Contouring

This technique involves plowing, tilling, planting, and cultivating across the slope of the land so that elevations along rows are as near constant as is practical. In conjunction with contouring, protected waterways must be used in all drains or drainageways, where gullies tend to start. Contouring, or contour farming, does best on fields that slope uniformly in one or both directions. Its effectiveness varies with the length and steepness of the slope and with storm intensity. This measure is inadequate as the sole conservation procedure for lengths greater than 18Q m at 1° steepness [11]. The allowable length declines with increasing steepness to 3Q m at 5.5° and 2Q m at 8.5°. Moreover, it provides almost complete protection against erosion from storms of low to moderate intensity, but little or no protection against occasional heavy storms that cause extensive

Table 4.9. Slope-length limits for contouring

Land Slope

Maximum Slop»

(%)

Length (m)

1-2

330

3-5

125

6-8

65

9-12

40

13-16

25

17-20

20

20-25

15

Source: [12].

overtopping and breaking of the contour lines. Contouring is most efficient in reducing erosion on gentle slopes. On steeper slopes, erosion becomes progressively more severe. Sometimes more erosion actually occurs in gullies developed in contoured areas than in the rills between crop rows on the noncontoured land. Ridge height is an additional factor affecting the effectiveness of contouring. Data from field studies indicate that contouring is more effective with high ridges, whereas limited heights may greatly reduce its effectiveness, especially under severe storms. Table 4.9 gives the slope-length data for successful contouring in the case of moderate ridge height and storm intensity [12]. The slope length varies with soil properties (length can be greater on more permeable soils), with type of crop grown (longer for more protective crops, such as small grains), and with the area's rainfall characteristics (longer with less intense storms). Experience with no-till and other reduced-tillage systems that leave the soil surface well protected with crop residue has shown that field lengths far in excess of those given in Table 4.9 can be used safely [4, 6, 8, 10].

Wherever possible, the upper and lower field boundaries should be changed to follow the contour. This reduces the number of short rows in the field and makes it more convenient to farm. Natural drainage ways should be prepared to handle the water that will be guided onto them by tillage marks and crop rows. Any water flowing onto the field from higher land should be intercepted by a diversion built along the upper field boundary to catch the foreign water and carry it away from the field. To meet this requirement, a new waterway has to be installed or an existing one widened and reshaped to accommodate the diverted water. On silty and fine sandy soils, erosion may be greatly reduced by storing water on the surface rather than letting it run off. Limited increases in storage capacity can be obtained by forming ridges at regular intervals according to the slope steepness. The ridge-furrow system is usually stable for gentle slopes of up to about 7° and for soils that have a relatively stable structure [13]. Contour ridging is generally ineffective on its own as a soil conservation measure on slopes steeper than 4.5°. Filed data collected from trial plots in Venezuela showed that contour ridges reduced soil losses under cabbage and cauliflower to 9.9 t/ha compared with 15 t/ha without ridges [14].

Contouring has no direct effect in controlling wind erosion unless ridges formed by tillage increase surface roughness. Thus, where the risk for wind erosion is much greater

Table 4.10. Types and functions of terraces

Type

Function

Diversion terraces Used to intercept overland flow on a hillside and channel it across a slope to a suitable outlet.

Magnum Formed by taking soil from both sides of an embankment.

Nichols Formed by taking soil from the upslope sides of an embankment only. Broad-based Bank and channel occupy width of 15 m.

Narrow-based Bank and channel occupy width of 3—4 m.

Retention terrace Level terraces used where water must be conserved by storage on the hillside.

Bench terraces Alternating series of shelves and risers used to cultivate steep slopes.

than that for water erosion, ridges formed by tillage should be at right angles to the prevailing wind direction without regard to the land slope.

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    What are theuses of contours agricultural engineering?
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