*3.3. Water Productivity*

The amount of irrigation water needed to obtain a given yield (kg m−3) is termed water productivity. The results obtained (Figure 3) clearly demonstrated that, for a given nitrogen treatment, water productivity decreases with increased irrigation dose. These results are in agreement with our previous study on potato growth using low discharge drip irrigation [14].

**Figure 3.** Water productivity (kg m<sup>−</sup>3) as a function of irrigation and fertigation regime. Values with the same letter are not significantly different (*p* < 0.05).

However, for a given irrigation dose, significant differences in water productivity could only be observed between the F0% treatment relative to the F50% and F100% treatments. The latter did not exhibit a significant difference, implying that for the nitrogen treatments employed in this study the limiting factor of water productivity in potato growth is the dose of irrigation water.

The comparison between the F0% treatment relative to the F50% and F100% treatments implied that both nitrogen and water doses should be considered. Therefore, further reduction in the nitrogen dose should be examined to evaluate the minimum nitrogen treatment from which water productivity is reduced. It was interesting to see that the total tuber yield obtained for the W80%F0% treatment was not significantly different from the one obtained for the W40%F50% and W60%F50% treatments. However, the other F0% treatments exhibited a significantly lower yield than the F50% and F100% treatments, suggesting that at low irrigation doses, nitrogen availability is the limiting factor for tuber yield. Concerning the W100%F0% treatment, we can assume that the additional water caused additional leaching of nutrients such as nitrogen.

If water and fertilizer costs are of a big concern for the farmer, one may apply the W40%F50% regime. However, in order to compensate for a relatively decreased yield, it is necessary to increase the field size in order to produce profitable amounts of potatoes.
