21. Potato Yields DISCONTINUED
DH Lister and S Subak

Annual average potato yields (t/ha) in England and Wales shown in relation to August rainfall in England and Wales

Specific Indicator
The yield of early potatoes (which are harvested before August) is not closely related to any single weather variable, and potato yields in Scotland are complicated by the fact that seed potato growers deliberately suppress the formation of large tubers. However, maincrop potato yields are clearly adversely affected by summer drought, particularly if they are not irrigated. The specific indicator chosen is therefore the annual average yield (in tonnes per hectare) of non-irrigated maincrop potatoes in England and Wales.
[Source: British Potato Council, Oxford Business Park South, Oxford OX4 2RT]

Relevance
Potatoes are one of the five most valuable crops in the UK, with an average annual producer value in excess of £500 million over the last ten years. In 1997, the UK potato crop covered 166,000 ha of which 130,200 ha was in England and Wales, mostly in the central and eastern lowland regions of England. Traditionally, potatoes have been a staple of the British diet and, with annual consumption at 105 kg/head, it is true to say that the population eats more than its own weight in potatoes each year. Demand is fairly inelastic, so that in drought years, when yields are low, prices tend to rise irrespective of the quality of the crop.
Although an increasing proportion of the crop is irrigated (see Indicator 20), about 60% of the maincrop is still rainfed, and maincrop potatoes (i.e.,those harvested after 31 July) make up 90% of the total.

Sensitivity to climatic and other factors
The potato plant has a high water requirement and a shallow rooting system, making it vulnerable to drought. Yields are markedly reduced after dry growing seasons, with particular problems from dry weather in the later stages of growth. During dry years, yields of non-irrigated potatoes are about 25% lower than in wet years. There is a particularly close relationship between the yield of non-irrigated maincrop potatoes and rainfall in August in England and Wales.
Potato crop yields are also adversely affected by high temperatures. However, some varieties withstand adverse conditions better than others. Thus, slow-maturing varieties, such as Cara, may grow well after peak summer temperatures and soil moisture deficits have passed.

Change over time
Over the eleven-year period shown in the figure, there is no apparent long-term trend. Exceptionally low yields are evident in 1989, 1990 and 1995 when summer rainfall totals were low. Yields were also low in 1991, despite reasonable summer rainfall, because rainfall in August was low. The lowest yields occurred in 1995, when it was both dry and hot in August (the highest temperatures in the present century).
Warmer and drier weather in late summer would be expected to reduce the quantity and quality of the non-irrigated maincrop potato harvest. However, any trend towards poorer yields is likely to be moderated by irrigation, a shift to higher rainfall areas and the use of varieties which mature either early or very late.