Since water availability is a major factor in the production of potatoes, it must also be a major factor in the testing of new varieties. This becomes increasingly important as the potato area under irrigation increases, and the trial system is based on local practice to ensure that results are relevant to current cultural methods. The percentages of NIAB Recommended List trials at present irrigated are: 56% first early, 38% second early and 32% maincrop trials. These closely reflect the national situation in the UK.

The usefulness of drought resistant varieties is obvious in dry situations. There is also a need to know the response of varieties to irrigation. Since irrigation is costly and sometimes limited, it is important to know the likely return of marketable yield from different varieties when planning irrigation schedules. It should be possible to consider irrigation water in the same way as other inputs like fertilizers, fungicides or aphicides.

Hitherto, the assessment of drought resistance has been based on yield results at centres affected by drought, so information depended on the seasons in which the variety was in trials. However, it is difficult to determine differential variety effects of drought and irrigation from the data. Therefore, in order to reduce dependence on standard field trial data, a two-bed system was used to develop a standard test (Flack & Richardson 1984).

The two bed system consisted of a variable depth bed lined with an impervious membrane to give a linear decrease in available soil water, and a variable irrigation bed with a sprinkler line designed to give a linear decrease in applied water.