Potato (Solanum tuberosum) originates in the Andes from the tropical areas of high altitude. The crop is grown throughout the world but is of particular importance in the temperate climates. Present world production is some 308 million tons fresh tubers from 19 million ha. (FAOSTAT, 2001).

Yields are affected by temperature and optimum mean daily temperatures are 18 to 20°C. In general a night temperature of below 15°C is required for tuber initiation. Optimum soil temperature for normal tuber growth is 15 to 18°C. Tuber growth is sharply inhibited when below 10°C and above 30°C. Potato varieties can be grouped into early (90 to 120 days), medium (120 to 150 days) and late varieties (150 to 180 days). Cool conditions at planting lead to slow emergence which may extend the growing period. Early varieties bred for temperate climates require a daylength of 15 to 17 hours, while the late varieties produce good yields under both long or short day conditions. For tropical climates, varieties which tolerate short days are required for local adaptation.

Potato is grown in a 3 or more year rotation with other crops such as maize, beans and alfalfa, to maintain soil productivity, to check weeds and to reduce crop loss from insect damage and diseases, particularly soil-borne disease. Potato requires a well-drained, well-aerated, porous soil with pH of 5 to 6. Fertilizer requirements are relatively high and for an irrigated crop they are 80 to 120 kg/ha N, 50 to 80 kg/ha P and 125 to 160 kg/ha K. The crop is grown on ridges or on flat soil. For rainfed production in dry conditions, flat planting tends to give higher yields due to soil water conservation. Under irrigation the crop is mainly grown on ridges. The sowing depth is generally 5 to 10 cm, while plant spacing is 0.75 x 0.3 m under irrigation and 1 x 0.5 m under rainfed conditions. Cultivation during the growing period must avoid damage to roots and tubers, and in temperate climates ridges are earthed up to avoid greening of tubers.

The crop is moderately sensitive to soil salinity with yield decrease at different levels of ECe: 0% at 1.7, 10% at 2.5, 25% at 3.8, 50% at 5.9 and 100°/ at ECe 10 mmhos/cm.

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

For high yields, the crop water requirements (ETm) for a 120 to 150 day crop are 500 to 700 mm, depending on climate. The relationship between maximum evapotranspiration (ETm) and reference evapotranspiration (ETo) is given by the crop coefficient (kc) which is: during the initial stage 0.4-0.5 (20 to 30 days), the development stage 0.7-0.8 (30 to 40 days), the mid-.season stage 1.05-1.2 (30 to 60 days), the late-season stage 0.85-0.95 (20 to 35 days), and at maturity 0.7-0.75.

Following figure shows growth periods of potatos (after W. C. Sparks, 1972)

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

This figure shows the relationships between relative yield decrease (1 - Ya/Ym) and relative evapotranspiration deficit for the individual growth periods.

Under evaporative conditions with ETm of 5 to 6 mm, the effect of soil water depletion up to 25 percent on yield is small (p = 0.25). Since potato has a shallow root system, normally 70 percent of the total water uptake occurs from the upper 0.3 m and 100 percent from the upper 0.4 to 0.6 m soil depth (D = 0.4-0.6 m). The uptake pattern will, however, also depend on the soil texture and structure.

Where rainfall is small and irrigation water supply is restricted, irrigation scheduling should be based on avoiding water deficit during the period of stolonization and tuber initiation (1b) and yield formation (3) Supply of water can be restricted during the early vegetative (1a) and ripening (4) periods. Savings can also be attained by allowing higher soil water depletion toward the ripening period (4) so that all available stored water in the root zone is used by the crop. This practice may also hasten maturity. Correct timing of irrigation may save 1 to 3 irrigation applications including the last irrigation prior to harvest.

Most common irrigation methods for potato are furrow and sprinkler. Yield response to frequent irrigation is considerable because the crop has a shallow root system and requires a low soil water depletion. For example, very high yields are obtained with the mechanized sprinkler systems where evapotranspiration losses 'are replenished each or every two days.

Water supply and scheduling are important in terms of quality. Water deficit in the early part of the yield formation period (3) increases the occurrence of spindled tubers, which is more noticeable in cylindrical than in round tubered varieties. Water deficit during this period followed by irrigation may result in tuber cracking or tubers with black hearts. Dry matter content may increase slightly with limited water supply during the ripening period (4). Frequent irrigation does reduce occurrence of tuber malformation.

Good yields under irrigation of a crop of about 120 days in the temperate and subtropical climates are 25 to 35 ton/ha fresh tubers and in tropical climates yields are 15 to 25 ton/ha. The water utilization efficiency for harvested yield (Ey) for tubers containing 70 to 75 percent moisture is 4 to 7 kg/m³.