwater table depth and soil surface strength has beenobserved, for all the fields in the study area. The over-all correlation of 0.69 was considered to be remarkablygood, especially when the other (often short term) fac-tors that can effect soil penetrability are considered.For Southlake it was, thus, possible to estimate soilsurface strength from the water table position. Linearregression was used to estimate the relationship be-tween the two variables, and the estimated pattern ofsoil surface strength was shown to follow the observed

pattern (Fig. 5). These data show the pattern of be-haviour that is expected, being harder in the late sum-mer, coming to their softest in the winter, and gradu-ally hardening again during the summer, this patternof behaviour being repeated for all the fields.

6. Estimation of surface flooding

The ecological requirements of some bird popula-tions (which are often taken as critical indicators) re-quires the deliberate maintenance of partially floodedareas (Tickner and Evans, 1991). Although it is pos-sible to calculate the form of the water table, either intwo dimensions or in three (Youngs et al., 1989) and


Fig. 4. Relationships between soil strength and water table depth, for the five monitored fields at Southlake Moor in the Somerset levelsand Moors ESA.

so estimate the soil water regime at all locations withinthe field, a simpler alternative is to assume that thewater table is approximately flat, and that the area ofthe field that is flooded is defined by the intersectionof this modelled water table height and the cumulativedistribution of heights. Data presented by Armstrongand Rose (1998a) demonstrate the essentially flat na-ture of the water table. The DITCH model, thus, sim-ply takes the predicted water table height in the centreof the field, and then identifies the intersection of thatheight value with the cumulative frequency curve, togive an estimate of the extent of surface flooding inthe field at any one time.

To implement this approach, it is essential to havesome measure of the microtopographic roughness ofthe area modelled, ideally, as a cumulative frequencycurve. Collection of these data requires measurementnot only of the gross topographic features of the site,but also measurement of the microtopography. It wasfound that this information is best given by a series

of closely measured transects, which give sampleestimates for the whole field, rather than systematicsurveys which tend to define only the macrotopo-graphic features. Fig. 6 shows the cumulative distribu-tions of surface heights for the six monitored fields inthe Broads ESA. These are roughly Gaussian in theirform, and so it is suggested that for more general ap-plication of the DITCH model, and in order to moveaway from the need to collect a large amount of to-pographic data, the cumulative frequency distributioncould be replaced by its mean and standard.

The DITCH model, thus, includes a prediction ofthe area flooded by considering the intersection of thewater table and the cumulative distribution of height.Unfortunately, it has not been possible to implementa scientific verification of the estimation of the extentof flooding, as there is no direct observation of thisphenomenon. Nevertheless, the results do correspondwith the subjective assessments of the site operatorsat both sites.


Fig. 5. Observed (*) values of soil penetrability and those predicted by the DITCH model, for the monitored fields, in Southlake Moor inthe Somerset Levels and Moors ESA, 1989–90.

7. Use of the model to evaluate managementoptions

The DITCH model thus produces information aboutsoil water regime, soil surface strength, and floodedarea. The model was then used to evaluate a num-ber of alternative management options in these twoareas.

7.1. Halvergate

The model was used to examine the effectiveness ofthe various tiers of management on the water regimesof the Halvergate soils. In the implementation of theDITCH model the predicted water tables and surfaceflooding conditions were plotted on one graph (Fig.7). In addition, to aid interpretation, the basic hydro-

logical variables of rainfall, evapotranspiration wereplotted, and also the drainage fluxes which immedi-ately identified periods of recharge and drainage.

The model was run for a 16 year period, from 1979to 1994, using the observed values for the hydraulicconductivity from the Halvergate area. and adoptingthree different ditch water level regimes representativeof the three tiers of management relating to the ESAprescriptions:• Tier 1. Ditch levels at 1.5 m below ground level from

1 January to 30 March, at 1.0 m below ground level1 April to 30 October, and at 1.5 m below groundlevel 1 November to 31 December.

• Tier 2. Ditch levels at 1.2 m below ground level from1 January to 1 March (i.e., with 30 cm more water inthe ditches than the comparable Tier 1 levels), risingto 45 cm below field level by 1 April, remaining


Fig. 6. Observed cumulative distributions of surface topographic heights for the six monitored fields in the Halvergate area of the BroadsESA.

at 45 cm below ground level until 30 October, andthereafter at 1.2 m below field level.

• Tier 3. Water levels are held at mean field level from1 January to 30 April; at 45 cm below field levelfrom 1 May to 30 October, then rising to mean fieldlevel by 1 December, and remaining at that leveluntil the end of the year.

The mean in-field water tables simulated by the modelover the 16 year run of rainfall data whilst subject tothe 3 different levels of management are shown in Fig.8, for each water management option. These resultsshow the dramatic effect on in-field water regimesthat are created by the different tiers of management.In particular, the adoption of high water levels in theTier 3 levels results in the water table fluctuating onlyin the upper, conductive, layers of the soil, so thatthe field and ditch levels are closely tied together. Bycontrast, where the ditch levels fall in summer, thezone of water movement becomes concentrated in thelower impermeable layers, and the field and ditch wa-ter levels become effective disconnected, as the soilis then unable to transmit sufficient water to maintain

the water levels in the face of continuing evaporativedemand.

7.2. Southlake

Lastly, the model was used to examine some of theeffects of the water management prescriptions in theSomerset Levels and Moors ESA (Fig. 9). It was con-sidered that the model had fitted the data sufficientlywell to be used to identify the impacts of these vari-ous regimes on the water levels within the field. Themodel was thus used to simulate the water table underboth Tier 3 and under ‘normal conditions’, i.e., un-modified by the current ESA agreements, and underthe current Tier 2 levels. The model was then run torepresent a 10 year period, to produce a sequence ofwater tables for the three conditions. The mean resultsfor all three water regimes are shown in Fig. 10, andas soil strengths in Fig. 11.

For Southlake Moor, the model simulates a smallbut consistent raising of the water table under the Tier3 regime, compared to either the Tier 2 or the ‘un-


Fig. 7. Example application of the DITCH model to the Halvergate area of the Broads ESA, showing observed ditch levels (dashed line),predicted water tables (solid line) and surface flooding percentages (histograms). To aid interpretation, rainfall, evapotranspiration anddrainage and recharge fluxes are also shown.

Fig. 8. Predicted mean water tables in response to the levels of ditch management (average of 20 modelled years) at the Halvergate sitein the Broads ESA.

altered’ regime, with the effect being greatest in thespring and early summer. In the later part of the sum-mer the effects of evapotranspiration become domi-nant, reducing the effect the ditch levels have on the

field water table. Although the three tiers representthree very different ways of managing the ditches,the effects on the mean in-field water tables are thusquite small. The dominant signal in the soil moisture


Fig. 9. Example application of the DITCH model to the Southlake area: ditch levels (––––) and predicted water tables (solid line). To aidinterpretation, rainfall, evapotranspiration and drainage fluxes are also shown.

Fig. 10. Mean water predicted for each of the three ditch management options.


Fig. 11. Mean soil strength at Southlake Moor in the Somerset levels and Moors ESA, predicted for each of the three ditch managementoptions.

regime is always the winter saturation and the sum-mer period of drying out. However, the effect in thecentre of the field in terms of changing from the ‘oldregime’ (Regime 2 in Figs. 10 and 11) to the new Tier3 regimes with raised water levels in the ditches, is todelay the start of the spring drying out phase by anaverage of 20 days.

8. Conclusions

This paper has shown that a hydrological budgetmodel can be used effectively to estimate the soil waterand ditch regimes where those ditch levels are beingmanipulated. The validation tests of the models usingthe two data sets suggest that the model offers a goodrepresentation of the system.

However, the results also show that the effects ofditch management options are not easily converted intoimpacts in the centre of the fields. The small impactsidentified in Figs. 10 and 11 indicate that although theditch regimes may alter, the effects in the centre ofthe fields are very much more subtle. Although theseeffects may seem small, they are concentrated at thecritical time of the year both for plants and for bird

life. Whether this effect is sufficient to achieve the re-quired ecological objectives is the subject of on-goingresearch. If the effects are not sufficient, then hydro-logical manipulation of sites to achieve or maintainwetland status may require more active interventionon the within-field regimes.

Acknowledgements

I am grateful to many colleagues who have con-tributed to this project of which this work is just apart. Particularly, I would like to thank Tim Daley ofADAS Taunton, Peter Woode of ADAS Bristol, ColinDennis and David Thomas of ADAS Norwich for thefield monitoring; and to the various landowners foraccess to their land to undertake this work. Thanksare also due to Andrew Portwood and Steve Rose ofADAS Gleadthorpe for their help in this work, andto Owen Mountford and Jo Treweek of ITE MonksWood and David Gowing of Silsoe College Cran-field University, for their many discussions relatingto this work. The Financial support of the UK Min-istry of Agriculture, Fisheries and Food, is gratefullyacknowledged.


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DITCH: a model to simulate field conditions in response to ditch levels managed for environmental...

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