1. INTRODUCTION

1.1    SCOPE AND PURPOSE OF THIS PAPER

This paper in effect replaces the excellent, but now long out of print booklet, entitled "Water Lifting Devices for Irrigation", by Aldert Molenaar, published by FAO as long ago as 1956 [1]. Since that time, little more than one generation ago, the human population has almost doubled. In the same short period, over twice as much petroleum, our main source of energy, has been consumed as in the whole of history prior to 1956. But there has also been a much wider awareness of the constraints which must force changes in technology.

The primary purpose of this paper is to provide a basis for comparing and choosing between all present and (near) future options for lifting irrigation water on small and medium sized . land-holdings (generally in the range 0.25 ha to say 25 ha). Small land-holdings in this size range are most numerous in many of the developing countries, and extension of the use of irrigation in this small farming sector could bring huge benefits in increased food production and improved economic well-being. It is also hoped that this paper will be useful to those seeking techniques for lifting water for purposes other than irrigation.

1.2    THE INCREASING IMPORTANCE OF IRRIGATION

Water has always been a primary human need; probably the first consideration for any community has always been the need for ready access to it. Irrigation water more specifically can offer the following important benefits:

1. increases land area brought under cultivation
2. improves crop yield over rain-fed agriculture three or four-fold
3. allows greater cropping intensity
4. produces improved economic security for the farmer
5. reduced drought risk, which in turn allows:

   -	use of high yield seeds
   -	increased use of fertilizer, pesticides and mechanization
   -	control of timing for delivery to market
   -	control of timing for labour demand

6. allows introduction of more valuable crops

Feeding the rapidly growing human race is an increasingly vital problem. There is no readily identifiable yield-increasing technology other than the improved seed-water-fertilizer approach. It is expected that in the next two decades about three quarters of all the increases in the output of basic staples will have to come from yield increases, even though during the past decade yield increases have only succeeded in supplying half the increase in output [2]. This is because there is less and less fertile but as yet uncultivated land available in the more densely populated regions. Irrigation of crops is a primary route to bringing more land under cultivation and to increasing yields from existing farm land. Irrigation will therefore be increasingly important in the future both to increase the yield from already cultivated land and also to permit the cultivation of what is today marginal or unusable land.

Table 1 indicates the irrigated regions of the world (adapted from [3]), and the principal developing countries where irrigation is currently practised. The majority of the land brought under irrigation since 1972 is mainly in countries where irrigation is already generally practised. Not many countries have significant areas of irrigated land and the two most populous countries, China and India, have about half of the entire world's irrigated land area within their borders. These two large and crowded countries will have to increase their irrigated land still further to improve their food production, while other countries facing similar population pressures on the land will have to do tomorrow what India and China do today.

Table 1 IRRIGATED AREAS OF THE WORLD (1972)

1.3    IRRIGATION AND THE "ENERGY CRISIS"

Water and good land can often be found in juxtaposition, but it is the provision of the necessary power for pumping which is so often the primary constraint. Human muscle power or domestic animals have been used since antiquity, and still are being used in many parts of the world, to lift and distribute water, but as will be explained later, these techniques are often extremely costly in real terms due to the low productivity that is achieved. Therefore, mechanized lift irrigation techniques are becoming increasingly important to meet the enormous predictable future demand.

The area of irrigated land in the world has been estimated to have increased by about 70% in the period 1952 to 1972 [3] and much of this expansion will have been through the increasing use of engine and mains-electrified pumps during that period of decreasing fuel and electricity prices (in real terms). However, since then the price of petroleum, and hence of electricity, has tended to rise, and this has reduced the margin to be gained by farmers from irrigation, since food prices have generally been prevented from rising in line with energy costs. Some governments attempt to mitigate this situation by subsidizing oil and rural electricity for use in agriculture, but many of these governments are the very ones that can least afford such a policy which exacerbates balance of payments deficits by encouraging the use of oil.

Despite present short-term fluctuations in oil prices, conventional oil-based engine-driven power sources and mains electricity are expected to continue to increase in the longer term. There are also major problems associated with maintenance of this kind of machinery.

There is therefore a considerable incentive in most of the poorer developing countries to discourage the use of oil, even though there is an equally strong incentive to encourage the increase of agricultural production, which so often demands pumped irrigation. As a result, there is an increasing need to find methods for energizing irrigation pumps that are independent of imported oil or centralized electricity.

1.4    SMALL-SCALE IRRIGATION AND DEVELOPMENT

Intensive irrigation of small-holdings is likely to become increasingly important and widely used during the next few decades, particularly in the developing countries. This is because the majority of land-holdings, particularly in Asia and Africa are quite small, under 2 ha [4]. Even in South America, where the maximum percentage of farmed land consists of very large land-holdings, the most numerous type of land-holding is under 5 ha.

Studies have shown that small land-holdings are often more productive, in terms of yield per hectare, than larger units. An. Indian farm management study [5], indicated that small family run land-holdings are consistently more productive than larger units, although they are more demanding in terms of labour inputs. A similar survey in Brazil [5], also showed better land utilization on small land-holdings; however this was achieved by applying between 5 and 22 times as much labour per hectare compared with large farms.

Small land-holdings also generally achieve better energy ratios than large ones; i.e. the ratio of energy available in the crop produced, to the energy required to produce it. Energy ratios for tropical subsistence and semi-subsistence agriculture are in the range 10 to 60 (i.e. the food product has 10 to 60 times as much energy calorific value as the energy input to grow it) [6]. Mechanised large scale commercial agriculture, which usually, but not necessarily produces a better financial return, generally has energy ratios in the range from about 4 to less than 1. Therefore, in a situation where commercial fuels will get both scarcer and more expensive, there is more scope for increasing food production through improving the productivity of small labour-intensive land-holdings which have the potential capability to produce most food from a given investment in land and energy.

Small-scale irrigation has been shown to offer positive results in alleviating poverty. For example, the introduction of irrigation can double the labour requirements per hectare of land [5], and raise the incomes thereby not only of the farmers but also of landless labourers. The same reference gives examples from actual surveys of the average percentage increase in income for farmers who practised irrigation compared with those who did not; examples of increases obtained were 469% in Cameroon, 75% in South Korea, 90% in Malaysia, and 98% in Uttar Pradesh, India. In the Malaysian case, the increased income for landless labourers resulting from the introduction of irrigation averaged 127%.

Finally, there is probably more scope for significantly increasing yields in the small farm sector through irrigation than with large farms. For example, the average rice yield in the poorer South and South East Asian countries is typically 2 t/ha, while in Japan, with sophisticated small-scale irrigation and land management, 6 t/ha is commonly achieved [7]. The Asian Development Bank has reported that a doubling of rice production per hectare should be possible in the region within 15 years [7]. Obviously irrigation is not the only factor necessary to achieve such improvements, but it is perhaps one of the primary needs.

1.5    THE CHOICE OF WATER LIFTING TECHNIQUE

There are many different types of human and animal powered water lift, some of which are better than others for different purposes. While the power source or prime-mover so often attracts most interest, the correct selection of water conveyance and field distribution system can often have a greater influence on the effectiveness (technically and economically) of any irrigation system than differences between pumping power sources. In fact the use of a well-optimized and efficient water distribution system is vital when considering certain renewable energy systems where the cost is closely related to the power rating, and therefore a minimum power system needs to be selected.

Before looking for radical new water lifting techniques, there is also much scope for improving traditional and conventional pumping and water distribution methods; for example, petroleum-fuelled engines are commonly badly matched to both the pump and the piping system used for water distribution, which can waste a considerable proportion of fuel used.

The wide range of options for providing power for pumping water include some traditional technologies, such as windmills, and some entirely new technologies owing their origins to very recent developments, such as solar photovoltaic powered pumps. There are also technologies which have been widely and successfully used in just one area but which remain unknown and unused elsewhere with similar physical conditions; an example is the hydro-powered turbine pump, which has been used in tens of thousands solely in China. There are also some interesting new (and some not so new) options which are currently being experimented with, some of which may become available for general use in the near future; for example, steam pumps, Stirling engine pumps, and gasifiers for running internal combustion engines. All of these can produce pumping power from agricultural residues or other biomass resources, perhaps in future even from fuel crops, and may become more important as oil becomes scarcer and more expensive.