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137Cs在土壤侵蚀中的应用

137Cs在土壤侵蚀中的应用
137Cs在土壤侵蚀中的应用

Using 137Cs technique to quantify soil erosion and deposition rates in an agricultural catchment in the black soil region,Northeast China

Haiyan Fang ?,Liying Sun,Deli Qi,Qiangguo Cai

Key Laboratory of Water Cycle and Related Land Surface Processes,Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing 100101,China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 13April 2011

Received in revised form 12March 2012Accepted 20April 2012

Available online 27April 2012Keywords:137

Cs technique Black soil Catchment

Soil erosion and deposition Spatial pattern

Soil erosion signi ?cantly affects the productive black soil region in Northeast China.Quanti ?cation of the soil erosion is necessary for designing ef ?cient degradation control strategies.137Cs measurements undertaken on 61sampling points collected within a 28.5ha agricultural catchment in the black soil region of Northeast China were used to establish the magnitude and spatial pattern of soil redistribution rates as well as sediment budget within the catchment.Estimated soil redistribution rates using the Mass Balance Model 2(MBM2)ranged from ?56.8to 171.4t ha ?1yr ?1for the sampling points that were veri ?ed by means of both runoff plot data and pedological investigation.Erosion generally occurred behind the shelterbelts,especially in the ephemeral gully susceptible areas,while deposition mainly occurred along the shelterbelts and at the catchment outlet.In the study catchment,69%of the eroded sediments came from the slopes and 31%the ephemeral gullies.Sediments deposited along the shelterbelts at a rate of ca.78t yr ?1and ca.33t yr ?1at the catchment outlet.The gross soil loss rate for the catchment was ?4.4t ha ?1yr ?1with a sediment delivery ratio of 53%.The mean rate of ?14.5t ha ?1yr ?1in the erosion areas was much higher than the tolerable value,suggesting that effective soil conservation measures are urgently required to reduce the severe black soil loss for sustainable management of the soil resource.

?2012Elsevier B.V.All rights reserved.

1.Introduction

The black soil region in Northeast China has been cultivated as farmland,and is considered essential for Chinese crop production (Xu et al.,2010).Nevertheless,severe soil erosion has occurred since large-scale land reclamation in the 1950s,and the thickness of the A-horizon of the black soils has decreased from 60–70cm in the 1950s to 20–30cm at present (Fan et al.,2005;Zhang et al.,2007).In some places the loess parent material has been exposed,reducing soil productivity (Wang et al.,2009).The severe soil loss is also a threat to water quality in local streams and rivers (Yu et al.,2003).Understanding the spatial pattern of soil redistribution and the analysis of sediment budget are thus very important for designing soil and water conservation programs,targeting remediation measures and for evaluating the bene ?ts of catchment management (Yang et al.,2006a ).

Using classical erosion techniques such as erosion plots and pre-dictive models for monitoring and assessing soil loss has many limita-tions:they are dif ?cult to handle,time consuming,and expensive (La ?en et al.,1991;Quine,1999;Du and Walling,2011).The use of 137

Cs measurements has attracted increasing attention as an approach to quantitatively estimate soil redistribution over agricultural land-scapes (Wakiyama et al.,2010).The arti ?cial radionuclide 137Cs (half-life:30.2years)was introduced into the environment by the atmo-spheric testing of thermonuclear weapons,primarily during the period from 1954to mid-1970s.After releasing into the stratosphere,the 137Cs was distributed globally and deposited as fallout,mainly in association with precipitation.Its use as a sediment tracer lies in its rapid and strong adsorption by ?ne particles (He and Walling,1996)so that in most ag-ricultural environments its subsequent redistribution is a direct re ?ec-tion of the erosion,transport and deposition of soil particles.Estimates of soil loss and/or gain rates can be derived from 137Cs measurements by comparing the inventories measured at a speci ?c point with the ref-erence inventory.The 137Cs technique provides retrospective estimates of medium-term (ca.50years)erosion rates (Walling and He,1999;Du and Walling,2011),and the spatial pattern of erosion and deposition rates can be evaluated and analyzed based on a single https://www.sodocs.net/doc/a29466440.html,ing 137Cs technique for estimating soil erosion and deposition rates has been ap-plied to a wide range of environments,and the basis of the technique is well established and documented (Ritchie et al.,1974;Elliot,et al.,1990;Nouira et al.,2003).

In recent years,the severe erosion of black soil in Northeast China has attracted increasing attention.Many studies have been conducted on environmental and anthropogenic impacts on soil erosion,and factors in ?uencing erosion rates (Jing et al.,2003;Liu et al.,2005;Yang et al.,2006b ),soil erosion types (Wang et al.,2010)and gully dynamics (Hu et al.,2007;Zhang et al.,2007;Wu et al.,2008)as well as the controlling strategies in the black soil region of Northeast China (Wang et al.,2003;Yang et al.,2005).However,limited

Geomorphology 169–170(2012)142–150

?Corresponding author.Tel.:+861064889036.E-mail address:fanghy@https://www.sodocs.net/doc/a29466440.html, (H.

Fang).

0169-555X/$–see front matter ?2012Elsevier B.V.All rights reserved.doi:

10.1016/j.geomorph.2012.04.019

Contents lists available at SciVerse ScienceDirect

Geomorphology

j o ur n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /g e o m o rp h

scienti?c data on the rates of erosion and deposition associated with agricultural land use and landform positions were mainly focused at the slope scale(Fang et al.,2006a,2006b;Wang et al.,2010).

Since the large-scale land reclamation,the black soil region in North-east China has been affected by severe soil erosion although shelterbelts there were established ca.40years ago(Zhang et al.,2006b).The snow-melt runoffs in spring and storms in the rainy season usually induce the development of ephemeral gullies even in a small catchment(Hu et al., 2007;Zhang et al.,2007;Wu et al.,2008).It will be helpful to imple-ment land use management for a small catchment through studying its soil redistribution pattern and sediment budget.However,limited quantitative data are available for assessing soil erosion characteristics of an agricultural catchment in the black soil region.

Therefore,a small catchment in the black soil region was selected for this study,in order to(i)evaluate the reliability of the137Cs technique for the quanti?cation of soil erosion;(ii)quantify the spatial pattern of soil redistribution by using137Cs technique;and(iii)estimate sedi-ment budgets at a medium(ca.50-year)time period for the catchment.

2.Materials and methods

2.1.Study catchment

The study catchment is located at the Heshan Farm,Northwest Heilongjiang Province,China(125°11′E,48°56′N;Fig.1).The catch-ment has an area of28.5ha with an elevation of320to360m.The slopes of cultivated land for the catchment range from0.4%to8.4% with an average value of4.2%.The region's climate is semihumid and continental with a long and cold winter.Mean January and July temperatures are?20°C and21°C,respectively(Zhang et al., 2007).Precipitation is534mm with a67%of annual rainfall from June to August(Wu et al.,2008).

The dominant soil association is classi?ed as Udic Argibborlll in the USDA Taxonomy.The parent materials of the Phaeozem are Qua-ternary lacustrine and?uvial sand beds or loess sediments that lie below the Phaeozem(Sun and Liu,2001).The main textural classes of the top soil are silt clay loam to clay loam(8–27%sand,29–66% silt,and26–40%clay)(Wu et al.,2008).

Most of the lands in Heshan Farm were historically covered by bush wood.However,intensive land reclamation was carried out in the black soil region with increasing people since the establishment of the People's Republic of China,and the newly cultivated land in Heilongjiang Province was up to5.93million ha by1978(Fan et al., 2005).Though historic records of the condition of the land in Heshan Farm are not extensive or speci?c,the reclamation history for the black soil region coupled with local people's interview survey in-formed us that the study catchment has been reclaimed for farmland since the1950s and the general environmental conditions have not substantively changed over the last50years(Hu et al.,2007),which is the time range associated with the137Cs measurements.

Soybean(Glycine max(L.)Merr.)is the major crop in the rotation with wheat(Triticum aestivem L.)and corn(Zea mays L.)for the

study

Fig.1.Map showing the locations of the study catchment,sampling points,shelterbelts and ephemeral gullies.The sampling points along the transects A–H were indicated with a transect name and a number,and the ephemeral gullies(EGs)were named as EG1to EG4.

143

H.Fang et al./Geomorphology169–170(2012)142–150

catchment.A single tillage operation is used with a cultivator harrow to a depth of about0.25m after harvesting in autumn(late September) or before sowing(early May)in spring.From October to April,the crop-land is left fallow with no vegetation cover for cold weather.Pinus shel-terbelts were established to protect crop against wind disaster.For the convenience of mechanical tillage,the tillage direction is usually paral-lel to the shelterbelt.The downslope cultivation easily accelerates soil erosion in the study catchment.

An ephemeral gully system was observed in the lower part of the study catchment(Fig.1).In the black soil region,ephemeral gullies develop twice a year.The?rst takes place usually in late April,as a re-sult of snowmelt runoffs,and the second occurs usually in late July resulting from rain storms(Zhang et al.,2007).The ephemeral gullies have been recognized as an important source of sediment eroded from croplands(Zhang et al.,2006a,2007).The presence of the ephemeral gullies may contribute greatly to the sediment budget of the catchment.

2.2.Soil sampling

From the catchment survey,the sampling strategy was based on a multiple-transect approach accompanied by the key points'sampling method.Eight transects were selected representing the landscape characteristics from the upper plateau to the lowest position of the catchment.The sites that represented major landforms but were not on the transects were also sampled.A reference site for determining the137Cs inventory was selected on an uncultivated second-growth Quercus mongolica forest land with a?at topography(Fig.1).It was judged to have been unaffected by soil erosion or deposition since 1950,and it is located ca.5km away from the investigated catch-ment.The137Cs soil samples were collected in late June,2010,using a5-cm-diameter hand operated core sampler.Soil cores were taken to a depth of40cm on the eroded cultivated land and60–100cm at the deposition sites to ensure that the core had penetrated to the full depth of the137Cs pro?le.The distances between sampling points along the transects ranged from50to100m and between the other sampling points depended on the landform characteristics.The sam-ples were sectioned with3–5cm intervals up to30cm to determine both the depth distribution and the137Cs inventory for one of the ref-erence sampling points.

https://www.sodocs.net/doc/a29466440.html,boratory analysis

The bulk and sectioned soil core samples were air-dried,weighted, and passed through a2-mm sieve for the measurements of137Cs ac-tivity.The radioactivity of137Cs in soil samples was measured by a hyper-pure coaxial Ge detector linked to a multichannel analyzer, detected at662keV peak with counting time over80,000s,providing a measurement precisions of±5%for137Cs at the95%con?dence level. The results were originally calculated on a unit mass basis(Bq kg?1) and were then converted to an inventory value(Bq m?2)using the total weight of the bulked core soil sample and the sampling area.

2.4.Estimation of soil redistribution rates using137Cs radionuclide

A number of approaches have been proposed for deriving estimates of soil redistribution rates from137Cs measurements in cultivated areas. Mass balance models have been frequently used to estimate137Cs loss and gain for speci?ed erosion and deposition rates and to establish cal-ibration relationships.In this paper,the Mass Balance Model2(MBM2; Walling and He,1999;Walling et al.,2002)was used to convert the areal activities of137Cs into soil redistribution rates(t ha?1yr?1).

For an eroding site,the change of the137Cs total inventory with time can be represented as:

dA teTdt ?1?Γ

eTI teT?λtP R

d

A teTe1T

where A(t)is the cumulative137Cs activity per unit area(Bq m?2);

R the erosion rate(kg m?2yr?1);t the time since the onset of radio-

nuclide inputs;d the cumulative mass depth representing the average

plough depth(kg m?2);λthe decay constant for137Cs(yr?1);I(t)

annual deposition?ux of137Cs at time t(Bq m?2yr?1);Γthe propor-

tion of the freshly deposited137Cs fallout removed by erosion before

incorporation into the plough layer;and P the particle size correction

factor.

For a depositional point where A(t)>A ref(the local reference

inventory),the corresponding soil deposition rate can be expressed as:

R′?

A ex

t

t0

C d t′

àá

e?λt?t′

eTdt′

e2T

where A ex is the excess137Cs inventory(Bq m?2),de?ned as the mea-

sured total inventory A(t)minus the local direct fallout input A ref,and

C d(t′)(Bq kg?1)the137Cs concentration of deposited sediment.

2.5.Measurements of the ephemeral gullies

In August2011,the ephemeral gullies of the catchment developed

well,and the measurements of the ephemeral gullies were conducted

using the method of Zhang et al.(2007).A portable hand-GPS devise

(Magellan Explorist500)was used in the?eld to determine the spa-

tial distribution of the ephemeral gullies by linking the GPS positions.

The developed ephemeral gullies were named FG1–FG4(Fig.1).At each

of the positions along the ephemeral gullies,one channel width and

three depths were measured.The length of each segment between

two GPS points was measured using a50-m-long surveyor's tape.The

eroded volume of each ephemeral gully was calculated using the

cross-sectional dimensions and the distances between cross-sections.

The soil losses from the ephemeral gullies were then calculated based

on the volume estimations and topsoil bulk density of1250kg m?3.

The major geomorphic characteristics and the calculated soil losses

from the ephemeral gullies are summarized in Table1.

2.6.Calculating soil loss and/or gain for the catchment

ArcGIS9.3software was used with the location data of?eld mea-

surements to create spatial distributions of137Cs inventories(Bq m?2)

and the pattern of soil redistribution(t ha?1yr?1)through the kriging

interpolation method.The erosion and deposition areas of the catch-

ment were extracted from the interpolated soil redistribution map.

The mean rates of soil loss and/or gain in the erosion and deposition

areas were obtained using ArcGIS software algorithms.The soil loss

and sediment gain were calculated through multiplying the mean rates

of erosion and deposition by corresponding soil redistribution areas.

Soil gain per year along the shelterbelts as well as that at the catchment

outlet was also obtained with the same method.

Table1

Main characteristics of the developed ephemeral gullies(Fig.1)formed in summer

(measured in August2011).

EG1EG2EG3EG4Total

Length(m)365345103237

Mean width(m)0.850.93 1.34 1.47 1.23

Mean depth(m)0.080.100.130.090.10

Volume(m3) 2.45 4.937.8413.6328.85

Soil loss(kg)30606161979917,03436,054

Note:Soil loss was estimated based on the volume estimations and a topsoil bulk

density of1250kg m?3.

144H.Fang et al./Geomorphology169–170(2012)142–150

2.7.Topographical factors for the sampling points

Slope gradient and curvature at each sampling point were com-puted by ArcGIS 9.3to study the impacts of topographical factors on soil redistribution rates.Slope gradient was described as a percentage.A positive curvature indicates an upwardly convex surface,while a neg-ative curvature indicates a concave surface,and a value of zero indicates a ?at surface.

3.Results 3.1.

137

Cs inventory in the reference site

The 137Cs inventories for the ?ve cores collected from the reference site are given in Table 2.The mean activity of 137Cs was 2506Bq m ?2with a standard deviation of 402Bq m ?2.The 137Cs inventories were quite close except the R 4sampling point,which represented a percent-age deviation of 31%higher than the mean reference value,whereas the reference value sampled at the R 1point was lower than all the other points with around 14.6%deviation from the mean.These variations result probably from the spatial variability of the radionuclide's inven-tories due to soil heterogeneity,such as soil micro-topography,argilla-ceous level,vegetation density and biological activity as well as by splash or runoff phenomena.Such phenomena have also been observed in other regions (Owens and Walling,1996;Nouira et al.,2003).

The depth distribution of 137Cs areal activity at the R 3point was used to validate the use of the mean value of these data as back-ground value.The 137Cs areal activity pro ?le showed a sharp decrease with increasing depth (Fig.2),and could be ?tted by an exponential function.The distribution was typical of an undisturbed site (Frissel and Pennders,1983).Most of the 137Cs was contained within the top 15cm soil layer,with 95%of the 137Cs accumulated in the upper 14cm and sharp drop in 137Cs areal activity below that depth.

In the black soil region of Northeast China,use of 137Cs technique to examine soil erosion has been conducted and the values of reference inventories have also been reported in several studies.In Jilin Province,Yan and Tang (2004)and Fang et al.(2006a)reported the 137Cs refer-ence inventories of 2464Bq m ?2in Jiutai County,and 2233and

2376Bq m ?2in Dehui County.In Keshan County of Heilongjiang Prov-ince,a reference inventory of 2500Bq m ?2that was quite close to the averaged reference value in the present study was also reported by Wang et al.(2010).Generally,global 137Cs fallout deposition in-creases with increasing latitude in the Northern Hemisphere,though it is also a strong function of the annual precipitation (Zheng and Wang,2002;Fang et al.,2006a ).The reference inventories of 137Cs mentioned above are all located in the south of the study catchment.This means the 137Cs reference value should be larger than the reported values.In conjunction with the 137Cs pro ?le distribution in Fig.2,the established reference 137Cs inventory of 2506Bq m ?2is regarded as reliable.3.2.Distribution of

137

Cs inventories for the catchment

The 137Cs inventory values for the 61bulk soil cores collected from the catchment varied greatly and ranged from 582to 10,420Bq m ?2,with a mean value of 2371Bq m ?2.A majority (68.9%)of the soil sam-ples had lower 137Cs inventory values than the established reference with a median of 1783Bq m ?2and a skewness of 3.3(Table 3),imply-ing that most of the catchment area could suffer from soil erosion.Correspondingly,31.1%of the 137Cs inventories had higher values than the reference with a high standard value of 1086.2Bq m ?2.The spatial pattern of 137Cs inventories indicates that the higher 137Cs inventories mainly appeared along the shelterbelts and at the catchment outlet.However,the lower 137Cs inventories were mainly located be-hind the shelterbelts,especially in the region where the ephemeral gullies occurred (Fig.3).

3.3.Soil redistribution and sediment budget

The soil loss and/or gain rates within the study catchment were computed by using the MBM2with site-speci ?c parameters:γ(propor-tion of the annual 137Cs fallout susceptible to erosion prior to incorpora-tion into the soil pro ?le by tillage)=0.6,H (relaxation mass depth of the initial distribution of fallout 137Cs in the soil pro ?le)=4kg m ?2,d (cumulative mass depth representing the average plough depth)=312kg m ?2(multiplied by tillage depth of 0.25m by soil bulk density of 1250kg m ?3),and p (particle size correction factor de ?ned as the ratio of 137Cs of the mobilized sediment to that of the original soil)=p ′(further particle size correction factor re ?ecting differences in grain size composition between mobilized and deposited sedi-ments)=1.0.The estimates of soil redistribution represent mean annual

Table 2

Measurement inventories for 137

Cs in the reference site (Bq m ?2).

Sampling points

R 1R 2R 3R 4R 5Mean Standard deviation 137

Cs activity

213923882270328124502506

402

A.U.(activity uncertainty)

3.22

7.14

4.70

8.84

6.69

Note:R 1–R 5represent reference points and R 3is the sectioned reference sampling point.

137

Cs Area activity (Bq m -2

)

D e p t h (c m )

Fig.2.Depth distribution of

137

Cs for R 3point.

Table 3

137

Cs inventories for the sampling points.

Total samples

Inventories less than the reference Inventories greater than the reference Minimum (Bq m ?2)582.0582.02557Maximum (Bq m ?2)10,420.0243510,420Mean (Bq m ?2)2371.11731.43785.8Standard deviation (Bq m ?2)

1437.5494.91086.2Median (Bq m ?2)1783

17833220

Skewness

3.3?0.5 3.1Number of samples

61

42

19

145

H.Fang et al./Geomorphology 169–170(2012)142–150

values for the past 50years.The estimated soil redistribution rates for the 61sampling points ranged from ?56.8t ha ?1y ?1(maximum erosion)to 171.4t ha ?1y ?1(minimum erosion),with an average of ?2.2t ha ?1y ?1and a median of ?6.9t ha ?1y ?1(Table 4).The

mean rate for the sampling points that had experienced erosion was ?15.0t ha ?1y ?1,and 26.1t ha ?1y ?1for those with deposition.The soil redistribution of the catchment had the same spatial pattern as that of the 137Cs inventories (Fig.4).Except for topography,the ero-sion and deposition areas were also affected by the shelterbelts and ephemeral gullies in the study catchment.For the sampling points on the eight transects that run through the catchment,the soil erosion/gain rates varied from ?47.5to 171.4t ha ?1yr ?1(Fig.5).Almost all the sampling points on transects A –D in Fig.5showed soil erosion,ex-cept for some depression sites and those located along the shelterbelts with soil gain rates of 8to 16t ha ?1yr ?1.However,in the ephemeral gully areas,all the sampling points presented soil loss with the largest erosion rate of ?47.5t ha ?1yr ?1(Fig.5E –F).Sediment deposition gen-erally occurred at the catchment outlet with soil gain rates of 32.63to 171.41t ha ?1yr ?1(Fig.5G –H).

Spatial integration of the soil redistribution pattern shown in Fig.4indicates a net erosion rate of 236t yr ?1in the erosion area

(i.e.,

Fig.3.Spatial distribution of

137

Cs inventories.

Table 4

Soil loss and/or gain rates for the sampling points.

Gross samples

Samples from areas of erosion Samples from areas of deposition Minimum (t ha ?1yr ?1)?56.8?56.80.9Maximum (t ha ?1yr ?1)171.4?1171.4Mean (t ha ?1yr ?1)?2.2?15.026.1Standard deviation (t ha ?1yr ?1)

30.712.939.3Median (t ha ?1yr ?1)?6.9?11.713.2Skewness

3.3?1.4 3.2Number of samples

61

42

19

https://www.sodocs.net/doc/a29466440.html,puted spatial patterns of soil redistribution rates.

146H.Fang et al./Geomorphology 169–170(2012)142–150

14.5t ha ?1yr ?1)and a net soil gain rate of 111t yr ?1in the deposi-tion area;hence a net soil loss of 125t yr ?1for the study catchment (Table 5).Using these data,the sediment delivery ratio (SDR )for the catchment is calculated to be 53%(Table 5).This means that a large part of the eroded soil was re-deposited within the catchment.

3.4.Slope,curvature and soil redistribution

The computed slopes at the sampling points ranged from 0.5%to 8.4%,and the curvatures ranged from ?0.5to 0.5m ?1.No signi ?-cant correlations are found between soil erosion and slope gradient

A l t i t u d e (m )

S o i l e r o s i o n r a t e s (t h a -1

y r -1

)

A l t i t u d e (m )

S o i l e r o s i o n r a t e (t h a -1

y r -1

)

A l t i t u d e (m )

S o i l e r o s i o n r a t e (t h a -1

y r -1

)

A l t i t u d e (m )

Sampling points on the transects

S o i l e r o s i o n r a t e (t h a -1

y r -1

)

Sampling points on the transects

Fig.5.Altitude above sea level and soil erosion rates (negative values indicate soil loss and positive values soil gain)for the sampling points along the transects A –H (Fig.1).(A)–(H)at the upper left of the graphs correspond to the transects A –H respectively.

147

H.Fang et al./Geomorphology 169–170(2012)142–150

(r 2=0.05,p =0.07)and between soil erosion and curvature (r 2=0.01,p =0.50;Fig.6),suggesting that the impacts of slope gradient and curva-ture on soil redistribution rates were very limited.4.Discussion

The reliability of 137Cs technique for estimating soil erosion is of great importance for soil loss control and land management.The rates of erosion estimated from the 137Cs measurements on the cores from the study catchment were compared with data from the runoff plot measurements.The plots were established in a catchment ca.8km away from the study catchment.

The estimated mean erosion rates of the runoff plots during 2003–2004range from 8.1to 28.2t ha ?1yr ?1(Liu et al.,2008).The 137Cs-method predicted a mean erosion rate of the same order of magnitude (14.5t ha ?1yr ?1).The measured sediment losses rep-resent net export of soil only from the plots,because the plots are iso-lated from the surrounding areas.In addition,they are valid only for the measurement period although rainfall erosion capacity during 2003–2004seems to be comparable to the mean rainfall erosion ca-pacity in the study catchment (Liu et al.,2008).

The soil redistribution rates from the 137Cs method represent time-integrated,medium-term averages for the last 50years,and are less in ?uenced by extreme events (Schuller et al.,2003).Therefore,they were compared with soil redistribution rates estimated from a pedolog-ical investigation by Cui (2007),which also re ?ected the cumulative effect of past processes.Both rates are at the same order of magnitude.Therefore,the soil redistribution rates from the 137Cs measurements seem to be reliable.

Soil redistribution rates greatly depend on the position of sam-pling points (Nearing et al.,2005).In Songhuajiang Town of the black soil region,Fang et al.(2006b)found that the topsoil of upper slope positions was eroded and produced soil was accumulated in lower slope positions.The same pattern was also found in Bajiamiao Village of Dehui City,Jilin Province (Yan and Tang,2005).In contrast,the erosion areas in the studied catchment were mainly located 0to 100m away from the shelterbelts,especially where ephemeral gullies occur.A large portion of eroded soils were deposited along the shel-terbelts and at the catchment outlet (Figs.4and 5).The spatial pattern of soil redistribution led to a lower SDR value (Table 5).By contrast,in the Lucky Hills catchment of the Walnut Gulch Experimental Catchment (WGEW),southeast Arizona,SDR estimated from 137Cs data was 89%(Nearing et al.,2005).Annual rainfall in the WGEW is around 300mm,which is less than that in our catchment,whereas slopes are much steeper in the WGEW,which may explain the higher SDR value.

Topographic factors such as slope gradient and curvature are usually regarded as main factors controlling soil redistribution (e.g.,Sadiki et al.,2007;Porto et al.,2011).However,our study,as well as Fulajtar (2003),Bujan et al.(2003)and Afshar et al.(2010)for Slovakia,Argentina and Iran suggested weak relations of soil redistribution rates with slope gra-dient and curvature.In our study area,soil redistribution may be more in ?uenced by slope length (Cui et al.,2007)as well as disturbance by the shelterbelts and ephemeral gullies (Figs.4and 5).Although the on-site impacts of slope gradient and curvature may be limited,the off-site impacts linked to sediment transfer may still be important,as suggested from the SDR values for our study catchment and the WGEW.Moreover,gentle and long slopes characterize the study catch-ment,and a large upslope drainage area is usually required for ephem-eral gully development in the black soil region.

The shelterbelts in the study catchment that split the long slopes into smaller ones were established in the 1970s (Zhu,1985).When soil erosion occurs,the sediment eroded from upslope is intercepted by the shelterbelts,and some sediment also accumulates behind the shelterbelts due to reducing soil erosion capacity when runoff passes across the shelterbelts (Fig.4;Cui et al.,2007).For the upper shelter-belt,the deposition areas in front of and behind the shelterbelt were 4.3and 1.0ha,with deposition rates of 6.8and 4.7t ha ?1yr ?1,re-spectively.For the lower shelterbelt,the deposition areas in front of and behind the shelterbelt were 3.6and 2.5ha,with deposition rates of 7.6and 8.1t ha ?1yr ?1,respectively.Annually,ca.34and 44t yr ?1sediments were deposited along the upper and lower shel-terbelts,respectively (Table 6).The estimate derived from the 137Cs measurements relates to the last 50years because the main period of 137Cs fallout was the early 1960s,whereas the shelterbelts were established later.Thus,the sediment deposition rates along the shel-terbelts should be much higher than those estimated from the 137Cs measurements.

The gullies developed in spring due to snowmelt runoff are elimi-nated by tillage activity when seeds are sowed,and reoccur at the same locations in the following rainy season (Casali et al.,2006;Meng and Li,2009).Although we did not measure soil losses from the ephem-eral gullies in spring,they can be very similar to those in the rainy sea-son because of similar gully sizes (Zhang et al.,2007).Therefore,annual soil loss from the ephemeral gullies was around 72t yr ?1(Table 1),with a gross erosion rate of 2.5t ha ?1yr ?1(i.e.,0.25kg m ?2yr ?1)for the whole catchment.The estimated erosion rate is comparable to those reported for other gully areas:0.045to 0.47kg m ?2yr ?1for 19

Table 5

Gross sediment budgets using the interpolated soil redistribution map.Locations Soil redistribution (t ha ?1yr ?1)Soil loss (t yr ?1)Sediment delivery ratio (%)Erosion area ?14.5

236.4–Deposition area 9.1111.0–All

?4.4

125.4

53

S o i l l o s s /g a i n r a t e s (t h a -1 y r -1

)

Slope (%)

S o i l l o s s /g a i n r a t e s (t h a -1 y r -1

)

Curvature (m -1

)

Fig.6.Relationships of soil loss/gain rates with (A)slope gradient and (B)slope curvature.

148H.Fang et al./Geomorphology 169–170(2012)142–150

US states (Casali et al.,2000),0.50kg m ?2yr ?1in the loess region of central Belgium (Poesen et al.,1996),and 0.1to 0.68kg m ?2yr ?1in southern Portugal (Vandaele and Poesen,1995).

It should be noted that soil losses estimated from only one mea-surement may not be comparable to average soil losses over the past 50years.However,the above estimates can be used to evaluate the contribution of gully development to the total soil loss.

To assess the main erosion and deposition processes in the study catchment,a schematic sediment budget was produced from Tables 1,5and 6.The result gives a total erosion of 236t yr ?1,a soil loss from the ephemeral gullies of 72t yr ?1(31%of the total soil erosion)and hence a net soil loss from the slopes of 164t yr ?1(69%).The sediment deposition rates along the upper and lower shelterbelts were 33t yr ?1(14%of the total deposition)and 44t yr ?1(19%),respectively.A sedi-ment deposition rate at the catchment outlet was 33t yr ?1(14%).Hence,a net soil loss from the catchment was ca.125t yr ?1(Fig.7).

Although most eroded soil was deposited within the studied catchment and SDR was signi ?cantly lower than that in the Loess Plateau (Fang et al.,2008)and other regions (e.g.,Nearing et al.,2005),the obtained mean erosion rate in the whole erosion area (15.0t ha ?1yr ?1)was much higher than the tolerable value (5.5t ha ?1yr ?1;Fan et al.,2006).The black soil on the slopes in the catchment was shallow (usually 20–30cm depth),and once topsoil was eroded below a certain thickness,it becomes unsuitable for agriculture and hard to be recovered.Therefore,effective soil conservation measures are required to reduce soil loss for sustain-able land use.

5.Conclusions

The 137Cs technique was used to assess soil erosion and deposition characteristics for a 28.5ha catchment in the typical black soil region

of Northeast China.The soil redistribution rates were estimated by using the MBM2for the 61sampling points,and were veri ?ed by means of both runoff plot data and pedological investigation.

Based on the 137Cs data and the resultant estimates of soil loss/gain rates,the spatial distributions of 137Cs inventories and soil redistribu-tion were mapped using the kriging interpolation.The gross soil loss for the whole catchment was ?4.4t ha ?1yr ?1.The soil redistribution was highly in ?uenced by the shelterbelts and the development of the ephemeral gullies,whereas the impacts of slope gradient and curvature were insigni ?cant at the 0.05level.Soil erosion mainly occurred behind the shelterbelts and in areas with ephemeral gullies,while deposition occurred along the shelterbelts and at the catchment outlet.The soil losses from the slopes and the ephemeral gullies were 164t yr ?1(69%of the total soil loss)and 72t yr ?1(31%),respectively,and sedi-ment gains along the shelterbelts and at the catchment outlet were 78and 33t yr ?1.Although only 53%of the eroded soil can be exported out of the catchment at the 50-year time-span,the net soil erosion of 14.5t ha ?1yr ?1in the erosional areas was much higher than the toler-able value.Effective soil conservation measures are required to reduce the severe soil erosion and realize a sustainable land-use management.

Acknowledgments

This work was ?nancially supported by the projects of the Nation-al Natural Science Foundation of China (grant numbers 40901133,41101261,41001165and 30901163),the Open Fund of Key Laborato-ry of Mollisols Agroecology,Northeast Institute of Geography and Agroecology,Chinese Academy of Sciences (CAS:grant number 2011ZKHT-15),and the Special Foundation of the CAS President.The authors are grateful to Dr.Kirstie Fryirs,Dr.Tongxin Zhu,Editor Takashi Oguchi,Master Kingsley Kyere-Donkor,Dr.Junyu Qi,and Dr.Zhenghong Tang for their constructive comments that improved this paper.

Table 6

Area and sediment gain along the shelterbelts based on the interpolated soil redistribution map.Upper shelterbelt Lower shelterbelt In front of shelterbelt Behind shelterbelt

In front of shelterbelt

Behind shelterbelt

Area (ha)Rate (t ha ?1

yr

?1

)Area (ha)Rate (t ha ?1

yr

?1

)Area (ha)Rate (t ha ?1

yr

?1

)Area (ha)Rate (t ha ?1yr ?1)4.3

6.8

1.0

4.7

3.2

7.6

2.5

8.1

Net slope erosion 164 t y -1

Ephemeral gully erosion 72 t y -1

Along the upper shelterbelt 34 t y -1

Along the lower shelterbelt 44 t y -1

At the catchment outlet 33 t y -1

Output 125 t y -1

Soil erosion

Sediment deposition Fig.7.Sediment budget including the main erosion and deposition processes based on values in Tables 1,5and 6.The soil loss from the slopes is estimated from the total soil loss in the catchment and the soil loss from the ephemeral gullies.

149

H.Fang et al./Geomorphology 169–170(2012)142–150

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新手魔方公式图解

b新魔方新手教程 前言 我们常见的魔方是3x3x3的三阶魔方,英文名Rubik's cube。是一个正6 面体,有6种颜色,由26块组成,有8个角块;12个棱块;6个中心块(和中心轴支架相连)见下图: (图1) 学习魔方首先就要搞清它的以上结构,知道角块只能和角块换位,棱块只能和棱块换位,中心块不能移动。 魔方的标准色: 国际魔方标准色为:上黄-下白,前蓝-后绿,左橙-右红。(见图2)注:(这里以白色为底面,因为以后的教程都将以白色为底面,为了方便教学,请都统一以白色为准)。 (图2)

认识公式 (图3)(图4)公式说明:实际上就是以上下左右前后的英文的单词的头一个大写字母表示 (图5)

(图6) (图7)

(图8) 步骤一、完成一层 首先要做的是区分一层和一面:很多初学者对于“一面”与“一层”缺乏清楚的认识,所以在这里特别解释一下。所谓一层,就是在完成一面(如图2的白色面)的基础上,白色面的四条边,每条边的侧面只有一种颜色,图(2). 如图(1)中心块是蓝色,则它所在面的角和棱全都是蓝色,是图(2)的反方向 图(3)和(4)则是仅仅是一面的状态,而不是一层! (1)(2) (3)(4) 注:图(2)和(4)分别是图(1)和(3)的底面状态 想完成魔方,基础是最重要的,就像建筑一样,魔方也如此,基础是最重要的。 由于上文提到过中心块的固定性,这一性质,在魔方上实质起着定位的作用,简单的说就是中心块的颜色就代表它所在的面的颜色。 一、十字(就是快速法中的CROSS) 第一种情况如图所示:

公式为R2 第二种情况如图所示: (白色下面颜色为橙色,为方便观察,特意翻出颜色) 橙白块要移到上右的位置,现在橙白块在目标位置的下面。但其橙色片没有和橙色的中心块贴在 一起。为此我们先做D’ F’ 即把橙色粘在一起,接着 R 还原到顶层,, F 是把蓝白橙还原到正确的位置(上面的F’ 使蓝白块向左移了九十度)。 公式为D’ F’ R F 图解: 当然,架十字不只只有上面两种情况,现我们在分析下其它的一些情况吧! 如下图: 橙白块的位置己对好,但颜色反了,我就先做R2化成第二种情况,然后用还原第二种情况的公式即可! (橙色下面颜色为白色,为方便观察,特意翻出颜色)

雨水对土地的侵蚀教案

雨水对土地的侵蚀教案 科学概念: 雨水和径流会把地表的泥土带走,使土地受到侵蚀。 侵蚀使地表的地形地貌发生改变。 过程与方法: 通过模拟实验来探究雨水对土地的侵蚀。 用文字、图画、符号记录实验结果,用口头和书面语言描述实验中的现象。 对实验结果做出自己的解释,在小组内交流结果和想法。 设计模拟实验,探究影响土壤被侵蚀程度的因素。 情感、态度、价值观: 关注自然界的侵蚀现象。

分组器材:湿润、混有少量沙石的土、一侧有孔的长方形塑料水槽、报纸、塑料薄膜、小铲子、降雨器(饮料瓶,瓶盖上扎孔)、水。 教师演示:雨水侵蚀土地的图片或录象、介绍实验操作的课件。 一、引入 1、下雨是一种经常发生的天气现象。下雨时,雨水降落到土地上。雨水会不会对土地产生影响?土地会发生什么变化?雨水会发生什么变化? 2、学生讨论交流。 3、好!今天这节课,我们就来探究一下这些问题。 二、雨如何影响土地 1、我们先来看两幅图。(出示雨水侵蚀土地的图片) 你能说说你看到的景象吗?

你平时看到过类似的景象吗? 你认为出现这样的景象的原因是什么呢? 2、模拟实验:下雨。 学生观察实验装置,明白实验器材所代表的含义。 教师课件出示介绍实验的基本操作方法,学生观看学习。 学生分组实验。要求:实验前仔细观察斜坡上的土地的形态;实验时注意观察“降雨”过程中的土地,以及“雨水”和“径流”的情况,并做记录;实验后描述实验中的现象并根据实验结果进行分析。 小组交流、汇报。 3、小结、拓展。 雨水会不会影响土地? 教师出示图片,学生观察并思考:斜坡上的这许多小细沟是怎样来的,这许多小细沟中汇集的水流又会怎样改变地形。

五年级科学上册《减少对土地的侵蚀》教学设计 教科版

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(图5) (图6)

(图7) (图8)步骤一、完成一层

首先要做的是区分一层和一面:很多初学者对于“一面”与“一层”缺乏清楚的认识,所以在这里特别解释一下。所谓一层,就是在完成一面(如图2的白色面)的基础上,白色面的四条边,每条边的侧面只有一种颜色,图(2). 如图(1)中心块是蓝色,则它所在面的角和棱全都是蓝色,是图(2)的反方向 图(3)和(4)则是仅仅是一面的状态,而不是一层 ! (1)(2) (3)(4) 注:图(2)和(4)分别是图(1)和(3)的底面状态 想完成魔方,基础是最重要的,就像建筑一样,魔方也如此,基础是最重要的。 由于上文提到过中心块的固定性,这一性质,在魔方上实质起着定位的作用,简单的说就是中心块的颜色就代表它所在的面的颜色。 一、十字(就是快速法中的CROSS) 第一种情况如图所示: 公式为R2 第二种情况如图所示: (白色下面颜色为橙色,为方便观察,特意翻出颜色) 橙白块要移到上右的位置,现在橙白块在目标位置的下面。但其橙色片没有和橙色的中心块贴在 (橙色下面颜色为白色,为方便观察,特意翻出颜色)

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0 《雨水对土地的侵蚀》说课稿 黄甫学校赵佳 一、教材分析 《雨水对土地的侵蚀》是教科版小学科学五上《地球表面及其变化》单元的第5课。《地球表面及其变化》单元教材,主要指导学生认识地球表面常见的地形及其变化和原因。属于小学科学课程标准中的“地球与宇宙”中“地表及变化”的范畴。外力作用引起的地形地貌变化是本单元的重点内容,特别是流水对土地的侵蚀和沉积作用而引起的地形地貌变化。流水对土地的侵蚀作用先从学生熟悉的降雨开始。《雨水对土地的侵蚀》便从学生经常看到的下雨现象开始,让学生模拟下雨,仔细观察雨水降落到地面,土地及雨水发生的变化,这是认识侵蚀的开始。本课重点指导学生认识流水对土地的侵蚀。教材分两部分。 第一部分:雨如何影响土地。 首先通过提出问题了解学生的想法。“雨水会不会影响土壤?土地会发生什么变化?流过土地的雨水变成什么样了?”学生常看到降雨,但是很多人可能从来没有想到它会侵蚀土地,会改变地形。这个活动的目的是引发学生的回忆,启发学生联系平常的生活经验谈谈自己的看法。学生可能会想到雨水会冲走土壤,使土壤流失,混有泥沙的水在蒸发后,里面的泥沙会留下来。 接着指导学生通过“下雨”的模拟实验,及对“雨水”降落时土地和径流的观察,认识流水对土壤的侵蚀作用。

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教科版科学五上《减少对土地的侵蚀》参考教案

教科版科学五上《减少对土地的侵蚀》参考教案教学目标 科学概念: 1.各种自然力量在重塑地表形状的过程中,也会阻碍人类的生产生活。 2.人类自身的活动也在改变着地表的形状,我们要尽量减少土地遭受腐蚀。 过程与方法: 1.通过模拟实验,检验自己的家园能否经住暴雨的侵袭。 2.对实验结果进行反思,并寻求改进的方法。 情感、态度、价值观: 1.认识到各种力量在重塑和改变地表的地势地貌时,也给人们的生产和生活带来正面或负面的阻碍。 2.认同人类活动要尊重自然规律,减少因自身活动带来负面阻碍的观点。 3.认识到土地对生命,以及人类生产生活的重要意义。 4.关注人类为防止水土流失,爱护土地不被破坏而采取的各种措施。 教学重点 通过模拟实验,检验自己的家园能否经住暴雨的侵袭。 教学难点 对实验结果进行反思,并寻求改进的方法。 教学预备 分组器材:长方形塑料水槽(或大盘子)、土、报纸、塑料薄膜、小铲子、降雨器、水、接水容器、建筑房屋的材料等。 教师演示:防治水土流失的图片、视频或课件。 教学过程 一、引入 1.流水、风、冰川、波浪和重力等都会腐蚀土地。腐蚀使某些地点的土壤流失,改变了地势地貌,同时对人类的生产生活也产生了极大的阻碍。为

了爱护自己的家园,人们想方法尽可能地减少腐蚀。出示预防水土流失的图片、视频或课件。 2.今天我们就运用我们在那个单元所学的知识,在一个大水槽里建筑一个有山地有平原的地势,然后选择地势中的一个地点建筑房屋。假如暴雨即今后临,我们的家园能经得住风雨的突击而不被破坏吗?我们就以小组为单位来比一比吧。 二、设计和建筑我们的家园 1.我们建筑什么样的地势,用什么材料?选择什么位置建筑房屋?小组讨论并画出设计图。 2.全班交流展现设计图,并描述什么缘故如此建筑地势,说明什么缘故如此选择家园的位置,以及选择建筑地势及房屋所需要的材料的理由。 3.评判、小结。 4.推测暴雨过后,我们的家园会如何样? 三、暴雨对我们的家园有什么阻碍 1.摸索:在竞赛实验中,每组的什么条件应该保持相同,什么缘故? 2.学生进行竞赛实验:用喷水器装水模拟暴雨降临,观看自己的家园能否经得住暴雨的突击。 3.交流讨论并评判竞赛实验结果。 4.反思:假如重新设计建筑自己的家园,我们会对地势做哪些改进?我们仍将房子建筑在最初选择的地点吗?什么缘故?我们将房子建筑在哪里? 四、总结、拓展延伸 1.我们差不多明白了哪些力量会改变地球表面的地势地貌? 2.地球的各种力量包括人类自身在改变地表的地势地貌时,会对我们造成哪些灾难? 3.调查一下,人们采取了哪些措施来防止这些灾难的发生,以及尽量减少灾难造成的危害。

三阶魔方公式口诀图解简版

步骤一、完成一层 (一)完成第一层十字 第一种情况如图所示:公式为R2 第二种情况如图所示:公式为D’F’R F 其它的一些情况 先做R2化成第二种情况,然后用第二种情况公式 前右要移到上后。R’D’ R D2。两种情况分别化为上面第一、二种情况。 如果刚开始时橙白块也还没对好,直接R’ D移到后下! (二)完成第一层角块 依然把十字放在顶层,还原角块时,我们首先在底层找有没有我们要还原的角,没有的话再到顶层去找!基本的两种情况为: 公式:D’R’ D R公式:R’D’ R 白色在底面!先做R’ D或D2 R就是上面第二种情况了! 最后还有两种情况,角块的位置已经对好,但颜色没对好,如下图::先做R’ D R化成第一种情况。 :先做R’D’R D化成第二种情况即可! 步骤二、完成第二层 开始还原第二层,把魔方倒过来,把做好的第一层放到底层 首先在顶层找有哪些块是可还原的。在顶层见到没有黄色的棱块均是要还原到第二层的。只有两种情况而已。 y’ R U R U R U' R' U' R' U’五顺五逆 R' U' R' U' R' U R U R U 五逆五顺 还有的情况就是位置正确但颜色没对好,或者已经在第二层但位置不对。先从顶层随便找个块“还原”到前右的位置 步骤三、完成顶层 (一)顶层十字 完成第二层后顶层会有以下三种情况:

针对上述三种情况,我们只需记住一个公式即:MUMUMUUM’UM’UM’UU:上顺上顺上顺顺下顺下顺下顺顺 我们最终的目的是使得顶面变成这样,如果你的魔方顶面已经是这样了,那这一步就可以直接跳过 状态1公式 状态2RB—公式—B’R’ 状态3公式整体转动魔方公式 (二)顶层平面 顶层拧完十字以后,只需学习以下两种左右公式: 第一种公式(左手公式): 经过公式L’U’LU’L’ U2L(上逆下逆上顺顺下)就变为第二种公式(右手公式): 经过公式RUR’URU2R’(上顺下顺上顺顺下)就变为7种情况 第一种:左手公式 第二种:右手公式 第三种:左手公式左手公式

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新魔方新手教程 前言 我们常见的魔方是3x3x3的三阶魔方,英文名Rubik's cube。是一个正6 面体,有6种颜色,由26块组成,有8个角块;12个棱块;6个中心块(和中心轴支架相连)见下图: (图1) 学习魔方首先就要搞清它的以上结构,知道角块只能和角块换位,棱块只能和棱块换位,中心块不能移动。 魔方的标准色: 国际魔方标准色为:上黄-下白,前蓝-后绿,左橙-右红。 (见图2)注:(这里以白色为底面,因为以后的教程都将以白色为底面, 为了方便教学,请都统一以白色为准)。 (图2)

认识公式 (图3)(图4)公式说明:实际上就是以上下左右前后的英文的单词的头一个大写字母表示 (图5)

(图6) (图7)

(图8) 步骤一、完成一层 首先要做的是区分一层和一面:很多初学者对于“一面”与“一层”缺乏清楚的认识,所以在这里特别解释一下。所谓一层,就是在完成一面(如图2的白色面)的基础上,白色面的四条边,每条边的侧面只有一种颜色,图(2). 如图(1)中心块是蓝色,则它所在面的角和棱全都是蓝色,是图(2)的反方向 图(3)和(4)则是仅仅是一面的状态,而不是一层! (1)(2) (3)(4) 注:图(2)和(4)分别是图(1)和(3)的底面状态 想完成魔方,基础是最重要的,就像建筑一样,魔方也如此,基础是最重要的。 由于上文提到过中心块的固定性,这一性质,在魔方上实质起着定位的作用,简单的说就是中心块的颜色就代表它所在的面的颜色。 一、十字(就是快速法中的CROSS)

第一种情况如图所示: (橙色下面颜色为白色,为方便观察,特意翻出颜色) 公式为R2 第二种情况如图所示: (白色下面颜色为橙色,为方便观察,特意翻出颜色) 橙白块要移到上右的位置,现在橙白块在目标位置的下面。但其橙色片没有和橙色的中心块贴在一起。为此我们先做D’F’即把橙色粘在一起,接着 R 还原到顶层,,F 是把蓝白橙还原到正确的位置(上面的F’使蓝白块向左移了九十度)。 公式为D’F’R F 图解: 当然,架十字不只只有上面两种情况,现我们在分析下其它的一些情况吧! 如下图: 橙白块的位置己对好,但颜色反了,我就先做R2化成第二种情况,然后用还原第二种情况的公式即可!

小学五年级上册科学 雨水对土地的侵蚀

《雨水对土地的侵蚀》 教学目标 科学概念 雨水和径流会把地表的泥土带走,使土地受到侵蚀。侵蚀使地表的地形地貌发生改变。 过程与方法 通过模拟实验来探究雨水对土地的侵蚀。用文字、图画、符号记录实验结果,用口头和书面语言描述实验中的现象。对实验结果做出自己的解释,在小组内交流结果和想法。设计模拟实验,探究影响土壤被侵蚀程度的因素。 情感、态度、价值观 关注自然界的侵蚀现象。在实验中培养合作、交流的探究精神。 教学重、难点 本课教学的重难点是“认识雨水对土地的侵蚀作用”,通过模拟实验来探究雨水对土地的侵蚀;在模拟实验的基础上,进一步设计自然界中其他因素对土地的侵蚀。 教学准备

湿润、混有少量沙石的土、一侧有孔的长方形塑料水槽、报纸、塑料薄膜、小铲子、饮料瓶,扎孔的瓶盖、水等。 教学过程 一、情境导入,初步感受,雨水对土壤的侵蚀作用。 1.谈话交流对土壤的认识 师:通过前面的学习,我们都知道土壤是地球上最有价值的资源,你们平时在哪些地方见到过土壤? 生1:爬山的时候在山上见到过土壤。 生2:公园的草地上,草的下面就是土壤。 师:讲的非常正确 生3:在种菜的田地里。 师:哦,那里存在也有是吧,你来说。 生4:在任何有植物的地方都有土壤。 师:这么说土壤跟植物紧密地联系在一起,是吧!就像同学们说得一样。 2.讨论下雨对土地的影响 师:我们的地球表面有各种地形,它们高低起伏,各不相同,但在这些地形之上都覆盖着一层厚厚的土壤。这个土壤覆盖在土地上,这就是平时所熟悉的土地。下面是一种我们非常常见的自然现象—下雨。下雨时候雨点就会洒落在土地之上,那么,土地会发生什么样的变化。(出示课件) 生1:土地变得很潮湿。 师:这是你想到的。

三阶魔方教程

三阶魔方教程

三阶魔方教程第一步:对好魔方白色面十字架 对于没接触过魔方的朋友,当面对着一个没有还原六面的魔方的时候也许会有点凌乱,这个我们不用着急,只要一点小小方法就可以排除这个困难了。下面这个图,就是我们要还原三阶魔方第一步最后对好了的十字架。 怎样对好上面这个图呢?魔方有6个面,我们从哪个面开始还原呢?所以我们就需要有一个参考点。魔方还原的方式有很多,也许每种还原方式都有不同还原方法,但其中原理是一样的。我们这个简单教程,就以白色中心块为参考点。 1、首先,我们魔方的握法:以上下两面中心块的颜色为参考点,白色中心块在下,黄色中心块在上。也就是说,我们握住魔方的方向就是等于,把魔方水平放在桌面,黄色中心块朝上白色中心块在底的握法。其他面颜色,左右前后这四面中心块朝哪边都无所谓。 2、为了对成我们第一步最后的样式,我们需要先对好如下图一样:以白色棱块和黄色中心块组成的一个小黄花平面。 3、为了要对好这个小黄花,我们先将魔方白色中心块朝下,黄色中心块朝上。其实对好这个小花很简单,只要把白色棱块调整到中间层就可以上去了。如果你想调的位置上面已经有白色棱块了,那么就水平方向转一转整个顶层,让开一个空位再调整就可以了。 举个例子:比如下面的图,我们要把白色棱块A替换到红色棱快B的时候,只要转2下就可以了。

前面层向右转一下右边层向后翻一下 完成。 4、对好小花朵后,接着很简单了。我们从魔方基本介绍中了解到:棱块是两面颜色的。我们刚刚调整的是棱块中的白色面,现在大家转动一下顶层可以发现(也就是刚才上面说的:由黄色中心块和白色棱块组成的小黄花),4个白色棱块的另一面颜色肯定能在中间层中心块找到同一个颜色,只是位置顺序不同而已。 我们现在要做的就是旋转顶层,找到与白色棱块另一面颜色一样的中心快,然后把前面层翻转180度,也就是把白色棱块A转到下面去,看下面的图:旋转顶层,注意:必须是转动顶层去找中心块,不能转动中间层去找顶层,否则永远也找不到正确位置的。找到白色棱块另一面颜色相同蓝色中心块,再把这一层翻转180度。 5、用同样的方法,将其他3个白色棱快都翻转下去,再把魔方倒过来拿,黄色中心块朝下,我们可以看到,我们已经完成第一步了,用小花朵对好了魔方白色面十字架,就像下面右边图片一样。

雨水对土地的侵蚀(研究课)

《雨水对土地的侵蚀》 研究课教案:参考饶正辉老师的教学设计 【教材分析】 《雨水对土地的侵蚀》是教科版小学科学五上《地球表面及其变化》单元的第5课。《地球表面及其变化》单元教材,主要指导学生认识地球表面常见的地形及其变化和原因。属于小学科学课程标准中的“地球与宇宙”中“地表及变化”的范畴。外力作用引起的地形地貌变化是本单元的重点内容,特别是流水对土地的侵蚀和沉积作用而引起的地形地貌变化。流水对土地的侵蚀作用先从学生熟悉的降雨开始。《雨水对土地的侵蚀》便从学生经常看到的下雨现象开始,让学生模拟下雨,仔细观察雨水降落到地面,土地及雨水发生的变化,这是认识侵蚀的开始。本课重点指导学生认识流水对土地的侵蚀。教材分两部分组成:第一部分,雨如何影响土地;第二部分,影响侵蚀的因素。教材这两部分的内容有紧密的逻辑关系,学生通过第一部分观察的现象是第二部分研究的地点。 【学生分析】 学生常看到降雨,但是很多人没有微观地、系统地考虑降雨对侵蚀土地、改变地形的作用。本课的开始,通过调动学生原有经验,学生可能会想到雨水会冲走土壤,使土壤流失,混有泥沙的水在蒸发后,里面的泥沙会留下来。学生缺乏的是降雨对土地侵蚀过程的感性经验。学生在四年级学习《天气》单元中测量降水量时,模拟过下雨,这对于本课的学习奠定了良好的技能基础。学习这一课不仅老师要从情感的角度思考:为什么要上这一课,更应该引导学生思考上这一课的意义。 【教学目标】 科学概念:雨水和径流会把地表的泥土带走,使土地受到侵蚀,从而使地表的地形地貌发生改变。 过程与方法: 1.通过模拟实验来探究雨水对土地的侵蚀,在实验中能用文字、图画、符号记录实验结果,用口头和书面语言描述实验中的现象; 2.根据对实验现象作出自己的解释,并依此推想影响土壤被侵蚀程度的因素; 3.设计模拟实验,对比研究影响土壤被侵蚀程度的因素。 情感态度价值观:关注自然界的侵蚀现象,进一步激发研究地形变化的兴趣。 【重难点】通过模拟实验来探究雨水对土地的侵蚀,认识雨水对土地的侵蚀作用。

三阶魔方公式图解、教程

三阶魔方公式、魔方图解、魔方教程,从零基础到精通! 魔方还原法Rubic's Cube Solution ————先看理论“ 魔方的还原方法很多 在这里向大家介绍一种比较简单的魔方六面还原方法。这种方法熟练之后可以在大约30秒之内将魔方的六面还原。 在介绍还原法之前,首先说明一下魔方移动的记法。魔方状态图中标有字母“F”的为前面,图后所记载的操作都以这个前面为基准。各个面用以下字母表示: F:前面 U:上面 D:下面 L:左面 R:右面 H:水平方向的中间层 V:垂直方向的中间层 魔方操作步骤中,单独写一个字母表示将该面顺时针旋转90度,字母后加一个减号表示将该面逆时针旋转90度,字母后加一个数字2表示将该面旋转180度。H的情况下,由上向下看来决定顺逆时针方向;V的情况下,由右向左看来决定顺逆时针方向。例如 U:将上层顺时针旋转90度 L-:将左面逆时针旋转90度 H2:将水平中间层旋转180度 目录 上层四角还原 下层四角还原 上下层八角还原 上下层边块还原 中层边块还原 上层四角还原 首先我们用最简单的几步使得上层的三个角块归位,暂不必考虑四周的色向位置)。还有一个角块存在五种情况,归位方法如下。 L D L- F- D- F D L2 D- L2 F L D- L- L- F- D F

下层四角还原 上层四角归位后,将上层放在下面位置上,作为下层。然后看上层和四周的颜色和图案排列,按照以下的操作使上层四个角块一次归位。共存在七种情况。 R2 U2 R- U2 R2 R- U- F- U F U- F- U F R R U R- U R U2 R- L- U- L U- L- U2 L R- U- F- U F R R U R- U- F- U- F R U- R- U- F- U F 上下层八角还原 要是上层和下层八个角块色向位置全部相同,存在下面五种情况: 当上下二层八个角块色向位置都不对时:按照(1)旋转。 当下层四个角块色向位置不对,上层相邻两个角块色相位置对时:将上层色向位置相同的两个角块放在后面位置上,按照(2)旋转。 当下层四个角块色向位置对,上层相邻两个角块色相位置也对时:将上层色向位置相同的两个角块放在前面位置上,按照(2)旋转后即变成第一种情况。 当下层四个角块色向位置对,上层四个角块色向位置不对时:按照(2)旋转后即变成第二种情况。 当下层相邻两个角块色向位置对,上层相邻两个角块色向位置也对时:将下层色向位置相同的两个角块放在右面位置上,上层色相位置相同的两个角块放在前面位置上,按照(2)旋转之后即变成第二种情况。 (1) R2 F2 R2 (2) R- D F- D2 F D- R 上下层边块还原 按照下图所示操作方法将上下层的边块归位。在上层边块归位时,要注意四周的色向位置。留下一个边块不必马上归位,留作下层边块归位时调整使用。 上层三个边块归位之后,将该层放在下面位置上作为下层,然后将上层的四个边块归位。操作时,为了不破坏下层已经归位的边块,必须将下层留下的一个未归位的边块垂直对着上层要归位的边块的位置。 R- H- R R H R- F H- F- V- D2 V F H- F2 H2 F

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