Forest Hydrology and Watershed Management - HydrologieForestiere et Amenagement des Bassins Hydrologiques(Proceedings of the Vancouver Symposium, August 1987; Actesdu Co11oque de Vancouver, Aout 1987):IAHS-AISHPub1.no.167,1987.

Simulation of water flow and soil erosionprocesses with a distributed physically. basedmodelling system

B~RGE STORM & GREGERS H. J~RGENSENDanish Hydraulic Institute, Agern Alle 5, DK-2970, Horsholm, DenmarkMERETE STYCZEN

Royal Veterinary and Agricultural University,Thorvaldsensvej 40, DK-187l Frederiksberg C,Denmark

ABSTRACT This paper describes a new soil erosion model(SEM), which has been developed based on a physicaldescription of the involved processes. The SEM modelincludes the processes of splash detachment of soil byrainfall, net transport of detached soil by overlandflow, overland flow detachment and net transport, and neterosion/deposition within each model grid square. SEMhas been incorporated as a separate component in thephysically-based, distributed hydrological modellingsystem SHE. The SEM/SHE model complex has been tested ona small research catchment with an area of 5.6 ha in theUnited States, and applied to two upland catchments inThailand of 45 km2 and 128 km2. Examples from the testsand applications are presented.

Simulation des processus d'ecoulement et d'erosion dessols au moyen d'un modele distribue base sur la physiquedu systemeRESUME Ce texte decrit un nouveau modele d'erosion des

sols (SEM) developpe selon la description physique desprocessus en jeu. Le modele SEM inclus le detachementdes particules de sols par l'impact des gouttes de pluie,le transport net de ces particules par le ruissellementde surface, le detachement des particules par ce memeruissellement et leur transport net, ainsi que1'erosion/deposition nette dans chaque cellule modelisee.Le modele SEM a ete incorpore dans le modele hydrologiquephysique distribue SHE. Le texte contient des exemplesdu test du modele combine SEM/SHE sur un petit bassin de

5.6 ha aux Etats-Unis, et de son ap~lication sur deuxbassins thallandais de 45 et 128 km .

INTRODUCTION

Catchments in many geographical areas have been subject to extensive

land-use changes during the recent decades. Human activities in-595

596 B~rge storm et al.

fluence the environment in forest areas. Commercial clearcutting,charcoal burning and the increasing demands for agricultural landare just a few examples of activities which have caused large-scaledeforestation. In water resources studies people have become in-creasingly concerned with the subsequent impact on the hydrologicalregime and the increased risk of severe soil erosion.

For this reason, the development of mathematical models has beendirected towards distributed and physically based models, whichreliably describe both the hydrological processes and the erosionand transport processes within the catchment. These models canguide the water resources planners in the assessment of the effectsof e.g. land-use changes.

A large number of soil erosion models have been developed in therecent years. They include the ARM-model (Donigian & Crawford,1979), the ANSWERS model (Beasley et al., 1980), the modifiedANSWERS model MODANSW (Park et al., 1982), and the CREAMS model(Knisel, 1980). Of these, only ANSWERS and MODANSW models arespatially distributed, dividing the catchment into smaller homoge-neous elements with uniform characteristics of soils, vegetation andtopography.

The ANSWERS and MODANSW models attempt a physical description ofsoil erosion by dividing the total process into its components ofrain splash detachment, overland flow detachment and transport ofdetached soil. However, due to lack of exact formulas describingeach component, simplified empirical relationships are applied inthe models. Some of the empirical factors used are derived frommeasurements of total erosion from standard runoff plots and thusrepresent a mixture of processes. Particularly the application ofsoil cover and soil erodibility factors C and K, developed for theUniversal Soil Loss Equation, USLE, (Wischmeyer & Smith, 1978), isquestionable when used in relationships for the single erosioncomponents.

The objective of this study was to develop ~ distributed soilerosion and sediment transport model (SEM). The representation ofthe individual processes of detachment and transport is physicallybased as far as possible, and the model parameters can therefore beinferred directly from available information about catchment proper-ties.

A precise simulation of the hydrological regime is of crucialimportance in soil erosion simulations. The SEM has therefore beendeveloped as a separate component "on top of" the hydrologicalmodelling system SHE (Abbott et al., 1986a,b). This paper presentssimulations of the total modelling system SEM/SHE from two studies.The first simulations are from a testing study of SEM/SHE on a smallresearch catchment. The subsequent simulations are examples from anongoing study in Thailand.

THE HYDROLOGICAL MODEL - SHE

The water movement in the catchment is modelled in SHE by a finitedifference representation of the partial differential equations ofmass, momentum and energy conservation. This comprises the proces-ses of surface flow and unsaturated and saturated flow. The canopy

Simulation of water flow and soil erosion 597

interception, the evapotranspiration and the snowmelt are based onempirical equations derived from independent experimental research.The catchment is discretized by two analogous horizontal grid squarenetworks for the overland flow and groundwater flow respectively.These are linked by a vertical column of nodes at each grid square(Fig. 1).

UNSATURATEDZONE MODEL

INCL. ROOT ZONE

CANOPY INTERCEPTION

MODEL

LAYERED SNOWMELT

MODEL

.r'''~,::,:::::<,::~:~t:Q?('<~;:;:::::::::::":~':'~:,:,::".

SATURATED FLOW MODEL (RECTANGULAR GRID)

FIG.l Schematic representation of the SHE.

The spatial distribution of the catchment properties and themeteorological inputs are maintained by assigning parameter valuesat each grid square and node point in the network.

A brief description of the components in SHE is given below. Fora detailed description and presentation of equations, see Abbott etal. (1986b) or Storm (1986).

Interception/evapotranspiration component

The interception process is modelled by a modified Rutter model(Rutter et al., 1971,1972). On the basis of meteorological input itcalculates the interception loss, the actual water storage on thecanopy and the net rainfall reaching the ground through the canopy.The latter is the sum of canopy drainage and throughfall. Thisdivision is important, and utilized in the simulation of the splashdetachment of the soil described in a subsequent section.

The actual evapotranspiration is calculated from a simplifiedversion of the Penman-Monteith equation (Monteith, 1965) where theactual evapotranspiration is based on the potential evapotranspira-tion and related to the soil moisture or soil pressure in the root-zone.

598 B~rge Storm et ai.

Overland and channel flow component

When the net rainfall rate exceeds the infiltration rate pondingoccurs. The overland flow is simulated in each grid square bysolving the two-dimensional diffusive wave approximation of the St.Venant equations in an explicit scheme.

For the channel flow the one-dimensional form of the above

tioned equation is supplied. It is solved in a separate nodelocated along boundaries of the grid squares, and modelled inimplicit finite difference scheme.

men-

systeman

Unsaturated zone component

The distribution of soil moisture in the unsaturated zone is calcu-

lated by solving the one-dimensional Richards equation (Jensen,1983). Extraction of moisture for transpiration and soil evapora-tion is introduced via sink terms at the node points in the rootzone (Jensen & J~nch-Clausen, 1981). Infiltration rates are deter-mined by the upper boundary which may be either flux controlled (netrainfall) or head controlled in case of ponding.

The lowest node point included in the finite difference schemedepends on the phreatic surface level, and allowance is made for theunsaturated zone to disappear in cases where the phreatic surfacerises to the ground surface.

Saturated zone component

The groundwater flow is modelled by using an implicit finite-dif-ference solution of the two-dimensional non-linear Boussinesq equa-tion for an unconfined aquifer. This equation uses the specificyield S of the soil. In cases of shallow water tables this is

inappropriate and the use of the effective specific yield Sef isrequired. Sef is related to the actual moisture profile above thephreatic surface. This difficulty has been overcome by introductionof an iterative procedure in the unsaturated zone component wherethe recharge rate to the groundwater is calculated in accordancewith the S/Sef ratio. This approach is inspired by the work ofBelmans et ai. (1983), but modified to suit the interaction betweenone-dimensional vertical flow and two-dimensional horizontal flow.

THE SOIL EROSION MODEL - SEM

The SEM is a physically based model which simulates the spatial andtemporal variations of the soil erosion and sediment yield in thecatchment. The model has been tailored to fit into the frame of the

SHE mode.l,and utilize the various catchment overlays establishedfor the SHE. It is assumed that the erosion and the transportprocesses do not affect the hydrological processes.

In each grid square the following processes are modelled:(a) splash detachment of soil by rainfall,(b) net transport of splashed soil by overland flow,

Simulation of water flow and soil erosion 599

overland flow detachment and net transport (flow entrain-(c)ment),

(d) s~diment transport capacity of overland flow,(e) total sediment load leaving the grid square to neighbor

squares of streams,(f) net erosion/deposition.The total sediment load leaving a grid square is limited by the

transport capacity of overland flow. Thus, if simulated transportis higher than the transport capacity, the flow entrainment and nettransport of splashed soil are both limited to yield a total sedi-ment load equal to the transport capacity.

In the following the main emphasis is made on the description ofthe processes of the rainfall splash detachment since the appliedapproach introduces some new features in soil erosion modelling. Adetailed description of the other processes is provided by Nielsenet al. (1986), and Nielsen (1986).

Rainfall splash detachment

The description of splash detachment is based on work by Styczen andH~gh-Schmidt (1986). For bare soil the splash detachment DS isexpressed by:

D 2DS = A(e) E ND P = A(e) MR

D(1)

where A(e) = soil erodibility factor; e = energy required to detach

an aggregate; ND = number of raindrops of a certain size class(diameter D) in the rainfall event; PD =momentum of a raindrop ofsize D; MR = squared momentum of the raindrops.

Including effects of canopy, mulch or close-growing vegetation,slope and water ponding on the ground, the net splash detachmentrate DR is simulated by

D 2DR = C1C2Khcosa A(e) E ND P

D(2)

where Cl = canopy factor; C2 = fraction of area affected(i.e. the fraction of ar~a not covered by mulch, stones,growing vegetation); Kh = water depth correction factor;surface slope.

The effect of the canopy on the splash detachment is accountedfor by the dimensionless factor Cl' which is calculated from thesquared momentums of the raindrops MR and the drops from the canopyMe respectively. Styczen & H~gh-Schmidt (1986) shows that Cl mayexceed 1.0 for canopy higher than approximately 1 m, due to thebigger size of drops falling from the canopy. This explains whysplash erosion may be higher on vegetated surfaces than for bare

soil. A fact that has been observed in many field tests, and uti-lized in an appropriate way in the SEM.

by splashor close-

a = ground

600 B~rge Storm et al.

The SEM model utilizes the relationship forthe MODANSW model (Park et al., 1982) based onrainfall intensity.

From the available information about vegetation cover and height,drop size distribution and rainfall intensity all of the parametersin equations (1) and (2) can be determined directly except for thesoil erodibility A(e). A(e) is estimated from calibration data.

the Kh factor used inwater depth and

Net transport of splashed soil

The splash detached soil particles or aggregates are transported bythe overland flow. Due to gravity aggregates are deposited again adistance downslope according to their fall velocities. Neglectingthe wash load the net transport qR is expressed by:

q = D gR R 2

f.E~i wi

(3)

where ~ = total splashed soil; q = the overland flow discharge; fi

= weight percentage of size fraction i; wi = fall velocity of sizefraction i.

Flow detachment and transport (flow entrainment)

For non-cohesive soils the flow entrainment of the soil by theoverland flow is equal to the transport capacity qT of the soil.This may be expressed by the Engelund-Hansen equation (Engelund &Hansen, 1967). This equation was also recommended by Julien &Simons (1985). For soil with a wide aggregate size distribution,the transport capacity, qTi, of each aggregate size fraction iscalculated from the transport equation and multiplied by the per-centage of the size fraction in the soil mixture.

The Engelund-Hansen transport equation is based on a fundamentalenergy equation for transport and deposition along a moveable bed.By introduction of an energy term describing the energy required tobreak the bonds between the soil aggregates, an extended energyequation for cohesive soils reveals. Through calculations similarto those which led to the Engelund-Hansen equation, the flow en-trainment of cohesive soils qE is described by

qE = 11 . qT (4)

where qT = sediment transport capacity and 11 = entrainment rate.the SEM model 11is used as a calibration factor.

In

Total sediment load and net erosion/deposition

The total sediment load SL leaving a grid square to neighbor squaresor streams is now determined as

qR + qE if qR + qE < qTSL = (5)

qT if qR + qE > qT

Simulation of water flow and soil erosion 601

where qR = net transport of splash detached soil; qE = flow entrain-

ment of soils; qT = transport capacity of the overland flow.

Assuming a grid square size of minimum 25 m, the sediment loadcan be assumed to be deposited within the next square for all ag-gregate sizes larger than approximately 2-5 ~m, due to the fallvelocity of the soil aggregates. When the neighbor square is astream, however, the concentrated flow can transport the aggregatestogether with the flow.

Size fractions less than 2-5 ~m are transported with the overlandflow as wash load, but this process is not accounted for in the SEM.For most soils the fraction less than 2-5 ~m is less than 5%, thusin most cases this model limitation has minor effect only on the

resulting total sediment yield.The net erosion/deposition during each time step is calculated as

the difference between the sediment load entering and leaving a grid

square.

MODEL APPLICATIONS

In the following two simulation studies with the SEM/SHE model aredescribed. In the first study a testing of SEM/SHE was carried out

using data from a small agricultural catchment with an area of5.6 ha in the Four Mile Creek catchment in Iowa. The catchment is

usually designated the Iowa State.University catchment Site 1 (ISUSite 1). The soils are predominantly silt loams and the vegetationduring the period of simulation is soybeans. The catchment haspreviously been subject to simulations with the ANSWERS and theMODANSW models, reported in Parks et al. (1982).

The simulations presented in the second application are from anongoing research study on two upland catchments in Thailand (Fig.3).The objective of the study is to predict the effects of land-usechange on the hydrological regime and the soil erosion process.Examples of preliminary simulations from one of these catchments,the 45 km2 Kgt27 catchment are shown.

The Kgt27 catchment is divided into two areas, a mountainous areawith steep gradients and covered by dense forest, and in lowerparts, plains with paddy fields for rainy season cultivation andscattered hill tops with grass and bushes. The soil is predominant-ly loamy.

Test simulations on the ISU Site 1 catchment

A network of 90 squares were used to describe the topographyISU Site 1 catchment. The area of each grid square was 25 mand a simulation timestep of 1 minute was used. Homogeneousand vegetation conditions were assumed.

Comparisons of observed and simulated hydrographs and sedimentconcentrations for two storms of different size are shown in Fig.2.

In both cases the observed and simulated hydrographs are wellmatched. The simulations of the sediment discharges are less satis-

factory. In both simulations the values ~ = 0.12 and A(e) = 4000

of thex 25 m,soil

602 B~rge Storm et al.

were applied. For the small storm event on 28 August, the sedimentyield is underpredicted which was not surprising.

For such small storms, which are of minor importance when evalua-

ting total annual sed~ment yield, the model was expected to give anunderestimation because only the smaller aggregates are in trans-port, while the present model assumes that all soil particles are intransport.

For the 15 August storm event the predicted sediment yield of4,400 kg comes quite close to the observed of 4,200 kg, however, themaximum yield is overpredicted.

FIG.2 Comparisons of simulated and observed hydrographsand sediment discharges for the storm of 15 August (a)and the storm of 28 August (b), at the ISU Site 1catchment.

Simulations on the Kgt27 catchment

A network of approximately 800 grid squares was superimposed on theKgt27 catchment, each square having the dimensions of 250 m x 250 m(Fig.3). The SHE was calibrated for a three months period June-August, 1986. The final calibration result appears in Fig.4.

The wet season usually begins in mid-May. At that time thecatchment is very dry and the phreatic surface is several metersbelow ground surface. Significant rise in the hydrograph due tooverland flow is only observed in heavy rain during the early periodof the wet season. Later overland flow becomes more frequent as thesoils saturate and the phreatic surface rises to the ground surface.

AUGUST 15 AUGUST 2B

DISCHARGE DISCHARGE(mm/hr) (mm/hr)

25- - MEASURED

0.5- SIMULATED

20

041

1\

,If15 0.3

10 0.2

5 0.1

0 0.00 20 40 60 BO 0 20 40 60 BO

TIME (min) TIME (min)SEDIM. DISCH. SEDIM. DISCH.

(kg/5) (kg/5)

B.of f\ 0 MEASURED 0.020- SIMULATED

60J f 000\0015 i

000 0

"4.0 0.010

2°L-j

0.005

0000J i/0.0 0 000 0 0 0 0, I ,0 20 4Q 60 BO 0 20 40 60 BO

TIME (min) TIME (min)

Simulation of water flow and soil erosion 603

At mid-July a state of saturation is observed in the low lying areaswhich even in periods with no rain are fuelled by subsurface flowfrom the high elevation areas in the catchment. In these highelevation areas the phreatic surface level is generally below theground surface.

FOREST ELEVATION RANGES

..~0IIIIII-~// ;%;1" /

B-m

> 567

421 - 567

356 - 421

278 - 356

221 - 278

149 - 221

70 - 149

33 - 7021- 33

< 21

FIG.3 3-D gray scale representation of the topographyin the Kgt27 catchment.

The patterns of the depth to the phreatic surface are illustratedfor two dates in Fig.5. It is seen that the soil is more wet at 10July than at 2 July. The general pattern compares well with theconditions expected on the basis of known catchment behaviour.

The storm events at the two above mentioned dates (marked with

arrows in Fig.4) were selected for simulations of the sedimentyield, and the comparisons of observed and simulated hydrographs andsediment discharges are shown in Fig.6.

The agreement shown in Fig.4 for these two storms seems lesssatisfactory when enlarged and shown for a period of two days. Thesimulated peak for the 2-) July storm arrives too early, whereas thetiming is correct in the second storm. It should be emphasized thatthese deficiencies in the hydrograph simulations influence thequality of the sediment yield simulation.

Peak flows usually occur a few hours after midnight in the Kgt27catchment, and measurement of actual sediment peak flow has not been

possible in practice. The observed sediment discharges in Fig.6 aretherefore based on a rating curve between streamflow discharge and

suspended sediment load. When comparing the simulatedand observedcurves, considerable uncertainty is embodied in the observed sedi-ment discharge.

604 B~rge Storm et al.

OISCHARGE (m3/ 5)

80.00

~

60.00

40.00

~

20.00

0.00JUN JUL

MEASURED HVDROGRAPH

- SIMULATED HVDROGRAPH

i.l

l:II!I

I

'I 1 ,

i~t ii

j

": :,I

~ 1 ,I' , '! .'.' '

.i, ,

.1, '

.! I ''I' [" , ,. ,"

V,

.IL !i. j~! '.

:, !'II ~"::,1

'\I~,\ I'," " '. i~\~J\''''-.J -~~ .

1986AUG

TIME

FIG.4 Comparison of simulated and observed hydrographsfrom 20 June-]l August, at Kgt27 catchment.

DEPTH BELOW

GROUND SURFACE

D0~~-

> 200 em100 - 200 em

50 - 100 em1 - 50 em

< 1 em

FIG.5 Spatial distribution of the depth to phreaticsurface at the two dates 2 July and 10 July, at Kgt27catchment.

Simulation of water flow and soil erosion 605

The parameter values n = 0.00055 and A(e) = 2,200 were used inboth simulations. Only the value of n was subject to adjustmentsduring the calibration, whereas the value of A(e) was kept constant.The small value of n reflects a larger resistance to soil erosion in

the forest catchment compared to the agricultural catchment of ISUSite 1.

The comparisons between the simulated and observed sedimentdischarges in Fig.5 show acceptable agreements considering theuncertainties involved. There is a general tendency of oversteepen-

ing of the simulated sediment discharge and some discrepancy in thetiming of the peaks is also seen.

To illustrate the capabilities of SEM/SHE to simulate the spatialdistribution of net erosion/disposition in the catchment, Fig.7shows the conditions on 10 July 1986. The following information canbe extracted from the figure:

(a) There is no appreciable erosion occurring in the paddyfields.

(b)areas.

Large erosion of steep slopes, in particular in the river

(c) A net disposition in the transition zones between steep andgently sloped areas.

DISCHARGE

(m3/s1

01 SCHARGE

(m3/s)

12.00

24.00

II

j

:I

\ ,\ I\ ,'- I

' ~

III\\\\\\ ,'-'-,- ---

48.00 - - MEASURED- SIMULATED

4800

3600 36.00

24.00

12.00

-----

2 3

110.00

2 3JUL 1986

SEDIMENT DISCHARGE

(kg/S)

9.60

~7.20

000 "

4.80

2.40

JUL 1986

FIG.6 Comparisons of simulated and measured hydrographsand sedimentdischargeat 2-3 July and 10-11 July 1986,in Kgt27 catchment.

0.0010

JUL 1986

SEDIMENT DISCHARGE

(kg/s)

48.00

36.00

24.00

12.00

0.00 J,--10

JUL 1986

606 B~rge Storm et al.

NET EROSION (+) I

DEPOSITION (-) IN KG.-IiiIm0-..

> 100003000 - 100001000 - 3000

-1000 - 1000-10000 - -1000

< -10000

FIG.7 Gray scale representation of the spatialdistribution of net erosion/disposition at 10 July 1986,in Kgt27 catchment.

SUMMARY AND CONCLUSIONS

The aim of this paper has been to present the first applications ofthe soil erosion model SEM, which has been incorporated in theexisting hydrological modelling system SHE. Combined they representa practical instrument in water resources and soil conservationplanning.

The distributed and physically-based description of the hydrolog-ical and sediment transport processes in the SEM/SHE model systemrequires the provision of a large amount of data which may be deter-mined from information about the catchment characteristics. Depend-ing on the availability of data, calibration of some parameters inthe SHE might be required. In the SEM two calibration parametersare included, both representing the erodibility of the soil.

The SEM/SHE was tested on a small agricultural research catchmentwith good agreement between simulated and observed hydrographs andsediment yield.

In a second study SEM/SHE was applied on upland catchments inThailand. This study illustrates the conditions which prevail inreal world applications. Despite constraints on the spatial re-presentation of meteorological input and soil data, the hydrologicalregime is adequately simulated. The simulated sediment dischargewas compared with computed values based on a sediment rating curve.Recognizing the uncertainty on these values, the comparisons showreasonable agreement.

Displays of spatial and temporal varying quantities such as thephreatic surface level and net erosion/disposition illustrates one

Simulation of water flow and soil erosion 607

of the strengths in the SEMjSHE. Although not verified in thepresent study, such maps may be very useful in the soil conservationplanning.

Currently the SEMjSHE model application on the Thailand catch-ments are continued and improved results may be obtained.

ACKNOWLEDGEMENTS The model developments have been financiallysupported by the Commission of the European Communities and nationalresearch councils. The SHE model has been developed jointly by theDanish Hydraulic Institute, the British Institute of Hydrology andSOGREAH (France). The data from the ISU-catchment were kindlyprovided by Mr Clinton Armstrong, Agricultural Engineering Dept.,University of Illinois. The simulations on the Kgt27 catchment wasperformed in cooperation with the Royal Irrigation Department (RID)in Thailand.

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Abbott, M.B., Bathurst, J.C., Cunge, J.A., O'Connell, P.E. &Rasmussen, J. (1986b) An introduction to the European Hydrologi-cal System - Syst~me Hydrologique Europ~en, "SHE", 2: Structureof physically-based, distributed modelling system. J. Hydrol.87, 61-77.

Belmans, C., Wesseling, J.G. & Feddes, R.A. (1983) Simulation modelof the water balance of a cropped soil: SWARTE. J. Hydrol. 63,271-286.

Donigian, A.D. Jr., & Crawford, N.H. (1976) Modelling pesticides andnutrients on agricultural lands. US EPA-600/2-76-043.

Engelund, F. & Hansen, E. (1967) A monograph on sediment transportin alluvial streams, Copenhagen.

Jensen, K. H~gh (1983) Simulation of water flow in the unsaturatedzone including the root zone. Institute of Hydrodynamics andHydraulic Engineering. Technical University of Denmark. SeriesPaper No. 33.

Jensen, K. H~gh & J~nch-Clausen, T. (1982) Unsaturated flow andevapotranspiration modelling as a component of the EuropeanHydrologic System (SHE) Proc. Int. Symp. on rainfall runoffmodelling, Mississippi, May 18-21, 1981, 235-252.

Julien, P.Y. & Simons, D.B. (1985) Sediment transport capacity ofoverlandflow. TransactionsASAE, 28(3),755-768.

Knisel, W.G. (ed.) (1980) CREAMS: A field-scale model for chemicals,runoff and erosion from agricultural management systems. USDACons. Dept. No. 26.

Monteith, J.L. (1965) Evaporation and environment. In: The State

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608 B~rge Storm et ai.

Nielsen, S.A. (1986) Soil Erosion, Phase I, Development of SoilErosion Model. Danish Hydraulic Institute.

Nielsen, S.A., Storm, B. & Styczen, M. (1986) Development of dis-tributed soil erosion component for the SHE hydrological model-ling system. Presented at the BHRA conference in Bournemouth,UK, June 1986.

Park, S.W., Mitchell, J.K. & Scarborough, J.N. (1982) Soil ErosionSimulation on Small Watersheds: A modified ANSWERS Model. Trans-

actions of the ASAE, 25(6), 1581-1588.Rutter, A.J., Kershaw, K.A., Robins, P.C. & Morton, A.J. (1971/72) A

predictive model of rainfall interception in forests, 1. Deriva-tion of the model from observations in a plantation of Corsicanpine. Agric. Meteoroi. 9, 367-384.

Storm, B. (1986) SHE. Systeme Hydrologique Europeen. A shortdescription. Danish Hydraulic Institute.

Styczen, M. & H~gh-Schmidt, K. (1986) A new description of therelation between drop sizes, vegetation and splash erosion.KOHYNO - NHP Seminar, Copenhagen.

Wischmeyer, W.H. & Smith, D.O. (1978) Prediction rainfall erosionloss. USDA, Agric. Handbook No. 537.