Stone cover and slope factors influencing hillside surface runoff...

$: Stone cover and slope factors influencing hillside surface runoff …web.hku.hk/~kshih/RJP/2000_JJS_HP.pdf · 2012-08-10 · in_ltration\ and interception and storage of ponds with$
HYDROLOGICAL PROCESSESHydrol[ Process[ 03\ 0718Ð0738 "1999#

Copyright © 2000 John Wiley & Sons, Ltd.Received 23 March 1999

Accepted 10 December 1999

Stone cover and slope factors influencing hillside surfacerunoff and infiltration: laboratory investigation

Jiin!Shuh Jean\0� Koe!Fe Ai\1 Kaimin Shih\0 and Chao!Chi Hung2

0 Department of Earth Sciences\ National Chen` Kun` University\ Tainan 690\ Taiwan "R[O[C[#1 Division of Hydro`eolo`y\ Central Geolo`ical Survey\ Taipei 124\ Taiwan "R[O[C[#

2 Satellite Geoinformatics Center\ National Chen` Kun` University\ Tainan 690\ Taiwan "R[O[C[#

Abstract]In this study\ laboratory rainfall simulation in an extensive area was used to study the in_ltration\ and

interception and storage from surface runo} in points with di}erent stone cover percentages "9\ 09\ 19 and

29)# and slopes "4>\ 09> and 19>#[ The experimental results of this study showed that the interrelationships

among the slope\ stone cover percentage\ groundwater level\ surface runo} amount\ and interception and

storage of the ponds were varied and irregular[ No systematic patterns were detected for the change in the

groundwater level\ surface runo} amount\ and interception and storage of the ponds with di}erent stone cover

percentages at di}erent slopes and no threshold values were apparent[ For a 4> slope\ if the stone cover

percentage was increased\ the amount of surface runo} was reduced\ the in_ltration and the groundwater level

experienced no signi_cant change\ and the interception and storage of the ponds increased[ For a 09> slope\ if

the stone cover percentage was increased\ the amount of surface runo} increased\ the in_ltration decreased\

the groundwater level experienced no signi_cant change or decreased slightly at certain points\ and the

interception and storage of the ponds increased[ For a 19> slope\ if the stone cover percentage was increased\

the amount of surface runo} increased\ the in_ltration decreased\ the groundwater level experienced no

signi_cant change or decreased slightly at certain points\ and the interception and storage of the ponds

increased[ With or without stone cover\ when the hydraulic conductivity of the top material is close to that of

_ne sand or laterite\ an increase in the slope gradient decreased the amount of surface runo} and increased the

storage amount of the ponds[ As for the stone distribution\ an interlaced style showed better performance in

the interception and storage of ponds than that of a regular style[ There was no signi_cant change in the

groundwater level[ Copyright Þ 1999 John Wiley + Sons\ Ltd[

KEY WORDS ponds^ rainfall simulation^ surface runo}^ interception^ storage^ stone cover percentage^ slope

INTRODUCTION

In the western part of Taiwan\ the annual rainfall in the mounts is about 1699 mm\ and this amountgradually decreases at lower altitudes\ _nally reaching 0999Ð0299 mm at the coast[ Only about 16) of thiswater is retained in reservoirs\ ponds and groundwater[ On the other hand\ almost 62) of the rainfall drainsquickly into the sea or evaporates directly[ With a sand tank model\ Jean and Hung "0887# laid out a seriesof ponds along a streambank in the laboratory to simulate conservation of water resources[ In that study\it was found that the runo} is reduced by the layout of arti_cial ponds\ but the e.ciency may dependlargely on the surface covering and geological conditions[ In addition\ the slope angle\ vegetation andhydrogeological characteristics also play important roles in the interception and storage of ponds[ Ai et al[

� Correspondence to] J[!S[ Jean\ Department of Earth Sciences\ National Cheng Kung University\ Ta!Hsueh Road\ Tainan\ Taiwan699 "R[O[C[#[ E!mail] jiinshuhÝsparc0[cc[ncku[edu[tw

J[!S[ JEAN ET AL[

Copyright Þ 1999 John Wiley + Sons\ Ltd[ Hydrol[ Process[ 03\ 0718Ð0738 "1999#

0729

"0888# used a laboratory rainfall simulation in a partial rainfall area to study the in~uence of slope degreeon the surface runo} with stone cover[

Most of the study about arti_cial ponds is concerned with irrigation design\ interception and storage\in_ltration and the e}ect on hillside hydrology[ From a laboratory test utilizing in_ltration pans\ Koon et

al[ "0869# indicated that available perimeter and e}ective width of cover particles are two variables a}ectingrainfall in_ltration rate[ Vittal et al[ "0877# proposed a study of interception and storage of surface runo} inponds in small agricultural watersheds in Andhra Pradesh\ India[ Poesen "0875# used _ne sand and silt asgeological samples and found that the arrangement and distribution of stone cover also a}ects the in_ltrationrate[ Lavee and Poesen "0880# suggested that for slopes of gradient under 3=4)\ with loamy sand soil\ themajor controlling factors of runo} are grain size\ distance between stones and the position of the stones "forinstance\ on top or embedded#[ The runo} rate increased with the grain size of the stone cover\ but decreasedwith the distance between stones[ On the other hand\ the runo} rate was higher for bare land than for landwith a low stone cover percentage and _ner grains[ At Walnut Gulch Experimental Watershed\ with 4> slopeangle\ Abrahams and Parsons "0880# found that the in_ltration also increased with a higher stone coverpercentage at the intershrub belt[ The laboratory work of Poesen "0875# found that a stone cover on top ofthe geological materials could increase the in_ltration and decrease the runo}^ whereas embedded stoneswould decrease the in_ltration\ thus\ increasing the runo} rate[ Agassi and Levy "0880#\ who performedlaboratory work with silt on a 4> slope\ suggested that "i# a stone cover will largely increase the in_ltrationand decrease the erosion^ "ii# the intensity of rainfall will not a}ect the in_ltration[ In the Fowlers Gap AridZone Research Station of New South Wales in Australia\ Dunkerley "0884# proposed that the parametersof stone cover area and edge length are more suitable for estimating the slope process and hillside hydrology[

In this study\ a rainfall simulator was used to carry out a laboratory simulation in an extensive rainfallarea "i[e[ rainfall simulation in the whole area of the soil tank# in order to understand the surface runo}\in_ltration\ and interception and storage of ponds with di}erent slope angles and stone cover percentages[As the stone cover is purely arti_cial in our sand tank model\ the model demonstrated here may representonly some typical and homogeneous geological conditions[ It is mainly for _nding out the general e}ects ofan individual factor\ and does not refer to any speci_c real case or site[

THEORY

Interception and stora`e

Figure 0 is an illustration of the conceptual stream~ow!generating mechanisms that might be operativeon a hillside feeding a short reach of stream[ In the diagram\ a rainfall event produces a stream~ow eventby providing lateral in~ows to the stream channel along its reach[ The lateral in~ow may be the result ofone or a combination of overland ~ows "Freeze\ 0867^ Anderson and Burt\ 0889#[ In Figure 0\ path 0represents overland ~ow "or surface runo}#^ path 1 represents subsurface storm ~ow "or subsurface runo}#^and path 2 represents groundwater ~ow[ The path by which water reaches a stream depends upon suchfactors as climate\ geology\ topography\ soils\ vegetation and land use[ An insight into the nature of thesubsurface ~ow regime is necessary for understanding the production of runo} by any of these threemechanisms "Freeze and Cherry\ 0868#[ If a series of ponds are laid out along a river and penetrate only theunsaturated zone above the water table\ the discharge from path 0 and path 1 can be intercepted[ If theponds are deep into the saturated zone\ which is below the water table\ the discharge from path 0 and path1 can be intercepted\ and the ponds can receive recharge from path 2 and path 3 as well[

The water storage rate in a pond "mL:min#\ S\ can be estimated such that

S�PA¦Qs¦Qb¦Qg−EA−I "0#

where S is the storage rate of the pond "mL:min#\ A is the surface area of the pond "m1#\ P is the precipitationper unit area "mm:h#\ E is the evapotranspiration per unit area "mm:h#\ Qs is the ~ow rate of surface runo}

SURFACE RUNOFF AND INFILTRATION


0720

Figure 0[ The mechanism of hillside runo} "Freeze\ 0863#

"mL:min#\ Qb is the ~ow rate of subsurface runo} "mL:min#\ Qg is the groundwater ~ow rate "mL:min# andI is the in_ltration rate "mL:min#[

Surface characteristics controllin` the hillside runoff

There are many factors controlling the type of surface runo}\ such as\ slope angle\ slope length\ slopeshape and slope direction\ which might a}ect wind and rain "De Lima\ 0877#[ The soil property\ climateand vegetation would determine the importance of these factors "Anderson and Burt\ 0889#[ The surfacecharacteristic of a hillside consists mainly of the surface materials\ vegetation and the property of the surfacematerials[

With a laboratory scale experiment\ Koon et al[ "0869# proposed that the in_ltration rate of runo} "I# ishighly related to the distribution of covering materials\ and the total circumference of grains "NP#\ ande}ective width "1D−L#[ The in_ltration rate per unit circumference "I:NP# is determined by e}ective width"1D−L#\ grain size "L#\ covering area and total area[ The relationship of these parameters shows

099I:NP�a¦b"1D−L# "1#

where a is the intercept of ordinate\ b is the slope and D is the distance from the mid!point of two stones tothe mid!point of the cover stone[

J[!S[ JEAN ET AL[


0721

LABORATORY SIMULATION

By applying the rainfall simulator in this study\ arti_cial rainfalls were performed on the simulated soil slopeof a semi!uncon_ned aquifer dug with _ve ponds of varying on!top stone cover percentages "9\ 09\ 19 and29)# and di}erent slopes "4>\ 09> and 19>#[ The surface runo} was herewith produced from the simulatedsoil slope[ The following were recorded]

0[ the water level change observed after the rain^1[ the surface runo} measured at _xed intervals after the rain stopped^2[ the change in water storage in the pond and the soil moisture content after the rain stopped[

Rainfall simulator

In this study\ an Arm_eld S09 Rainfall Hydrograph "Figure 1# "Arm_eld Technical Education Co[\ 0877#was used for the rainfall simulation experiment[ This equipment includes a soil tank and rainfall facility[

Soil tank "or catchment tank#[ The soil tank\ which is made of steel plate\ was adjusted to 4>\ 09> and 19>in the laboratory experiment of this study[ The tank is 019 cm long\ 79 cm wide and 19 cm high "Figures 1Ð3#[ The inlet is in the front end of the tank and equipped with a rotameter "Figure 1# to control the in~owrate and thus the groundwater level[ The outlet is in the rear end of the tank "Figures 1 and 2# and connectedto a measuring tank "or a collector# "Figure 1#[ The outlet can be visualized as a river course "Figure 2# in areal situation[

Rainfall facility[ The rainfall facility is 094 cm in height\ with a 274 mm:h rainfall intensity but without avibration screen[ As shown in Figure 1\ the clearance of the facility to the top of the soil tank is about 39cm[ In this study\ an adjustable ~ow!control meter and eight more nozzles "Full jet\ BSPT 0:7HH 5 SQ\ bySpray Systems Co[# were added to the facility[ In total\ nine nozzles were used in this facility[ Thus\ the

Figure 1[ The Arm_eld S09 Rainfall Hydrograph was used in this study[ A soil tank "or catchment tank#\ rainfall facility and measuringtank "or collector# are included "Arm_eld Technical Education Co[\ 0877#



0722

Figure 2[ The layout of the soil tank for the rainfall facility in this study

Figure 3[ The soil pro_le in the soil tank for the rainfall!hydrograph equipment in this study

J[!S[ JEAN ET AL[


0723

rainfall amount could be controlled at a steady state or adjusted as desired[ A 83=4 mL:s rainfall rate\ or0=71 mm:min rainfall intensity\ for each nozzle spray was used to simulate the rainfall in the extensive area"i[e[ the whole area of the tank#[ The rainfall area was 54 cm×79 cm "Figure 2#\ which would correspond toabout 39 m1 in the _eld situation[

Materials used in experiments

Surface soil[ The top of the soil tank "or catchment tank# was _lled with an 7!cm!thick laterite as thesurface soil or the simulated semi!uncon_ning bed "i[e[ _ne sand#[ The laterite consisted of 47) silt\ 26)_ne sand "×49 mm# and 4) clay[ After the laterite sample was taken from the _eld to the laboratory\ someof it had become agglomerated and was screened using a no[ 4 sieve "3 mm diameter in the mesh size#[ Thenthe laterite was placed and compacted in the soil tank for the rainfall simulation experiment\ in which themeasured in_ltration rate was the maximum rate for the laterite "the grain size is equivalent to _ne sandwith a coe.cient of permeability between 09−2 and 09−3 cm:s#[

Aquifer[ The 09!cm!thick\ clean medium!size sands "the grain size ranging from 9=314 to 9=74 mm# wereplaced beneath the surface soil as the simulated aquifer materials[ We combined the simulated aquifer andits overlying simulated semi!uncon_ning bed "i[e[ _ne sand# to form a semi!uncon_ning aquifer[

Stone cover[ On top of the surface soil\ subangular to subrounded gravel with grain size ranging from 9=84to 1 cm was chosen to be the covering layer[ This grain size would correspond to about 7=7Ð06=5 cm "i[e[coarse gravel to cobbles# in the _eld situation[ In the rainfall simulation experiment\ di}erent stone coverpercentages "9\ 09\ 19 and 29)# were used[

Experimental procedures

The processing and remoulding of the soil sample and the experimental procedure are as follows]

0[ The 09!cm!thick\ clean medium!size sand "grain size ranging from 9=314 to 9=74 mm# was put into thesoil tank "or catchment tank# as the simulated aquifer[

1[ The 7!cm!thick laterite\ free from agglomerated materials\ was placed on top of the simulated aquifer\and then compacted by sprinkling water layer by layer to be remoulded into the laterite slope "e[g[ 4>\09> or 19>#[ The remoulded laterite slope was dried naturally in order to be in accordance with the _eldsituation[ The laterite samples were collected at the end of the dry season[

2[ On top of the surface soil of laterite\ gravel of grain size ranging from 9=84 to 1 cm was chosen to be thecovering layer\ with cover percentages of 9\ 09\ 19 and 29)[

3[ Five ponds were dug into the upper and lower parts of the slope face\ two on the upper and three onthe lower parts\ in the con_guration shown in Figure 2[ The ponds were dug in a bowl shape[ Each pondwas 7=4 cm in diameter and 5 cm in depth[ The surface area of each pond was 45=64 cm1\ thus 172=64cm1 in total\ which took up 2=83) of the total area of surface soil in the soil tank[ The total volume ofwater for each pond was 293=4 mL[ Each pond was simulated to be impermeable using a plastic liningand thus _lled up by surface runo} only[

4[ Five plastic pipes were placed at the upper\ mid! and lower slopes "Figures 2 and 3# to observe thegroundwater level[ These _ve wells reached all the way to the bottom of aquifer[ The screen length ofeach well was 09 cm from the bottom of the well\ covering the whole aquifer thickness "09 cm thick#[

5[ Soil moisture meters were set up separately at the upper\ mid! and lower slopes[ Each meter consistedof several gypsum!made soil moisture blocks\ which were buried vertically downward at di}erent depthsof 0\ 5 and 09 cm[ These blocks did not react quickly enough to the change of soil moisture[

6[ Nine nozzles were spaced equally around the top of the tank to simulate the rainfall in the extensivearea[ The simulated rainfall rate from the nine nozzles was 83=4 mL:s[ The rainfall intensity from thenine nozzles was 0=71 mm:min[



0724

7[ Prior to each experiment\ the laterite slope of the simulated semi!uncon_ning aquifer was sprinkled with499 mL of tap water so that the surface soil of the laterite was at the saturated condition[ The rainfallsimulation experiment began when the initial moisture content of the surface soil was 19) and thegroundwater levels at wells 0 and 1 were 5 cm above the bottom of aquifer[

8[ The rainfall for each experiment lasted 19 s\ thus a total of 0789 mL of rain fell in the extensive area"i[e[ the whole area of the soil tank#[

09[ The surface runo} amount was measured at the outlet by the collector "or measuring tank^ Figures 1Ð3# every 09 s for a period of 2=4 min[ After the rainfall stopped\ the groundwater level at each well wasmeasured after 2=4\ 4\ 09\ 04\ 19 and 29 min[ The storage amount of each pond was measured after 3=4\5\ 00\ 05\ 19 and 29 min[ The soil moisture content at di}erent depths of 0\ 5 and 09 cm was measuredafter 9\ 6\ 01\ 06\ 11\ 21\ 31\ 41 and 51 min[

RESULTS

It should be noticed that each experimental result in the laboratory rainfall simulation test of this studyrepresents a mean of three test runs[ The di}erence in each value of three test runs is less than 09) error[

The hydraulic conductivity of aquifer

According to Darcy|s Law\ the hydraulic conductivity of an aquifer can be calculated by

Q�−KiA "2#

where Q is the discharge rate per unit time "mL:s#\ K is the hydraulic conductivity of aquifer "cm:s#\ i is thegradient of groundwater level and A is the cross!sectional area of the aquifer "cm1#[

The aquifer materials used in this simulation were clean sands\ 9=314 to 9=74 mm in diameter[ Before thesimulation started\ the groundwater levels of observation wells 0 and 1 were controlled at a steady state\ 5cm up from the aquifer bottom[ The discharge rate "Q# from the outlet of the catchment tank\ the gradientof the groundwater level "i#\ and the cross!sectional areas of the aquifer "A# were measured[ Thus\ the valueof hydraulic conductivity "K# is acquired by replacing the values of A\ i and Q in Equation "2#[ The averagevalue of K was 2=62×09−1 cm:s at a 4> slope\ 0=76×09−1 cm:s at a 09> slope and 2=19×09−1 cm:s at a 19>slope[ The values of K all belong to the range of hydraulic conductivity of coarse sand "p[ 07\ table 0\ DeRidder and Kruseman\ 0872#[

The hydraulic conductivity of surface soil

The values of hydraulic conductivity of surface soil "laterite# were determined from the measurements ofsoil moisture in the simulation and calculated from the time when the liquid passed a particular point[ Priorto the simulation\ the laterite slope was sprinkled with 499 mL of tap water and thus the initial soil moisturewas close to saturation point[ The hydraulic conductivity of surface soil "laterite# used in this study wasdetermined by

I�d:t "3#

where I is the hydraulic conductivity of the surface soil "cm:s#\ d is the distance between the soil moisturemeter and the surface "cm#\ and t is the time when the liquid reached a particular point "s#[

The time when the soil moisture meter reacted to the change of soil moisture at 0 cm below the surfacewas between 6 and 11 min[ Hence\ the range of hydraulic conductivity was from 0=28×09−2 cm:s to6=47×09−3 cm:s[ At 5 cm below the surface\ the time was between 01 and 11 min[ Hence\ the range ofhydraulic conductivity was from 7=22×09−2 cm:s to 3=44×09−2 cm:s[ These _gures "_ne sand# were thesame as the results of the in_ltration research of Hsieh and Wu "0874#[

J[!S[ JEAN ET AL[


0725

Table I[ The amounts of runo}\ storage and in_ltration for di}erent stone cover percentages for slopes of 4>\ 09> and19>

Variable Stone cover "4> slope# Stone cover "09> slope# Stone cover "19> slope#statistic

9) 09) 19) 29) 9) 09) 19) 29) 9) 09) 19) 29)"0# "1# "2# "3# "0# "1# "2# "3# "0# "1# "2# "3#

Surface runo} "mL#

Meana 594=9 427=2 411=9 403=9 364=9 407=9 456=4 591=9 172=6 275=6 307=9 305=9"20=7# "17=2# "16=4# "16=0# "14=9# "16=2# "18=8# "20=6# "03=8# "19=3# "11=9# "10=8#

SD 23=0 15=6 1=9 0=9 07=9 0=9 0=4 9=9 07=1 05=1 7=4 09=6F value 6=17� 65=30�� 30=02��Sche}e test Signi_cant di}erence pairs] Signi_cant di}erence pairs] Signi_cant di}erence pairs]

"0\2#\ "0\3# "0\1#\ "0\2#\ "0\3#\ "1\2#\ "1\3#\ "2\3# "0\1#\ "0\2#\ "0\3#

Storage amount after 3=4 min "mL#

Meana 605=2 734=6 746=9 740=9 655=4 762=4 782=4 757=9 651=2 714=2 744=9 709=2"26=6# "33=4# "34=0# "33=7# "39=2# "35=9# "36=9# "34=6# "39=0# "32=3# "34=9# "31=5#

SD 08=4 18=4 00=9 28=9 23=4 40=4 14=4 5=9 40=4 5=5 03=1 05=9F value 04=5�� 7=59�� 2=68Sche}e!test Signi_cant di}erence pairs] Signi_cant di}erence pairs] Ð

"0\1#\ "0\2#\ "0\3# "0\1#\ "0\2#\ "0\3#

Storage amount after 29 min "mL#

Meana 436=2 649=6 635=9 635=9 593=4 670=9 653=4 644=9 523=2 580=9 649=9 618=9"17=7# "28=9# "28=2# "28=2# "20=7# "30=0# "39=1# "28=6# "22=3# "25=3# "28=9# "27=9#

SD 03=7 25=7 19=9 19=9 14=4 20=9 7=4 9=9 14=2 08=9 05=2 29=1F value 27=94�� 36=67�� 8=34��Sche}e!test Signi_cant di}erence pairs] Signi_cant di}erence pairs] Signi_cant di}erence pairs]

"0\1#\ "0\2#\ "0\3# "0\1#\ "0\2#\ "0\3# "0\2#\ "0\3#

In_ltration "mL#

Meana 467=6 405=9 410=9 424=9 547=4 497=4 328=9 329=9 743=9 577=9 516=9 562=9"29=9# "16=1# "16=3# "17=1# "23=6# "15=7# "12=0# "11=5# "34=9# "25=1# "22=9# "24=9#

SD 15=4 42=8 8=9 27=9 45=4 49=4 13=9 5=9 53=0 11=9 11=4 4=2F value 0=39 10=97�� 04=21��Sche}e!test Ð Signi_cant di}erence pairs] Signi_cant di}erence pairs]

"0\1#\ "0\2#\ "0\3# "0\1#\ "0\2#\ "0\3#

� p ³ 9=94^ �� p ³ 9=90^ �� p ³ 9=990[a Total precipitation � 0899 mL^ percentage of total precipitation in parentheses[

Laboratory rainfall simulation

For a 4> slope\ the ANOVA analysis showed that the surface runo} amount under the di}erent stonecover percentages from 9 to 29) decreased signi_cantly\ as shown in Table I "F value�6=17\ p³ 9=94#[ Byperforming the Sche}e test\ there were signi_cant decreases between the stone cover percentages of 9Ð19)and 9Ð29) "i[e[ the signi_cant di}erence pairs "0\2# and "0\3##[ In Figure 4\ a peak appeared\ but the runo}amount decreased after the peak owing to the existence of stone cover[ The in_ltration amount experiencedno signi_cant change with an increase in stone cover percentages "F value�0=3 in Table I#[ Four and a halfminutes after the rainfall started\ the storage amount of ponds was raised by 5=7Ð6=3) with an increase inthe stone cover percentage "F value�04=5\ p³ 9=90#[ Thirty minutes after the rainfall started\ the storageamounts of ponds increased with the increase of stone cover percentages "F value�27=94\ p³ 9=990#[ InFigure 5\ the interception and storage amounts of the ponds located at the upper slope "D¦E# were greater



0726

Figure 4[ The runo} with a slope of 4> and various stone cover percentages

than those located at the lower slope "A¦B¦C#[ The di}erence was about 03=0Ð03=5) of total precipitation"�0899 mL#[ From the ANOVA analysis\ the groundwater levels under the stone cover percentages from 9to 29) showed no signi_cant changes at wells 0Ð4 "Table II#[

For a 09> slope\ the ANOVA analysis showed that the surface runo} amount under the di}erent stonecover percentages from 9 to 29) increased signi_cantly\ as in Table I "F value�65=30\ p³ 9=990#[ Byperforming the Sche}e test\ signi_cant increases were apparent between the stone cover percentages of 9Ð09)\ 9Ð19)\ 9Ð29)\ 09Ð19)\ 09Ð29) and 19Ð29) "i[e[ the signi_cant di}erence pairs "0\1#\ "0\2#\ "0\3#\"1\2#\ "1\3# and "2\3##[ In Figure 6\ for a stone cover of 19)\ the peak rate of surface runo} hydrograph wassigni_cantly higher than that of other percentages[ In the post!peak period\ except for the hydrograph under19) stone cover being lower than that of 9) cover\ the others are similar to the hydrograph of 9) stonecover[ The lag time was reduced owing to the existence of stone cover[ In Table I\ the in_ltration ratedecreased with an increase in stone cover "F value�10=97\ p³ 9=990#[ Four and a half minutes after therainfall started\ the storage amount of ponds increased by 4=3Ð5=6) with an increase in the stone coverpercentage "F value�7=59\ p³ 9=90#[ Thirty minutes after the rainfall started\ the storage amount of pondsincreased when the stone cover percentage was greater than 09) "F value�36=67\ p³ 9=990#[ In Figure 7\the storage amounts of the ponds located at the upper slope "D¦E# were similar to those located at the

J[!S[ JEAN ET AL[


0727

Figure 5[ The pond storage with a slope of 4> and various stone cover percentages] "a# ponds A¦B¦C\ "b# ponds D¦E and "c# allponds



0728

Table II[ The groundwater levels in the observation wells under conditions of di}erent stone cover percentages andslopes of 4>\ 09> and 19>[ Mean represents the average of the water levels at 9\ 2=4\ 4\ 09\ 04\ 19 and 29 min

Well Statistic Stone cover "4> slope# Stone cover "09> slope# Stone cover "19> slope#

9) 09) 19) 29) 9) 09) 19) 29) 9) 09) 19) 29)"0# "1# "2# "3# "0# "1# "2# "3#

0 Mean "cm# 5=132 5=060 5=046 5=046 5=118 5=175 5=118 5=103 5=160 5=160 5=160 5=103SD "cm# 9=073 9=005 9=094 9=094 9=037 9=044 9=977 9=035 9=017 9=017 9=017 9=035F value 9=47 9=21 9=17Sche}e!test Ð Ð Ð



3 Mean "cm# 4=746 4=699 4=632 4=603 3=846 3=732 3=632 3=660 3=932 3=960 3=918 2=818SD "cm# 9=057 9=096 9=007 9=013 9=049 9=039 9=989 9=017 9=007 9=017 9=092 9=027F value 0=66 2=18� 0=43Sche}e!test Ð No signi_cant di}erences Ð

4 Mean "cm# 3=546 3=318 3=360 3=360 1=118 0=732 0=646 0=632 9=660 9=499 9=260 9=232SD "cm# 9=127 9=027 9=017 9=027 9=072 9=057 9=962 9=073 9=977 9=965 9=969 9=938F value 1=13 01=28�� 33=03��Sche}e!test Ð Signi_cant di}erence pairs] Signi_cant di}erence pairs]

"0\1#\ "0\2#\ "0\3# "0\1#\ "0\2#\ "0\3#\ "1\2#\ "1\3#

� p ³ 9=94^ �� p ³ 9=990[

lower slope "A¦B¦C#[ From the ANOVA analysis\ as shown in Table II\ for the groundwater levels underthe stone cover percentages from 9 to 29)\ there were no signi_cant changes at wells 0Ð2[ However\ therewere signi_cant changes in the groundwater levels at well 3 "F value�2=18\ p³ 9=94# and well 4 "Fvalue�01=28\ p³ 9=990#[ By performing the Sche}e test\ there were no signi_cant changes in the ground!water levels between the stone cover percentages of 9Ð09)\ 09Ð19) and 19Ð29) at well 3\ but at well 4there were signi_cant decreases between the stone cover percentages of 9Ð09)\ 9Ð19) and 9Ð29) withrespect to the signi_cant di}erence pairs "0\1#\ "0\2# and "0\3#[

For a 19> slope\ the ANOVA analysis showed that the surface runo} amount under the di}erent stonecover percentages from 9Ð29) increased signi_cantly\ as in Table I "F value�30=02\ p³ 9=990#[ Byperforming the Sche}e test\ signi_cant increases were apparent between the stone cover percentages of 9Ð09)\ 9Ð19) and 9Ð29) "i[e[ the signi_cant di}erence pairs "0\1#\ "0\2# and "0\3##[ The peak of the surfaceruno} hydrograph increased and the lag time was reduced under the existence of stone cover "Figure 8#[ Thein_ltration rate decreased by 7=7Ð01) "F value�04=21\ p³ 9=90\ as in Table I#^ and the higher the stonecover percentage\ the lower the in_ltration rate[ Four and a half minutes after the rainfall started\ the storageamount of ponds showed no signi_cant change "F value�2=68# with an increase in the stone coverpercentage[ Thirty minutes after the rainfall\ the storage amounts of ponds increased with an increase in thestone cover percentage "F value�8=34\ p³ 9=90#[ Comparison with the laboratory simulation in the partialrainfall area "Ai et al[\ 0888# shows that the pond storage volume decreased with the higher slope angle\

J[!S[ JEAN ET AL[


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whereas it increased with the higher slope angle in the extensive rainfall area of this study[ In Figure 09\ theinterception and storage amounts of the ponds located at the upper slope "D¦E# were lower than thoselocated at the lower slope "A¦B¦C#[ The di}erence was about 4=5Ð5=3) of total precipitation[ From theANOVA analysis\ for the groundwater levels under the stone cover percentages from 9 to 29)\ there wereno signi_cant changes at wells 0Ð3[ However\ there were at well 4 "F value�33=03\ p³ 9=990#[ By performingthe Sche}e test\ signi_cant changes were apparent between the stone cover percentages of 9Ð09)\ 9Ð19)\9Ð29)\ 09Ð19) and 09Ð29) with respect to the signi_cant di}erence pairs "0\1#\ "0\2#\ "0\3#\ "1\2# and"1\3#\ as shown in Table II[

Surface runoff of 9) stone cover[ For a 19> slope\ the stream hydrograph was close to ~at "Figure 8#[ Thestudy of Abrahams and Parsons "0880# indicated that when the slope is greater than 01> "sin u× 9=1#\ thesurface runo} coe.cient decreases abruptly[ The study of Poesen "0875#\ who used _ne sand as the surfacematerial\ showed that the surface runo} amount is greater for a 1> slope than a 04> slope[ As illustrated inTable I and Figures 4\ 6 and 8\ the higher the slope gradient\ the smaller the amounts of surface runo}[ Inaddition\ the peak rate is lower\ and the lag time is longer[ The results of the simulation in this study arecomparable to the papers mentioned above[ As for the storage amount\ the higher the slope gradient\ thegreater the storage amounts "Table I#[

The distribution of styles of stone cover[ There were two distribution styles of stone cover in the simulation^A had a regular distribution and B had an interlaced distribution\ as illustrated in Figure 00[ The impact ofdistribution styles of stone cover to the surface runo} is illustrated in Table III[ For a 4> slope\ the surfaceruno} amount is higher in style B than in style A and the in_ltration amount is higher in style A than instyle B\ whereas for a 09> slope the surface runo} amount is higher in style A than in style B and thein_ltration amount is higher in style B than in style A[ By performing a t!test analysis\ the water levels atwells 0Ð4 showed no signi_cant changes under stone cover distribution styles A and B for the 4> slope and



0730

Figure 7[ The pond storage with a slope of 09> and various stone cover percentages] "a# ponds A¦B¦C\ "b# D¦E and "c# all ponds

09) stone cover "Table IVa# and the 09> slope and 29) stone cover "Table IVb#[ Four and a half minutesafter the rainfall started\ for 4> and 09> slopes the interception and storage amounts of the ponds slightly

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increased under the B distribution style compared with the A distribution style at the upper and lower slopes"Figures 01 and 02#\ in which ponds A¦B¦C are at the lower slope and ponds D¦E are at the upper slope"Figure 2#[

DISCUSSION

The in~uence of stone cover percenta`e on ponds

In general\ if the stone cover percentage is increased\ the interception and storage of ponds is alsoincreased^ and the larger the stone cover percentage\ the greater the interception and storage of ponds[ Laveeand Poesen "0880# indicated that the overland ~ow is in an inverse relationship with the distance betweenstone covers[ In this paper\ the distances between stone covers are 5 cm "cover 09)#\ 2=4 cm "cover 19)#and 1=4 cm "cover 29)#[ Under the conditions of rainfall in an extensive area "i[e[ the whole area of the soiltank#\ a decrease in the distance between stone covers reduced the surface runo} amount and increased theinterception and storage amounts of ponds for a 4> slope\ whereas for 09> and 19> slopes\ a decrease in thedistance between stone covers increased the surface runo} amount\ and increased the interception andstorage amounts of ponds "Table I#[ The simulation results are di}erent from the ones of rainfall in a partialarea "Ai et al[\ 0888#[

The in~uence of stone size on surface runoff\ in_ltration and stora`e ponds

The research of Valentin and Casanave "0881# indicated that a medium value of diameter equal to 9=918m was a threshold[ If the value was smaller than 9=918 m\ the ratio of in_ltration rate to non!embeddedstone cover percentage was positive\ whereas greater than 9=918 m\ the ratio was inverse[ Hence\ the greaterthe in_ltration rates\ the smaller the surface runo} amount[ In this study\ the sizes of stone in the cover arebetween 9=84 and 1 cm\ i[e[ smaller than the threshold of 9=918 m[ The research of Ai et al[ "0888# showedthat under the condition of rainfall in a partial area\ the greater the stone cover percentage\ the greater thein_ltration rate[ From the simulation results of this paper\ however\ it is an inverse ratio under the conditionof rainfall in an extensive area "i[e[ the whole area of the soil tank#[ In addition\ the interception amountand the long!term storage of ponds increase under the condition of rainfall in an extensive area "Figures 5\7 and 09 and Table I#[ Under the condition of rainfall in a partial area\ this phenomenon takes place onlyfor a 4> slope "Ai et al[\ 0888#[ Under both conditions of rainfall in partial and extensive areas\ the surfaceruno} amount decreased for a 4> slope and increased for a 19> slope[

The in~uence of stone distribution on ponds

Di}erent styles of stone cover distribution were simulated in the following two scenarios] 09) stone coverfor a 4> slope and 29) stone cover for a 09> slope[ The results show that the interlaced distribution style



0732

Figure 09[ The pond storage with a slope of 19> and various stone cover percentages] "a# ponds A¦B¦C\ "b# ponds D¦E and "c# allponds

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Figure 00[ The distribution styles of stone cover

Table III[ The impact of the stone cover distribution styles on the amounts of surface runo}\ storage and in_ltrationat di}erent slopes and stone cover percentages^ A\ regular distribution style^ B\ interlaced distribution style[ Total

precipitation � 0899 mL^ percentage of total precipitation in parentheses

Slope Stone Distribution Amount of surface Amount of storage Amount ofcover style runo} "mL# after 3=4 min "mL# in_ltration "mL#

4> 09) A 427=2 "17=2# 734=6 "33=4# 405=9 "16=1#09) B 450=4 "18=5# 755=9 "34=5# 361=4 "13=8#

09> 29) A 591=9 "20=6# 757=9 "34=6# 329=9 "11=5#29) B 457=9 "18=8# 774=4 "35=5# 335=4 "12=4#

"B# performed better in the interception and storage by ponds than the regular distribution style "A# did[The storage amount of the former increased slightly by 0) "Table III and Figures 01 and 02#[ This mayresult from the in~uence of di}erent stone cover distribution styles on the ~ow condition of surface runo}[

Other factors that have an in~uence on ponds

The factors in this study include slope gradient\ stone cover percentage and the existence of ponds\ butthe simulated results show lots of variation[ As surface runo} is a}ected by factors such as in_ltration\interception and stone cover before it reaches the river course or the storage ponds on the surface\ thisindicates that the storage amount of ponds and the ~ow condition of surface runo} are also signi_cantfactors[ The di}erent results from the two rainfall conditions\ in a partial area "Ai et al[\ 0888# and in anextensive area\ suggest that the distance that the surface runo} ~ows through may be a signi_cant factor[

The surface runo} may become laminar ~ow or turbulent ~ow as a result of the stone cover\ and hencea}ect the ~ow velocity\ the ~ow route and the ~ow style[ The runo} also has an e}ect on the capability oferosion or the movement of eroded material[ The erosion factor was not considered in this study\ but duringthe simulations it was observed that some eroded material was generated with the surface runo}[ The runo}moved with the eroded material\ and when a stone obstacle was encountered\ the eroded material wouldstop[ The velocity of ~ow decreased slightly and then it kept on moving downhill[ This phenomenon a}ectedthe results of our simulation\ and caused variations in surface runo}\ in_ltration and storage amount[

The on!top placement style of stone cover was used in this study[ According to global research results\this can increase the in_ltration capability[ From the simulated results under the conditions of rainfall in apartial area\ the in_ltration amount was increased "Ai et al[\ 0888#[ In this paper\ however\ under theconditions of rainfall in an extensive area\ the in_ltration amount was reduced[ This may be due to thedi}erent distances that surface runo} ~ows through[ In addition\ an increase in the slope gradient may cause



0734

Table IV[ The groundwater levels of the observed wells with the di}erent distribution styles] A\ regular distributionstyle^ B\ interlaced distribution style

Time after rainfall Water level "cm#started "min#

Well 0 Well 1 Well 2 Well 3 Well 4

A B A B A B A B A B

"a# 4> slope and 09) stone cover9 5=9 5=9 5=9 5=0 5=4 5=5 4=4 4=4 3=1 3=22=4 5=2 5=2 5=2 5=2 5=7 5=7 4=7 4=7 3=5 3=44 5=2 5=2 5=3 5=2 5=7 5=7 4=7 4=8 3=5 3=509 5=2 5=1 5=2 5=1 5=6 5=6 4=7 4=7 3=4 3=404 5=0 5=1 5=0 5=0 5=5 5=5 4=6 4=6 3=3 3=319 5=0 5=0 5=0 5=0 5=5 5=5 4=6 4=6 3=3 3=229 5=0 5=0 5=0 5=0 5=5 5=5 4=5 4=5 3=2 3=2Mean 5=060 5=060 5=075 5=060 5=546 5=560 4=699 4=603 3=317 3=303SD 9=005 9=092 9=025 9=977 9=094 9=977 9=096 9=014 9=027 9=002t!test value 9 9=11 −9=15 −9=10 9=19

"b# 09> slope and 29) stone cover9 5=9 5=0 5=9 5=9 6=1 6=0 3=5 3=4 0=5 0=42=4 5=3 5=3 5=3 5=3 6=4 6=4 4=9 3=8 1=9 1=94 5=3 5=3 5=3 5=3 6=4 6=5 3=8 3=8 1=9 1=909 5=2 5=2 5=2 5=2 6=3 6=4 3=7 3=8 0=7 0=804 5=1 5=2 5=0 5=1 6=3 6=3 3=6 3=7 0=6 0=819 5=0 5=1 5=0 5=1 6=2 6=3 3=6 3=7 0=5 0=729 5=0 5=0 5=0 5=0 6=2 6=3 3=6 3=6 0=4 0=6Mean 5=103 5=199 5=199 5=117 6=260 6=303 3=660 3=675 0=632 0=718SD 9=035 9=020 9=040 9=027 9=092 9=035 9=017 9=025 9=073 9=056t!test value −9=45 −9=23 −9=48 −9=08 −9=74

p × 9=94[

a decrease in the surface runo}\ and hence an increase in the in_ltration[ This indicates that the slopegradient has an e}ect on the surface runo} amount "Table I#\ and thus a}ects the in_ltration rate[ Thisresult di}ers from the paper of Chen and Yiou "0883#\ which concluded\ {In slopes smaller than 29 degrees\the in_ltration behavior has no big di}erence from the horizontal slope|[ This might be due to the di}erentsurface materials used in the simulation[ The material used in the research of Chen and Yiou "0883# wasquartz sand "smaller than no[ 19 mesh#\ whereas the upper material used in this study was the laterite"equivalent to _ne sand in diameter# and the lower material was medium!size sand "9=314 to c[ 9=74 mm indiameter#[

Findin`s

When the hydraulic conductivity of surface material is equivalent to _ne sand or laterite\ the slope is asigni_cant factor that a}ects the surface runo}\ the in_ltration amount\ and the interception and storage[Combined with the research results of Poesen "0875# and Abrahams and Parsons "0880#\ the conclusion canbe made that if the slope gradient is increased\ the surface runo} amount will decrease\ the in_ltrationamount will increase\ and the interception and storage of ponds will be raised[

Basically\ stone cover can increase the interception and long!term storage of ponds[ Furthermore\ thisstudy shows that the distribution styles of stone cover have an e}ect on the interception and storage ofponds[ The distance that the surface runo} ~ows through can a}ect the surface runo} and the in_ltrationamounts[ According to the study of De Lima "0877#\ on a cement plate\ the greater the slope length\ the

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Figure 01[ The pond storage of A and B distribution styles\ with a slope of 4> and 09) stone cover percentage] "a# ponds A¦B¦C\"b# ponds D¦E and "c# all ponds



0736

Figure 02[ The pond storage of A and B distribution styles\ with a slope of 09> and 29) stone cover percentage] "a# ponds A¦B¦C\"b# ponds D¦E and "c# all ponds

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greater the surface runo} amount owing to the impermeability of the cement plate[ In this study\ the surfacematerial of the simulations was laterite with a hydraulic conductivity equivalent to _ne sand[ Thus\ if theslope length was increased\ the in_ltration rate was raised\ and the surface runo} amount was reduced owingto the permeability of the laterite[

As the result of an increase in the stone cover percentage\ the simulated results of surface runo} amountand groundwater level were opposite for 4> and 19> slopes[ The simulated result of this study for a 09> slopeindicates that because of the layout of stone cover\ a transition zone where surface runo} might increase ordecrease is produced[ This tallies with the research results of Abrahams and Parsons "0880#\ who state that{the surface runo} amount is reduced abruptly when the degrees of slope are greater than 01|[

CONCLUSIONS

For a 4> slope\ under the conditions of rainfall in a partial area and in an extensive area\ if the stone coverpercentage is increased\ the amount of surface runo} is reduced\ the groundwater level remains the same\and the interception and storage of the ponds is increased[ However\ the in_ltration increased in a partialarea\ whereas it did not change signi_cantly in an extensive area[

In comparison with the simulated results for 4> and 19> slopes\ the hydrological condition for a 09> slopeis transitional\ i[e[ surface runo} may increase or decrease[ Under the conditions of rainfall in a partial area"Ai et al[\ 0888#\ if the stone cover percentage was increased\ the amount of surface runo} was reduced\ thein_ltration increased\ the groundwater level increased\ and the interception and storage of the pondsdecreased[ Under the condition of rainfall in an extensive area\ however\ if the stone cover percentage wasincreased\ the amount of surface runo} increased\ the in_ltration decreased\ the groundwater level remainedalmost the same or slightly decreased at certain points\ and the interception and storage of the pondsincreased[

If the stone cover percentage was increased\ the amount of surface runo} increased for a 19> slope underthe conditions of rainfall in partial and extensive areas[ However\ the in_ltration increased\ the groundwaterlevel increased and the interception and storage of the ponds decreased under the condition of rainfall in apartial area\ whereas the in_ltration decreased\ groundwater level was almost the same or slightly decreasedat certain points\ and the interception and storage of the ponds increased under the condition of rainfall inan extensive area[ The layout of ponds can decrease the surface runo} amount and increase the groundwaterstorage e.ciency\ but the in~uence of stone cover varies with the gradient of the slope[

The experimental results of this study show that the interrelationships among the slope\ stone coverpercentage\ groundwater level\ surface runo} amount\ and interception and storage of the ponds are variedand irregular[ No systematic patterns were detected for the change in the groundwater level\ surface runo}amount\ and interception and storage of the ponds with di}erent stone cover percentages at di}erent slopes\and no threshold values were apparent[

With or without stone cover\ when the hydraulic coe.cient of surface material is close to _ne sand orlaterite\ an increase in the slope gradient decreases the amount of surface runo} and increases the storageamount of ponds[

There are two types of stone cover placement[ One is to place a stone cover on top\ which was used inthis study\ and the other is to have an embedded stone cover[ In the latter condition\ the interrelationshipsamong the stone cover and the surface runo}\ in_ltration\ slope\ the interception and storage amount ofponds are still unknown and could be a subject of future study[

ACKNOWLEDGEMENTS

The authors would like to express our special thanks to Professors Ben!Shan Yiou and Tsao!Shien Liao andMr Chi!Chung Chien for their valuable comments[



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Agassi M\ Levy GI[ 0880[ Stone cover and rain intensity] e}ects on in_ltration\ erosion and water splash[ Australian Journal SoilResearch 18] 454Ð464[

Ai KF\ Jean JS\ Hung CC\ Shih K[ 0888[ Stone covering factors in~uencing hillside pond interception and storage in a partial rainfallarea[ Journal of the Geoloìcal Society of China 31] 274Ð397[

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Stone cover and slope factors influencing hillside surface runoff...

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