Yuming ZHAO , Hongtao JIANG , Shijie WANG

1. College of Resource and Environment Science, Pingdingshan University, Pingdingshan 467000, China;

2. State Key Laboratory of Earth Surface Processes and Resource Ecology, School of Geography, Beijing Normal University,Beijing 100875, China;

3. College of Geographical Science, Inner Mongolia Normal University, Hohhot 010020, China

In previous studies on soil and water conservation benefits of ridge tillage, contour ridge tillage has been paid more attention to. In the 1940s,Smith and Whitt[12-13]studied contour P factor, and the results show that when the slope gradient is zero, current direction is uncertain, so contour P factor is 1.0;contour P factor is also equal to 1.0 as the slope gradient is above 25%; when the slope gradient is 2%,the factor is 0.6[14]; as the slope gradient is 7%,the factor is 0.5[15].In recent years, slope ridge tillage (there is an obvious acute angle between ridge direction and contour) has been paid more attention to gradually. Takken et al.[16-19]suggest that it is very important to establish an accurate model for runoff prediction, and calculation of runoff direction only according to landform principle may lead to a big difference between the predicted and measured values,so that there is a big difference between the predicted erosion mode and observed erosion mode.Moreover,they point out that the existence of ridge direction will affect runoff greatly, so that current flows along ridge direction instead of surface slope,and then the actual slope gradient of current flowing along the ground reduces obviously;current flows slowly,and dispersion action of soil by current and sediment carrying capacity of current also decrease evidently,thereby conserving soil and water.

With the increasingly close combination of soil erosion models and computer, it is more and more common to use grid to extract terrain factors. In 2006, Jenness et al. proposed the concept of directional slope gradient[20],and the slope gradient in a specific direction can be calculated according to the difference of elevation between grids and horizontal distance in the direction. Therefore, it is possible to calculate slope gradient in ridge direction according with field actual situation, but only eight directional slope gradients adjacent to the central point can be calculated, which is inconsistent with actual ridge direction in the field and has limited its application.In 2007, Zhang Tianyu[21]analyzed the relationship among surface slope gradient, slope gradient in ridge direction and the angle between ridge direction and contour and put forward their conversion formula, and slope gradient in ridge direction can be calculated based on surface slope gradient and the angle between ridge direction and contour. In comparison to the algorithm of directional slope gradient,slope gradient in all directions can be calculated by using the latter method,so the method is simple and practical.In 2009, Liu Xuejun et al.[22]analyzed the relation among surface slope gradient, slope gradient in ridge direction and the angle between ridge direction and surface slope direction and obtained the similar conversion formula.The angle between ridge direction and contour as well as the angle between ridge direction and surface slope direction are complementary angles, so the two formulas are consistent. The scholars above have proposed various methods to calculate directional slope gradient or slope gradient in ridge direction, but these methods have not been applied in a large region.According to the American Universal Soil Loss Equation (USLE)[23], China’s Soil Loss Equation (CSLE)[24]and the method of calculating slope gradient in ridge direction[21],based on the investigation data of soil and water loss in Binxian County of Heilongjiang Province in 2007, the distribution of ground slope gradient, ridge direction and slope gradient in ridge direction in different terrain conditions and soil and water conservation benefits in the black soil area of northeast China were analyzed.

General Situation of the Study Area and Methods

General situation of the study area

Binxian County(126°55′41″-128°19′17″ E, 45°30′37″-46°01′20″ N) is located in the middle of Heilongjiang Province, the east of Songnen Plain,and east of the typical black soil area,and it is wide from the east to the west but narrow from the south to the north.It has a typical cold temperate continental monsoon climate, with characteristics of long and cold winter, hot and rainy summer, and short and changeable autumn. Its annual average temperature is 3.9 ℃,and ≥10 ℃accumulated temperature is 2 826 ℃.Generally speaking,soil is frozen from late October to next March or April,and the depth of frozen soil varies from 1.10 to 4.93 m.

In Binxian County, there are mountains in the east and south, hills in the middle and the north,and plains in other regions. The primary vegetation is mixed coniferous broad leaved forest mainly composted of Pinus koraiensis,but there is natural secondary forest mainly composted of Larix gmelinii and Pinus sylvestnis now. Except there is a big forest region in the south and eastern mountainous areas,there is a large area of farmland in most central and northern areas where there is a small area of scattered secondary forest.Dark brown earth, black soil, meadow soil and albic soil are common here. Among them, dark brown earth is mainly distributed in southern mountainous areas, and its area accounts of 38.0%of total area of the county; black soil is mainly distributed in hilly areas, accounting for 28.5%;meadow soil is mainly distributed in the alluvial plain, accounting for 25.4%; there is scattered albic soil at the junction of mountainous and hilly areas,accounting for 4.7%.

Extraction of field investigation units and making of investigation map

According to Fig.1, 168 field cells were extracted by stratified sampling method[25-26],and concrete steps,points for attention and making of investigation map are shown in the study of Zhao Yuming et al[27].

Field investigation and data processing

Detailed descriptions of field investigation and data processing are shown in the study of Zhao Yuming et al[27].Among them,land was divided into various land use types according to the National Land Classification issued by the Ministry of Land and Resources on April 21,2001.Plot division was conducted based on the principle that plots in the same group have thesame use, coverage and canopy density,soil and water conservation measures and are continuous in space;cultivated land plots have the same crop,coverage, soil and water conservation measures and ridge direction and are continuous in space.Land use borders were drawn based on the boundary point information detected by GPS.Data of ridge direction are course data of GPS (0° is due north; 90° is due east; 180° is due south; 270° is due west; 360° is also due north). Slope gradient factor was calculated according to the formulas proposed by Mc-Cool et al.[28]and Liu Yuanbao et al[29].Slope gradient in ridge direction and ridge tillage factor P was calculated based on the formula used in the study of Zhao Yuming et al[27].

Results of field investigation

According to relative height difference, the investigation units were grouped into plain (＜200 m), hilly(200-500 m) and mountainous areas(＞500 m).By using the software of Arcmap and SPSS, landscapes of 167 investigation units(unit 59 is located in the Songhua River and was not surveyed)and their areas were analyzed.According to Table 1,about 90%of the investigation units are distributed in plain and hilly areas. Average area of investigation units in mountainous areas is the largest, followed by plain areas, while it was the smallest in hilly areas. The maximum and minimum area of investigation units in mountainous areas are also the largest,and there is no obvious difference between plain and hilly areas. There is a big height difference in mountainous areas,and a small basin is large in area;in plain areas, investigation units were extracted from artificially divided regions with an area of 1 km2, so that its average area is middle; in hilly areas,most investigation units were extracted from various basins, while few investigation units were extracted from artificially divided regions with an area of 1 km2, so its average area is the smallest, and the maximum and minimum are close to that of plain areas.

余蔭山房的布局整體上基本遵循傳統(tǒng)的建筑語法，建筑物與景觀按十字交叉軸線分布。余蔭山房的軸線系統(tǒng)由一根水平的軸線作為主軸線，貫穿兩根垂直軸線構(gòu)成。在水平軸線上，自左到右布置了童子拜觀音山、方形水池、浣紅跨綠橋、玲瓏水榭、獅山。垂直軸線上分別布置了深柳堂與方形水池和臨池別館、玲瓏水榭與兩樹池（圖2）。除了遵循傳統(tǒng)的建筑語法，余蔭山房的布局還運(yùn)用了對比的修辭手法。西區(qū)空間是接待空間，建筑物占園林空間的比重大，布局方正整齊，空間開敞明亮。東區(qū)是休閑空間，建筑物占園林空間的比重小，建筑物與配景的布置靈活自由，空間幽深，層次豐富。西區(qū)與東區(qū)在建筑比重和空間品質(zhì)方面形成了鮮明的對比。

Table 1 Basic characteristics of field investigation units in different terrains

Results and Analysis

Distribution of ground slope gradient

Ground slope gradient, namely down-slope gradient,refers to the gradient of flow direction under natural conditions and the maximum of all surface slope gradients. Cultivated land is dominant in the study area, so studying the distribution of ground slope gradient of cultivated land can help to comprehend the situation of land use in the study area. As shown in Table 2,with the increase of relative height difference from plain to mountainous areas, the average, maximum and standard deviation of ground slope gradient of cultivated land rise obviously, and they are much smaller in plain areas compared with that of hilly and mountainous areas;the minimums are 0° in the three kinds of terrains.

As shown in Fig.2, in plain areas,most ground slope gradients of cultivated land are below 5° , especially below 1°; few ground slope gradients of cultivated land vary from 5° to 10 °;very few ground slope gradients are above 10°. In hilly and mountainous areas, most ground slope gradients are less than 15°, and their ground slope gradients are distributed similarly.That is, the proportion of ground slope gradient below 1°is slightly high;with the increase of ground slope gra-dients from 1 to 5°, the proportions of the ground slope gradients increase;the proportions of ground slope gradients above 5° decrease with the increase of the ground slope gradients.However, the proportions of ground slope gradients smaller than 10° in hilly areas are higher than that of mountainous areas; the proportions of ground slope gradients from 10 to 15°in hilly areas are lower than that of mountainous areas; the proportion of ground slope gradient higher than 15°is only about 5%,far lower than that of mountainous areas.

According to Table 2 and Fig.2,the average ground slope gradient of cultivated land is the smallest in plain areas,accounting for 31%and 23%of that of hilly and mountainous areas;the average ground slope gradient of cultivated land in hilly areas accounts for 75% of that of mountainous areas;it is the largest in mountainous areas.With the increase of relative height difference, the distribution range of ground slope gradient of cultivated land is more and more big, that is, the proportion of ground slope gradient below 5°reduces, while the proportion of ground slope gradient above 5° rises.It is clearly seen that more steep slopes are utilized. Most ground slope gradients of cultivated land are smaller than 5° in plain areas; ground slope gradients in hilly and mountainous areas improve greatly,and most of them are below 15°.Moreover,ground slope gradients in mountainous areas are distributed evenly, and ground slope gradient above 15° accounts for a considerable proportion.

Table 2 Basic characteristics of ground slope gradient of cultivated land in different terrains°

Distribution of the angle between ridge direction and contour

When the angle between ridge direction and contour is about 0°, ridge direction and contour coincide, that is,it is contour cultivation; as the angle between ridge direction and contour is about 90°, ridge direction is perpendicular to contour, that is, it is downslope cultivation; when the angle between ridge direction and contour is 0-90°, there is an obvious acute angle between ridge direction and contour,that is,it is slope tillage.

As shown in Fig.3, the angle between ridge direction and contour α is divided into seven grades, including-1°≤α＜0° (as ground slope gradient is 0°, α is -1°), 0°≤α＜15°, 15°≤α＜30°, 30°≤α＜45°, 45°≤α＜60°, 60°≤α＜75°and 75°≤α≤90°.Proportion of the angle between ridge direction and contour -1° (ground slope gradient is 0°) is 30.6% in plain areas, 9.8% in hilly areas and 3.8% in mountainous areas, namely decreasing with the increase of relative elevation. In the three kinds of slope cropland, each proportion of the angle between ridge direction and contour rises with the increase of the angle,but the increasing trend is slow in plain areas, and the trends are similar in hilly and mountainous areas. That is, the angle between ridge direction and contour below 45°is distributed evenly,and each proportion in hilly areas is slightly lower than that of mountainous areas; each proportion of the angle between ridge direction and contour above 45° improves obviously, especially in hilly areas. In a word, proportion of the angle between ridge direction and contour-1° decreases with the increase of relative height difference from plain to mountainous areas. In the three kinds of slope cropland, ridge tillage is optional, and the trend towards downslope tillage is not obvious in plain areas; in hilly and mountainous areas,the angle between ridge direction and contour 0- 45° is distributed evenly,and each proportion of the angle between ridge direction and contour above 45° rises gradually with the increase of the angle, that is, the trend towards down-slope tillage is obvious.

Distribution of slope gradient in ridge direction

In comparison with ground slope gradient, slope gradient in ridge direction can reflect actual situations of slope and hydrology during farm cropping more accurately, and research into slope gradient in ridge direction is conductive to the accurate estimation of soil and water loss of slope cropland. According to Table 3,with the increase of relative height difference, the average, maximum and standard deviation of slope gradient in ridge direction of cultivated land rise obviously, and they are much smaller in plain areas compared with that of hilly and mountainous areas, which is similar to the change of ground slope gradient; the minimums are 0° in the three kinds of areas.

As shown in Fig.4,the proportions of slope gradients in ridge direction decrease with the increase of slope gradients in ridge direction in plain,hilly and mountainous areas. However,in plain areas,most slope gradients in ridge direction of cultivated land arebelow 5°,especially below 1°;6.2%of slope gradients in ridge direction vary from 5 to 10 °; very few slope gradients in ridge direction are above 10°.

In hilly and mountainous areas,most slope gradients in ridge direction are below 15°, and the proportions of slope gradients in ridge direction reduced greatly with the increase of slope gradients in ridge direction;slope gradient in ridge direction above 15° can be ignored in hilly areas, but its proportion reaches 8.4% in mountainous areas.

As shown in Table 3 and Fig.4,the average slope gradient in ridge direction of cultivated land is the smallest in plain areas, accounting for 29%and 21%of that of hilly and mountainous areas; the average slope gradient in ridge direction in hilly areas accounts for 70% of that of mountainous areas; it is the largest in mountainous areas. With the increase of relative height difference, the distribution range of slope gradient in ridge direction becomes increasingly large, that is, the proportion of slope gradient in ridge direction above 5° decreases gradually,while the proportion of slope gradient in ridge direction below 5°improves; most slope gradients in ridge direction of cultivated land are smaller than 5° in plain areas. In hilly and mountainous areas, the proportions of slope gradients in ridge direction decrease obviously with the increase of slope gradients in ridge direction, and the decrease is very distinct in mountainous areas where slope gradient in ridge direction above 15° accounts for a considerable proportion.

With the increase of relative height difference from plain to mountainous areas, the proportion of slope gradient in ridge direction below 5°drops,and it is far higher in plain areas than that of hilly and mountainous areas;the proportion of slope gradient in ridge direction 5-10° increases; the proportion of slope gradient in ridge direction above 10° rises obviously,and it is far lower in plain areas than that of hilly and mountainous areas.

In plain, hilly and mountainous areas, the average slope gradients in ridge direction of cultivated land are much smaller than the average ground slope gradients in the same terrain condition. In plain areas, the average slope gradient in ridge direction accounts for about 70% of the average ground slope gradient; in comparison with ground slope gradient, the proportion of slope gradient in ridge direction below 5°increases by 4.9%,while the proportions of slope gradients in ridge direction 5-10° and above 10°decrease by 3.8% and 1.1% respectively.In hilly areas, the average slope gradient in ridge direction accounts for about 73% of the average ground slope gradient; in comparison with ground slope gradient, the proportion of slope gradient in ridge direction below 5° rises by 15.7%, while the proportions of slope gradients in ridge direction 5-10°, 10-15° and above 15°reduce by 7.8%, 5.1% and 3.0% respectively. In mountainous areas, the average slope gradient in ridge direction accounts for about 78%of the average ground slope gradient; in comparison with ground slope gradient,the proportion of slope gradient in ridge direction below 5°improves by 17.4%,while the proportions of slope gradients in ridge direction 5-10°, 10-15°and above 15° decline by 8.9%, 5.8%and 2.7% respectively. In a word, with the increase of relative height difference, the ratio of average slope gradient in ridge direction to average ground slope gradient rises from 70%to 78% ; in comparison with ground slope gradient,in the same terrain condition, the proportion of slope gradient in ridge direction below 5° increases distinctly, while the proportion of slope gradient in ridge direction above 5°decreases greatly. With the increase of relative height difference from plain to mountainous areas, the increase in the proportion of slope gradient in ridge direction below 5° rises from 4.9% to 17.4%; the decrease in the proportion of slope gradient in ridge direction above 5° is increasingly large, that is, the decrease in the proportion of slope gradient in ridge direction 5 -10° improves from 3.8% to 8.9%,and the decrease in the proportion of slope gradient in ridge direction above 10° increases from 1.1% to 8.5%.

Table 3 Basic characteristics of slope gradient in ridge direction of cultivated land in different terrains °

Distribution of relative soil and water conservation benefits of slope gradient in ridge direction

Relative soil and water conservation benefit factor PLof slope gradient in ridge direction is the ratio of slope gradient in ridge direction to ground slope gradient, varying from 0 to 1.0.The smaller the PL, the better the soil and water conservation benefits of slope gradient in ridge direction; thelarger the PL, the worse the soil and water conservation benefits of slope gradient in ridge direction; when PLis 1.0, there are no soil and water conservation benefits of slope gradient in ridge direction. According to PL, the soil and water conservation benefits of slope gradient in ridge direction are grouped into five grades, including very good (0-0.2), good (0.2-0.4),moderate (0.4-0.6), bad (0.6-0.8),and very bad (0.8-1.0).When ground slope gradient is 0°,PLis 1.0, so there are no soil and water conservation benefits.

According to Table 4,the average PL of slope cultivated land is 0.55 in plain areas, 0.69 in hilly areas and 0.67 in mountainous areas. In other words, relative soil and water conservation benefits of slope gradient in ridge direction are moderate in plain areas but bad in hilly and mountainous areas.From plain to hilly areas,PLrises,so relative soil and water conservation benefits of slope gradient in ridge direction are increasingly bad;from hilly to mountainous areas, PLreduces slightly, so relative soil and water conservation benefits of slope gradient in ridge direction increase slightly.

According to Fig.5, in plain areas,when PLis smaller than 0.8, the proportions of PL0-0.2,0.4-0.6 and 0.6-0.8 are 5.2%, 13.9% and 63.4% respectively;the proportion of PL＞0.8 reduces to 7.4%. In hilly and mountainous areas, the proportion of PLincreases slowly with the increase of PLfrom 0 to 0.6 and then rapidly with increase of PLfrom 0.6 to 1.0. In short,except that the proportion of PL＞0.8 reduces fast in plain areas, the proportion of PLincreases gradually with the increase of PLin different terrains.In plain areas,the proportion of PL0.6-0.8 is 63.4%; in hilly and mountainous areas, the proportions of PL0.6-1.0 are 73.7%and 66.1%respectively.

Table 4 Basic characteristics of PL of slope cultivated land in different terrains

Conclusions and Discussion

(1)In plain,hilly and mountainous areas, most ground slope gradients of cultivated land are below 5°,below 15°and below 15° respectively, and the average ground slope gradients are 1.92° , 6.20° and 8.27° respectively.With the increase of relative height difference,the proportion of ground slope gradient below 5° reduces, while the proportion of ground slope gradient above 5° rises. It is clearly seen that more steep slopes are utilized.

(2) Proportion of the angle between ridge direction and contour -1°(ground slope gradient is 0°) is 30.6%in plain areas, 9.8% in hilly areas and 3.8% in mountainous areas, namely decreasing with the increase of relative elevation. In the three kinds of slope cropland (ground slope gradient is above 0°), ridge tillage is optional,and the trend towards down-slope tillage is not obvious in plain areas; in hilly and mountainous areas,the angle between ridge direction and contour 0-45° is distributed evenly, and each proportion of the angle between ridge direction and contour above 45° rises gradually with the increase of the angle, that is, the trend towards downslope tillage is obvious.

(3)In plain,hilly and mountainous areas, most slope gradients in ridge direction of cultivated land are below 5°, below 10° and below 15° respectively; the average slope gradients in ridge direction are 1.33°, 4.52° and 6.45° respectively, which account for about 70%,73%and 78%of the average ground slope gradients in the same terrain condition. With the increase of relative height difference,the distribution range of slope gradient in ridge direction becomes increasingly large,

(4)In the present ridge tillage condition, the proportion of PL0.6-0.8 is 63.4% in plain areas; in hilly and mountainous areas, the proportions of PL0.6-1.0 are 73.7% and 66.1% respectively. Except that the proportion of PL 0.8-1.0 reduces fast in plain areas, the proportion of PLincreases gradually with the increase of PLin different terrains. In plain, hilly and mountainous areas,the relative quantities of soil erosion in the present ridge tillage condition account for about 55%, 69% and 67% respectively of that in down-slope ridge tillage conditions, so the present ridge tillage has obviously relative soil and water conservation benefits.

In addition, slope ridge tillage is common in the black soil area of north China, and the reasons are shown as follows. Firstly, terrain is broken in the black soil area of north China, especially in hilly areas, so it is difficult to conduct contour tillage. Secondly, a plot is big in the black soil area of north China,and a piece of farmland is often composed of a whole slope, along which many secondary slopes form,so contour ridge tillage will change int

o down-slope or slope ridge tillage along the secondary slopes. Thirdly, when ground slope gradient is large, a huge machine will lean during the process of contour tillage but will be stable during the process of down-slope tillage, so farmers tend to conduct down-slope tillage instead of contour tillage.

According to the phenomena above, some suggestions are put forward. Firstly, it is suggested to return the grain plots to forestry in regions with a gradient of above 15°, because a steep slope with a gradient of above 15° is not suitable for tillage; the proportion of slope gradient above 15° is very small,only about 6.2%in hilly areas and 11.1% in mountainous areas,and their area accounts for about 3.9%of total area of surveyed regions; returning the grain plots to forestry has very small effects on grain yield. Secondly, if a piece of farmland is composed of a whole slope, the farmland should be divided into many plots along the ditch valley, so that farmers can conduct contour tillage. Thirdly, in greatly undulating regions, some projects should be implemented to make slopes straighter.

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