Tomato (Lycopersicon esculentum) is the second most important vegetable crop next to potato. Present world production is about 100 million tons fresh fruit from 3.7 million ha.(FAOSTAT, 2001).

Tomato is a rapidly growing crop with a growing period of 90 to 150 days. It is a daylength neutral plant. Optimum mean daily temperature for growth is 18 to 25ºC with night temperatures between 10 and 20ºC. Larger differences between day and night temperatures, however, adversely affect yield. The crop is very sensitive to frost. Temperatures above 25ºC, when accompanied by high humidity and strong wind, result in reduced yield. Night temperatures above 2OºC accompanied by high humidity and low sunshine lead to excessive vegetative growth and poor fruit production. High humidity leads to a greater incidence of pests and diseases and fruit rotting. Dry climates are therefore preferred for tomato production.

Tomato can be grown on a wide range of soils but a well-drained, light loam soil with pH of 5 to 7 is preferred. Waterlogging increases the incidence of diseases such as bacterial wilt. The fertilizer requirements amount, for high producing varieties, to 100 to 150 kg/ha N, 65 to 110 kg/ha P and 160 to 240 kg/ha K.

The seed is generally sown in nursery plots and emergence is within 10 days. Seedlings are transplanted in the field after 25 to 35 days. In the nursery the row distance is about 10 cm. In the field spacing ranges from 0.3/0.6 x 0.6/1 m with a population of about 40,000 plants per ha. The crop should be grown in a rotation with crops such as maize, cabbage, cowpea, to reduce pests and disease infestations.

The crop is moderately sensitive to soil salinity. Yield decrease at various ECe values is: 0% at ECe 2.5 mmhos/cm, 10% at 3.5, 25% at 5.0, 50% at 7.6 and 100'/. at ECe 12.5 mmhos/cm. The most sensitive period to salinity is during germination and early plant development, and necessary leaching of salts is therefore frequently practised during pre-irrigation or by over-watering during the initial irrigation application.

The graph below depicts the crop stages of tomato, and the table summarises the main crop coefficients used for water management.

Total water requirements (ETm) after transplanting, of a tomato crop grown in the field for 90 to 120 days, are 400 to 600 mm, depending on the climate. Water requirements related to reference evapotranspiration (ETo) in mm/period are given by the crop factor (Kc) for different crop development stages, or: during the initial stage 0. 4-0. 5 (10 to 15 days), the development stage 0. 7-0.8 (20 to 30 days), the mid- season stage 1.05-1.25 (30 to 40 days), the late-season stage 0.8-0.9 (30 to 40 days) and at harvest 0.6-0.65.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the total growing period are shown in the figure below.

The relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growing periods are shown in the figure below.

The plant produces flowers from bottom to top during the active development of the stem. Fruits can be harvested while the plant is still flowering at the top. Some-times three flowering periods related to three harvests can be distinguished. However, for mechanical harvesting where the fruits are used for tomato paste, only one picking is made. Water supply needs to be adjusted according to the use of the product, c. g. for salad or paste.

Highest yields of salad tomatoes are obtained by frequent, light irrigation. Where mechanical harvesting is used, heavy, infrequent irrigation is more appropriate with the last irrigation applied long before harvest.

Following table presents the growth periods of tomato at the first harvest

 For subsequent harvest periods, 2, 3 and 4 will overlap and an additional 20 to 30 days are required for each harvest.

The relationship between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit (1 - ETa/ETm) is given in the following figure. The crop is most sensitive to water deficit during and immediately after transplanting and during, flowering (2) and yield formation (3). Water deficit during the flowering period (2) causes flower drop. Moderate water deficit during the vegetative period (1) enhances root growth.

For high yield and good quality, the crop needs a controlled supply of water throughout the growing period. Whereas under water limiting conditions some water savings may be made during the vegetative (1) and ripening (4) periods, water supply should preferably be directed toward maximizing production per ha rattler than extending the cultivated area under limited water supply.

The crop has a fairly deep root system and in deep soils roots penetrate up to some 1. 5 m. The maximum rooting depth is reached about 60 days after transplanting. Over 80 percent of the total water uptake occurs in the first 0.5 to 0.7 m and 100 per-cent of the water uptake of a full grown crop occurs from the first 0.7 to 1.5 m (D = 0.7 - 1.5 m). Under conditions when maximum evapotranspiration (ETm) is 5 to 6 mm/ day water uptake to meet full crop water requirements is affected when more than 40 percent of the total available soil water has been depleted (p = 0.4).

The crop performance is sensitive to the irrigation practices. In general a prolonged severe water deficit limits growth and reduces yields which cannot be corrected by heavy watering later on. Highest demand for water is during flowering. however, withholding irrigation during this period is sometimes recommended to force less mature plants into flowering in order to obtain uniform flowering and ripening. Care should be exercised in this to avoid damage to the mature plants.

Excessive watering during the flowering period (2) has been shown to increase flower drop and reduce fruit set. Also this may cause excessive vegetative growth and a delay in ripening. Water supply during and after fruit set must be limited to a rate which will prevent stimulation of new growth at the expense of fruit development. Heavy, irregular irrigations or dry periods alternating with wet periods should be avoided. For production of salad tomato with more than one harvest, the crop flourishes best under light, frequent irrigation, well-distributed over the growing period with the soil depletion level during the different growth periods remaining below 40 percent (p < 0.4). This promotes optimum growth during the total growing period and results in high yield of good quality. With one harvest uniform ripening is required and the depletion level during this period may increase to 60 to 70 percent.

When water supply is limited, application for a salad crop can be concentrated during periods of transplanting, flowering (2) and yield formation (3). For a crop grown for paste production, a more extensive irrigation may be applied with last heavy irrigation applied prior to flowering.

Surface irrigation by furrow is commonly practised. Under sprinkler irrigation the occurrence of fungal diseases and possibly bacterial canker may become a major problem. Further, under sprinkler, fruit set may be reduced with an increase in fruit rotting. In the case of poor quality water, leaf burn will occur with sprinkler irriga-tion; this may be reduced by sprinkling at night and shifting of sprinkler lines with the direction of the prevailing wind. Due to the crops specific demands for a high soil water content achieved without leaf wetting, trickle or drip irrigation has been successfully applied.

Frequent light irrigation improve the size, shape, juiciness and colour of the fruit, but total solids (dry matter content) and acid content will be reduced. However, the decrease in solids will lower the fruit quality for processing. In selecting the irrigation practices consideration must therefore be given to teh type of end product required. Prolonged water deficits leads to fruit cracking. Where fruit rot is a problem, frequent sprinkler irrigation should be avoided during the period of yield formation.

A good commercial yield under irrigation is 45 to 65 tons/ha fresh fruit, of which 80 to 90 percent is moisture, depending on the use of the product. the water utilization efficiency for harvested yield (Ey) for fresh tomatoes is 10 to 12 kg/m³