Text 6. Field measurements

6.1. The evaluation of surface irrigation at the field level is an important aspect of both management and design. Field measurements are necessary to characterize the irrigation system in terms of its most important parameters, to identify problems in its function, and to develop alternative means for improving the system.

System characterization necessitates a series of basic field measurements before, during, and after the irrigation. The objectives of the evaluation will dictate whether the field measurements are comprehensive or are simplified for special purposes. In some cases, there are alternative methodologies and equipment for accomplishing the same ends. The selection provided herein is based on a limited selection found to be most useful during numerous field evaluations and, in some measure, the practicality in the international sense.

Five classes of field measurements are presented: (1) field topography and configuration; (2) water requirements; (3) infiltration; (4) flow measurement; and (5) irrigation phases.

6.2All field evaluations should include a relatively simple assessment of the field topography and layout. These measurements are well enough known that only their brief mention is required. There is first of all the field's primary elevations. This information requires that a surveying instrument be used to measure elevations of the principal field boundaries (including dykes if present), the elevation of the water supply inlet (an invert and likely maximum water surface elevation), and the elevations of the surface and subsurface drainage system if possible. These measurements need not be comprehensive nor as formalized as one would expect for a land levelling project.

The field topography and geometry should be measured. This requires placing a simple reference grid on the field, usually by staking, and then surveying the elevations of the field surface at the grid points to establish slope and slope variations. Usually one to three lines of stakes placed 20-30 metres apart or such that 5-10 points are measured along the expected flow line will be sufficient. For example, a border or basin would require at most three stake lines, a furrow system as little as one, depending on the uniformity of the topography. The survey should establish the distance of each grid point from the field inlet as well as the field dimensions (length of the field in the primary direction of water movement as well as field width). There are important items of information that should be available from the survey: (1) the field slope and its uniformity in the direction of flow and normal to it; (2) the slope and area of the field; and (3) a reference system in the field establishing distance and elevation changes.

It is also worthwhile at this stage of the evaluation to record the location and extent of major soil types (this may require sampling and some laboratory analyses). The cropping pattern should be determined and, if a crop is on the field at the time of the evaluation, any obvious differences in growth and vigour should be noted. Similarly, the cultivation practices should be recorded.