|Handbook for Agrohydrology (NRI)|
|Chapter 6: Catchment characteristics|
Field orientation is particularly important with regard to water conservation measures that may be attempted on agricultural land. Fields are often defined according to convenience, exploiting useful land marks such as the position of roads and access, rivers and natural features. They are rarely oriented with runoff losses and methods of runoff prevention in mind. Figure 6.11 shows a typical semi-arid agricultural landscape.
A study of the photograph and the features of drainage shows that most fields are oriented so that one corner is at the highest topographic elevation. Few of the boundaries are parallel or at right angles to the overall land slope and the natural drainage. In practical terms, this means that when a farmer ploughs, he or she will always plough such that ridges and furrows provide channels that encourage runoff. To plough along the contour would necessitate a start in the highest or lowest corner, and ploughing for very short distances. The length of ploughing would gradually increase until the full diagonal width of the field was attained, then the distances would decrease until the farmer eventually reached the opposite corner from where he or she had started. This would be a very difficult and inefficient exercise from the viewpoint of ploughing, but cultivation would be on the contour, disregarding local variations, and would inhibit the natural flow direction of runoff. If contour bunding were to be practiced, similar difficulties would be encountered.
The boundaries set for fields, in areas of agricultural activity where the effects of urbanisation are small, are often those of roads and tracks. The directions of these roads and tracks are not often exactly along the contour. They remove natural vegetation, cross natural drainage systems, redirect runoff and concentrate it into ditches, under culverts and bridges. This leads to the disruption of the natural drainage, the concentration of flow and in many cases, serious problems of soil erosion.
The problems of field orientation are complex. They involve land ownership, the freedom of access and many other social issues, as well as a consideration of the physical environment and the behaviour of drainage. The field studies of most projects will be sited upon land that is already allocated and used for farming, so few opportunities for the implementation of new allocations will exist. However, the influence of field orientation is an important factor to note when field sites are being selected and where the opportunity exists, serious consideration should be given to the siting of new fields with a favourable aspect to natural drainage. The problems of contour cultivation and the effects of local microtopography on such practices are discussed in more detail in chapter 7, Water Harvesting.