Equipment. Simultaneous observation of the performance of several mole drains in heavy grassland requires recording flowmeters. Simple instruments recording instantaneous rate of flow have been designed and found satisfactory. A recording rain gauge is a necessary adjunct.

Plot size and edge effects. Where the land is laid up in ridge and furrow, even though rudimentary, the ridge forms an effective barrier between furrows. One drained furrow at the edge of a plot is a sufficient, but not always necessary, guard against the neighbouring plot. Where the land is farmed on the flat, a single catch-water furrow forms adequate protection.

The nature of British rainfall. Rainstorms producing drainage are almost invariably of discontinuous type, consisting of well-defined intense showers separated by periods of almost complete quiescence. Thus a storm of 8 hr. duration may consist of as many as six quite distinct showers.